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Chapter 1

Corrosion and Its Control by CoatingsGordon P. Bierwagen

Department ofPolymers and Coatings, North Dakota State University,Fargo, ND 58105

Corrosion protection of metallic substrates has long been one of the key rolesperformed by organic coatings. Such coatings remain one of the most cost-effectivemeans of providing practical protectionfromcorrosion to easily corrodible metallic(and sometimes non-metallic) structures and objects. This choice of a coating by(material + application cost) only has created a mentality so widespread that littlebasic research has been done in recent years towards significantly improving theperformance of corrosion control coatings or developing new measurement methodsfor their assessment. There are several technical organizations besides the ACS towhich the corrosion control by organic coatings is very important. The NationalAssociation of Corrosion Engineers (NACE), Steel Structures Painting Council(SSPC), the Electrochemical Society (ECS), and the Federation of Societies forCoatings Technology (FSCT) are all very much interested in this topic and holdregular symposia on this topic. But, this book based on a Symposium held by theACS-Polymeric Materials: Science & Engineering (PMSE) Division is unique in thequality and insight into the chemistry of howandwhy coatings control corrosion, andillustrates the need for more PMSE Symposia in this area. During the sessions atwhich most of the chapters published in this book were presented orally, theattendance was quite high, indicating, even in oral presentation, these papers eliciteda considerable interest with large attendance at the Symposium.

Fromthe user's point of view, corrosion control by coatings is very important,especially for those objects and items that are subject to environments that causecorrosion. Many users would like to have to paint/coat an object only once forcorrosion protection, and then assume appearance and function will maintain. This,of course, does not occur, but when failure in corrosion protection of a coatingoccurs, the function of the coated object can be threatened. The main goal of users ofcorrosion protective coatings is to provide protection of the coated object as long aspossible. Often this desire and need for corrosion protection by a coatingextendsbeyond just the intact coating, as the userwishes the coating to protect areas of thecoated object that have undergone minor damage in handlingor use. Thus, thecoating is not just a barrier layer between the object and its environment, but should

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In Organic Coatings for Corrosion Control; Bierwagen, G.;ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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alsoact to protect small, local areas of substrate exposed by damage, as well asstopthe spread of the damage.

A major difficulty for coating users is toassessthe present stateof protectionagainst corrosion in a coated system after it has been exposed to its environment.Visual observation is very difficult for many parts of objects, and certain items, suchas underground pipelines and storage tanks, are impossible to view without muchspecial effort. A user would like to know a specific time to carefully check theperformance of his corrosion protective system, either by a known lifetime ofperformance of the coating or by a measurement that can be made on an intactcoating systemthat will predict remaining lifetime. Most coating users alsodesire acoating that generates no hazardouswaste in applicationand removal.

Developers/designers of corrosion protective coatings need new materials toreplace hazardous materials in coatings and new test methods for evaluating newcoatings and new materials. Many of the more efficient materials used in the past forcorrosion control are considered to be toxic or hazardous. There is a considerablelist, but recently the elimination of these materials makes it difficult to use pastcoating formulation practice as any guide to new developments. Working withtotally new materials also makes lifetime prediction for new coatings very difficult,as there is no past history of performance to use as a guide for such predictions.Further, salt fog testing according to ASTM 117 has been shown to be a poorpredictor of performance ), but until very recently, no substitute has been put forth,and many coating specifications still requirea certain performance in this test.

Fundamental research on corrosion control by coatings has been addressingboth the issues of new testmethods and new materials. This has been difficult in anera where there has been decreasing support for long-term research. Withinmanycompanies, long-term research has been severely curtailed, especially among metalmakers and coatings manufacturers. Support for fundamental research on corrosioncontrol by coatings has been limited, but the focus both in government labs andacademia has been to find better methods of predictingprotective lifetimes. Couplingelectrochemical methods and cyclic exposure methods has shown promise(2). Also,several questions remain unanswered: such as why and how do chromate pigmentsand pretreatments work, what can replace chromtes - especially for protection ofAlalloys, what gives true wet adhesion to metals in coatings, and what measurement(s)give the best predictions of in-field performance of coatings.

However, the situation concerning corrosion control coatings is now rapidlychanging. The long pending imposition of rules and regulations severely limiting oreliminating chromate-based metal pretreatments and chromate pigments in coatings iscoming to pass. Restrictions on the handling of hazardous materials is makingmanufacture, application, and removal of coatings containing hexavalent chromiumin either the coating pigmentation or in the metal pretreatment very difficult, soon tobe almost impossible. For example, the USAir Force (USAF) has a goal of Cr-free(pretreatment + coating) systems by the year 2000. Current USAF aircraft coatingsare based on SrCr04 pigmented primers and chromate-based anodizing as theaerospace aluminum alloy pretreatment. A new pretreatment and pigmentationparadigm is needed to replace the current Cr-based systems, especially for structuralaerospace Al alloys.

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Further, the ubiquitous continuous salt fog test for corrosion control performanceby coatings is acknowledged by most workers in corrosion control assessment to beof little value for the prediction of the performance of environmentally compliantcoating systems. Yet, most government and many commercial specifications forcoatings have historically requiredpassing up to 2000hours of this test. Why such atest method, apparently designed for the assessment of lead and chromate basedpigmentation of solvent-bome alkyd coatings, remains so widely incorporated intospecifications in place today is a mystery to manyand a problem for many users andsuppliers of corrosion control coatings. Most of the newer environmentallycompliant coating technologies such as powder coatings, high solids and water-bornesystems perform worse in regular salt-spray testing than their equivalent solvent-bome coatings, but under use conditions, they perform better. This is beingacknowledged in the move within users and manufacturers of corrosion controlcoatings in the shift to cyclic testing such as Prohesion and the objective predictiveelectrochemical methods of electrochemical impedance spectroscopy (EIS) andelectrochemical noise methods (ENM).Corrosion Control by CoatingsCorrosion of metal objects occurs by electrochemical reactions at the surfaceinvolving the oxidation of the metal in the presence of water, electrolyte andoxygen.Most metals, except for the so-called noble metals, are most stable as oxides undermost ambient conditions. Coatings are often used as a protective layer over the metalsubstrate to prevent the substrate from oxidizing in a manner deleterious to thefunction and appearance of anobject. They do so in several ways (5). First, they actas a barrier limiting the passage of current necessary to connect the areas of anodicand cathodic activity on the substrate. This occurs especially if the coatingwets thesubstrate surface very well and has good adhesion in the presence of water andelectrolyte. Coatings do notreally stop oxygen sufficiently to make its concentrationat the surface rate limiting, and they do not completely stop water ingress intocoatings. However, a good barrier coating slows water and electrolyte penetrationand is not displaced by water at the substrate/coating interface. Barrierproperties cancome mainly fromthe polymer or frompigment volume concentration effects. Aspigments block diffusion of water and oxygen below the critical pigment volumeconcentration (CPVC), increasing the pigment volume concentrations improves thebarrierpropertyof coatings. If the CPVC is exceeded, even locally, voids allow easypassage of water to the substrate surface and the barrier properties are lost. Coatingsthat act as barriers usually give better protection as their film thickness increases(without imperfections) or they are applied in multiple layers.

Second, coatings can act to release inhibitor materials that passivate thesubstrate or block the corrosion reactions. These are usually primer coatings thatcontain inhibitive pigments such as chromtes, phosphates or molybdates. Coatingssuch as this will protect damaged areas of coatings by stopping corrosionreactions onlocal areas of the surface exposed by physical damage. Some coatings use solubleorganic inhibitors, but these often leach out the film too rapidly to give long termprotection.

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4Third, coatings can provide cathodic protection to a substrate if they are

formulated with a metal pigment that is more electroactive than the substrate. This ismost commonly done with zinc powder used over steel or iron. Other metal powdersthat might provide cathodic protection are too reactive in particulate form (Mg) orformoxide films that prevent electrical contact between particles. The metal pigmentvolume concentration must exceed the CPVC to have all of the particles touching andalso in contact with the metal surface. These are the so-called Zn-rich coatings oftenused as primers for steel objects where galvanizing cannot be used. This type ofcoating provides protection for damaged areas, but must be overcoated by a topcoatto keep the metal pigments frombeing directly oxidized byatmospheric exposure.Coatings Used for Corrosion ControlThere are many coatings on the market today that offer some form of corrosionprotection to metal substrates. Major use areas where corrosion protective propertiesof coatings are a preeminent requirement for so-called Original EquipmentManufacture (OEM) or factory-applied coatings are: automotive coatings systems;appliance coatings; metal coil coatings; powder coatings for heavy duty use,especially pipeline coatings (usually identified as a class as fusion-bonded epoxycoatings); farm and construction equipment coatings; and general use coatings forobjects used in exterior exposure, such as lawn furniture, metal window frames, etc.This list is not all-inclusive, but identifies major OEM areas where corrosionprotection by coatings is important. Field applied coatings where corrosionprotection is of primary concern are aircraft coatings, pipeline coatings in the field,marine coatings, railroad car coatings, the general area called industrial maintenancecoatings - general purpose coatings for exterior protection- bridges, decks, industrialplants, storage tanks, exterior metal structures, etc. These coatings are often multiplelayer systems with the primer coatings (first layer next to the metal substrate and itspretreatment) usually designed to provide the corrosion protection in damaged areasand the overcoat(s) providingbarrier andUV protection to underlying layers.Polymers that are most successfully used forcorrosionprotective coatings are epoxy-based materials;polyurethane based polymers, urethane topcoats over epoxyprimers,some cross-linked polyester materials, and some melamine cross-linked polymers.One thing in common among these polymers is their ability to wet and adhere tometal oxides, plus their stability in the presence of water and basic conditions.Coatings that can be cross-linked at relatively high temperatures in thin films giverelatively good performance. One other characteristic of successful corrosionprotective coatings is that they can be applied in relatively defect free films. Filmthickness uniformity is very important for corrosion protection (4). There is muchinformationfromsuppliers and tradepublications about the relative merits of variouscoating systems, and it is suggested that the reader see these sources fordetails.Coating systems work successfully only when the metal surface is well cleaned. Infactory use, when the metal often receives a pretreatment to form a protective oxideor related material. This is often done using chromate baths for Al alloys andphosphate-based pretreatment for steels. These baths are often acidic to remove priorsurface oxides and leave a controlled oxide surface with chromate or phosphateincorporated. Again, this is a field withmuch information on field use available from

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suppliers and trade literature. One issuethat usersof thesepretreatments must face isenvironmental legislation, especially on waste material fromthese systems, whichcanbe toxic, especially chromate baths. Dry pretreatments or environmentally benignsystems that put down thin adherent protective layers are now being examined toreplace earlier systems. Plasma cleaning and deposition as well as sol-gel chemistryare being examined, especially in systems with high maintenance and refinishingcosts. In production, %ths of the coating line space and cost is often devoted tocleaning and pretreatment. Also, galvanizing is being used on many sheet steelsystems, especially for automobile, siding and appliance use. This often provides amore uniform surface for coating than untreated steel, as well as providing cathodicprotection to thesteel.

Measurement ofCorrosionProtection by CoatingsThe area of corrosion protection by coatings is that is currently undergoing the mostchange is the areaof testing of performance. It is safe to stick withproventests, andmany specifications have existing tests included. However, the single most used testmethod, the continuous salt spraytesthas significant weaknesses. It is in the processof being replaced by other tests, which have been shown to be better predictors ofperformance. As new technologies have developed to provide coatings that protectagainst corrosion while reducingVOC and the use of toxic pigments and inhibitors,older testmethods have not been always able to identify correctly thosenew coatings,which provide improved protection. There is a new generation of testmethods thathave been developed that provide objective, numerical characterization of coatingperformance, or improvedranking types of tests that while still subjective, providebetter predictionof new coating performance. The numerical testmethods are basedon electrochemical methods, and they include Electrochemical ImpedanceSpectroscopy (EIS) (5), and Electrochemical Noise Methods (ENM) ( , amongothers. Many of the chapters of thisbook include work based onthesemethods, andhow they are being used for the study of organic coatings over metals.Other developments in test protocols for determining coating performance againstcorrosion have included cyclic testing methods (7) such as the Prohesion cabinettest, alternating wet-dry cycling of coatings, and using UV exposure in the cyclicexposure of coatings. These exposures are now being coupled to some of theelectrochemical methods just mentioned for more realistic studies of coatingperformance.Lifetime and Cost Issues inCoatings forCorrosion ProtectionAs stated above, coatings remain one of the most cost-effective means of providingpractical protection from corrosionto easily corrodible metallic (andsometimes non-metallic) structures and objects. But often they are chose by initial investment costonly. The use of organic coatings for corrosion control (the term control is usedbecause, in a thermodynamic sense, corrosion can never be eliminated, onlycontrolled to a low enough rate as to be ignored) is so pervasive in oursociety that istoo often taken for granted. Quality and effectiveness of corrosion control bycoatings is assumed by manyusersto be lowcost andeasy to achieve. For these and

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other reasons, users of corrosion control coatings often choose coatings only by(material + application) cost and appearance, not by cost effectiveness as measuredby their true performanceand lifetime ofthat performance. However, withhigh laborcosts and difficulties in recoating large, buried, difficult to reach or complex objects,more sophisticated coatings users are focusing on the total costs of corrosionprevention andcontrol. This leads to a realization that a coating system that provideslong use life but is somewhat more expensive initially for initial application will payfor itself in reduced maintenance costs and reduced need for expensive recoating.The more this reasoning is followed in analyzing the cost of corrosion protection bycoatings, the more the research into measuring and predicting the protectiveproperties of coatings will be performed. Analyzing the coating by its initial costalone makes a coating that significantly increases the lifetime of protection at asomewhat higher cost not well accepted in the marketplace. The payoff ondeveloping systems based on true lifetime costs has been shown in the automobileindustry, where the use of two sided galvanized steel + enhanced corrosion protectiveED primers has raised the average lifetime until noticeable rust damage on cars toabout 10 years.New TechnologiesNew technologies and materials for corrosion protection by coatings are coming intothe coatings science from other areas. The possibility of providing corrosionprotection by incorporating the use of conductive polymers, such as dopedpolyaniline, is being actively pursues by researchers. The extension of the thin filmtechnologies developed for the semi-conductor electronics industry to surfacepreparation of substrates for protective coatings is being pursued. Plasma cleaningand plasma deposition of thin films for subsequent coatings is being examined, aswell as the use of sol-gel thinfilmsfor surface pretreatment. Both of these latter mayreplace Chromate-based pretreatments for metals. The in-situ sensing of the state ofcorrosion protection in a system by implanted electrodes, and some other nondestructive testing method is being considered by large users of coated metals asanother way of doing maintenance on a need basis, not only on a regular cycle.Because of the cost of the objects that they protect and the large costs ofmaintenanceand repainting, the corrosion protective properties of organic coatings are moreimportant than ever. Any added lifetime of use of objects and materials that coatingscanadd is in actualitya significant contribution to the economy and the environment.SummaryCorrosion protection is a key property of organic coatings, and their use for thispurpose is a key contribution that coatings make to the world economy. If coatingscontinue to receive the minimal attention from the many users and developers ofcoatings that treat coatings for corrosion protection as almost commodities, thecontinued investment of coatings suppliers and research agencies will significantlyslow. This will increase the burden that corrosion already has on our economy andalso increase the need for maintenance and repair. If we continue and increase ourinvestment in understanding and preventingcorrosionby well designed new coatings,

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7everyone will benefit. One can see this effect already in the drastically increasedlifetimes against corrosion of cars and household appliances vs. 10-15 years ago,effects due to a combination of improved substrates andcoatings.The symposium contains a good number of papers examining these newer testmethods, showing the value of electrochemical testing in combination of cyclicexposure testing, and also, showing how environmentally compliant coatings arerapidly displacing regularcoatings in corrosioncontrol use. Many of thechapters of this symposium book are devoted to the electrochemical testing ofcoatings, and the examination of the testmethods with respect to predicting coatinglifetimes, or at least ranking coatings within a cohort of candidate coatings. Thesepapers are Chapters 1-8, 11-12, 14, 34. The emphasis within these papers is toimprove numerical ranking of corrosion performance; to develop measurement toolsthat provide insight into what is happening at the metal/coating interface, as well aswithin the coating during exposure; and to provide insight into the mechanisms thatlead to the failure of the protection the coating affords to the metal. Some of theissues addressed in these papers are what is the proper exposure for the testing, whatshould the composition of the immersion electrolyte be for electrochemical testing,and what accelerating factors are valid to give failure within a laboratory testprocedure in a manner that properly emulates field service failure. Thestatistics ofsampling to predict coating failure is also considered (Ch. 18).

Localized measurement methods for examination of defect areas in coatingsare also described, including scanning acoustic microscopy (Ch. 10), localizedelectrochemical impedance spectroscopy (Ch. 2.), and SEM . These are all utilized togive insight into local failures in the coatings. There is also a paper discussing therelationship between defects, localfluctuations n filmthicknesses and other coatingproperties, and corrosion protection (Ch. 16). Another paper gives predictivemodeling for the formation of a common local defect noted in corrosion failure ofcoatings, blister formation (Ch. 17). Several papers address the important issue ofwater uptake anddiffusion in protective coatings, andhow water transport in coatingsand its effects on coating properties is a key issue that requires attention vis a viscorrosion control (Ch 12-13).Several papers consider new thin-film technologies for corrosion protection. Onepaper (Ch. 21) considers self-assembled monolayers and multi-layers asfilmsfor theprotection of copper. Two papers address the formation of thin plasma-polymerprotective layers for improvement of the subsequent adhesion of thicker, standardcorrosion protective films (Ch. 19-20). Another examines the properties of electro-polymerized thinfilms(Ch. 23). There were also two papers addressing the still yetunresolved issues about the potential corrosion protection afforded by poly(aniline)films to metals (Ch. 30-31). This latter is an area of extreme interest, because therehave been indications that chemically doped conductive poly(aniline) can providecorrosion protection without the need for pigmentation, but solely due toelectrochemical effects. Two papers address the issues of microbial inducedcorrosion and its assessment in coated systems (Ch.25-26)The other papers consider a diverse range of problems in the use of corrosionprotective coatings. There are two papers that consider the specific problems ofaircraft protective coatings (Ch. 23 & 24), and one paper focusing on the protectionof concrete (Ch. 27), a topic closed allied to the protection of metals. Also examined

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are coatings for use in oil fields (Ch. 15) and marine anti-corrosion coatings (Ch. 28).There are also papers that address issues of the development of substitutes forchromtes in pigmentation (Ch. 32 & 33) and metal pretreatment (Schulman paper),new polymer matrices for corrosion protective coatings (Ch. 29), environmentallycompliant coatingsfromnatural products (34), and a chapter on M0S2 in protectivepoly(ethylene) films.Organizing and participating in this symposium has been a very satisfying experiencefor me, and I wish to take this opportunity to thank all of the presenters of papers atthe symposium and those that prepared the papers that make up the chapters of thisbook for their contributions. I wish to thank ACS Books for the opportunity toorganize and publish this symposium and the help that they have provided in makingthis book possible.Literature Cited(1). Skerry, B.S&Simpson,C.H. "Accelerated Test Method for Assessing Corrosion

and Weathering of Paints forAtmosphericCorrosion,"Corrosion 1993, 49B663674.

(2) ASTM D5894, Annual Book ofASTMStandards; Amer. Soc. Testing &Materials, West Conshohaken, PA, 1997.

(3) Bierwagen, G.P. "Reflections on CorrosionControl byCoatings," Prog. OrganicCoatings 1996, 28, 43-48.

(4) Bierwagen, G P. "Defects &Heterogeneities inCorrosionProtective OrganicCoatings Films and Their Effects on Performance" ACS Symposium Book,Corrosion andItsControl By Coatings, G.P. Bierwagen, ed.

(5) Fredrizzi, L .; Deflorian,F.;Boni, G.; Bonora, P.L. andPasini, E. "EISStudy ofEnvironmentallyFriendly Coil CoatingPerformances,"Prog. Org. Coatings1996, 29, 89-96.

(6) Mills, D. J .; Bierwagen, G. P.; Tallman, D.E. and Skerry, B.S. "Investigation ofAnticorrosive Coatings by the ElectrochemicalNoise Method," MaterialPerf.,1995, 34, 33.

(7) Appleman, B.R. "Cyclic Accelerated Testing:The Prospects for ImprovedCoatingPerformance Evaluation" J. Protective Coatings &Linings Nov. 1989,71-79. AndAppleman, B.R., "Surveyof Accelerated Test Methods for Anti-Corrosive CoatingPerformance" J. Coatings Tech. 1990, 62, (#787), 57-67.

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