Gray, 2001 Reviw

LJournal of Alloys and Compounds 336 (2002) 88–113www.elsevier.com/ locate / jallcom

Review article

Protective coatings on magnesium and its alloys — a critical review*J.E. Gray, B. Luan

Integrated Manufacturing Technologies Institute, National Research Council Canada, 800 Collip Circle, London, Ontario, N6G 4X8 Canada

Received 6 August 2001; accepted 12 October 2001

Abstract

Magnesium and its alloys have excellent physical and mechanical properties for a number of applications. In particular its highstrength:weight ratio makes it an ideal metal for automotive and aerospace applications, where weight reduction is of significant concern.Unfortunately, magnesium and its alloys are highly susceptible to corrosion, particularly in salt-spray conditions. This has limited its usein the automotive and aerospace industries, where exposure to harsh service conditions is unavoidable. The simplest way to avoidcorrosion is to coat the magnesium-based substrate to prevent contact with the environment. This review details the state of the art incoating and surface modification technologies, applied to magnesium based substrates for improved corrosion and wear resistance. Thetopics covered include electrochemical plating, conversion coatings, anodizing, gas-phase deposition processes, laser surface alloying/cladding and organic coatings. Crown copyright 2002 Published by Elsevier Science B.V. All rights reserved.

Keywords: Surfaces and interfaces; Thin films; Metals; Electrochemical reactions; Vapor deposition

1. Introduction Unfortunately, magnesium has a number of undesirableproperties including poor corrosion and wear resistance,

Magnesium is the 8th most abundant element on the poor creep resistance and high chemical reactivity thatearth making up approximately 1.93% by mass of the have hindered its widespread use in many applications.earth’s crust and 0.13% by mass of the oceans [1]. It also One of the main challenges in the use of magnesium,has some advantageous properties that make it an excellent particularly for outdoor applications, is its poor corrosionchoice for a number of applications. Magnesium has a high resistance. Magnesium and its alloys are extremely suscep-strength:weight ratio with a density that is only 2/3 that of tible to galvanic corrosion, which can cause severe pittingaluminum and 1/4 that of iron. Magnesium also has high in the metal resulting in decreased mechanical stability andthermal conductivity, high dimensional stability, good an unattractive appearance. Corrosion can be minimized byelectromagnetic shielding characteristics, high damping the use of high purity alloys that maintain heavy metalcharacteristics, good machinability and is easily recycled impurities such as iron, nickel and copper below a[1]. These properties make it valuable in a number of threshold value. The elimination of bad design, fluxapplications including automobile and computer parts, inclusions, surface contamination, galvanic couples andaerospace components, mobile phones, sporting goods, inadequate or incorrectly applied surface protectionhandheld tools and household equipment. Magnesium has schemes can also significantly decrease the corrosion rateeven been suggested for use as an implant metal due to its of magnesium alloys in service [3].low weight and inherent biocompatibility [2]. Due to One of the most effective ways to prevent corrosion is tolimited fossil fuel stores and environmental problems coat the base material. Coatings can protect a substrate byassociated with fuel emission products, there is a push in providing a barrier between the metal and its environmentthe automotive industry to make cars lighter in order to and/or through the presence of corrosion inhibiting chemi-decrease fuel consumption. The use of magnesium alloys cals in them. In order for a coating to provide adequatecan significantly decrease the weight of automobiles corrosion protection, the coating must be uniform, wellwithout sacrificing structural strength. adhered, pore free and self-healing for applications where

physical damage to the coating may occur. One of theproblems with magnesium is its chemical reactivity. As*Corresponding author.

E-mail address: [email protected] (B. Luan). soon as it comes in contact with air or water an oxide /

0925-8388/02/$ – see front matter Crown copyright 2002 Published by Elsevier Science B.V. All rights reserved.PI I : S0925-8388( 01 )01899-0

J.E. Gray, B. Luan / Journal of Alloys and Compounds 336 (2002) 88 –113 89

hydroxide layer forms on the surface which can have a electroless copper, Ni–P, Ni–Pd–P or Ni–B has beendetrimental effect on coating adhesion and uniformity. shown to increase the thermal conductivity /diffusivity,Thus, the precleaning process plays a critical role in the fatigue strength and lifetime of these alloys which aredevelopment of a good protective coating on magnesium being developed for use as negative hydride electrodesand its alloys. [7,8]. An immersion process that microencapsulates

There are a number of possible coating technologies Mg Ni alloy powders in copper using a solution of copper2

available for magnesium and its alloys, each with their sulfate and nitric acid is extremely promising due to itsown advantages and disadvantages. The purpose of this simplicity, cost effectiveness and elimination of toxicreview is to provide a clear picture of the technologies chemicals [9].currently available.

2.1.1. ChallengesThere are a number of challenges to be met in order to

2. Coating technologies — state of the art develop a useful plating process for magnesium and itsalloys. Magnesium is classified as a difficult to plate metal

There are a number of technologies available for coating due to its high reactivity. This means that in the presencemagnesium and its alloys. These include electrochemical of air magnesium very quickly forms a passive oxide layerplating, conversion coatings, anodizing, hydride coatings, that must be removed prior to metal plating. The rapidorganic coatings and vapor-phase processes. Each of these formation of the oxide layer necessitates an appropriatewill be described in detail in the following sections. In this pretreatment of the surface to introduce a surface layer thatreview the term magnesium is used to refer to magnesium prevents oxidation but which can easily be removed duringand its alloys unless otherwise specified. The chemical the plating process. The extreme reactivity of magnesiumcomposition of some common magnesium alloys is listed metals also means that care must be taken to ensure thatin Appendix A. the metal reduction pathway is the preferred electroless or

electrochemical process, since in many cases loose immer-2.1. Electrochemical plating sion layers, which inhibit good adhesion, form readily on

the surface by displacement [10]. Care must be taken toIt is often desirable to alter the surface properties of a develop non-traditional plating baths since magnesium

workpiece in order to improve its corrosion and wear reacts violently with most acids and dissolves in acidresistance, solderability, electrical conductivity or decora- media.tive appearance. This can be accomplished by coating the Magnesium is also prone to galvanic corrosion becausepart with a metal that has the desired properties necessary most other metals have a more noble electrochemicalfor the specific application. One of the most cost effective potential. Electrolytic contact with another metal can causeand simple techniques for introducing a metallic coating to the formation of local corrosion cells on the surfacea substrate is by electrochemical plating. The plating leading to pitting. Therefore, the metal coating must beprocess can be subdivided into two types: electroplating pore free otherwise the corrosion rate will increase [11]. Aand electroless plating. In both cases a metal salt in minimum coating thickness of 50 mm has been suggestedsolution is reduced to its metallic form on the surface of to ensure pore-free coatings of Cu–Ni–Cr for outdoor usethe workpiece. In electroplating the electrons for reduction [4]. Another challenge in the plating of magnesium is thatare supplied from an external source. In electroless or the quality of the metal coating depends on the alloy beingchemical plating the reducing electrons are supplied by a plated. This requires the development of different pretreat-chemical reducing agent in solution or, in the case of ment processes for different alloys. Alloys are especiallyimmersion plating, the substrate itself. difficult to plate because intermetallic species such as

Plating of magnesium has been shown to be useful in a Mg Al are formed at the grain boundaries, resulting in ax y

number of applications. Cu–Ni–Cr plating has been shown non-uniform surface potential across the substrate, andto have good corrosion resistance in interior and mild therefore further complicating the plating process.exterior environments [4]. However, a plating method has Electroplating poses additional challenges due to annot been developed that can produce coatings to withstand uneven distribution of current density in the plating bath,marine or salt splash conditions, thereby limiting the use of which results in non-uniform plating of complex shapes,magnesium in the automotive, aerospace and marine particularly in holes and recessed areas. The technique alsoindustries. Electroless nickel coatings have proven useful uses a lot of electricity, this can significantly increase thein the computer and electronics industries for corrosion cost of the plating process. The electroless plating processand wear resistance, improved solderability and creation of does not suffer this disadvantage and coats complex shapesstable electronic contacts [5]. Ni–Au coatings on mag- uniformly even inside holes. Another advantage of electro-nesium have been used in a variety of space applications less plating is that second phase particles such as carbides,for improved electrical conductivity and optical reflectance diamonds or PTFE can be co-deposited during the plating[6]. The microencapsulation of Mg Ni alloy powders by process to improve the hardness, abrasive properties orx

90 J.E. Gray, B. Luan / Journal of Alloys and Compounds 336 (2002) 88 –113

lubricity of the final composite coating [12]. The deposi- layers. A uniform coverage is especially difficult totion of alloys with enhanced wear properties and higher achieve on magnesium alloys. The existing processeshardness values is also readily achieved. require many steps and are laborious, time consuming and

Electroless deposition processes have a limited bath life must be precisely controlled to achieve acceptable adhe-and the problem is aggravated in the case of magnesium sion and corrosion resistance. They are also alloy specificplating due to its high reactivity. An example reported is and do not work very well on, for example, AZ91 which isthe process developed by PMD (UK) that is capable of six commonly used in the automotive industry for die castregenerations, each with a 45-min turnover time if a strict magnesium parts.replenishment schedule is followed [13]. The short lifetime The pretreatment requirements vary for different alloysof the baths is a serious limitation both from cost and and different plating baths. There are currently two generalenvironmental perspectives. Research on increasing the processes for treating magnesium prior to plating. Thesebath life and eliminating toxic chemicals is necessary in are zinc immersion and electroless nickel plating from aorder to create ‘green’ plating processes for coating fluoride containing bath [17]. These two general processesmagnesium. are outlined schematically in Fig. 1. Some specific pre-

One criticism of using metal coatings to protect mag- treatment processes will be outlined in the next section.nesium from corrosion is that the presence of heavy metals Although, there are many variations the current processesreduces the recyclability of the metal. However there are a predominantly follow one of the two general schemesnumber of ways that these metals can be stripped from the outlined in Fig. 1. The purpose of each of the individualsurface and these are summarized in Table 1. The pro- pretreatment steps is outlined in Table 2 [10].cesses outlined in Table 1 demonstrate that while metalplating may complicate recycling of the metal it is 2.1.2.1. Zinc immersion [18]. The zinc immersion pre-certainly not prohibitive. treatment process has been criticized for the precise control

that is required to ensure adequate adhesion. In many cases2.1.2. Pretreatment processes non-uniform coverage of the surface is seen with spongy

The most difficult part of plating magnesium is develop- non-adherent zinc deposits on the intermetallic phase ofing an appropriate pretreatment process, once a suitable the base alloys [19]. The copper cyanide strike that mustundercoating is in place many desired metals can be plated. follow has also been criticized for a number of reasonsTo date zinc and nickel have been directly plated onto [19]. The first is that it is an electroplating process, whichmagnesium, and are used as undercoatings for subsequent means that it is more difficult to coat complex shapes.metal deposition. The undercoatings must be non-porous Copper deposits slowly in the low current density areas,since porosity in underlayers promotes porosity in over- which allows attack of the zinc by the plating solution.

Table 1Stripping processes for metal coatings on magnesium

Applicable metal Stripping conditionsaZinc immersion layer Soak at room temperature in a solution of

10–15% hydrofluoric acid (70%)aChromium Apply reverse current in hot alkaline solution

aCopper Hot alkaline polysulfide and cyanide solutionaNickel 15–25% HF12% NaNO at 4–6 V3

aCopper, nickel, tin, cadmium, zinc 1:2:2 mixture of HNO –HF–H O3 2

aGold, silver, copper, nickel Proprietary alkaline chemical strippers containingcyanide at 20–608C. Addition of NaOHprevents dissolution of magnesium

Copper, nickel etc. Plated magnesium is set as the anode and electrolyzedOnly the plating film is dissolved [15] in an aqueous solution of 180–220 ml / l phosphoric

acid140–50 g/ l hydrofluoric acid or 150–220 ml / lphosphoric acid190–110 g/ l ammonium bifluorideAn iron sheet is used as the cathode

Copper Immerse in a mixture of ammonia solutionA protective ammonium magnesium and ammonium persulfatesulfate surface layer protects thebase alloy from dissolution [16]

a Ref. [14].


Fig. 1. Generalised pretreatment processes.

This in turn allows attack on magnesium by the plating patent [20] attempts to improve this process by eliminatingsolution resulting in non-adherent copper depositing by the copper cyanide step from the pretreatment process. Thedisplacement directly on the magnesium surface. The copper cyanide electroplating is replaced by a zinc elec-deposits in these areas are porous and have poor corrosion troplating step followed by copper deposition from aresistance. The second criticism leveled at the copper pyrophosphate bath after the zinc immersion. This patentcyanide plating process is the high cost treatment of waste claims that by creating a uniform zinc film of at least 0.6generated by the use of a cyanide containing bath. A recent mm in thickness, adherent plating films can be obtained on

Table 2Function of individual pretreatment steps in plating processes

Surface cleaning Remove soil, debris, oil or grease

Alkaline cleaning Mg is passive in alkaline media — wet surface, removesoils, greases, etc.

Acid pickle Remove gross surface scale or oxides and replace with apreferred oxide to be removed later. Etching treatments alsoprovide surface pits to act as sites for mechanical interlockingto improve adhesion

Acid activation Remove residual oxide, minimum etching, minimizes the effectof local corrosion cells by creating an equipotentialized surface

Zinc immersion Dissolves oxides, hydroxides-form thin zinc layer to preventreoxidation of Mg

Copper strike Zn is very reactive therefore most metals cannot be plateddirectly on it. Copper acts as a base for subsequent plating

Fluoride Removes oxide and replaces it with a thin layer of MgF2

Activation Fluoride treatment is believed to control the rate of zinc ornickel deposition and thus produce more adherent deposits [24]This step has a similar role to acid activation

Electroless A nickel based alloy is deposited on the surface to act as a basenickel plating for further electroless plating or electroplating


any magnesium alloy using the disclosed process. The zinc shaped and high aspect ratio samples. A slight variation ofelectroplating step can occur simultaneously with the zinc the pretreatment uses a copper cyanide plating bath thatimmersion process or in a separate step. The process is as contains a soluble silicate [26]. The authors claim to obtainfollows good adhesion on magnesium with this process. Zinc

immersion prior to tin plating of magnesium has also beenDegrease → Alkaline Clean → Acid Clean → Activation →explored [27]. A magnesium alloy is treated with a

Zinc Immersion → Zinc Electroplate → Copper Plating conventional zinc immersion pretreatment and then zincplated in an aqueous zinc pyrophosphate bath. Tin is

A number of processes based on the zinc immersion subsequently plated to improve the tribological propertiespretreatment process have been developed. The three main of the plated alloy.processes are the Dow Process, the Norsk-Hydro processand the WCM Canning Process [19,22]. One criticism of 2.1.2.2. Direct electroless nickel plating. Difficulties inall of these processes is that they do not produce good plating the AZ91 alloy have been cited for the zincdeposits on magnesium alloys with an aluminum content immersion pretreatment processes. In an attempt to addressgreater than 6–7% [21]. The general pretreatment se- this issue a pretreatment process in which electrolessquence for each of these is outlined below for comparison nickel is plated directly onto magnesium alloy AZ91 die[19,22] castings was developed by Sakata et al. [19]. The authors

state that uniform, adherent coatings were obtained. InDow process general the pretreatment is as followsDegrease→Cathodic Cleaning→Acid Pickle→

Pretreat → Degrease → Alkaline Etch → Acid ActivationAcid Activation→Zincate→Cu Plate→ Alkaline Activation → Alkaline Electroless NickelNorsk-Hydro processStrike → Acid Electroless Nickel PlatingDegrease→Acid Pickle→Alkaline Treatment→

Zincate→Cu PlateThis process has been criticized [11] for using an acid

WCM processelectroless nickel treatment that can result in corrosion of

Degrease→Acid Pickle→Fluoride Activation→the underlying magnesium if any pores are present in the

Zincate→Cu Plate.nickel strike layer.

A simpler process has been developed by PMD (UK)The Dow process was the first to be developed but has [13,28,29]. The basic sequence of this pretreatment is as

been shown to give uneven zinc distributions as well as followspoor adhesion in many cases. A modified version of the

Pretreat → Alkaline Clean → Acid Pickle → FluorideDow process [8] introduces an alkaline activation follow-Activation → Electroless Nickel Platinging the acid activation step. This results in good adhesion

of Ni–Au films on AZ31 and AZ91 alloys. The authorshave also significantly shortened the pretreatment time, The authors determined that the etching, conditioningwhich is important in a manufacturing setting. The Norsk- and plating conditions had a large effect on the adhesionHydro process has been shown to improve the quality of obtained. An insufficient etch or fluoride conditioningthe zinc coating on AZ61 alloy in terms of adhesion, resulted in poor adhesion. It was also determined that usingcorrosion resistance and decorative appearance. Deposits hydrofluoric acid for conditioning led to a wide platingof Cu–Ni–Cr, on samples pretreated with this process, window while ammonium bifluoride resulted in a muchhave been shown to exceed the standards for outdoor use narrower (pH 5.8–6.0 and temperature 75–778C) window[23]. Dennis et al. [22,24] show that samples treated with for acceptable adhesion. The chromic acid treatment wasboth the Dow and Norsk-Hydro processes give porous zinc found to heavily etch the surface and leave behind a layercoatings and perform poorly in thermal cycling tests. It of reduced chromium. The fluoride conditioning was foundwas found that the WCM process resulted in the most to remove chromium and control the deposition rate byuniform zinc film and was the most successful in terms of passivating the surface. The passivating effect of fluorideadhesion, corrosion and decorative appearance. However, was also exploited in the plating of magnesium alloypreferential dissolution of magnesium rich areas on the MA-8 [30]. In this case the nickel plating bath containedalloys occurred with all three processes, which could limit fluoride to inhibit corrosion of the substrate during plating.the effectiveness of any of these pretreatment methods. The authors report strong adhesion of the nickel film

A similar process has been used as an undercoating for however, the bath life is too short to be industriallysamples to be plated with a series of metals by electroless applicable. The addition of a complexing agent, glycine,and electroplating techniques [25]. The authors claim to was shown to improve the stability of the plating bath.obtain uniform plating films with good corrosion resist- Another proposed process [31] involves treatment of theance, solderability and electrical conductivity on intricately sample with a chemical etching solution containing pyro-


phosphate, nitrate and sulfate, avoiding the use of toxic widely varying surface chemical composition. The pre-chromium ions. The process sequence is as follows treatment process was altered to create a uniform surface

for nickel plating. An AC 5 V electrolytic treatment wasChemical Etching → Fluoride Treatment → Neutralization added during the fluoride activation stage. The final→ Electroless Nickel Plating process is outlined below

Clean → Acid Pickle → HF dip → AC electrolyticThe electroless nickel plating bath does not contain anytreatment → electroless nickel plating from a fluoride bathchloride or sulfate. The plated samples achieved have high

adhesion and corrosion resistance. One obstacle to coating → nickel electroplating → gold electroplatingmagnesium with nickel is that most conventional nickelplating baths are acidic and can attack or corrode the This strategy was successfully applied to the plating ofmagnesium surface. This problem has been addressed by satellite components, which exceeded their predicted lifethe development of an aqueous acidulated nickel bifluoride span of 2 years.electroplating bath that contains a polybasic acid [32]. This Another possible pretreatment process for plating mag-bath has been shown to not corrode magnesium. nesium alloys with silver or gold [37] involves an anodic

treatment of the alloy, followed by coating the surface with2.1.3. Noble metal plating for space applications an electrically-conductive resin film. Subsequent plating

Nickel and gold plating on magnesium have both proven can then be achieved as desired. It is suggested that thisuseful for space applications. Successful gold plating has plating method can be applied to objects such as satellitebeen achieved using the following sequence on mag- embarking devices.nesium–lithium alloys [33] Nickel coatings on magnesium have also been proposed

for use in aerospace applications. Direct electroless nickelDegrease → Alkaline Cleaning → Chromic Acid Pickle →plating on magnesium alloy ZM21 has been shown to

Electroplated Nickel → Electroless Nickel → Gold Strike produce coatings with good mechanical, environmental,→ Electroplated Gold optical and soldering properties [36]. The electroless nickel

deposition was carried out after fluoride pretreatment andThe initial electrodeposited nickel film is porous but acts followed by chromium trioxide passivation. Annealing the

as an activator for uniform electroless nickel plating and samples resulted in precipitation hardening and improvedproved to be a suitable basis for gold plating. Gold plating adhesion.of magnesium alloy AZ31 has also been achieved usingthe zinc immersion pretreatment process followed by 2.1.4. Alternative processeselectroless nickel plating and subsequent gold plating [34]. There are a few plating processes for magnesium that doAn adherent gold deposit with good mechanical, thermal not use either of the two conventional pretreatment pro-and optical properties as well as good environmental cesses. An immersion process whereby magnesium and itsstability was obtained. It was determined that the immer- alloys can be coated with tin has been described [38]. Thission zinc produced a fine, adherent coating for subsequent process produces a tin oxide coating with good corrosionelectroless nickel deposition. An acid gold bath was found resistance and is comprised of the following treatmentto give the best gold coating. The surface morphology of procedurethe coating was a homogeneous-grain structured zinc layer

Degrease → Immerse in chromate solution → Immersion infollowed by a hard porous microcracked electroless nickeldibutyl-tin-dilaurate in ethyl cellusolve → Anneal at 5008Cfilm with a uniform phosphorous distribution. The final

gold layer was crack-free.Gold plating on magnesium alloy RZ5 has been investi- Another invention [39] involves acid activation of the

gated using a variety of techniques [35]. Plating nickel substrate through contact with an aqueous solution con-from non-aqueous solutions, as a base for gold plating, was taining HF, which can be derived from NH HF , NaF or4 2

attempted. However, this is expensive and since all the LiF, and a metal salt of Ni, Fe, Ag, Mn, Pd or Co. Thepretreatment steps are aqueous there is a risk of moisture mineral acids, monocarboxylic acids or oxides of thecontamination of the bath. The plating bath used was 30 aforementioned metals are preferred. The metal salt mustg / l nickel sulfate1200 ml / l dimethylformamide at pH 4, be soluble in HF, be displaced by the substrate and

225–308C and 10 mA/cm . The zinc immersion and direct catalytic to electroless nickel. Current may be applied toelectroless nickel plating showed poor results. In the case increase the deposition rate but is not essential. Electrolessof zinc immersion, a non-uniform zinc coating was ob- nickel plating from an amino borane bath in the presenceserved. With direct electroless nickel plating non-uniform of an organosulfur compound is then undertaken followeddeposition, excessive corrosion and exfoliation of the by heat treatment at 150–3008C to improve the adhesion.deposit occurred. The poor quality of the deposits was The films formed by this treatment are both adherent andattributed to coarse, non-uniform substrate grains with continuous. The authors claim that this process works for


aluminum, magnesium and zinc however, all cited exam- coatings protect the substrate from corrosion by acting asples are for aluminum. an insulating barrier of low solubility between the metal

A method for plating silver on magnesium has been surface and the environment [45] and/or by containingproposed [40]. The process involves electrolytic (pulsed corrosion inhibiting compounds.current) cleaning of the substrate in an alkaline solution There are a number of different types of conversionfollowed by acid etching and electroplating in a silver coatings including chromate, phosphate /permanganate andcontaining bath. The authors claim that an adherent and fluorozirconate treatments. As with all surface treatments,continuous coating can be achieved. cleaning and pretreatment of the sample is crucial to

A non-aqueous process for aluminizing magnesium has obtaining a good conversion coating.also been suggested [41]. The process involves pretreat- One of the main disadvantages of conversion coatings isment by liquid drop impact erosion with a hydraulic jet the toxicity of the treatment solutions. The conventionalspray of finely divided abrasive particles suspended in an conversion coatings are based on chromium compoundsinert, anhydrous aprotic media such as oil. A second that have been shown to be highly toxic carcinogens [44].pretreatment step involves the removal of a thin layer of The development of an environmentally friendly process ismagnesium by an anodic treatment in an aprotic organo– a necessity due to the more stringent environmentalaluminum electrolyte medium. This occurs via the reaction protection laws currently in effect or being proposed. TheMg12R→MgR where R is an ethyl or methyl from the coating of alloys also represents a significant challenge due2

aluminum electrolyte. For magnesium this process must be to their non-uniform surface composition. The conversioncarried out in an inert atmosphere. The process can be coating must therefore be capable of coating all thesummarized as follows elements present in the alloy uniformly [44].

aprotic pretreatment → degrease → toluene rinse → anodic2.2.1. Chromate conversion coatings

treatment → direct immersion in aluminizing bath → Chromate conversion coatings can be used as pretreat-cathodic aluminum electroplating (inert atmosphere) ments prior to a final sealing process or as ‘post-treat-

ments’ after a plating process to improve corrosion resist-ance, paint or adhesive bonding properties or to provide aThe aluminizing bath consists of an electrolyte with thedecorative finish [46].general formula MX AlR9R0R in a suitable solvent. Suit-n

There are a few general rules that should be followedable solvents are aromatic hydrocarbons such as toluene orwhen applying a conversion coating, these include [45]xylene and ethers such as THF, dipropyldibutyl ether or

dioxane.1 1 1 1 1. Substrates with fine grained microstructure respond bestM5Na , K , Rb , Cs or a quaternary ions with N, P,

to chromating.As or Sb as the central atom or a tertiary ion with S, Se or2. Co-deposition of other metals is detrimental to theTe as the central atom.

coating process.2 2 2 2 2 2 22X 5 F , Cl , Br , I , CN , N or 1/2 SO n 5 2 or 33 4 3. Proper cleaning and pretreatment of the surface isnecessary to ensure optimum coatings.

R 5 organyl radical (ethyl or methyl) 4. After chromating the substrate should be properlyrinsed to remove any residual acid or base which could

2 2 2 2 2R9 5 R or H or F or Cl or CN or N3 react with the coating.5. The coating should be air dried at low temperature

R0 5 R9 or selected from same class as R9 (708C) for a maximum of 10 min.

The deposits are 99.99% pure aluminum and offer a The mechanism of formation of chromate conversionhigh degree of corrosion protection and a silver-bright coatings [44,45,47] is believed to be due to the dissolutiondecorative appearance. They also have good conductivity, of the metal surface, with a corresponding reduction ofsuperior ultrasonic weldibility and high reflectivity. water or oxygen to form hydroxyl ions, which causes an

increase in pH at the liquid–metal interface. This in turn2.2. Conversion coatings causes precipitation of a thin complex chromium–metal

gel on the surface that contains both hexavalent andConversion coatings are produced by chemical or elec- trivalent chromium compounds [45,47,48]. Pure chromic

trochemical treatment of a metal surface to produce a acid solutions are not used to form conversion coatingssuperficial layer of substrate metal oxides, chromates, because the deposition rate is too slow [44]. Other anionsphosphates or other compounds that are chemically bonded in solution that can act as a catalyst for deposition areto the surface [42,43]. On magnesium, these coatings are required [46], some of these include acetate, formate,typically used to provide corrosion protection and good sulfate, chloride, fluoride, nitrate, phosphate and sulfamatepaint-base properties to the metal [43,44]. Conversion ions [48]. The pH of the solution is the most important


factor in controlling the formation of chromate films [48]. through an increase in chromate concentration in the bathThe as-deposited gel is very soft but hardens upon drying leads to improved protective properties of the coating. Theto become hydrophobic, less soluble and more abrasion addition of zinc cations to the chromating solution wasresistant [48]. These coatings provide corrosion protection also shown to improve the properties of the chromateby presenting a non-reactive barrier to the environment, coatings.through their self-healing properties and by the inhibiting The structure of chromate conversion coatings formedeffect of Cr(VI) that is slowly leachable in contact with using the commercial processes, MX1, MX3 and MX7 onmoisture [45,47,48]. The hexavalent chromium is reduced AZ91D has been reported [51]. The MX1 process wasduring corrosion to form an insoluble trivalent chromium found to produce a magnesium chromate film while MX7species that terminates the oxidative attack [44]. The was shown to produce magnesium phosphate at theprotection afforded by the coating is proportional to the surface. The MX3 process produced an amorphouscoating thickness [48]. In order to maintain its protective chromium oxide film containing some oxides and fluoridesproperties the coating cannot be subjected to high tempera- of magnesium and aluminum. Corrosion studies showedture (.668C) since this would decrease the coating that conversion films with a significant amount of oxygenthickness and eradicate the self-healing ability of the and chrome, such as MX3, gave the best corrosioncoating [45,47,48]. The coating retains its self-healing protection. Another study [52] investigated the structure ofcharacteristics as long as it remains in its hydrated form. surface films formed by the Dow 7 commercial conversionThe stability of the films at higher temperature can be coating process on die cast AZ91D and pure magnesium.improved by sealing or painting on top of the conversion On AZ91D the structure of the film is granular while onlayer [45,47,48]. pure magnesium a porous cell structure was observed

Chromate conversion coatings on magnesium have been which was separated from the substrate by a thin barriershown to have the following structure [49]: a dense layer layer at the interface. This structure resembles the structureof Mg(II) and Cr(III) hydroxides covers the magnesium of films formed by anodizing. The films produced in bothsurface with a porous overlayer of Cr(OH) . This over- cases were composed of MgF , MgO (OH) , NaMgF ,3 2 x y 3

1layer results from selective dissolution of Mg(OH) from Cr O and NH . Additional compounds such as2 2 3 4

the dense layer. Increasing the thickness of the dense layer AlO (OH) , FeO (OH) and Mn(IV) were also found inx y x y

has been shown to inhibit the corrosion of magnesium in the film formed on AZ91D.chloride solutions [49]. It has been suggested [50] that the A chromate conversion coating has been developed forrate of formation of the coating is controlled by the magnesium–lithium alloys [47]. The coatings were founddiffusion of chrome(VI) through the deposited coating. to be around 8–11 mm in thickness with excellent adhesionThe protective properties of the coating can be increased even under humidity and thermal cycling tests. The opticalby reducing the porosity of the overlayer through the and paint-base properties of the coating were also unaffect-precipitation of coating components that are insoluble in ed by humidity and thermal cycling tests.alkali [50]. The authors [50] also suggest that the deposi- A number of patents that discuss chromate conversiontion rate of chromate conversion coatings on pure mag- coatings are also available for review [53–56]. These arenesium can be increased by introducing copper ions into all based on the technology discussed above with variousthe bath. These metal ions are inert to magnesium but coating baths. One novel variation involves the depositionprecipitate on the deposited chromium species and act as of a chromium–silicon coating from a chromite bath thatcathodic sites to enhance deposition. contains a soluble silicate [55]. The coating produced is

These coatings have been shown to provide good porous which renders it suitable for further processing suchcorrosion protection for magnesium and its alloys in mild as painting. However, the corrosion resistance of the initialservice conditions. However due to the significant en- conversion coating is poor.vironmental hazards associated with the use of chromatecompounds there is a push to find alternative coating 2.2.2. Phosphate–permanganate conversion coatingsprocess. The application of chromate conversion coatings Phosphate–permanganate treatments are being exploredfor magnesium is discussed below. as an alternative to conventional chromate conversion

Chromate conversion coatings have been shown to coatings [57–60]. These treatments are more environmen-significantly reduce corrosion on AZ31C, AZ63A and tally friendly and have been shown to have corrosionAZ91C magnesium alloys in salt spray tests [43]. How- resistance comparable to chromate treatments. A sys-ever, the coatings are thin and are generally not suitable as tematic study on phosphate–permanganate treatment ofa final coating for outdoor use. AZ91D and WE43A alloys [57] using a bath containing

A study on the composition and protective properties of potassium permanganate and sodium phosphate has shownchromate conversion coatings on magnesium [49] has that homogeneous, non-powdery and uniform coatings candemonstrated that the protective properties of these coat- be achieved. The phosphate concentration and pH of theings are due to the presence of Cr(OH) in their structure. conversion bath were found to have the most effect on the3

An increase in the amount of Cr(OH) in the dense layer quality of the final coating. The corrosion resistance3


observed was comparable to chromate coatings. For AZ91 properties for subsequent powder coating. Another patentalloys a filiform corrosion morphology with phosphate– [62] recommends treatment of magnesium with a solutionpermanganate was observed which minimizes the depth of containing sodium ions, condensed phosphate ion andpitting. For WE43 the corrosion behaviour was the same borate ions as a treatment prior to powder coating.for both chromate and phosphate–permanganate coatings. Magnesium phosphate conversion coatings have also been

Another study investigated the use of phosphate–per- shown to protect the internal surface of a magnesium basedmanganate treatments for the conversion coating of combustion engine piston [63]. A thin layer of magnesiumAM60B die cast magnesium alloys [58]. After appropriate phosphate is formed on the surface of the piston uponpretreatment the samples were treated with a phosphate dipping in a solution of iron phosphate. This layer thenpermanganate process and E-coated (cathodic epoxy acts as an adhesive for the combustion products of fuel andelectrocoated). It was shown that the treatment provides lubricants in the engine, which in turn form a protectivegood paint adhesion. The most important factor in produc- coating on the piston surface. Conversion coatings com-ing best quality conversion coatings was found to be the posed of zinc phosphate have also been shown to havecontrol of the pH. Another study with the same magnesium adhesion and corrosion resistance comparable to coatingsalloy [59] demonstrated that with proper sample pretreat- formed by conventional anodic oxidation or chromatingment, i.e. sufficient cleaning and acid pickling, magnesium [64,65]. These films are formed in a solution containingsamples with good paint-base performance could be zinc ions, manganese ions, phosphate ions and hydrofluoricachieved with a phosphate–permanganate bath. The per- acid. Paint-adherent and corrosion resistant coatings com-manganate is said to contribute manganese to the coating posed of magnesium phosphate and magnesium fluorideand acts as an accelerator without depositing metallic have also been developed [66]. These are formed frommanganese on the magnesium surface. The coatings were phosphate ion and fluoride ion containing solutions atshown to have good corrosion resistance, paint base neutral pH. The development of coatings made up ofperformance and were composed of an agglomerate of P–Mn, Mn–N and other nitrogen containing species haswell-formed crystalline phosphate compounds. In a further been described [67,68]. These coatings are formed from apaper [60] the authors tested this treatment on the AZ91D slightly acidic bath containing a phosphorous containingmagnesium alloy. They determined that the phosphate– acid, divalent manganese ions and an amine compound.permanganate treatment can be operated over a wide range The amine component is believed to prevent excessiveof conditions without affecting the corrosion inhibition of etching of the substrate in the acidic media [67]. Thesethe coating produced when pretreatment included either films were shown to be corrosion-resistant, highly rustgrinding or phosphoric acid pickling. Samples that had inhibiting and strongly paint-adherent [68].been alkaline cleaned showed extremely high corrosionrates perhaps due to non-uniform deoxidation of the 2.2.3. Fluorozirconate conversion coatingsmagnesium surface. They found that samples pretreated Fluorozirconate treatments have also shown promise aswith a phosphoric acid pickle and then phosphate–per- potential pretreatments for magnesium and magnesiummanganate gave excellent corrosion resistance and paint- alloy materials. The group IV-A elements such as titanium,base properties. Another study [51] examined the possi- hafnium and zirconium are believed to form continuousbility of producing manganese type conversion coatings on three-dimensional polymeric metal or metalloid–oxidethe surface of magnesium alloy AZ91D. After cleaning and matrices, from aqueous solutions, in a similar manner assurface activation the samples were immersed in a solution chromium [69]. This makes them an attractive alternativecontaining potassium permanganate and either nitric or for environmentally friendly conversion coatings. Thesehydrofluoric acid. The coating formed in the HF containing coatings may provide corrosion protection through abath was very thin and was shown to be amorphous in galvanic setting or may act as a physical barrier to thestructure. It contained magnesium fluoride, hydroxides and environment [69]. This has been exploited to producemanganese oxides. The coatings formed in the bath corrosion resistant coatings, by exposing a metal substratecontaining nitric acid were substantially thicker and crys- to an aqueous acidic solution of zirconium ions, stabilizedtalline manganese oxide was observed. The corrosion in solution by organic or inorganic oxy-anion compoundsresistance of these coatings was equivalent to the protec- [69]. Upon drying a continuous polymeric zirconium oxidetion afforded by a standard chromate treatment. layer becomes fixed on the surface. In a related patent a

A number of phosphate based conversion coatings can conversion coating system composed of mixtures of groupbe found in the patent literature. One of these involves IV-A and group III-A elements is proposed [70]. Thetreating a magnesium-based sample with an aqueous bath inventors believe that these coatings provide enhancedcontaining diammonium hydrogen phosphate [61]. The corrosion resistance over simple zirconium oxide systemsinventors suggest that this results in the formation of due to a redox component in the coating that mimics themagnesium phosphate, its hydrate and magnesium hydro- chromate redox model. The preferred embodiment of thisgen phosphate on the surface without any prior pretreat- patent uses cerium and zirconium in combination.ment. This coating was found to have good adhesive One difficulty with this type of treatment is its higher


sensitivity to contamination with hard water or constituents studies are warranted. In a more recent paper [76] theof the pretreatment baths that necessitates the use of authors examined this process on magnesium, ZC71 alloy,demineralized water rinsing prior to the fluorozirconate WE43 alloy and a metal matrix composite with a ZC71treatment [71]. In a recent study [72] samples of mag- alloy matrix and 12% (v/v) silicon carbide particles. Theynesium alloy AZ91D and AM50A were treated with a found that crystalline MgSnO was formed on all four3

commercial fluorozirconate solution followed by powder substrates by a similar mechanism as discussed above.coating with an epoxy–polyester powder. This treatmentwas shown to have similar paint adhesion properties as 2.2.5. Other processeschrome conversion coatings. The samples showed reason- A conversion coating on AZ91D that uses a solutionable corrosion resistance in mild corrosive environments containing a food additive and an organic acid has beenbut extremely poor resistance to stone chipping, which shown to passivate the metal surface [77]. After degreas-limits its use in more severe service conditions. ing, the magnesium alloy is immersed in a solution

Another study on fluorozirconate treatment of mag- containing sodium benzoate, sodium glucosate and annesium alloy AZ91 HP [71] has shown that the coating organic acid. The films produced were iridescent withcontains predominantly Zr–Mg–Al oxides and hydroxides. netlike cracks and were shown to have slightly betterMorphological studies concluded that the coating is com- corrosion resistance than a chromate treated sample. Theposed of two layers. The first layer covers the metal morphology of the conversion coatings provides a goodsurface continuously and is porous and amorphous. The base for painting which can further improve the corrosionsecond layer consists of individual crystalline particles rich resistance of the treated magnesium part.in MgO/Mg(OH) . Good corrosion resistance was ob- In an alternative patented process [78], a chemical2

served and drying temperatures up to 2008C were shown to treatment involving acid pickling in a hydroxy acetic acidhave no effect on the quality of the coating. solution followed by conversion coating with an or-

A recent article concluded that fluorozirconate treat- ganofunctional silane compound has been explored. Thisments were not suitable for use in severe corrosive process was shown to maintain good paint adhesion andenvironments [73]. The use of fluorotitanate treatments corrosion resistance in salt spray tests for coatings onwas investigated for this purpose. It was found that the AZ91D alloy.fluorotitanate treatment alone did not provide adequate The corrosion protection of cerium, lanthanum andprotection against corrosion in severe corrosive environ- praseodymium conversion coatings on magnesium andments. However a combination of fluorotitanate1E-coat1 magnesium alloy WE43 has been investigated [79]. Thepowder paint was shown to be effective in mild corrosive samples were polished, cleaned in water and methanol andenvironments. dried prior to immersion in a Ce(NO ) , La(NO ) or3 3 3 3

Pr(NO ) solution. A visible, adherent but easily removed3 3

2.2.4. Stannate treatments coating was produced on the surface. The authors demon-A process for creating stannate and zincate based strated that these coatings provide an increase in corrosion

conversion coatings on magnesium alloy AZ91B10.5% Si resistance for magnesium and its alloys. However, thehas been developed [74]. Following pickling and activa- coatings deteriorate on prolonged immersion in the testtion, the samples were treated with one of a variety of buffer solution so their protective effect is short term.immersion tinning solutions or a zincate solution. The A process for creating cerium-based coatings on variouscoatings showed some corrosion resistance but in both metals has been suggested [80]. Optimum coatings arecases only thin layers were formed. Details on the corro- formed in a pH 2.5 solution of cerium chloride andsion resistance of these coatings were not given, and hydrogen peroxide. The inventors demonstrated that thefurther studies are warranted. best corrosion resistance on aluminum is obtained with

A study on stannate treatment of ZC71 and a metal high cerium concentration and 3% hydrogen peroxide. Thematrix composite of ZC71112% SiC particles has been addition of organic brighteners was observed to further

2undertaken [75]. After mechanical finishing and pickling decrease the corrosion rates from 7 mg/m /s for bare2the samples were immersed in a stannate bath for selected aluminum to 1.5 mg/m /s for treated aluminum. The

periods of time. The treatment resulted in the formation of inventors suggested that this coating technique may also bea 2–3 mm thick, continuous and adherent, crystalline applicable to magnesium.coating of MgSnO on both materials. The nucleation and Cobalt conversion coatings have also been proposed3

growth of the coating is completed in about 20 min. The [81]. However, there has been little work done in this areainitial nucleation was found to occur at cathodic sites on and all examples cited have been performed on aluminum.the surface with crystal growth to a grain size of about 2–5 The coating, which is composed of Al O , CoO, Co O2 3 3 4

mm until they coalesced. There was an increase in the and Co O (on aluminum), is formed by contacting the2 3

corrosion potential of the magnesium surface as the film substrate with an aqueous solution of a Co(III) hexacoordi-formation proceeded indicating that the coating does have nated complex of the form X [Co(NO ) ] where X5Na,3 2 6

a passivating effect on the surface. Further corrosion K or Li. The unsealed coating has a porous structure that


Table 3 2.4. AnodizingPreferred treatment conditions for hydride coating formation

pH 10–14 Anodizing is an electrolytic process for producing aTemperature (8C) 40–80 thick, stable oxide film on metals and alloys [45]. These

2Cathodic current density (mA/cm ) 20–100 films may be used to improve paint adhesion to the metal,Cathodic current frequency 0.1–3

as a key for dyeing or as a passivation treatment [45]. The(biased square wave or intermittent) (Hz)stages for processing include [45]; (1) mechanical pretreat-ment, (2) degreasing, cleaning and pickling, (3) elec-trobrightening or polishing, (4) anodizing using d.c. or a.c.

provides good paint adhesion. In order to achieve im- current, (5) dyeing or post-treatment and (6) sealing. Theproved corrosion resistance it is necessary to seal these films have a thin barrier layer at the metal–coatingcoatings. Sealing is discussed in more detail in Section 2.4. interface followed by a layer that has a cellular structure.

Conversion coatings have been known for some time, Each cell contains a pore whose size is determined by thebut it should be mentioned that a great deal of the work type of electrolyte and its concentration, temperature,done on conversion coating of magnesium substrates is current density and applied voltage. Their size and densityproprietary in nature. Thus, there is still a great deal of determine the extent and quality of sealing of the anodizedresearch to be done to better understand the surface film [45]. Coloring of anodized films can be achieved byreactions between magnesium based substrates and coating absorbing organic dyes or inorganic pigments into the filmmaterials. immediately after anodizing, by a second-step electrolytic

deposition of inorganic metal oxides and hydroxides intothe pores of the film or by a process called integral color

2.3. Hydride coating anodizing. The latter is achieved by adding organicconstituents to the anodizing electrolyte that decompose

A technique for producing a magnesium hydride coating during the process and form particles which becomeon magnesium and its alloys by electrochemical means has trapped in the film as it grows [45,84]. The color may alsobeen developed as an alternative to Cr-based conversion be controlled by a process called interference coloring.coatings [82,83]. This process involves treating the mag- Interference coloring involves control of the pore structurenesium substrate, which acts as the cathode, in an alkaline to produce color by interference of the light reflected fromsolution prepared by adding alkali metal hydroxide, am- the top and bottom of the pores [84]. This process ismonium salts or similar alkaline materials. A supporting difficult to control in production.electrolyte may also be added to decrease the solution’s Sealing of the anodized film is necessary in order toresistance. However, the authors caution against the use of achieve an abrasion and corrosion resistant film. In this

2chlorides since Cl poses the potential of corrosion of the step the porous oxide film is sealed off by the precipitationworkpiece. Prior to cathodic treatment the samples are of hydrated base metal species inside the pores. This canmechanically polished, degreased with acetone and acid be accomplished by boiling in hot water, steam treatment,etched. The preferred treatment conditions and some dichromate sealing and lacquer sealing [45,84]. Thesespecific treatment processes are shown in Tables 3 and 4, films are often inadequate as the only surface treatment butrespectively. The hydride coating thus produced has been they provide an excellent paint base for a corrosionfound to decrease the corrosion rate of AZ91D alloy by protection system.1 /3 which is comparable to the dichromate treatment. The wear resistance and hardness of anodized films can

Table 4Treatment process for hydride coating formation

Stage Operation Condition

1 Mechanical polishing No. 600 emery paper

2 Solvent degrease Acetone

3 Acid etch10% HF (w/w) or 30 s, room temperature10% HNO (w/w) 10 s, room temperature3

4 Cathodic treatment 20–608C, intermittent cathodic current2(250 mA/cm , 0.1–0.5 Hz), 30 min, pH 12

NaOH 0.01 MNa SO 0.1–0.2 M2 4


be improved by operating at a decreased electrolyte strongly alkaline bath consisting of an alkali metal hy-temperature and an increased current density. The process droxide and a fluoride or iron salt or a mixture of the twois referred to as hard anodizing [84]. The properties of the [87]. This process produces a two-phase, two-layer coat-hard anodized coatings can be further improved through ing. The first layer is deposited at a lower voltage andthe incorporation of solid film lubricants such as PTFE or results in a thin, approximately 5 mm, light green coating.molybdenum disulfide [84]. The overlayer is formed at a higher voltage. It is a thick

One of the main challenges for producing adherent, dark green, approximately 30.4 mm, layer that has goodcorrosion resistant, anodic coatings on magnesium results abrasion resistance, paint base properties and corrosionfrom the electrochemical inhomogeneity due to the phase resistance. The corrosion resistance of AZ91D alloyseparation in the alloy. The presence of flaws, porosity and treated with this technique has been subjected to a 3-yearinclusions from mechanical pretreatment can also result in atmospheric exposure test. Superior corrosion resistanceuneven deposition. It can also be difficult to achieve compared to tested conversion coatings was reported [51].uniform throwing power over all areas of shapes featuring The composition, structure and growth mechanism ofdeep recesses, narrow cavities or sharp corners. Enhanced anodic films on pure magnesium and magnesium alloycorrosion can occur if the coating contains defects [85]. AZ91D produced using the Dow 17 process, have beenAnother disadvantage of this technique is that the fatigue studied [88]. The study confirms that the film has a thinstrength of the base metal can be affected by localized barrier layer at the interface with an overlying, cylindricalheating at the surface during the treatment [86], especially pore structure, layer. On pure magnesium, the authorsin thicker films. Yet another disadvantage is that the assume that film growth proceeds by the formation ofcoatings produced are a brittle ceramic material that may magnesium fluoride and magnesium oxyhydroxide at thenot have appropriate mechanical properties for all applica- metal–film interface and dissolution of the film at the basetions. On the other hand, incorporation of PTFE type of the pores. The crystallization of magnesium fluoride andparticles in hard anodized coatings may result in coatings NaMgF proceeds in the overlying porous layer. The films3

with some technically superior functional properties. formed on AZ91D alloy were uneven, most likely due toIn the following sections some specific anodizing treat- the presence of intermetallic Mg–Al particles located at

ment processes are outlined. grain boundaries and the surface porosity. However, thegrowth mechanism of the coating was similar and a porous

2.4.1. Modified acid fluoride anodizing cell structure with crystalline particles of MgF and2

Anodizing of magnesium alloy ZM21 has been per- NaMgF was also found in these films.3

formed in an anodizing bath containing ammonium bifl-uoride, sodium dichromate and phosphoric acid [85]. The 2.4.3. Anomag processcoating is formed by a chemical reaction between the The anomag process is a proprietary treatment inventedmagnesium alloy surface and hexavalent chromium in by Magnesium Technology Licensing Ltd. The anodizingwhich magnesium is oxidized by the hexavalent bath for this process consists of an aqueous solution ofchromium, which is in turn reduced to the trivalent state ammonia and sodium ammonium hydrogen phosphate

[89]. The coating produced is a mixed MgO–Mg(OH)22 1 2 31 2Cr O 1 14H 1 6e → 2Cr 1 7H O2 7 2 system with the possibility of additional compounds suchas Mg (PO ) depending on any additives present in the21 2 3 4 2Mg → Mg 1 2ebath. Due to the presence of ammonia in the system, spark

An alternating current is required to ensure replenishment formation is repressed, which eliminates the need forof the reactant concentrations at the metal–electrolyte cooling equipment. The coatings produced are semitran-interface. The coating thus produced is composed of sparent to pearl colored depending on the presence andhexavalent and trivalent chromium. It also contains mag- concentration of certain additives such as fluoride andnesium chromate, magnesium phosphate, magnesium hy- aluminate. The properties of these coatings have beendroxide and magnesium bifluoride. investigated by a number of authors and their findings are

The anodic coatings produced by this technique were discussed below.found to be highly stable when subjected to high humidity, A study by Guerci [73] found that the thickness andhigh temperature, thermal cycling tests and thermovacuum properties of the oxide layer produced using the Anomagtests. They were also deemed suitable for some space process depend on the bath composition, temperature,applications due to their high solar absorptance, high IR current density and treatment time. The coating producedemittance and good optical properties. has a cellular microstructure similar to that observed for

other anodizing processes. The authors found that die-cast2.4.2. Dow 17 process [14] magnesium samples treated with the Anomag process,

Chemical treatment no. 17, developed by Dow Chemi- followed by powder coating had good paint adhesioncals, can be applied to all forms and alloys of magnesium. properties and excellent corrosion protection. FurtherThe anodizing bath employed in this treatment is a development in the areas of cost reduction, choice of


chemicals used in the process and layer thickness optimi- resistance [95]. The coating has been shown to consist ofzation was proposed. Another study on coating AZ91 alloy mainly MgAl O . This process is capable of producing2 4

with this process [90] also showed the formation of a uniform coatings even on edges and cavities. The coatingsporous film with a pore size of about 6 mm and porosity of produced provide wear and corrosion protection although13%. Sealing and painting were shown to reduce the pore dyeing has been shown to result in a decrease in corrosionsize and porosity to 3 mm and 4%, respectively. The resistance.chemical composition of the film was found to consistmainly of Mg (PO ) Samples that had been anodized, 2.4.5. HAE process [14]3 4 2.

anodized–painted and anodized–sealed all showed a sig- This treatment is effective on all forms and alloys ofnificant reduction in general corrosion rates. The best magnesium provided no other metals are inserted orresults occurred for magnesium treated with the Anomag attached to the magnesium workpiece. The treatmentprocess followed by painting and sealing. These samples produces a two phase coating as in the Dow 17 process. Atshowed a 97% reduction in the general corrosion rate a lower voltage a 5-mm thick, light tan subcoating iscompared to the bare metal. The three-step treatment was produced. At a higher voltage a dark brown, thicker (30also shown to reduce galvanic corrosion rates. However mm) film is produced. Upon sealing the HAE treatmentanodizing or anodizing–painting without sealing did not provides excellent corrosion resistance. The dark brownreduce the rate of galvanic corrosion. coating is hard with good abrasion resistance but it can

Fatigue testing of AZ91 samples treated with the adversely affect the fatigue strength of the underlyingAnomag process has shown that there is no change in the magnesium, particularly if it is thin. The corrosion resist-fatigue properties compared to the untreated samples [91]. ance of AZ91D treated with this technique has been testedThe corrosion fatigue properties of these coatings have by a 3-year atmospheric exposure experiment. Superioralso been studied [91]. The results were not conclusive corrosion resistance compared to conversion coatings washowever it appeared that at low applied stress levels the observed [51].coating seemed to provide resistance to corrosion fatiguewhile at higher stress levels no change compared to 2.4.6. Galvanic anodizing [14]uncoated samples was observed. An interesting observa- This technique produces a protective black coating withtion was that sealed Anomag coatings performed extremely good paint-base characteristics on all forms of magnesium.poorly in these tests. The authors propose that this could be This process does not require an external source of current.due to an effect of the thermal cure process on the base Very thin films are produced with little dimensional changemetal. to the workpiece.

2.4.4. Magoxid-coat process [92,93] 2.4.7. Cr-22 treatment [14]This proprietary process, invented by GmbH Ltd., is an This process is a high-voltage process that is not

anodic plasma–chemical surface treatment that forms an currently in use. Green and black coatings can be producedoxide ceramic layer on magnesium materials. The plasma on all alloys by varying the solution composition, tempera-is discharged by an external power source in a slightly ture and current density. These coatings provide goodalkaline electrolyte near the surface of the workpiece, corrosion resistance for unpainted parts when properlywhich acts as the anode of the system. The oxygen plasma sealed but have been used primarily as a paint base. Hardgenerated causes partial short-term surface melting and protective black coatings have also been produced in anultimately the formation of an oxide–ceramic layer. The aqueous anodizing bath containing chromate, vanadate,anodizing bath for this process is free of chloride and may phosphate and fluoride compounds [96].contain inorganic anions such as phosphate, borate, sil-icate, aluminate or fluoride [94]. The bath may also contain 2.4.8. Tagnite surface treatmentorganic acids such as citrate, oxalate and acetate. A source The Tagnite surface treatment, invented by Technologyof cations is also present and may be chosen from alkali Application Group is a chromate-free, anodic electrodepo-ions, alkaline earth ions or aluminum ions. Finally a sition surface treatment [97]. The surface treatment in-stabilizer such as urea, hexamethylenediamine, volves a pretreatment step to create a firmly bondedhemethylenetetramine, glycol or glycerin is also added. protective base layer that is compatible to deposition of theThe coating consists of three layers, a thin (100 nm) barrier anodic coating [98]. The pretreatment can consist of (1)layer at the metal surface followed by a low porosity oxide immersion in an ammonium fluoride bath [98] or (2)ceramic layer and finally a higher porosity ceramic layer. electrolytic treatment in an aqueous bath containing hy-The final layer acts as a good base for paint adhesion and droxide and a fluoride compound [99]. The anodizationimpregnation treatments. Impregnation of the coating with bath consists of an aqueous solution containing hydroxide,particles of fluorine polymers has been shown to sig- fluoride and silicate species. This produces a ceramic likenificantly improve the load bearing properties of the coating of SiO at the surface. This process has been2

coatings while maintaining good adhesion and corrosion shown to successfully coat the internal passages and blind


holes of a gearbox [97]. The abrasion resistance, wear containing an alkali metal silicate and an alkali metalresistance, paint adhesion and corrosion resistance were all hydroxide [108]. In the second case the fluoride is part ofimproved compared to the Dow and HAE processes even the anodizing bath [109]. In both instances the coating isprior to organic finishing [97–100]. Surface sealing of the formed when sparks are discharged at the surface. Thiscoating was shown to further improve its corrosion resist- causes the surface to melt with simultaneous deposition ofance [97]. a fluoride containing silicate coating.

2.4.9. Miscellaneous processes2.5. Gas-phase deposition processes

A number of patents detailing anodizing procedures withvarious bath compositions have been granted. Protective

All the processes discussed thus far have been wetcoatings with no inherent color can be easily colored and

chemical surface treatments. Protective coatings can alsoprovide a good paint base. These coatings can be produced

be produced from the gas phase. These are typicallyfrom an alkali rich aqueous bath containing borate or

metallic coatings but can include organic coatings such assulfate anions and phosphate and fluoride, chloride or

thermal spray polymer coatings and diamond like coatings.aluminate ions [101]. During the anodizing process a direct

All of these processes have the advantage that they havecurrent is applied then briefly turned off or polarity is

little negative environmental impact. However, the capitalincompletely reversed to allow formation of magnesium

costs associated with these techniques are usually high. Inphosphate and magnesium fluoride, magnesium chloride or

the sections to follow a variety of gas phase surfacemagnesium aluminate [101]. The performance of this bath

modification technologies and their application to mag-can be improved by the addition of a buffering agent in the

nesium protection will be discussed.form of an amine compound [102]. The wear resistancewas determined to be less than 20 mg/10 000 revolutionson a Taber abrasive system, while the good corrosion 2.5.1. Thermal spray coatingsresistance was demonstrated by the appearance of less than In this process the coating material which can be metal,

210 corrosion points /cm , after 240 h salt spray testing ceramic, cermet or polymeric is fed to a torch or gun[102]. where it is heated to above or near its melting point. The

An anodic treatment with an aqueous bath containing resulting droplets are accelerated in a gas stream onto thesilicate, carboxylate, alkali hydroxide, and one or more substrate and the droplets flow into thin lamellar particlesborate, fluoride or phosphate compound has been reported and adhere to the surface [110]. A number of coatingto have superior decorative appearance, corrosion and techniques fall under this umbrella including flame spray-abrasion resistance compared to HAE and Dow 17 [103]. ing, wire spraying, detonation gun deposition, plasmaThis process produces a glassy white oxide coating with spray and high velocity oxyfuel.the chemical composition 2MgO:SiO . Some of the advantages of this technique include the2

Another type of process involves anodizing in an ability to create a coating of virtually any material thataqueous solution containing a polybasic organic acid such melts without decomposing, minimal substrate heatingas polyvinylphosphonic acid [104–106]. The coating pro- during deposition and the ability to strip and recoat wornduced is an insoluble metal oxide–organic complex, or damaged coatings without changing the properties orwhich, under optimized conditions, shows no porosity at dimensions of the part [110].55 0003 magnification and has good corrosion resistance. One major disadvantage is that the process is line ofHowever, although the inventors suggest that this process sight and small deep cavities cannot be coated, especiallyshould work on magnesium, all examples cited for this if the surface lies parallel to the spray direction. Thesework used aluminum as substrate. coatings also require sealing due to their inherent porosity

An anodizing process for forming a chemically stable and mechanical finishing to obtain a smooth finish. Oneand hard spinel compound of MgO–Al O on magnesium final disadvantage of this technique are the health and2 3

surfaces has been disclosed in a patent by Gillich et al. safety issues generated by the production of dust, fumes,[107]. The anodizing bath consists of aluminate, alkali noise and light radiation during treatment.hydroxide and at least one boron, phenol, sulfate or iodine As with most surface treatments, in order to ensurecompound. The coating is white in color, resistant to adequate adhesion, the substrate must be properly pre-corrosion and abrasion and could be easily colored in a pared. The substrate must be both cleaned and roughenedconventional dyeing bath. prior to the application of the thermal spray coating.

An anodic treatment that produces a hard, durable, The process has been used to coat a magnesium alloy touniform, adherent and corrosion resistant, fluoromag- be used as part of a satellite [111]. Prior to application of anesium–silicate coating has also been reported [108,109]. thermal spray coating the substrate was washed andThere are two variations of this process: in the first case roughened by blasting. An aluminum coating is thenthe magnesium substrate is pickled in an aqueous hydro- applied by thermal spraying followed by sealing with afluoric acid bath prior to anodizing in an aqueous bath chromate conversion coating. The coatings were shown to


withstand corrosive environments and have good electrical titanium oxide, aluminum oxide, zirconium oxide,conductivity. chromium oxide or silicon oxide. The aluminum precursor

and metal oxide precursor must be able to decompose at2.5.2. Chemical vapor deposition temperatures less than 4308C. A final sealing of the surface

Chemical vapor deposition (CVD) can be defined as the to reduce porosity can be obtained by immersion in adeposition of a solid on a heated surface via a chemical boiling aqueous bath for 30 min. The authors claim that thereaction from the gas phase. Advantages of this technique protective films formed on any magnesium alloy areinclude deposition of refractory materials well below their corrosion resistant and well adhered. Another study claimsmelting points, achievement of near theoretical density, that SiO films produced on AZ91 alloy by a CVD process2

control over grain size and orientation, processing at show no visible corrosion spots after 240 h immersion inatmospheric pressure and good adhesion [112]. This NaCl [82]. This film was also found to be stable in acidprocess is not restricted to line of sight like most physical solutions and organic solvents.vapor deposition (PVD) processes so deep recesses, highaspect ratio holes and complex shapes can be coated. Due 2.5.3. Diamond-like carbon filmsto the high deposition rate that can be achieved, thick Diamond like carbon films can be produced using acoatings can be produced. However, this process is limited number of different processes such as PVD, CVD and ionto substrates that are thermally stable at $6008C. Efforts implantation. These coatings are desirable for many appli-are underway to reduce the high temperature requirements cations due to their high hardness, low friction coefficient,and plasma and organometallic CVD processes offset this electrical insulation, thermal conductivity and inertnessproblem somewhat. A further disadvantage of this process [117]. Plasma enhanced CVD has been used to produceis the toxic nature of the chemical precursors, which amorphous hydrogenated SiC and diamond-like coatingsnecessitates the use of a closed system. Toxic solid on magnesium [117]. Corrosion resistance studies bybyproducts can also be produced which introduces waste anodic polarization in the presence of sodium chloridedisposal costs. The energy cost of this process can also be showed both a decrease in the corrosion current density athigh due to the need for high deposition temperatures and all potentials and a large positive shift of the potential atsometimes low efficiency of the process [112]. A variety of which the corrosion current becomes large in chlorideCVD processes used to coat magnesium substrates are containing solutions. The most promising films weredescribed below. obtained with defect free diamond-like coatings, these

Molybdenum coatings from an organometallic chemical showed no sign of corrosion upon microscopic examina-vapor deposition precursor have been described [113]. It tion. Diamond-like carbon films on magnesium alloys withwas determined that crack-free, adherent coatings on good lubricity, corrosion resistance, adhesion and smooth-magnesium could be obtained at a decomposition tempera- ness have been produced using a CVD process [118]. Theture of 4008C with a thickness ,0.5 mm. An increase in surface of the alloy is subjected to a CVD treatment usingcorrosion potential from 21.457 V (SCE) to 20.74 V methane and hydrogen to form a diamond-like carbon(SCE) was observed in NaCl. The coating underwent coating and then treated with a high-frequency plasmapartial dissolution but remained homogeneous during CVD process using carbon tetrafluoride. Finally, diamond-polarization. It was concluded that provided the coating is like carbon films using methane and a plasma source ionhomogeneous, non-porous and crack-free, the resistance of implantation method, to produce adherent films on mag-magnesium to general corrosion in neutral chloride solu- nesium, have been reported [119]. This process involvestion was improved. CVD has also been used to coat creating a graded carbon composition interface betweenmagnesium particles with nickel for use in a magnesium the metal and the diamond-like coating via ion implanta-hydride storage device [114]. tion.

A plasma-assisted CVD technique has been successfullyused to deposit TiCN and ZrCN layers on magnesium 2.5.4. Physical vapor deposition processesalloys AZ91 and AS21 [115]. The coatings were deposited PVD involves the deposition of atoms or moleculesat low temperature (below 1808C) from an organometallic from the vapor phase onto a substrate. This processtetrakis(diethyl)-amino metal complex. The topography of includes vacuum deposition, sputter deposition, ion plat-the deposited layers was shown to be smooth to domed ing, pulsed-laser deposition and diffusion coatings.with a dense to columnar fracture surface. The authorsdemonstrated that adherent layers with hardnesses of 1400 2.5.4.1. PVD on magnesium. The role of PVD pro-HK0.01 and 1530 HK0.01 for ZrCN and TiCN, respective- cesses in magnesium surface finishing can be divided intoly could be obtained. two sections, the deposition of wear and corrosion protec-

A patented process for producing a protective film on tion coatings and the creation of bulk magnesium alloysmagnesium containing substrates has been disclosed [116]. with unique corrosion resistant properties.The coating process involves CVD of an intermediate There are a few challenges to overcome in the PVDaluminum layer, followed by a metallic oxide layer of coating of magnesium substrates. The deposition tempera-


ture must be below the temperature stability of magnesium the titanium or titanium alloy target. The vapor is de-alloys (1808C) and good adhesion must be obtained despite posited on the magnesium or magnesium alloy substrate tothis low temperature [120]. The coating must also have form a thin film. One drawback of this technique is that it

26 28good corrosion resistance. For most PVD processes the must be done under low pressure conditions (10 –10substrate must be in the range of 400–5508C however, it Torr), and is also a line of sight process.has been shown that the deposition temperature can be Another patented process [126], invented by DaimlerCh-significantly reduced by applying a pulsed bias voltage rysler, describes a process for creating an anticorrosionduring deposition [120]. TiN coatings produced by this coating on magnesium materials. The corrosion protectiontechnique on magnesium alloy AZ91 have been shown to layer described in this patent can be applied as a coatingbe adherent and pore free [120]. The load-carrying capaci- by flame spraying, plasma spraying or sputtering. Thety of these samples was significantly improved by intro- layer can also be formed on a surface area of theducing an intermediate layer of electroless Ni–P [120]. magnesium material by coating the casting mould prior toCorrosion studies have not been reported for this system to pouring the magnesium, by co-extrusion or by plating. Thedate. Another recent article reports the PVD of Cr and CrN magnesium material to be protected is highly pure mag-multilayer films on magnesium alloy AZ91 [121,122]. nesium that does not contain iron, nickel or copper. TheThese coatings were found to have good adhesion and coating consists of an alloy containing titanium, zirconiumwear properties but poor corrosion resistance due to the or magnesium as the base metal and certain metal addi-presence of pores in the coating. Upon comparison of tives. The additives are selected from the group consistingmonolayered Cr and multilayer CrN coatings a slight of alkali metals, alkaline earth metals, rare earth metals,increase in corrosion protection was observed for multi- yttrium, metals from group 12 to 15 of the fourth or alayers. Another approach involves coating magnesium higher period of the periodic table and manganese. Thesealloys with purified magnesium or magnesium alloys metals can be divided into two categories. The alkali[123]. The corrosion resistance in this case is thought to metals, alkaline metals, rare earth metals and yttrium havecome from the decrease in alloying elements that cause a lower quiescent potential than magnesium and thereforegalvanic corrosion. Table 5 shows the weight loss after act to cathodically protect the underlying metal fromimmersion tests in 1% NaCl for various alloys and AZ31 corrosion. Metals from group 12 to 15 of the 4th or higheralloy samples that had been coated with purified mag- period and manganese have a high hydrogen evolutionnesium is shown below. overvoltage and protect the magnesium metal by poisoning

The corrosion protection afforded by the purified mag- the cathodic half reaction. The thickness of the corrosionnesium coating is comparable to corrosion of the purer, protection layer should be at least 0.2 mm in order to bemore corrosion resistant magnesium alloys AZ91 and 6N– effective. The patent sites examples of negative contactMg. In a more recent article [124], these authors investi- current density measurements between the AM50A mag-gated the microstructure of PVD deposited magnesium 3N nesium alloy and Mg–Mn, Mg–Pb and Mg–In samples.or AZ91 on magnesium AZ31 substrates. They determined The use of multilayer materials [127] has also beenby atomic force microscopy that the deposited coating was proposed for anti-wear, anti-erosion and anti-abrasioncomposed of faceted magnesium particles which existed as coatings on lightweight metals. These coatings involve theisolated species in the early stages of deposition but grew deposition of an intermediate layer consisting of Cr, Mb,to cover the surface as deposition continued. Heteroge- Ni, Ti, Zr, their nitrides and carbides or solid solutions ofneous corrosion in 1% NaCl was not observed on these C and N in these metals. This intermediate layer cansamples although there was a slight swelling of the grains consist of a single layer or a stack of several layers withthat was attributed to a homogeneous corrosion mechanism individual thicknesses of 0.5–5 mm. The final layer(s) is athrough the formation of a Mg(OH) on the surface. tungsten based deposit such as W, WC, WN and tungsten2

In a patented process [125] PVD–PLD has been used to alloys that is 5–60 mm thick. In scratch tests of anodizedcoat magnesium substrates with titanium or titanium alloy aluminum coated with tungsten and anodized aluminummaterial. A focused laser beam is used to heat and vaporize with an intermediate layer of titanium followed by tung-

sten deposition the critical loads were 5 N and 12 N,respectively. The use of intermediate layers prior toTable 5tungsten coating was also shown to improve performanceWeight loss of magnesium alloys after immersion in 1% NaClof the aluminum alloys in thermal cycling and erosionSample Weight loss

2 testing.(mg/cm )

3N–Mg 752.5.4.2. Deposition of surface alloys. PVD can also beAZ31 3.5used to create new magnesium alloys in bulk form or as aAZ91 0.7

6N–Mg 0.2 coating material. The addition of alloying elements toCoated AZ31, evaporation source: AZ91E 0.3 conventionally processed alloys results in the formation ofCoated AZ31, evaporation source: 3N–Mg 0.6 second phase, galvanically active particles that increase


corrosion problems [128]. Deposition from the vapor phase through the removal of surface oxide and creation of ancan be used to provide a high extension of solid solution, ion implanted intermixed layer prior to metal deposition. Aeven in non-miscible systems [128]. Binary alloys such as recent article describes this process for depositingMg–Zr, Mg–Ti, Mg–V, Mg–Mn and Mg–Cr have been chromium on magnesium with a film thickness from 200 tosuccessfully deposited from the vapor phase [128–136]. 300 nm [139]. The corrosion behavior of these films wasCorrosion studies on these binary alloys have shown that found to be strongly influenced by the formation of anthe corrosion rate of vapor deposited magnesium can be intermixed layer and the particle content of the film. As thereduced by alloying with manganese, zirconium or particle content decreased the corrosion potential becametitanium while chromium and vanadium have a detrimental more noble indicating increased passivation of the sample.effect on the corrosion resistance of pure magnesium This has been attributed to a reduction in the number of[129–136]. The vapor deposited alloys are columnar in film defects.structure and have a high degree of porosity. In situ A primary limitation of this technique is its line-of-sightmechanical working during deposition has been found to nature that makes processing of complex shapes difficult.reduce the porosity and eliminate the columnar micro-structure around the flail line [130,131]. However, the 2.6. Laser surface alloying /cladding [82,140]corrosion resistance of the flailed alloys was inferiorcompared to pure magnesium [131]. Laser surface melting can create metastable solid solu-

tions at metal surfaces where the cooling rate can be as102.5.5. Diffusion coatings high as 10 K/s. This process is a form of rapid

Diffusion coatings can be deposited by heating the solidification processing but only the surface region iscomponent to be treated in contact with a powdered modified. The advantages of this technique include thecoating material in an inert atmosphere. This process ability to treat complex geometries, up to millimeters depthproduces alloy coatings at high temperatures by the inward of treatment, lower operation cost and greater control ofdiffusion of the coating material into the substrate material the concentration of the modified layer. This process has[137]. Recently this technique has been used to create an been investigated for improved corrosion resistance onaluminum diffusion coating on AZ91D magnesium alloy Mg–Li and Mg–Zr alloys.[138]. The diffusion coating was formed by heat treatment For laser surface alloying a metallic coating and theof the magnesium alloy in aluminum powder at 4508C for underlying substrate are melted using a high power laser.1 h, under an inert gas atmosphere. An Al–Mg inter- The rapid melting, mixing and resolidification cause themetallic compound, 750 mm thick, was formed on the coating and substrate to alloy. Cu, Al and Cr coatings havesurface. The intermetallic layer formed was shown to have been shown to have more noble pitting potentials andsurface cracks but no cracks or pores were observed near therefore more promising corrosion protection. Improvedthe interface of the reacted layer and the magnesium alloy wear resistance has also been reported for commercialsubstrate. The surface layer produced was found to consist purity magnesium, AZ91 and WE54 alloys that have beenmainly of d phase magnesium and g phase Al Mg . surface alloyed with Al1Ni and Al1Si [141]. The wear12 17

resistance of these substrates was not improved when2.5.6. Ion implantation [82] alloyed with Al1Cu. This process was shown to have

Ion implantation involves exposing a surface to a beam improved wear resistance over laser cladding with mag-of ionized particles. This results in ions being embedded nesium containing powders particularly when SiC particlesand neutralized at interstitial positions in the substrate to were added to reinforce the surface coating.form a solid solution. Bulk properties are not modified. In another related process the injection of hard particlesThere is very little information available on this process of TiC and SiC into a molten pool of magnesium metal,with magnesium. However a study on the corrosion created by a laser beam, has been used to modify mag-

1protection of AZ91 implanted with N demonstrated that nesium alloy AZ91 to improve resistance to sliding wear2

Mg corrosion could be suppressed with an appropriate ion [142]. The surface was shown to consist of TiC and SiCdose. particles dispersed throughout the cladding layer. These

surfaces were shown to have improved wear resistance2.5.7. Metal plasma immersion ion implantation and however opposing materials used for wear testing showeddeposition (MPIID) significant wear. The injection of fine particles of Mg Si2

In MPIIID the substrate is immersed in a metal plasma was also studied. These materials showed high wearand a pulsed negative high voltage is applied to the resistance and little wear of opposing materials.sample. Ions are extracted from the plasma, accelerated to In another closely related technique, a coating is ther-the substrate and implanted. During the pulse pauses low mally sprayed onto the substrate and subsequently re-energy metal ions are deposited on the surface. The metal melted with a 2 kW Nd:YAG laser [143]. This techniqueplasma is usually produced by a cathodic arc discharge has been used to coat a 17% (v/v) SiC particulate[139]. MPIIID is typically used to enhance film properties reinforced ZK60 magnesium composite with an Al-12.5


wt.% Si alloy. The as-sprayed coating was loosely adhered pretreatment, solvent cleaning or alkaline cleaning. Clean-but following laser remelting complete fusion of the ing is typically followed by a pickling or an etching stepcoating to the substrate was observed. Despite this im- coupled with a chemical treatment, such as conversionproved adhesion, laser remelting did not adequately im- coating or anodizing. These treatments roughen and chemi-prove the corrosion resistance due to excessive diffusion of cally modify the surface so that the organic coating willmagnesium into the laser-melted surface. It was also have good adhesion to the surface.determined that careful control of the processing parame- Prior to applying an organic coating to a magnesiumters is necessary to provide an adequate coating. casting, all moisture and air must be removed from the

Laser cladding of AlSi30 on magnesium alloys AZ91 pores of the cast part. The presence of moisture orand WE54 has been shown to decrease volume loss by 38 entrapped air results in the formation of holes in theand 57%, respectively, in wear tests [141]. The authors coating due to degassing during the curing process [145].determined that wear is significantly reduced in vacuum An appropriate primer coating must also be applied toand that the primary wear mechanism in air is loss of ensure adequate adhesion of the coatingadhesion followed by oxidation of wear particles to form A variety of treatment processes for producing specifichard, abrasive oxide particles. decorative effects on magnesium alloys are outlined in

Table 6 [14].2.7. Organic /polymer coatings Another technique for treating magnesium surfaces prior

to the application of an organic coating involves exposingOrganic finishing is typically used in the final stages of a the material to an aqueous solution containing an organic

coating process. These coatings can be applied to enhance compound [146] after appropriate cleaning and picklingcorrosion resistance, abrasion and wear properties or for procedures. The compound must have a particular structuredecorative purposes. An appropriate pretreatment process XYZ, where X and Z are both polar functional groups andis required in order to produce coatings with superior Y is a straight chain structure with 2–50 carbon atoms.adhesion, corrosion resistance and appearance. Magnesium Some examples of these include 1-phosphonic acid-12-(N-surfaces must be free of surface contamination, smut and ethylamino)dodecane, 1-phosphonic acid-12-hydroxy-loose silicates, oxides and intermetallic compounds [144]. dodecane, p-xylylene diphosphonic acid and 1,12-Cleaning processes for magnesium can involve mechanical dodecane diphosphonic acid. These compounds react with

Table 6Organic surface finishing processes for magnesium surfaces

Decorative effect Finishing for interior use Finishing for exterior use

Bright metal Buff1ferric nitrate pickle Buff1ferric nitrate pickle1

1clear epoxy or acrylic clear epoxy or acrylic

Satin finish Wire brush1ferric nitrate Wire brush1ferric nitratepickle1clear epoxy or acrylic pickle1clear epoxy or acrylic

Tinted clear Ferric nitrate pickle1tinted Not recommendedepoxy or acrylic

Dyed clear Ferric nitrate pickle1clear Not recommendedepoxy or acrylic1dye dip

Metallic Chrome pickle or dilute Chrome pickle or dilutechromic acid1epoxy, acrylic, chromic acid1polyvinylpolyvinyl butyral or vinyl butyral primer1epoxy orpigmented with metal powder acrylic pigmented with metalor paste powder or paste

Wrinkle Chrome pickle or dilute Not generally used inchromic acid1standard outdoor servicewrinkle finish

High-gloss enameled Chrome pickle or dilute chromic Chrome pickle or dilute chromic acidacid1epoxy, acrylic, polyurethane 1polyvinyl butyral primer1acrylic,or alkyd enamel alkyd or polyurethane enamel

Smooth leatherette Chrome pickle or dilute chromic Chrome pickle or dilute chromicacid1vinyl cladding acid1vinyl cladding

Textured leatherette Chrome pickle or dilute chromic Chrome pickle or dilute chromic acidacid1vinyl organosol 1polyvinyl butyral or vinyl

primer1vinyl organosol


the hydroxide groups on the metal surface through the acid must be uniform and well-adhered to the substrate. Or-groups to form a chemical bond. There is also a reaction ganic coatings that do not meet this criteria often undergobetween the remaining functional groups and the sub- filiform corrosion. In filiform corrosion, filamentous corro-sequent paint coating. These coatings are said to sig- sion products appear under a coating [150]. There are anificantly improve paint adhesion and to inhibit corrosion. number of difficulties in preventing corrosion with organicAnother patented process [147] also improves paint adhe- coatings. Some of these include, difficulty in applying asion and corrosion resistance for lightweight metals. In this perfectly uniform film, non-uniform crosslink density inprocess an ester cross-linked polymer coating is formed by the film, nonuniformity in local pigment volume con-reaction between a polymer rich in carboxylic groups and a centration and swelling or degradation of the polymer uponpolymer rich in hydroxyl groups. This ester cross-linked exposure to various gases or liquids [151]. These problemspolymer system is then combined with a group IV element can be minimized by using a multiple layer coating systemthrough the addition of one of fluozirconic acid, fluotitanic consisting of a topcoat, that is typically the most hydro-acid, fluohafnic acid or their corresponding salts. One main phobic and UV resistant coating, and primer and midcoatsadvantage of these two processes is that they are chromate that have high crosslink density and wet adhesion to thefree. substrate and each other [151]. With a multiple layer

Organic coating systems can include a variety of differ- system it is unlikely that defect areas will overlap, thisent processes that make use of organic polymers, such as ensures that the substrate is completely coated with organicpainting, powder coating, E-coating (cathodic epoxy material. This has been exploited in a patented system thatelectrocoating) and the application of lacquers, enamels makes use of a multiple layer coating on a magnesiumand varnishes. These systems can be based on a variety of wheel for improved corrosion and chipping resistancecoating resins such as acrylic, alkyd, butyrate, cellulose [152]. This system consists of sequential layers of a cationacetate, cellulose acetate butyrate, chlorinated polyethers, electrodeposition coating, liquid coating, powder coating,epoxies, fluorocarbons, nitrocellulose, nylon, polyesters, an additional liquid coating and finally a top coat.polyethylene, polypropylene, polyurethanes, rubber resins, Another process for eliminating pinholes or defects insilicones and vinyls [144]. Traditionally organic coatings organic coatings involves the application of an organichave been solvent based which poses a significant en- sealant to the coating [153]. This is accomplished byvironmental concern with their use. However alternative application of a low viscosity UV curable resin to theprocesses that eliminate this problem are available. Some sample followed by low temperature curing with UV light.of these include powder coatings, the use of compliance One application of organic coatings is in the protectionsolvents and waterborne solvents [144]. of US naval aircrafts from the operational and environmen-

The primary function of organic coatings is to act as a tal conditions they are exposed to. In 1989 the standardbarrier between the metal substrate and its environment. It coating system consisted of an epoxy primer followed by ais important that these coatings provide resistance to polyurethane topcoat with a service life of approximatelytransport of ions, water, oxygen and charge through the 4–6 years [154]. The primer consists of a highly cross-film to the substrate [151]. In applications where physical linked epoxy–polyamide system containing titanium oxidedamage is likely to occur it is also important that the as a pigment and strontium chromate as a corrosioncoating have self-healing characteristics. This can be inhibitor [154]. This layer has excellent adhesion andaccomplished by the presence of corrosion inhibiting chemical resistance. The topcoat is a two componentpigments or additives in the coating or by the use of a polyurethane that is flexible but durable and chemicalsacrificial anodic compound in the film [151]. One resistant [154]. In some cases a sealant is also appliedpatented process [148] demonstrates that the addition of between the primer and topcoat. Its elastic nature offsetszinc chromate to a phenolic resin primer can significantly the brittleness of the epoxy and minimizes cracking of theimprove corrosion resistance. The inventors showed that paint system. There are some challenges associated withsamples treated with this process have no evidence of this type of system including the use of environmentallycorrosion after 1000 h salt spray testing while samples unfriendly materials such as VOCs and chromate com-coated with a conventional polymer coating were severely pounds, inadequate rain-erosion properties and lack of highcorroded after 24 h. In another process corrosion inhibition temperature resistance. A coating with improved rainof magnesium is said to be improved by addition of both a erosion resistance has been developed with a two com-corrosion inhibiting leachable pigment (chromate) and an ponent system consisting of a pigmented poly-ion reactive pigment in the form of small spherical (ether)urethane and a ketimine crosslinker /chain extenderaluminum particles [149]. The metal spheres are believed [154]. These coatings are elastomeric and can absorb andto react with corrosive ions and help maintain the pH in dissipate impact energy however, a topcoat is still requiredthe alkaline region while the chromate provides self-heal- to improve chemical resistance and weathering properties.ing properties by leaching out at defect sites and forming a Some of the environmental concerns have been met by theprotective coating. development of water borne paints and high solids technol-

For an organic coating to act as an effective barrier, it ogy such as powder coating. A one-coat system has been


developed that addresses some of the environmental appearance of the paint film. The paint coated surfacesconcerns [154]. The Unicoat system is a self-priming were shown to have excellent corrosion resistance sincetwo-part polyurethane that adheres directly to the substrate. even after 4000 h salt spray testing and a 3-year atmos-It contains titanium oxide and vesiculated beads that act to pheric exposure test there were no signs of blisters orwhiten and reduce gloss, respectively. Zinc molybdate, corrosion.zinc phosphate and organo–zinc salt pigments are also A recent paper [157] describes the effect of addingpresent as corrosion inhibitors rather than the traditional conductive polypyrrole (CP) to an acrylic paint, on thechromate compounds. A number of silicone based paints corrosion protection afforded by the organic coating. CPsthat are resistant to temperatures up to 6508C have also are believed to react with the metal at defects in thebeen developed. The use of natural polymers such as coating under certain electrochemical conditions. This canpolysaccharide graft polymers has also been suggested act to protect the metal but in some cases corrosion can be[155] as an environmentally friendly alternative. These accelerated. The protection afforded by polypyrroles is notcoatings are produced by the hydrolysis–condensation fully understood; four possible mechanisms have beenreaction of polysaccharide, such as commercial starches proposed; (1) the coating may act as a barrier (2) the CPand celluloses, with an antimicrobial agent such as halogen acts as a sacrificial anode (3) the coating acts as a reservoirsubstituted silanes. The examples cited show that on for corrosion inhibiting ions that are released as the CPaluminum these coatings have excellent corrosion resist- changes redox state (4) CP could act to stabilize a passiveance. oxide layer. Further research is required to precisely

In the following sections, examples of organic coating determine the mechanism of corrosion inhibition. How-systems that have been applied to magnesium or mag- ever, this study did show an improvement in corrosionnesium alloy substrates are reviewed. resistance due to the addition of small amounts of CP to

the paint system.2.7.1. Painting

One of the most important steps in painting of mag- 2.7.2. Powder coatingnesium is choosing an appropriate primer. Primers for Powder coating is a process in which a pigmentedmagnesium should be alkali-resistant and based on resins resinous coating powder is applied to the substrate andsuch as polyvinyl butyral, acrylic, polyurethane, vinyl then heated to fuse the polymer together in a uniform,epoxy and baked phenolic [14]. The addition of zinc pinhole-free film [144]. Powder coating is an excellentchromate or titanium dioxide pigments is commonly used alternative to traditional painting processes since it is notfor corrosion prevention [14]. detrimental to the environment and uniform thick coatings

A study of the corrosion resistance of die cast AZ91D can be obtained in a single operation even on roughmagnesium alloys with paint finishing has been reported surfaces or edges. There is also little loss of coating[156]. Prior to painting, the samples were treated with material during application and even basic resins that areeither a conversion coating or an anodizing process. The not readily soluble in organic solvents can be appliedsurface treatments studied are shown in Table 7. The paint [144]. However, there are a few inherent disadvantages tofilm was applied in two layers. The first layer was a primer this techniquecontaining an epoxy resin applied to a thickness of 25–30mm and cured at 1708C for 20 min. The final coat was an 1. The powder must be maintained in a very dry, pulver-acrylic paint resin applied to a thickness of 25–30 mm and ized form.cured at 1508C for 20 min. 2. Thin coatings are difficult to obtain.

It was determined [156] that conversion films with a 3. Color matching and color uniformity can be difficult tosignificant amount of oxygen and chrome on the surface maintain.(JIS-1 and JIS-3 baths) and anodized films with a thick 4. Coating in recessed areas can be difficult.coating (Dow-17, HAE-B, U-5) gave the best adhesion and 5. High temperatures required for curing may be unaccept-

able for some substrates.

Table 7As with any coating process, appropriate pretreatment ofSurface treatments applied to magnesium alloy AZ91D prior to painting

the surface is critical for obtaining the best surface finish.Name Bath composition

This challenge was met by Applied Coating TechnologyJIS-1 Na Cr O , HNO2 2 7 3 who found that the use of an alternative chromate processJIS-3 Na Cr O , CaF2 2 7 2 in the pretreatment of magnesium alloys was ineffective asJIS-7 Mn(H PO ), NaF, Na Cr O2 4 2 2 7 a base for powder coating [145,158]. They developed aDOW 22 Na Cr O , KMnO , H SO2 2 7 4 2 4

process that used an organometallic, titanium based con-DOW 17 NH HF , Na Cr O , H PO4 2 2 2 7 3 4

HAE-A KOH, KF, Al(OH) , KMnO , Na PO version coating to achieve the desired results. This process3 4 3 4

HAE-B KOH, KF, Al(OH) , KMnO , Na PO3 4 3 4 is outlined in Table 8.U-5 Na SiO , carboxylate, fluoride2 3 Powder coating has also been used to coat magnesium


Table 8 and submerging it in a tank that contains positivelyProcess for powder coating magnesium alloys charged paint. The paint is attracted to the metal to form aStage Operation Condition uniform coating that is subsequently cured by baking

[162]. These coatings provide some protection against1 Alkaline clean 3–5 min, 140–1608C2 Tap water rinse chipping, cracking, abrasion and corrosion. However, the3 Etch primer 1–2 min coating is quite thin therefore it should be combined with a4 Tap water rinse thicker top coat. Air pocket formation can also be a5 Conversion coating 2 min, Alodine 5200 (Henkel)

problem with this technique therefore parts should be6 Water rinse Deionizeddesigned to eliminate blind holes.7 Blow-off oven

8 Dry-off oven 2508F In a study to improve the protective coatings on9 Powder coating magnesium aircraft components [163] it was determined

that coatings that had a total thickness of 2 mil (550.8mm) performed the best in salt fog corrosion tests. Theauthors determined that the best corrosion protection of the

castings of engine valve covers. This company discovered aerospace magnesium alloys ZE41A-T5 and WE43-T6 wasthat pretreatment with a process using chromic acid was obtained using the following schemenecessary in order to achieve acceptable adhesion of the

Non-chromate conversion coating → E-coat → Primercoating [159].Powder coatings can be applied in a number of ways → Resin Sealer

including electrostatic powder spraying, fluidized bed orflame spraying of thermoplastic powders. Flame spraying This process eliminates the environmental hazard posedhas been used in the application of ethylene acrylic acid by using traditional chromate containing conversion or(EAA) copolymers on a variety of substrates [160]. In this anodized coatings and was deemed to have excellentprocess the plastic powder is propelled through a flame corrosion resistance due to the high impedance values ofthat heats and melts the polymer and the surface so that the the coating although atmospheric corrosion studies had yetcoating particles coalesce and flow into a continuous to be completed at the time of publication.coating. The EAA polymers have been shown to haveexcellent adhesion to metals due to the acrylic acidfunctional groups, which promote adhesion by hydrogen 2.7.4. Sol–gel processand ionomeric bonding to the substrate. The surface Synthesis of gels by the sol–gel process involves thepreparation is critical for obtaining optimum coating hydrolysis and condensation polymerization of metal al-adhesion. The more free metal present, the more ionomeric koxides. This process can be used to produce polymericbonding and consequently the more adhesion. On mag- networks of inorganic–organic composite materials. It isnesium this poses a problem due to the very fast oxidation possible to form adherent, uniform coatings on metalof the surface in air. Any oil or chemical contamination of surfaces by the addition of components, to the reactionthe surface can also lead to a loss of adhesion or pinholes mixture, that are reactive with the surface that is to beformed by degassing of the contaminant during heating. coated. This process has been shown to produce corrosion-The processing conditions must be carefully controlled in protective coatings on aluminum alloys by a simple wetorder to achieve the desired adhesion, corrosion resistance, coating technique through the formation of a stable tailoredlow temperature flexibility and elongation and abrasion interface [164]. It is also possible to produce coatings withresistance. Factors such as particle size and distribution, high scratch and abrasion resistance through the in situresidence time in the flame, proper blending of additives generation of nano-particles in the coating [165]. Theseand degree of cross-linking have a significant effect on the coatings were tested on magnesium, aluminum and zincedquality of the coating produced. steel. The coatings produced were transparent with excel-

Satisfactory corrosion resistance in salt spray tests and lent adhesion, scratch and abrasion resistance, and corro-combined corrosion cycle tests has been demonstrated with sion protection. These coatings were shown to be superiorthe use of an epoxy based powder coating system on to an epoxy coating due to the formation of a very stablemagnesium substrates [161]. For this process an epoxy tailored metal /nanocomposite interface and the presence ofbased powder containing a curing agent comprised of a the inorganic backbone in combination with in situ gener-polyester resin with carboxyl groups or dibasic acid ated particles. One of the primary advantages of thisdihydrazide groups was found to be most effective. technique is the excellent adhesion obtained with a mini-

mum of sample pretreatment. The metal surfaces studied inthese examples were simply degreased, rinsed and dried

2.7.3. E-coat prior to dip-coating in the sol–gel mixture. Samples areE-coat or cathodic epoxy electrocoat is a process for then cured at relatively low temperature (100–2208C) to

painting metal surfaces by charging the metal part negative give the final product [164,166]. It has also been demon-


strated that the coatings can be pigmented to give a colored 3. Coating applications in the automotive industrycoating [164,165].

In order to obtain the best surface finish in terms ofcorrosion and wear protection, decorative appearance and

2.7.5. Polymer platingmanufacturing ability, it is often necessary to encompass

Polymer plating is the electrochemical polymerization ofseveral of the coating techniques discussed. This approach

a polymer film on the surface of a substrate that functionshas been successfully applied to the production of a Mg–

as one of the electrodes in the electrochemical cell. ThisAl hybrid hatchback for Volkswagen’s 3-l car [170]. The

process has been used to deposit triazine disulfide poly-inner component of the hatchback is fabricated from die

mers on the surface of magnesium alloy AZ91D [167,168]cast magnesium alloy AM50 and covered with an

by the electrochemical processing of triazine dithiols at thealuminum panel from the outside. In order to prevent

surface of a magnesium anode. Studies on the structure andgalvanic corrosion it was necessary to precoat the mag-

composition of the films showed that oriented films with anesium component, which was then bonded and folded

layered structure composed of organic polymer, Mg(OH)2 with the bare aluminum part. The magnesium part under-and MgO were produced. It was determined that the

went cleaning, acid pickling, chromating, E-coating andhighest polymer deposition rate with a minimum of side

finally powder coating prior to bonding and folding to thereactions could be achieved by using 1 M NaOH as the

pickled aluminum part. The whole component was thenelectrolyte and performing the plating operation at a

cleaned, phosphated and E-coated to provide maximumtemperature of 208C. The polymer films produced were

protection. This study demonstrates how magnesium canfound to suppress corrosion by suppressing the transfer of

be adequately protected from corrosion for use as automo-water, oxygen and electrons to the magnesium surface.

tive body components.Corrosion protection was found to depend on the thickness

A multicomponent coating process for finishing mag-of the film produced and the type of alkyl substituent on

nesium die castings with class A surface quality has alsothe monomer. The best corrosion protection was observed

been developed [171]. Class A refers to the glossy, smoothfor the N(C H ) substituent and was most likely due to6 13 2 appearance that is required for readily visible, outerthe alkyl chains in the polymer film, which were highly

surfaces of automobiles. Die castings are particularlyoriented and closely packed. Corrosion protection was also

difficult to finish due to the presence of flow marks, hotfound to increase with increasing film thickness.

tears and contraction cavities. These cavities tend to outgasA minimum number of pretreatment steps were required

during thermal curing of coatings creating pinholes in theprior to polymer plating. In this instance the samples were

final film. It is possible to eliminate these by grinding ormechanically polished followed by degreasing in acetone

sanding the surface prior to coating however this poses aand hot air drying. The corrosion resistance of these

serious environmental hazard due to the explosive naturecoatings was evaluated by electrochemical impedance

of magnesium dust. The standard coating system currentlyspectroscopy (EIS) however, the adhesion, salt-spray

applied to outer automobile surfaces does not meet class Aresistance and scratch and abrasion resistance of the

requirements when used on magnesium alloys. The authorscoatings was not evaluated. This process is still in its

[171] have developed a coating system that shows goodinfancy and will require more study before a marketable

paint compatibility, adhesion, corrosion resistance andprocess can be developed.

decorative appearance for die cast magnesium substrates.The process involves applying a conversion coating (chro-

2.7.6. Plasma polymerization mate, phosphate–permanganate or fluorozirconate) to thePolymeric coatings can be applied from the gas phase by surface followed by the application of a filler and finally

exposing a substrate to a reactive gas in the presence of a painting with a basecoat and clearcoat. The filler used is aglow discharge plasma. This has been used to coat silicone-modified polyester resin that has improved level-magnesium used as actuator arms in disk drives [169]. The ing behaviour and good corrosion inhibition.process outlined in this patent involves exposing themagnesium to a fluorinated alkane, under vacuum, in thepresence of a glow-discharge plasma. This results in the 4. Conclusionsformation of a thin uniform fluorocarbon coating on thesurface. The low surface energy coating produced is The examples described in the previous section demon-resistant to atmospheric water, chlorine and oxygen. It is strate that it is possible to develop appropriate coatingalso tough, scratch-resistant and smooth. These coatings schemes for the protection of magnesium for use inwere not evaluated for their corrosion resistance in salt- automotive components. However, to date, no singlespray conditions and while they are most likely adequate coating technology has been developed which functions tofor use in a computer system, a more rigorous surface adequately protect magnesium from corrosion in harshtreatment is most likely required for harsher service service conditions. The current coating schemes are com-conditions. plex, multilayer systems that incorporate many different


technologies and must be conducted very carefully in order by protecting the substrate at defect sites in overlyingto achieve optimum results. layers.

There are a number of patents that claim to have coating Anodizing is the most widely commercially used coatingprocesses for magnesium and its alloys. While some of technology for magnesium and its alloys. This process isthese have direct evidence to support the use of these technologically more complex than electroplating or con-technologies on magnesium, there are a large number of version coating but is less sensitive to the type of alloyclaims that only provide direct evidence that the coating being coated. It does involve more capital investment duetechnology works on aluminum and its alloys. The chemis- to the need for cooling systems and/or high powertry of aluminum is quite different from that of magnesium consumption but this may be balanced by the decreasedand it is therefore possible that these coatings may not cost of waste disposal. While there are anodizing processesperform as well on magnesium. that make use of chromates, there are many alternative

There are a number of factors to be considered when treatments that are much more environmentally friendly.developing a coating process for an industrial application. The coatings produced by anodizing are porous ceramic-These include capital investment, ease of manufacturing, like coatings. These properties impart good paint-adhesioncoating performance and environmental issues. characteristics and excellent wear and abrasion resistance

In the case of electrochemical plating, the capital to the coating. However, without further sealing, they areinvestment is relatively small. However, there are some not adequate for use in applications where corrosionserious concerns over waste disposal. The most successful resistance is of primary importance. The coatings producedexample of electroless plating technology for magnesium are brittle ceramic, insulating materials and are not appro-has a maximum turnover rate of 6, when a strict replenish- priate for load bearing applications or applications wherement schedule is followed. Further research is required to electrical conductivity is required.enhance the longevity of plating baths and to decrease The use of gas-phase coating processes and laser surfacewaste generation. The use of toxic chemicals such as melting /alloying/cladding to modify the surface or createchromium compounds, cyanide compounds and fluoride coatings on magnesium is an excellent alternative withcompounds in the pretreatment and plating baths also respect to its environmental impact. These techniquesnecessitates further research into the development of produce very little and in some cases no hazardous waste.‘green’ plating technologies. Another challenge associated However, the capital cost associated with these techniqueswith electroless plating on magnesium is the narrow is much higher than solution phase coating technologies.window for operating conditions in order to obtain op- These coating processes also tend to be line-of-sight whichtimum coatings. The difference in the surface chemistry of makes it difficult to uniformly coat complex shapes andvarious magnesium alloys also represents a significant inside holes or deep recesses. Finally, the corrosion,challenge in development of uniform, pore-free coatings. adhesion and wear properties, of these coatings, on mag-Despite these challenges, this technology does have the nesium, have not been widely documented.potential to produce uniform, corrosion and wear resistant Organic coatings are extremely versatile and can becoatings with good electrical conductivity and solderabili- applied to many metals provided an appropriate pretreat-ty, at a low cost. Electroplating does not have as much ment can be developed for the substrate. The adhesion andenvironmental impact due to the relatively long bath life of corrosion resistance of these coatings are inadequatethe plating solutions. However, achieving uniform coatings without pretreatment. Organic coatings are typically theon complex shapes can be extremely difficult due to last step(s) in a coating system. They may be applied for auneven throwing power of the current required for metal purely decorative effect or to enhance the corrosiondeposition. resistance of the overall coating system. It is usually

Conversion coatings also represent a minimum capital necessary to apply multiple layers of these coatings toinvestment, however the most widely used type of conver- provide optimum corrosion and wear resistance due tosion coatings are chromate conversion coatings. These difficulties in obtaining perfectly uniform, pore-free coat-represent a serious environmental risk due to the presence ings. Some organic coating technologies such as polymerof leachable hexavalent chromium in the coatings. A plating, sol–gel coating or plasma polymerization require anumber of chromate free conversion coatings are under minimum of pretreatment steps prior to deposition. How-development but this technology is still in its infancy. ever, the adhesion, and corrosion and wear resistance ofSimilar to electrochemical plating, the difference in surface these coatings have not been widely documented. Therechemistry of various magnesium alloys represents a signifi- are some environmental concerns with the use of solventcant challenge in development of uniform, pore-free borne organic coatings but the development of water-bornecoatings. Conversion coatings do not provide adequate and powder coating technologies has led to a decrease incorrosion and wear protection from harsh service con- the use of these chemicals.ditions when used alone. However, they can act as a good There are a large number of coating technologiesbase for producing adherent organic coatings and act to available for protecting magnesium and its alloys. How-enhance corrosion resistance of a combined coating system ever, the widespread use of magnesium in the automotive


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Gray, 2001 Reviw