ASM Handbook, Volume 5B, Protective Organic Coatings Copyright # 2015 ASM InternationalW K.B. Tator, editor All rights reserved asminternational.org Coating Deterioration Kenneth B. Tator, KTA-Tator, Inc. PAINTS AND COATINGS of all types are deteriorate and lose their protective or aesthetic the formulation to enhance application and per- widely used to provide color and pleasing aes- function as a result of old age, combined with formance properties. These diverse ingredients, thetics, and to prevent deterioration of the exposure to aggressive environments. along with the ways molecules react with each underlying substrate when exposed to various In this article, coating failures due to specifi- other and with the substituent ingredients, pro- environments. Besides protection and beauty, cation errors, poor surface preparation or appli- vide the variability in the molecular structure however, coatings provide light reflectivity, cation, deficient film thickness, or another of a coating. camouflage surfaces, reflect and absorb heat, abnormality during application are not dis- When a molecule crosslinks with another and provide a variety of other functions. cussed, even though, to a greater or lesser molecule, the reactive sites of each of the react- However, in order to provide these functions, extent, all of them affect deterioration and ing molecules must align and come within very the protective coating must remain intact and resulting substrate corrosion during the normal close proximity to each other (generally within adherent on the surface to which it has been service life of any coating. 3 to 5 angstroms (A˚ =1Â 10–10 m) for the applied. The vast majority of all protective Rather, the deteriorating effects of exposure chemical crosslinking reaction to occur (Ref 1). coatings perform admirably until an old age, environments and their interaction with the For example, in an epoxy resin that is cross- at which time natural deterioration and degra- paint or coating are discussed. This discussion linked with a polyamide copolymer, the molec- dation occur. However, a coating can fail provides an introduction to the mechanism of ular sizes of each co-reactant material are prematurely, preventing its aforementioned premature corrosion of a metallic substrate relatively large, and the reactive functional functions from being realized. when that substrate has been properly coated groups are interspersed along the ends or mid- The major reasons for the occurrence of cor- with a suitably resistant coating system in a chain of the molecule. Stoichiometric (com- rosion usually are poor or deficient surface given environment. plete theoretical crosslinking) reactions are preparation, or insufficient coating thickness. This article discusses some of the environ- rare, and quite often the reacting groups do There are, of course, many other reasons why mental influences on a protective coating film not come into sufficient proximity to react. This coatings deteriorate and corrosion occurs: that can result in deterioration: is because the coating resin is dispersed in a solvent that evaporates, reducing mobility of Energy: solar, heat A paint or coating is incorrectly formulated the molecules of the reactants. Additionally, or manufactured by the coating supplier. Permeation: moisture, solvent, chemical, low reactant temperatures reduce molecular and gas An unsuitable coating is specified for a mobility. The presence of pigments and other given environment. Stress: drying and curing-internal stress; ingredients also separate the reacting molecular vibration- external stress; impact and abrasion Environmental conditions are different than chains. Because there are billions of reactive that understood by the specifier. Biological influences: microbiological, mil- sites, and because formulators add excesses of There is improper, or insufficient, mixing of dew, and marine fouling reactive moieties as appropriate to ensure suitable the coating at the time of application. reactions do occur at room temperature (or what- There are adverse ambient conditions when These generalized categories of environmental ever the design reactive temperature is), suitable the coating system is applied. influences unfortunately do not act singly, but crosslinking generally occurs. However, there The drying and/or curing of the coating after in combination, sometimes with unpredictable can be tens of millions of unreacted moieties application is impaired. catastrophic results. remaining in the crosslinked coating resin. Also, There is chemical, physical, and/or mechan- resin molecular reactivity often initiates at dis- ical damage to the coating system during crete localized areas and progresses from these exposure. Variability within a Properly areas in a manner similar to the formation of frost Applied Coating Layer on a window. The intersection of one reaction These causes of failure are relevant only when area with another results in an interstitial bound- a premature coating failure occurs. As men- Coating materials—even when thoroughly ary with different properties than that of the tioned, however, premature coating failure is mixed, applied, dried, and cured properly— reacted area. Similarly, the resin reactions around extremely rare: of the hundreds of millions of gal- have, from a molecular point of view, great pigment particles and other paint constituents lons of paint manufactured and applied each year variability in their compositional makeup. also have a different crosslinking density than in the United States alone, it is estimated that only The articles “Elemental Chemistry Introduc- that of the pure resin reaction. a small fraction—less than one one-hundredth of tion” and “Composition of a Paint Coating” in Solvents in solvent-borne coatings, and water one percent—of these coatings ever fail prema- this Volume describe coating resins, the way in latex or waterborne coatings, evaporate after turely. Instead, most protective coatings are suc- atoms form molecules, and how the molecules application, leaving micropores, microcracks, cessfully specified and applied to a properly react with other molecules to form a coating. or capillaries within the coating. If evaporation prepared surface to the appropriate thickness. Various ingredients such as pigments, fillers, is impeded, due to low temperature or other These coatings perform as intended, but over time co-reactants, and surfactants are included in reasons, the solvent or water can accumulate Coating Deterioration / 463 and cause a void within the coating cross sec- as corrosion inside buildings, corrosion in box 3B: Chemical atmospheric exposure, neutral tion. The inner or outer surface of the void can girders, and various stresses such as chemical, (pH 5.0 to 10.0) provide a means of moisture penetration into the mechanical, condensation, temperature, and 3C: Chemical atmospheric exposure, alka- dried film. Similarly, pigment agglomerations, stress combinations. line (pH 10.0. to 12.0) which are pigment particles in contact with each The Society for Protective Coatings (SSPC) 3D: Chemical atmospheric exposure, pres- other, can impede resin wetting, leaving a micro- has also defined environmental zones for coat- ence of mild solvents, intermittent contact void or discontinuity in the crosslinked coating. ing systems (Ref 4): with aliphatic hydrocarbons and their deriva- All of these result in the apparent presence tives (mineral spirits, lower alcohols, gly- of inhomogeneities and phase separations in a 0: Dry interiors where structural steel is cols, etc.) crosslinked coating film. Even if the film is ther- embedded in concrete, encased in masonry, 3E: Chemical atmospheric exposure, severe. moplastic, and not crosslinked, such inhomo- or protected by membrane or noncorrosive Including oxidizing chemicals, strong sol- contact type of fireproofing geneities and phase separations still are present, vents, extreme pHs, or combinations of these and for the same reasons. 1A: Interior, normally dry (or temporary with high temperatures The presence of low-molecular-weight regions protection). Very mild (oil-base paints do not last six years or more) For the most part, these environmental in coating films has been demonstrated by elec- tron and light microscopy studies. Films made 1B: Exteriors, normally dry (includes most descriptions are somewhat similar to the extent of epoxy, phenolic, and phthalate resins were areas where oil-based paints last six years that they progress from a relatively mild, non- or more) corrosive environment to a relatively aggressive observed to consist of micelles or granules of high-density segments separated by narrow 2A: Frequently wet with freshwater. environment. The more benign mild environ- boundary regions of low-molecular-weight mate- Involves condensation, splash, spray, or fre- ments are generally warmer, dryer, and less rial. At the film-substrate interface, the low- quent immersion. (Oil-based paints now last polluted. The more severe environments gener- five years or less.) ally have more moisture, or are in immersion, molecular-weight material exists as a thin contin- uous film or as channels between micelles, 2B: Frequently wet by saltwater. Involves and have salts or chemical constituents. thereby providing pathways for easy entry of condensation, spray, or frequent immersion. Moisture, salts, and chemicals are primary water to the interface (Ref 2). (Oil-based paints now last three years or influences in the corrosion process on steel and less.) most metals