Defects and Heterogeneities in Corrosion ... - ACS Publications

Chapter 10

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 20, 2015 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch010

Defects and Heterogeneities in Corrosion Protective Organic Coatings Films and Their Effects on Film Performance

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Gordon P. Bierwagen , Dennis E. Tallman , Joel Zlotnick , and Carol S. Jeffcoate 1

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Departments of Polymers and Coatings and Chemistry North Dakota State University, Fargo, ND 58105 Department of Chemistry, Case-Western Reserve University, Cleveland, OH 44106

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A limiting feature of many corrosion protective coatings is how well (defect-free) and how uniformly they can be applied and cured on the metal substrate they are to protect. Failure in coatings/metal systems is very rarely global, and usually occurs at one small site of weakness in the film, and then progresses to give appearance failure or performance failure of the substrate. Careful analysis of film performance often shows that films do not fail in areas where the coating is uniformly applied at the film thickness recommended by the coating manufacturer. The distinct effects of coating heterogeneities have been observed in this laboratory (1) in studies of a multiple sample set of marine coatings by electrochemical noise methods (ENM). It is something which must be considered carefully in choosing the size of the sample set in experimental studies of coating failures (2). One good coating sample definitely does not constitute a representative sample for durability studies. Film thickness fluctuations, local pigment volume concentration fluctuations, local variations in film cross-link density, fluctuations in chemical composition (for example, from a poorly mixed two component system), and locally uncured areas are all local heterogeneities that can cause significant problems. Sampling for lifetime testing of corrosion protective coatings should include coatings with defects that are a representative of those that might be expected in actual practice. Similar arguments have been put forth for the proper analysis of automotive paint film samples that are subjected to exterior exposure testing (3). 4

Summer undergraduate research associate at North Dakota State University.

©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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metal or coating. If the corrosion is due to local issues in the metal, its is often due to heterogeneitities in alloys (¥), or local undesirable metal impurities in a metal or Corrosion of metal substrates in systems protected by organic coatings is almost always a local failure issue, as corrosion occurs at a weak/susceptible point in the alloy. However, in coated metal systems, the corrosion failure can very often be traced to a local defect in the organic coating. As commonly seen in practice, in what appears to be a fairly, uniformly coated system, failure occurs at sharp edges of the object. The film thickness is locally low due to Laplace pressure effects across areas of high radius of curvature (5'6). The coating is also mechanically damaged more easily after application at these sharp edges. Figure 1, takenfromPierce and Schoof (6) , illustrates how surface-tension-driven-flows at a sharp edge can generate local thin spots in a coating. Another common source of corrosion failure is blistering and related defects (7) . These often are caused by a coating being applied over a poorly cleaned and prepared substrate or by phase separation and entrapment of solvent during the film formation process in coatings. Because they are often associated with a pre-existing film problem, blistering and subsequent corrosion problems are considered to be caused by film imperfections. It is well known that for successful application and use of corrosion protective coatings, much effort must be made to prepare the surface for coating. Both physical and chemical cleaning are used. A rule of thumb is that 75% of an OEM painting line should be devoted to cleaning and 25% to paint application processes.

Figure 1. Schematic of Typical Thinning Points in Film-Corrosion Often Occurs at Thin Point at Sharp Edge (with permissionfromPierce & Schoof (6)). Defects can also occur during the application process. Pinholes are caused by foam bubbles trapped in a film during the drying process. Craters are caused by surfactant spreading during the coating application and drying/film formation processes (5'8). Table I gives a summary of defects often seen in coatings and their probable causes. Two or more layers are often used in corrosion protective systems because they significantly reduce the chance of coating imperfections (due to application processes) yielding an imperfection which penetrates directly to the metal substrate. This is illustrated in Fig. 2 (2). Imperfections that



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Figure 2. Schematic of Multilayer Coating and Imperfections [with permission from Bierwagen, Prog. Organic Coatings, 28 (1996) 43-48 (ref. 2)]. penetrate all the way to the metal surface are often termed "holidays', and there is a significant market for devices called "holiday detectors" which are used to examine coatings after application, but prior tofielduse (9). Fluctuations in film thickness from application processes are a significant cause of poorfilmperformance. These can occur due to poor leveling,ribbingin coil coating processes, and a myriad of other reasons. Film thickness is a very difficult property to measure and control over the entire surface of a coating. Some mapping has been done of fluctuations in film thickness in typical coatings, and these data indicate significant fluctuation in surface profiles (10Ί1). A qualitative description of film thickness effects on coating corrosion protection was given recently (72), but no quantitative information was provided. Neal gives a discussion offieldmeasurement of corrosion protective coatings (75). Babic, et al (14), discuss briefly the effect of film thickness on film performance, but they are considering only globally average thickness, not local fluctuations. Film resistance should be linearly related to film thickness if the normal relations observed in resistors hold true and the thickness for example, is equivalent to the length of a resistive wire. The simple relationship (75) for resistance applies R = pL/A, where R is thefilmresistance, ρ thefilmresistivity, L thefilmthickness, and A the surface area of thefilmover which the measurement is made. Since film corrosion protective performance is often related to resistance, as first noted by Bacon and Rugg, (7(5), a direct relationship between film performance and thickness should hold. This means that areas of locally lowfilmthickness will give locally poor performance. There is much concern about film thickness control in field use of corrosion protective coatings (77). In general, film thickness variations and the various defects described in Table 1 are the most severe problem faced by the users of corrosion protective coatings. Performance degradation caused by film nonuniformities. Funke has considered much of this in earlier publications (18-19).


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Table 1. Film Thickness/Application-Related Defects Reference Cause Defect Pinholes Craters Ribbing Scratches Solvent Popping Edge Curvature Thinning Brushmarks De-Wetting Blisters Uneven Film Thickness

Bubbles, Foaming Surface Contamination Flow Instability Mechanical Damage Solvent Entrapment Laplace Pressure Effects Leveling Problems Surface Incompatibility Sub-Film Contamination Poor Application

8 5,8 5 9 3,7 5,6,7 5 8,8 7 8

Another defect in coatings related to corrosion failure is the porosity that occurs in pigmented coating films above their critical pigment volume concentration (CPVC). It is very well known that there is a sharp increase in corrosion in coated metals as one increases the pigment volume concentration of the coating (PVC) above the CPVC (20). The rapid development of corrosion in salt spray testing of panels for which the coating was above its CPVC was used as one of the earliest measurements of pigmented coating CPVC. The cause of this effect was assigned to the voids that form in the coating above the CPVC which act as pathways for the electrolyte and oxygen to get directly to the metal/coating interface and support rapid corrosion. By comparison at PVC values below the CPVC the coating has intact barrier properties and materials have to diffuse to the interface through a continuous, closed coating, giving a much reduced corrosion rate. This was more recently considered and discussed by Skerry, et al. (21), in a symposium on CPVC related topics. New work of Fishman, et. al.,(22-23) has shown that local fluctuations in PVC can generate local areas where the PVC exceeds the CPVC, even though the globally PVC

Defects and Heterogeneities in Corrosion ... - ACS Publications

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