Smart Coatings

8/9/2019 Smart Coatings

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SMART COATINGS FOR MAGNESIUMALLOYS

ABSTRACT

This project reviews the use of smart coatings for corrosion protection. Smart coatings are

coatings that respond to the environment. In general this response requires a trigger from theexternal environment. Triggers include the presence of moisture, pH, chloride ion concentration,mechanical damage, and temperature and redox activity. Diverse methods of self repair areoutlined as are a variety of ways of holding and releasing corrosion inhi!itors. Differing methodsof sensing in coatings !oth using responsive molecules and "ano sensors are outlined. #astly,the current and future needs for smart coatings are discussed.


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INTRODUCTION

$orrosion is a naturally occurring process, which is defined as the degradation or deterioration of a su!stance and%or its properties, usually a metal, over a period of time due toenvironmental exposure.

&mong various lightweight metals, 'g alloys are expected to !e used in the manufacturing of structural components for aircraft, automotive, electronics, and construction industries !ecause of superior physical and mechanical properties such as good electromagnetic shielding and highstrength%weight ratio ()% *+. oor corrosion resistance of these metals, however, hinders their use on alarger scale

-tili ation of light weight 'agnesium alloys as engineering materials has !een limited due totheir high affinity for oxygen and water, and hence vulnera!ility to corrosion. 'any efforts have !eenattempted to address the corrosion issue of 'g alloys to pave a path for wider implementation of 'g invarious applications, such as automotive, airplane, electronic devices and !iomaterials

SMART COATINGS FOR CORROSION PROTECTION

/ver the past decade there has !een very significant progress in the development of smartcoatings. Such coatings have the a!ility to !oth dramatically extend the life of structures and toadd additional functionalities to coated systems. Indeed it can !e argued that smart coatings are

fundamentally changing the way coatings systems are developed. 0ather than act as a rigid !arrier !etween the su!strate and the environment, smart coatings are designed to respond to theenvironment and, through that response, enhance the coating 1 s life and functionality.

TYPES OF SMART COATING

& review of the literature in this area would lead most readers to conclude that there is nosingle classification system used for smart or intelligent coatings. Indeed different texts classifythe literature differently. Some classification systems that could !e used might !e !ased onapplication, function, material types, level of complexity and responsiveness, among others. artof the complexity of defining new developments in this area is inherited from current coatingtechnology, which is classified on polymer type, functionality and end use, !ut with noindividual coating uniquely fitting into any of these classifications individually. It would !euniversally accepted that a smart coating will respond to triggers from the external environment.In theory this response to a trigger can !e automatic or can !e mediated !y some intelligentanalysis of the triggers. In practice the vast majority of research has concentrated on automatic or intrinsic responses where the coating will, !ecause of its inherent design or materials, respondwithout any additional (to the trigger+ external actions. To some extent an intrinsic response isalready seen in current chromate loaded coatings since solu!ility of chromate compounds is pH


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dependent, so larger doses of chromate will !e delivered at low pH typical of the acidicenvironment that develops as a result of attac2 of the metal. & mediated response would !ewhere sensing within the material allows information to !e processed within or external to thematerial and then the properties of the material are either altered automatically !y the material or through external intervention. This type of response is not yet developed although sensing

developments are providing tools which may in the future provide the !asis for such a response.To simplify the discussion, this chapter is confined to corrosion protection of a metal su!strate !ut includes coatings that may have a !uilt in signaling response to indicate to maintenance staff that some remedial action needs to !e underta2en. Signaling response not directly connected tocorrosion will also !e included as the mediated response outlined a!ove may integrate triggersfrom a variety of environmental factors. In 3ig. *.) a representation of a smart coating !ased onan automatic response is given. In the diagram a conventional coating is depicted consisting of a

primer and top coat. The diagram illustrates that -4 degradation could !oth degrade the top coatand act as a trigger that promotes the release of healing agent from capsules in the top coat torepair any damage in the top coat. Similarly water or ion transport through the paint fi lm to themetal surface !oth provides conditions that can lead to corrosion !ut also permits the release of

agents encapsulated in the primer that can heal any su!sequent corrosion. In 3ig. *.* arepresentation of a smart coating that responds through a mediated response is given. 3igure *.*a represents a response from within the material and 3ig. *.* ! a response external to thematerial. In 3ig. *.* a signal reaches sensory elements, either through defects in the coating or !y

penetrating through the coating. 5hen the sensory elements in a local area have detected acritical level of the triggering agent they can pass a signal on to a actuating agent when can thenrespond (for example !y releasing a repair agent+. 3igure *.* ! represents the case when theresponse occurs external to the fi lm. In this case when the sensory elements have detected acritical level of triggering agent they emit a signal to an external device that then promotes anexternal response.


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MOST COMMON METHODS AND TECHNOLOGIES FOR PRODUCTION OFSMART COATINGS

The advanced materials that have !een developed recently, such as composite materials,nanostructured materials, newly developed magnesium and aluminum alloys, requireincreasingly sophisticated coatings for improved performance and dura!ility to fulfill the modernindustry requirements. 5ith the environmental crisis due to increased industrial pollution such as


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glo!al warming, environmental compati!ility !ecomes an aspect that gains much attentionduring the design phase of new materials. Therefore, the process involving toxic hexavalentchromate coatings certainly would not match the target. 3urthermore, while most of theconventional protective coatings are just passive !arriers that prevent the attac2 of corrosivespecies with the metallic su!strate, the ultimate dream for the future is to engineer 6smart1

protective coatings that can provide multi functionalities including self healing capa!ilities, selfcleaning, antifouling and anti friction, etc. The following sections highlight the most commonmethod and technologies used for the preparation of smart coatings for corrosion protection.

) $hemical conversion coatings* "ano capsule and microcapsule !ased polymer coatings

7 #ayer !y layer (#!#+ self assem!ling molecule deposition8 Shape memory (S'+ and self healing coatings9 $ar!on nano tu!es: $lay nano tu!es; "ano porous titania interlayer < Self healing ion perm selective conducting polymer coating= Self healing and self cleaning super hydropho!ic coatings)> 5ater !orne smart coatings

! CHEMICAL CONVERSION COATINGS

The corrosion of metals is one of the main destructive processes that lead to hugeeconomic losses. olymer coating systems are normally applied to a metal surface to provide adense !arrier against the environmental species in order to protect metal structures fromcorrosive attac2. 5hen the !arrier is damaged and the corrosive agents penetrate the metalsurface, the coating system cannot stop the corrosion process. The most effective solution so far for the initial com!at for active protection of metals to improve the corrosion protection of themetals and alloys is to employ chromate containing conversion coatings.

$hromate has !een used since the early )=>>s as a way of controlling the corrosion of active metals. It is the most common type of conversion coating applied to improve the corrosion

resistance of many ferrous and nonferrous metals and their alloys. 'ajor reasons for thewidespread use of chromating are its self healing nature, the ease of application, high electricconductivity and high efficiency%cost ratio. These advantages have made it a 6standard1 methodof corrosion protection. $hromate has also !een considered as the 6pioneer1 smart coating due toits outstanding self healing capa!ilities to repair damage and corrosion in several metals andalloys. Self healing or active corrosion protection (&$ + involves the release of chromate fromthe coating, transport through solution and action at the site of damage namely pits or microcrac2s.


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"! NANO CAPSULE AND MICROCAPSULE#BASED POLYMER COATINGS

There are several techniques that have !een developed for repairing visi!le or detecta!ledamage on coatings. However, these conventional techniques are generally costly, time

consuming and complicated. In addition, most of them have a limited a!ility to deal with theinternal and invisi!le damage. &ccordingly, the development of self repairing coatings isexpected to fill this technological gap. & su!stantial amount of the current research activity isdevoted to polymer coating of self healing functionalities. The first attempt goes !ac2 to )=9:when S warc, pu!lished in Nature his article entitled 6#iving polymers1, predicting a new classof polymer of self repairing functionality or what we call nowadays 6self healing coatings1.Since that time, many efforts have !een invested to prepare such 2ind of polymers asanticorrosion protective coatings for industrial application. The whole concept of 6smart1coatings that react as a response to external stimuli (such as pH, humidity changes, or coatingdamage+ and repair themselves has experienced a tremendous !oost with the advent of nanotechnology. "anotechnology has !een used, for example, to improve sol?gel coatings.

Direct corrosion?inhi!itor incorporation in a sol?gel hy!rid matrix has proved difficult as selfhealing coatings due to the negative effect of the corrosion inhi!itor (mostly !en otria ole+ onthe sta!ility and !arrier properties of the sol?gel layer. In a recent study, #ama2a et al . showedthat Ti/ * nanostructured reservoir layer, which contains rutile and anatase phases, loaded withcorrosion inhi!itors enhances the self healing properties to the sol?gel films over &&*>*8 .0esults proved that deposition of a titanium oxide layer loaded with inhi!itors followed !y sol? gel fi lm is the most effective way to improve the corrosion protection of &&*>*8.

$! LAYER#BY#LAYER (LBL) SELF#ASSEMBLING MOLECULE DEPOSITION

"ano containers with controlled release of the corrosion inhi!itor have !een fa!ricatedusing layer !y layer (#!#+ deposition. The #!# process involves the stepwise electrostaticassem!ly of oppositely charged species (e.g. polyelectrolytes and inhi!itors or nanoparticles+ ona su!strate surface to form a coating of controlled permea!ility and release with multiplefunctionalities. The permea!ility of the polyelectrolyte multilayers depends on the nature of the

polyelectrolytes. /ther functions of the #!# film can !e adjusted !y changing the pH, ionicstrength, temperature or !y magnetic fields. It was reported that storage of corrosion inhi!itors inthe polyelectrolyte multilayer for possi!le application in protective coatings will have two

advantages@ ()+ isolate the inhi!itor avoiding its negative effect on the integrity of the coatingand (*+ provide intelligent release of the corrosion inhi!itor !y regulation of the permea!ility of polyelectrolyte assem!lies changing local pH and humidity. $hanging the pH is the most pro!a!le driving force to initiate the release of inhi!itor at the cathodic and anodic ones due tocorrosion. Therefore, a 6smart1 coating can 6feel1 corrosion and start the healing action.

% SHAPE MEMORY (SM) AND SELF#HEALING COATINGS


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CORRISION PREVENTION METHODS FOR MAGNESIUM ALLOYS-

There are two straightforward methodologies to reduce corrosion progress of 'g alloys. &dditionof alloying elements, such as &l, n etc, into the !ul2 'g matrix may lead to favora!le microstructure,grain si e or 2inetic restriction to corrosion (with attendant changes in properties and cost+. The other intensively exploited approach is applying sta!le and inert coatings onto the surface of 'g alloys as a

!arrier to provide protection functionality. $ompared to alloying, surface properties will !e altered rather than the fundamental properties of the !ul2 materials and desira!le protection can !e o!tained !y meansof simply and efficient processing procedures

The most successful commercial coatings are chromate !ased, however, the extreme$arcinogenicity to organisms and environments has led to a strict !an on the utili ation of chromatecoatings glo!ally. Therefore, there is an urgent need to develop alternative coatings to tac2le the corrosionissue of 'g alloys as chromate did.

There are a num!er of technologies availa!le for coating magnesium and its alloys. Theseinclude

*.) electrochemical plating *.* conversion coatings, *.7 hydride coatings, *.8 anodi ing, *.9 gas phase deposition processes, *.: laser surface alloying %cladding, *.; /rganic%polymer coatings.


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"! ! ELECTROCHEMICAL PLATING

It is often desira!le to alter the surface properties of a wor2 piece in order to improve itscorrosion and wear resistance, solder a!ility, electrical conductivity or decorative appearance.This can !e accomplished !y coating the part with a metal that has the desired propertiesnecessary for the specific application. /ne of the most cost effective and simple techniques for introducing a metallic coating to a su!strate is !y electrochemical plating. The plating processcan !e su!divided into two types@ electroplating and electro less plating. In !oth cases a metalsalt in solution is reduced to its metallic form on the surface of the wor2 piece. In electroplatingthe electrons for reduction are supplied from an external source. In electro less or chemical

plating the reducing electrons are supplied !y a chemical reducing agent in solution or, in the

case of immersion plating, the su!strate itself.


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"!"! ! CHROMATE CONVERSION COATINGS

$hromate conversion coatings can !e used as pretreatments prior to a final sealing process or as 6post treatments1 after a plating process to improve corrosion resistance, paint or adhesive !onding properties or to provide a decorative finish

There are a few general rules that should !e followed when applying a conversion coating, theseinclude

). Su!strates with fine grained microstructure respond !est to chromating.*. $o deposition of other metals is detrimental to the coating process7. roper cleaning and pretreatment of the surface is necessary to ensure optimum coatings.8. &fter chromating the su!strate should !e properly rinsed to remove any residual acid or

!ase which could react with the coating.9. The coating should !e air dried at low temperature (;> min.

"!"!"! PHOSPHATE . PERMANGANATE CONVERSION COATINGS

hosphate?permanganate treatments are !eing explored as an alternative to conventionalchromate conversion coatings. These treatments are more environmentally friendly and have

!een shown to have corrosion resistance compara!le to chromate treatments. & systematic studyon phosphate?permanganate treatment of & =)D and 5A87& alloys using a !ath containing


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potassium permanganate and sodium phosphate has shown that homogeneous, non powdery anduniform coatings can !e achieved. The phosphate concentration and pH of the conversion !athwere found to have the most effect on the quality of the final coating. The corrosion resistanceo!served was compara!le to chromate coatings. 3or & =) alloys a filiform corrosionmorphology with phosphate permanganate was o!served which minimi es the depth of pitting.

3or 5A87 the corrosion !ehaviour was the same for !oth chromate and phosphate? permanganate coatings.

"!"!$! FLUOROZIRCONATE CONVERSION COATINGS

3luoro irconate treatments have also shown promise as potential pretreatments for magnesium and magnesium alloy materials. The group I4 & elements such as titanium, hafniumand irconium are !elieved to form continuous three dimensional polymeric metal or metalloid? oxide matrices, from aqueous solutions, in a similar manner as chromium . This ma2es them anattractive alternative for environmentally friendly conversion coatings. These coatings may

provide corrosion protection through a galvanic setting or may act as a physical !arrier to theenvironment. This has !een exploited to produce corrosion resistant coatings, !y exposing ametal su!strate to an aqueous acidic solution of irconium ions, sta!ili ed in solution !y organicor inorganic oxy anion compounds. -pon drying a continuous polymeric irconium oxide layer

!ecomes fixed on the surface. In a related patent a conversion coating system composed of mixtures of group I4 & and group III & elements is proposed. The inventors !elieve that thesecoatings provide enhanced corrosion resistance over simple irconium oxide systems due to aredox component in the coating that mimics the chromate redox model. The preferredem!odiment of this patent uses cerium and irconium in com!ination.

"!"!%! STANNATE TREATMENTS

& process for creating stannate and incate !ased conversion coatings on magnesiumalloy & =)B)>.9E Si has !een developed . 3ollowing pic2ling and activation, the samples weretreated with one of a variety of immersion tinning solutions or a incate solution. The coatingsshowed some corrosion resistance !ut in !oth cases only thin layers were formed. Details on thecorrosion resistance of these coatings were not given, and further studies are warranted

"!$! HYDRIDE COATING

& technique for producing a magnesium hydride coating on magnesium and its alloys !yelectrochemical means has !een developed as an alternative to $r !ased conversioncoatings.This process involves treating the magnesium su!strate, which acts as the cathode, in anal2aline solution prepared !y adding al2ali metal hydroxide, ammonium salts or similar al2alinematerials. & supporting electrolyte may also !e added to decrease the solution1s resistance.However, the authors caution against the use of chlorides since $l poses the potential of corrosion of the wor2piece. rior to cathodic treatment the samples are mechanically polished,degreased with acetone and acid etched. The preferred treatment conditions and some specifictreatment processes are shown in Ta!les 7 and 8, respectively. The hydride coating thus produced


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has !een found to decrease the corrosion rate of & =)D alloy !y )%7 which is compara!le to thedichromate treatment.

"!%! ANODIZING

&nodi ing is an electrolytic process for producing a thic2, sta!le oxide film on metalsand alloys . These films may !e used to improve paint adhesion to the metal, as a 2ey for dyeingor as a passivation treatment. The stages for processing includeF ()+ mechanical pretreatment, (*+degreasing, cleaning and pic2ling, (7+ electro!rightening or polishing, (8+ anodi ing using d.c. or a.c. current, (9+ dyeing or post treatment and (:+ sealing. The films have a thin !arrier layer at themetal?coating interface followed !y a layer that has a cellular structure. Aach cell contains a porewhose si e is determined !y the type of electrolyte and its concentration, temperature, currentdensity and applied voltage. Their si e and density determine the extent and quality of sealing of the anodi ed film. $oloring of anodi ed films can !e achieved !y a!sor!ing organic dyes or inorganic pigments into the film immediately after anodi ing, !y a second step electrolyticdeposition of inorganic metal oxides and hydroxides into the pores of the film or !y a process

called integral color anodi ing. The latter is achieved !y adding organic constituents to theanodi ing electrolyte that decompose during the process and form particles which !ecometrapped in the film as it grows. The color may also !e controlled !y a process called interferencecoloring. Interference coloring involves control of the pore structure to produce color !yinterference of the light reflected from the top and !ottom of the pores G


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In this process the coating material which can !e metal, ceramic, cermet or polymeric isfed to a torch or gun where it is heated to a!ove or near its melting point. The resulting dropletsare accelerated in a gas stream onto the su!strate and the droplets flow into thin lamellar particlesand adhere to the surface. & num!er of coating techniques fall under this um!rella including

flame spraying, wire spraying, detonation gun deposition, plasma spray and high velocity oxyfuel.

Some of the advantages of this technique include the a!ility to create a coating of virtually any material that melts without decomposing, minimal su!strate heating duringdeposition and the a!ility to strip and recoat worn or damaged coatings without changing the

properties or dimensions of the part. /ne major disadvantage is that the process is line of sightand small deep cavities cannot !e coated, especially if the surface lies parallel to the spraydirection. These coatings also require sealing due to their inherent porosity and mechanicalfinishing to o!tain a smooth finish. /ne final disadvantage of this technique are the health andsafety issues generated !y the production of dust, fumes, noise and light radiation during

treatment.

"!&!"! CHEMICAL VAPOR DEPOSITION

$hemical vapor deposition ($4D+ can !e defined as the deposition of a solid on a heatedsurface via a chemical reaction from the gas phase. &dvantages of this technique includedeposition of refractory materials well !elow their melting points, achievement of near theoretical density, control over grain si e and orientation, processing at atmospheric pressureand good adhesion. This process is not restricted to line of sight li2e most physical vapor deposition ( 4D+ processes so deep recesses, high aspect ratio holes and complex shapes can !ecoated. Due to the high deposition rate that can !e achieved, thic2 coatings can !e produced.However, this process is limited to su!strates that are thermally sta!le at :>>


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coatings, these showed no sign of corrosion upon microscopic examination. Diamond li2ecar!on films on magnesium alloys with good lu!ricity, corrosion resistance, adhesion andsmoothness have !een produced using a $4D process. The surface of the alloy is su!jected to a$4D treatment using methane and hydrogen to form a diamond li2e car!on coating and thentreated with a high frequency plasma $4D process using car!on tetrafluoride. 3inally,

diamondpolari ation li2e car!on films using methane and a plasma source ion implantationmethod, to produce adherent films on magnesium, have !een reported. This process involvescreating a graded car!on composition interface !etween the metal and the diamond li2e coatingvia ion implantation.

"!&!%! PHYSICAL VAPOR DEPOSITION PROCESSES

4D involves the deposition of atoms or molecules from the vapor phase onto asu!strate. This process includes vacuum deposition, sputter deposition, ion plating, pulsed laserdeposition and diffusion coatings .

"!&!%! ! PVD ON MAGNESIUM!

The role of 4D processes in magnesium surface finishing can !e divided into twosections, the deposition of wear and corrosion protection coatings and the creation of !ul2 magnesium alloys with unique corrosion resistant properties

There are a few challenges to overcome in the 4D coating of magnesium su!strates.The deposition temperature must !e !elow the temperature sta!ility of magnesium alloys()>?99>


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line. However, the corrosion resistance of the flailed alloys was inferior compared to puremagnesium.

"!&!&! DIFFUSION COATINGS

Diffusion coatings can !e deposited !y heating the component to !e treated in contactwith a powdered coating material in an inert atmosphere. This process produces alloy coatings athigh temperatures !y the inward diffusion of the coating material into the su!strate material .0ecently this technique has !een used to create an aluminum diffusion coating on & =)Dmagnesium alloy The diffusion coating was formed !y heat treatment of the magnesium alloy inaluminum powder at 89> C% s. This process is a form of rapid solidification processing

!ut only the surface region is modified. The advantages of this technique include the a!ility totreat complex geometries, up to millimeters depth of treatment, lower operation cost and greater


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"! ! ! PAINTING

/ne of the most important steps in painting of magnesium is choosing an appropriate primer. rimers for magnesium should !e al2ali resistant and !ased on resins such as polyvinyl !utyral, acrylic, polyurethane, vinylepoxy and !a2ed phenolic . The addition of inc chromate or

titanium dioxide pigments is commonly used for corrosion prevention.& study of the corrosion resistance of die cast & =)D magnesium alloys with paint finishing has !een reported rior to painting, the samples were treated with either a conversion coating or ananodi ing process. The surface treatments studied are shown in Ta!le ;. The paint film wasapplied in two layers. The first layer was a primer containing an epoxy resin applied to athic2ness of *9?7> mm and cured at );> min. The final coat was an acrylic paint resinapplied to a thic2ness of *9?7> mm and cured at )9> min.

"! !"! PO,DER COATING

owder coating is a process in which a pigmented resinous coating powder is applied to thesu!strate and then heated to fuse the polymer together in a uniform, pinhole free film . owder coating is an excellent alternative to traditional painting processes since it is not detrimental tothe environment and uniform thic2 coatings can !e o!tained in a single operation even on roughsurfaces or edges. There is also little loss of coating material during application and even !asicresins that are not readily solu!le in organic solvents can !e applied However, there are a fewinherent disadvantages to this technique

). The powder must !e maintained in a very dry, pulveri ed form.2. Thin coatings are difficult to o!tain.3. $olor matching and color uniformity can !e difficult to maintain.4. $oating in recessed areas can !e difficult.

5. High temperatures required for curing may !e unaccepta!lefor some su!strates.


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"! !$! E#co/t

A coat or cathodic epoxy electrocoat is a process for painting metal surfaces !y chargingthe metal part negative and su!merging it in a tan2 that contains positively charged paint. The

paint is attracted to the metal to form a uniform coating that is su!sequently cured !y !a2ing .

These coatings provide some protection against chipping, crac2ing, a!rasion and corrosion.However, the coating is quite thin therefore it should !e com!ined with a thic2er top coat. &ir poc2et formation can also !e a pro!lem with this technique therefore parts should !e designed toeliminate !lind holes .

In a study to improve the protective coatings on magnesium aircraft components it wasdetermined that coatings that had a total thic2ness of * mil (99>.


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' "a/H as the electrolyte and performing the plating operation at a temperature of *>


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CURRENT PROSPECTS AND FUTURE TRENDS

Innovations in automotive and aerospace materials and their protective coatings are of increasing interest in !oth industry and academia due to competition among the larger manufacturers in the automotive and aerospace industries.

Two major needs are facing the coating industry and the industries that rely on protectivecoatings@

J The need to remove toxins and environmentally damaging chemicals from coating solutions.J The need for !oth fixed and mo!ile infrastructure for increasing periods as airplanes, power

plants, chemical plants are used long past their original design life.

The removal of environmentally unfriendly chemicals, including volatile organicchemicals (4/$+, chromate inhi!itors and fluoride from protective coatings, is !eing driven !y

!oth regulation and consumer sentiment. In many respects the replacement of chromate has !eenthe most challenging issue, since it performs extremely well as an inhi!itor and it has thus !eendifficult to find another inhi!itor that can replace it. The search for a replacement for chromatehas spawned a significant effort in developing high throughput inhi!itor assessment techniquesvia electrochemical and other means. The approach has changed from finding an individualchromate replacement to finding synergies !etween chemicals that, together, can !e used toreplace chromate. The use of mo!ile and fixed infrastructure present the protective coatingindustry with particular challenges as in many cases the painted structures are very difficult toget to or would require partial deconstruction of the o!ject. Thus it would !e of great advantageif the paint life could match that of the infrastructure. Such life extension could occur when theself healing technologies outlined in this chapter are commerciali ed.

3uture trends that may impact on the protective coating industry include@

J The need for weight reduction in our transport fleets to reduce costs and greenhouse gasesFJ The development of paint films with multiple functionalities (e.g. insulation, odour a!sorption,energy generation+FJ Development of intelligent maintenance systems for !oth fixed and mo!ile infrastructureFJ 0evision upwards of the design life of infrastructure.

Kreater fuel economies are !eing required (sometimes !y regulation+ of !oth land and air transport. /ne means of o!taining such economies is the replacement of heavy metals with lightalloys (particularly aluminium and 'agnesium+ not just for isolated components !ut for


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structural components and, where possi!le, engine parts. This will require the development of long life protective coatings for these suscepti!le metals and thus will provide opportunities for innovative smart coating systems. In part for weight reduction !ut also for production savingsthere are strong pressures to com!ine multiple functionalities in the one coating system. Thisaspect has not !een explored in detail in this chapter !ut many of the systems discussed that

allow coatings to change their forms or properties could !e used to promote such multifunctionality. Both military and civilian users of infrastructure are loo2ing for means to reducethe cost of maintenance while improving safety. Systems that can automatically diagnose thestate of a vehicle will !e critical to such intelligent maintenance systems and this presents a greatopportunity for !uilt in coating condition monitoring as detailed in this chapter. 0eali ing thatinfrastructure is currently used past its design life, future specifications are li2ely to mandatelonger design lives, providing opportunities for smart coatings.

CONCLUSIONS

The examples descri!ed in the previous section demonstrate that it is possi!le to developappropriate coating schemes for the protection of magnesium for use in automotive components.However, to date, no single coating technology has !een developed which functions toadequately protect magnesium from corrosion in harsh service conditions. The current coatingschemes are complex, multilayer systems that incorporate many different to achieve optimumresults.

There are a num!er of patents that claim to have coating processes for magnesium and itsalloys. 5hile some of these have direct evidence to support the use of these technologies onmagnesium, there are a large num!er of claims that only provide direct evidence that the coating

technology wor2s on aluminum and its alloys. The chemistry of aluminum is quite different fromthat of magnesium and it is therefore possi!le that these coatings may not perform as well onmagnesium.

There are a large num!er of coating technologies availa!le for protecting magnesium andits alloys. However, the widespread use of magnesium in the automotive industry is still deterred

!y the lac2 of appropriate protective coatings that can withstand harsh service conditions. & greatdeal of research is still required to develop !etter, simpler, cheaper coating technologies so wecan ta2e advantage of the lower weight and excellent mechanical properties of this material


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REFERENCES

Hand!oo2 of Smart $oatings for 'aterials rotection

Adited !y &!del Salam Hamdy 'a2hlouf 0eview article rotective coatings on magnesium and its alloys L a critical review M.A. Kray, B. #uanNwww.elsevier.com%locate%porgcoat M.'. #eu inger, $onversion coating of magnesium alloy surfaces D. 'a2oto, S. 'itsuo, T. Susumu, $oating pretreatment and coating method formagnesium alloy product "ovel smart stannate !ased coatings of self healing functionality for & =)D

magnesium alloy &!del Salam Hamdya, , D. . Butt!4anadia !ased coatings of self repairing

functionality for advanced magnesium Ale2tron A8) 'g? n?rare earth alloy

&!del Salam Hamdy a,!, , I. Doench a, H. 'Ohwald a Smart self healing anti corrosion vanadia coating for magnesium alloys

&!del Salam Hamdya,!, , I. Doench!, H. 'Ohwald! 0ecent progress in corrosion protection of magnesium alloys !y organic coatings

0ong Kang Hua, Su hanga, Mun 3u Bua, $hang Mian #ina, , Kuang #ing Song!, rotective coatings on magnesium and its alloys L a critical review

M.A. Kray, B. #uanN
http://www.elsevier.com/locate/porgcoathttp://www.elsevier.com/locate/porgcoat

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