Corrosion and Its Control by Coatings - ACS Symposium Series (ACS

Chapter 1

Corrosion and Its Control by Coatings

Downloaded by CHRISTIAN ALBRECHTS UNIV KIEL on November 7, 2014 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch001

Gordon P. Bierwagen Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105

Corrosion protection of metallic substrates has long been one of the key roles performed by organic coatings. Such coatings remain one of the most cost-effective means 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 little basic research has been done in recent years towards significantly improving the performance of corrosion control coatings or developing new measurement methods for their assessment. There are several technical organizations besides the ACS to which the corrosion control by organic coatings is very important. The National Association of Corrosion Engineers (NACE), Steel Structures Painting Council (SSPC), the Electrochemical Society (ECS), and the Federation of Societies for Coatings Technology (FSCT) are all very much interested in this topic and hold regular symposia on this topic. But, this book based on a Symposium held by the ACS-Polymeric Materials: Science & Engineering (PMSE) Division is unique in the quality and insight into the chemistry of how and why coatings control corrosion, and illustrates the need for more PMSE Symposia in this area. During the sessions at which most of the chapters published in this book were presented orally, the attendance was quite high, indicating, even in oral presentation, these papers elicited a considerable interest with large attendance at the Symposium. From the user's point of view, corrosion control by coatings is very important, especially for those objects and items that are subject to environments that cause corrosion. Many users would like to have to paint/coat an object only once for corrosion protection, and then assume appearance and function will maintain. This, of course, does not occur, but when failure in corrosion protection of a coating occurs, the function of the coated object can be threatened. The main goal of users of corrosion protective coatings is to provide protection of the coated object as long as possible. Often this desire and need for corrosion protection by a coating extends beyond just the intact coating, as the user wishes the coating to protect areas of the coated object that have undergone minor damage in handling or use. Thus, the coating is not just a barrier layer between the object and its environment, but should

©1998 American Chemical Society

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

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2 also act to protect small, local areas of substrate exposed by damage, as well as stop the spread of the damage. A major difficulty for coating users is to assess the present state of protection against 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, such as underground pipelines and storage tanks, are impossible to view without much special effort. A user would like to know a specific time to carefully check the performance of his corrosion protective system, either by a known lifetime of performance of the coating or by a measurement that can be made on an intact coating system that will predict remaining lifetime. Most coating users also desire a coating that generates no hazardous waste in application and removal. Developers/designers of corrosion protective coatings need new materials to replace hazardous materials in coatings and new test methods for evaluating new coatings and new materials. Many of the more efficient materials used in the past for corrosion control are considered to be toxic or hazardous. There is a considerable list, but recently the elimination of these materials makes it difficult to use past coating formulation practice as any guide to new developments. Working with totally 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 Β117 has been shown to be a poor predictor of performance^), but until very recently, no substitute has been put forth, and many coating specifications still require a certain performance in this test. Fundamental research on corrosion control by coatings has been addressing both the issues of new test methods and new materials. This has been difficult in an era where there has been decreasing support for long-term research. Within many companies, long-term research has been severely curtailed, especially among metal makers and coatings manufacturers. Support for fundamental research on corrosion control by coatings has been limited, but the focus both in government labs and academia has been to find better methods of predicting protective lifetimes. Coupling electrochemical methods and cyclic exposure methods has shown promise(2). Also, several questions remain unanswered: such as why and how do chromate pigments and pretreatments work, what can replace chromâtes - especially for protection of Al alloys, 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 rapidly changing. The long pending imposition of rules and regulations severely limiting or eliminating chromate-based metal pretreatments and chromate pigments in coatings is coming to pass. Restrictions on the handling of hazardous materials is making manufacture, application, and removal of coatings containing hexavalent chromium in either the coating pigmentation or in the metal pretreatment very difficult, soon to be almost impossible. For example, the US Air Force (USAF) has a goal of Cr-free (pretreatment + coating) systems by the year 2000. Current USAF aircraft coatings are based on SrCr04 pigmented primers and chromate-based anodizing as the aerospace aluminum alloy pretreatment. A new pretreatment and pigmentation paradigm is needed to replace the current Cr-based systems, especially for structural aerospace Al alloys.



3 Further, the ubiquitous continuous salt fog test for corrosion control performance by coatings is acknowledged by most workers in corrosion control assessment to be of little value for the prediction of the performance of environmentally compliant coating systems. Yet, most government and many commercial specifications for coatings have historically required passing up to 2000 hours of this test. Why such a test method, apparently designed for the assessment of lead and chromate based pigmentation of solvent-bome alkyd coatings, remains so widely incorporated into specifications in place today is a mystery to many and a problem for many users and suppliers of corrosion control coatings. Most of the newer environmentally compliant coating technologies such as powder coatings, high solids and water-borne systems perform worse in regular salt-spray testing than their equivalent solventbome coatings, but under use conditions, they perform better. This is being acknowledged in the move within users and manufacturers of corrosion control coatings in the shift to cyclic testing such as Prohesion™ and the objective predictive electrochemical methods of electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM). Corrosion Control by Coatings Corrosion of metal objects occurs by electrochemical reactions at the surface involving the oxidation of the metal in the presence of water, electrolyte and oxygen. Most metals, except for the so-called noble metals, are most stable as oxides under most ambient conditions. Coatings are often used as a protective layer over the metal substrate to prevent the substrate from oxidizing in a manner deleterious to the function and appearance of an object. They do so in several ways (5). First, they act as a barrier limiting the passage of current necessary to connect the areas of anodic and cathodic activity on the substrate. This occurs especially if the coating wets the substrate surface very well and has good adhesion in the presence of water and electrolyte. Coatings do not really stop oxygen sufficiently to make its concentration at the surface rate limiting, and they do not completely stop water ingress into coatings. However, a good barrier coating slows water and electrolyte penetration and is not displaced by water at the substrate/coating interface. Barrier properties can come mainly from the polymer or from pigment volume concentration effects. As pigments block diffusion of water and oxygen below the critical pigment volume concentration (CPVC), increasing the pigment volume concentrations improves the barrier property of coatings. If the CPVC is exceeded, even locally, voids allow easy passage of water to the substrate surface and the barrier properties are lost. Coatings that 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 the substrate or block the corrosion reactions. These are usually primer coatings that contain inhibitive pigments such as chromâtes, phosphates or molybdates. Coatings such as this will protect damaged areas of coatings by stopping corrosion reactions on local areas of the surface exposed by physical damage. Some coatings use soluble organic inhibitors, but these often leach out the film too rapidly to give long term protection.



4 Third, coatings can provide cathodic protection to a substrate if they are formulated with a metal pigment that is more electroactive than the substrate. This is most commonly done with zinc powder used over steel or iron. Other metal powders that might provide cathodic protection are too reactive in particulate form (Mg) or form oxide films that prevent electrical contact between particles. The metal pigment volume concentration must exceed the CPVC to have all of the particles touching and also in contact with the metal surface. These are the so-called Zn-rich coatings often used as primers for steel objects where galvanizing cannot be used. This type of coating provides protection for damaged areas, but must be overcoated by a topcoat to keep the metal pigments from being directly oxidized by atmospheric exposure.

Coatings Used for Corrosion Control There are many coatings on the market today that offer some form of corrosion protection to metal substrates. Major use areas where corrosion protective properties of coatings are a preeminent requirement for so-called Original Equipment Manufacture (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 epoxy coatings); farm and construction equipment coatings; and general use coatings for objects used in exterior exposure, such as lawn furniture, metal window frames, etc. This list is not all-inclusive, but identifies major OEM areas where corrosion protection by coatings is important. Field applied coatings where corrosion protection is of primary concern are aircraft coatings, pipeline coatings in the field, marine coatings, railroad car coatings, the general area called industrial maintenance coatings - general purpose coatings for exterior protection- bridges, decks, industrial plants, storage tanks, exterior metal structures, etc. These coatings are often multiple layer systems with the primer coatings (first layer next to the metal substrate and its pretreatment) usually designed to provide the corrosion protection in damaged areas and the overcoat(s) providing barrier and UV protection to underlying layers. Polymers that are most successfully used for corrosion protective coatings are epoxybased materials; polyurethane based polymers, urethane topcoats over epoxy primers, 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 to metal 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 give relatively good performance. One other characteristic of successful corrosion protective coatings is that they can be applied in relatively defect free films. Film thickness uniformity is very important for corrosion protection (4). There is much informationfromsuppliers and trade publications about the relative merits of various coating systems, and it is suggested that the reader see these sources for details. Coating systems work successfully only when the metal surface is well cleaned. In factory use, when the metal often receives a pretreatment to form a protective oxide or related material. This is often done using chromate baths for Al alloys and phosphate-based pretreatment for steels. These baths are often acidic to remove prior surface oxides and leave a controlled oxide surface with chromate or phosphate incorporated. Again, this is a field with much information on field use available from



5 suppliers and trade literature. One issue that users of these pretreatments must face is environmental legislation, especially on waste material from these systems, which can be toxic, especially chromate baths. Dry pretreatments or environmentally benign systems that put down thin adherent protective layers are now being examined to replace earlier systems. Plasma cleaning and deposition as well as sol-gel chemistry are being examined, especially in systems with high maintenance and refinishing costs. In production, %ths of the coating line space and cost is often devoted to cleaning and pretreatment. Also, galvanizing is being used on many sheet steel systems, especially for automobile, siding and appliance use. This often provides a more uniform surface for coating than untreated steel, as well as providing cathodic protection to the steel.

Measurement of Corrosion Protection by Coatings The area of corrosion protection by coatings is that is currently undergoing the most change is the area of testing of performance. It is safe to stick with proven tests, and many specifications have existing tests included. However, the single most used test method, the continuous salt spray test has significant weaknesses. It is in the process of being replaced by other tests, which have been shown to be better predictors of performance. As new technologies have developed to provide coatings that protect against corrosion while reducing VOC and the use of toxic pigments and inhibitors, older test methods have not been always able to identify correctly those new coatings, which provide improved protection. There is a new generation of test methods that have been developed that provide objective, numerical characterization of coating performance, or improved ranking types of tests that while still subjective, provide better prediction of new coating performance. The numerical test methods are based on electrochemical methods, and they include Electrochemical Impedance Spectroscopy (EIS) (5), and Electrochemical Noise Methods (ENM) ( φ , among others. Many of the chapters of this book include work based on these methods, and how they are being used for the study of organic coatings over metals. Other developments in test protocols for determining coating performance against corrosion have included cyclic testing methods (7) such as the Prohesion™ cabinet test, alternating wet-dry cycling of coatings, and using UV exposure in the cyclic exposure of coatings. These exposures are now being coupled to some of the electrochemical methods just mentioned for more realistic studies of coating performance.

Lifetime and Cost Issues in Coatings for Corrosion Protection As stated above, coatings remain one of the most cost-effective means of providing practical protection from corrosion to easily corrodible metallic (and sometimes nonmetallic) structures and objects. But often they are chose by initial investment cost only. The use of organic coatings for corrosion control (the term control is used because, in a thermodynamic sense, corrosion can never be eliminated, only controlled to a low enough rate as to be ignored) is so pervasive in our society that is too often taken for granted. Quality and effectiveness of corrosion control by coatings is assumed by many users to be low cost and easy to achieve. For these and



6 other reasons, users of corrosion control coatings often choose coatings only by (material + application) cost and appearance, not by cost effectiveness as measured by their true performance and lifetime of that performance. However, with high labor costs and difficulties in recoating large, buried, difficult to reach or complex objects, more sophisticated coatings users are focusing on the total costs of corrosion prevention and control. This leads to a realization that a coating system that provides long use life but is somewhat more expensive initially for initial application will pay for itself in reduced maintenance costs and reduced need for expensive recoating. The more this reasoning is followed in analyzing the cost of corrosion protection by coatings, the more the research into measuring and predicting the protective properties of coatings will be performed. Analyzing the coating by its initial cost alone makes a coating that significantly increases the lifetime of protection at a somewhat higher cost not well accepted in the marketplace. The payoff on developing systems based on true lifetime costs has been shown in the automobile industry, where the use of two sided galvanized steel + enhanced corrosion protective ED primers has raised the average lifetime until noticeable rust damage on cars to about 10 years.

New Technologies New technologies and materials for corrosion protection by coatings are coming into the coatings science from other areas. The possibility of providing corrosion protection by incorporating the use of conductive polymers, such as doped polyaniline, is being actively pursues by researchers. The extension of the thin film technologies developed for the semi-conductor electronics industry to surface preparation of substrates for protective coatings is being pursued. Plasma cleaning and plasma deposition of thin films for subsequent coatings is being examined, as well as the use of sol-gel thinfilmsfor surface pretreatment. Both of these latter may replace Chromate-based pretreatments for metals. The in-situ sensing of the state of corrosion protection in a system by implanted electrodes, and some other nondestructive testing method is being considered by large users of coated metals as another 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 of maintenance and repainting, the corrosion protective properties of organic coatings are more important than ever. Any added lifetime of use of objects and materials that coatings can add is in actuality a significant contribution to the economy and the environment.

Summary Corrosion protection is a key property of organic coatings, and their use for this purpose is a key contribution that coatings make to the world economy. If coatings continue to receive the minimal attention from the many users and developers of coatings that treat coatings for corrosion protection as almost commodities, the continued investment of coatings suppliers and research agencies will significantly slow. This will increase the burden that corrosion already has on our economy and also increase the need for maintenance and repair. If we continue and increase our investment in understanding and preventing corrosion by well designed new coatings,



7 everyone will benefit. One can see this effect already in the drastically increased lifetimes against corrosion of cars and household appliances vs. 10-15 years ago, effects due to a combination of improved substrates and coatings. The symposium contains a good number of papers examining these newer test methods, showing the value of electrochemical testing in combination of cyclic exposure testing, and also, showing how environmentally compliant coatings are rapidly displacing regular coatings in corrosion control use. Many of the chapters of this symposium book are devoted to the electrochemical testing of coatings, and the examination of the test methods with respect to predicting coating lifetimes, or at least ranking coatings within a cohort of candidate coatings. These papers are Chapters 1-8, 11-12, 14, 34. The emphasis within these papers is to improve numerical ranking of corrosion performance; to develop measurement tools that provide insight into what is happening at the metal/coating interface, as well as within the coating during exposure; and to provide insight into the mechanisms that lead to the failure of the protection the coating affords to the metal. Some of the issues addressed in these papers are what is the proper exposure for the testing, what should the composition of the immersion electrolyte be for electrochemical testing, and what accelerating factors are valid to give failure within a laboratory test procedure in a manner that properly emulates field service failure. The statistics of sampling to predict coating failure is also considered (Ch. 18). Localized measurement methods for examination of defect areas in coatings are also described, including scanning acoustic microscopy (Ch. 10), localized electrochemical impedance spectroscopy (Ch. 2.), and SEM. These are all utilized to give insight into local failures in the coatings. There is also a paper discussing the relationship between defects, localfluctuationsin film thicknesses and other coating properties, and corrosion protection (Ch. 16). Another paper gives predictive modeling for the formation of a common local defect noted in corrosion failure of coatings, blister formation (Ch. 17). Several papers address the important issue of water uptake and diffusion in protective coatings, and how water transport in coatings and its effects on coating properties is a key issue that requires attention vis a vis corrosion control (Ch 12-13). Several papers consider new thin-film technologies for corrosion protection. One paper (Ch. 21) considers self-assembled monolayers and multi-layers asfilmsfor the protection of copper. Two papers address the formation of thin plasma-polymer protective layers for improvement of the subsequent adhesion of thicker, standard corrosion protective films (Ch. 19-20). Another examines the properties of electropolymerized thinfilms(Ch. 23). There were also two papers addressing the still yet unresolved 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 there have been indications that chemically doped conductive poly(aniline) can provide corrosion protection without the need for pigmentation, but solely due to electrochemical effects. Two papers address the issues of microbial induced corrosion and its assessment in coated systems (Ch. 25-26) The other papers consider a diverse range of problems in the use of corrosion protective coatings. There are two papers that consider the specific problems of aircraft protective coatings (Ch. 23 & 24), and one paper focusing on the protection of concrete (Ch. 27), a topic closed allied to the protection of metals. Also examined



8 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 for chromâtes in pigmentation (Ch. 32 & 33) and metal pretreatment (Schulman paper), new polymer matrices for corrosion protective coatings (Ch. 29), environmentally compliant coatingsfromnatural products (34), and a chapter on M0S2 in protective poly(ethylene) films. Organizing and participating in this symposium has been a very satisfying experience for me, and I wish to take this opportunity to thank all of the presenters of papers at the symposium and those that prepared the papers that make up the chapters of this book for their contributions. I wish to thank ACS Books for the opportunity to organize and publish this symposium and the help that they have provided in making this book possible.

Literature Cited (1). Skerry, B.S & Simpson, C.H. "Accelerated Test Method for Assessing Corrosion and Weathering of Paints for Atmospheric Corrosion," Corrosion 1993, 49B 663674. (2) ASTM D5894, Annual Book of ASTM Standards; Amer. Soc. Testing & Materials, West Conshohaken, PA, 1997. (3) Bierwagen, G.P. "Reflections on Corrosion Control by Coatings," Prog. Organic Coatings 1996, 28, 43-48. (4) Bierwagen, G P. "Defects & Heterogeneities in Corrosion Protective Organic Coatings Films and Their Effects on Performance" ACS Symposium Book, Corrosion and Its Control By Coatings, G.P. Bierwagen, ed. (5) Fredrizzi, L.; Deflorian, F.; Boni, G.; Bonora, P.L. and Pasini, E. "EIS Study of Environmentally Friendly Coil Coating Performances," Prog. Org. Coatings 1996, 29, 89-96. (6) Mills, D. J.; Bierwagen, G. P.; Tallman, D.E. and Skerry, B.S. "Investigation of Anticorrosive Coatings by the Electrochemical Noise Method," Material Perf., 1995, 34, 33. (7) Appleman, B.R. "Cyclic Accelerated Testing: The Prospects for Improved Coating Performance Evaluation" J. Protective Coatings & Linings Nov. 1989, 71-79. And Appleman, B.R., "Survey of Accelerated Test Methods for AntiCorrosive Coating Performance" J. Coatings Tech. 1990, 62, (#787), 57-67.


Corrosion and Its Control by Coatings - ACS Symposium Series (ACS

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