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˚ =1 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 and other materials. These influ- Accordingly, there is great variability in the crosslinking density of a coating, even if it is 2C: Freshwater immersion ences, and the degrading influences of other envi- 2D: Saltwater immersion ronmental effects can loosely be categorized as formulated, mixed, applied, and cured properly. Figure 1 illustrates some of this variability. 3A: Chemical atmospheric exposure, acidic energy related (solar, heat/cold, and nuclear radi- Relative to the properly pigmented, dried, (pH 2.0 to 5.0) ation); permeation related (moisture, solvents, and cured portions of the cross section of a coating layer, the areas of the deficiencies men- Water droplet tioned are quite small, both in area and in cross- sectional dimension. Thus, multiple layers of a Permeating water coating system is not likely to provide an over- ABCD lap of deficient areas, and a porosity or area of E Pigment particles moisture penetration in one layer almost cer- tainly will not coincide with that in another Crosslinked resin layer, even though the weak areas for moisture penetration remain. A three-coat system usually Penetrating moisture is better than a two-coat system, and both are better than a one-coat system, even if that layer Substrate is relatively thick. Fig. 1 Crosslinking density of a coating varies greatly, resulting in very different degrees of moisture penetration: (A) No moisture penetration/high crosslink density. (B) Penetration through or over void. (C) Penetration Environmental Effects Resulting through a pigment agglomeration. (D) Penetration through a microcrack or capillary. (E) Penetration through area of in Coating Deterioration low crosslink density

Characterizing an environment is a daunting, Table 1 Atmosphere corrosivity categories and examples of typical environments almost impossible task. The environment at one end of a bridge can be different than that at Corrosivity category Exterior environment Interior environment another end, and both may be different that than ... of a center span that is suspended over water or C1 very low Heated buildings with clean atmospheres, e.g., offices, shops, schools, hotels high in the air. Similarly, an exterior environ- C2 low Atmospheres with low levels of pollution. Mostly Unheated buildings where condensation may occur, e.g., ment at the top of building can be different than rural areas depots, sports halls that near the bottom of the building relative to C3 medium Urban and industrial atmospheres, moderate sulfur Production rooms with high humidity and some air sunlight, wind intensity and direction, and even dioxide pollution. Coastal areas with low salinity pollution, e.g., food processing plants, laundries, breweries, dairies temperature. Ships hauling cargos have differ- C4 high Industrial areas and coastal areas with moderate Chemical plants, swimming pools, coastal ship- and ent environments, not only within the cargo salinity boatyards tanks, but above and below the waterline. C5-I very Industrial areas with high humidity and aggressive Buildings or areas with almost permanent condensation The International Standards Organization high atmosphere and with high pollution Standard has attempted to define principle (industrial) environments for coatings (Ref 3). The environ- Categories for water and soil mental categories and descriptions are pre- Im 1 Fresh water River installations, hydroelectric plants sented in Table 1. In addition, the standard Im 2 Sea or brackish water Harbor areas with structures such as sluice gates, locks, jetties; offshore structures discusses metal loss/year for each category Im 3 Soil Buried tanks, steel piles, steel pipes and time of wetness; special conditions such 464 / Coating Analysis and Evaluation chemicals, and gases); mechanically related spectrum as emitted and as absorbed on the sur- meter). This naturally occurring energy from (internal crosslinking and curing stresses, exter- face of the earth (Ref 5). the sun has a shorter wavelength than visible nal vibration and flexibility stresses, and impact/ Electromagnetic radiation with the shortest light (400 to 780 nm) and accordingly is more abrasion); and biological (microbiological and wavelengths has the greatest energy. However, energetic. Ultraviolet light has sufficient energy macrobiological—mildew and fungus). the shorter wavelengths are more readily to disrupt and break covalent bonds of organic Unfortunately, these categories are not all absorbed and have less penetrating effects than molecules. The UV light range is from approx- inclusive, and most importantly, are not mutu- longer wavelengths (Ref 6), as can be seen in imately 10 to 400 nm and is divided into three ally exclusive. In any environment, most if not Fig. 3. Radio waves can be transmitted over subcategories: UV A, 320 to 400 nm; UV B, all of the environmental influences are present long distances compared with shorter wave- 280 to 320 nm; UV C, 10 to 280 nm. The detri- to varying degrees, along with perhaps other length television and radar, which allow trans- mental effects of UV radiation to paints was influences not mentioned here. It is the synergis- mission generally along a line of sight. Very believed, approximately ten years ago, to start tic effect of the combinations of these and other short radiation types such as cosmic rays, and at 295 nm and extend to approximately 400 nm. environmental influences that degrade the coat- a and b nuclear radiation, although high Recently, however, experience has shown that ing, or for that matter any material, resulting in energy, cannot penetrate even the thickness of there is sufficient radiation and penetration of UV loss of suitability for its intended purpose. Each a sheet of paper. X-ray and g radiation are not light as low as 280 nm to cause deterioration of of these environmental categories is discussed found in solar radiation but are man-made by paint. Ultraviolet radiations below this wavelength subsequently. bombardment of certain elements with elec- are not considered detrimental, because they are trons, or concentration of certain naturally generally absorbed by moisture and other small radioactive elements (such as uranium). These molecules in the atmosphere and therefore are of lit- Energy Related Degradation high-energy shortwave radiations are powerful tle consequence. Moreover, they have little ability enough to ionize gases, readily cleave chemical to penetrate into the surface of an organic material. Energy acting on a coating (or material) can bonds, and induce potentially deadly chemical The frequencies of radiation that are most harm- degrade a material by breaking or interfering changes in human and animal tissues. ful to polymeric systems are those from the blue with the chemical bonds holding the resin (or Ultraviolet light falls within the wavelength part of the visible light spectrum and the near-UV a molecule) together and to a substrate. The influ- from 10 to 400 nm (1 nm is one-billionth of a light spectrum. The longer wave lengths are not ence of energy in virtually every case makes an energetic enough to harm molecules, and most of organic molecule more susceptible to degradation the other potentially harmful high-frequency rays by other environmental influences (i.e., perme- are screened out by the atmosphere of the earth. 30 ation, mechanical, and biological). The primary 2 1 The breaking of molecular bonds and the for- energy influences are solar radiation, heat (and mation of free radicals by UV energy results in cold), and to a much lesser extent, nuclear 3 a shortening of the molecular chain group of an 20 radiation. 4 atom and, accordingly, a reduction in its molec- Solar Energy. The sun was formed 4.5 bil- ular weight. 5 lion years ago and is composed of 91.2% H 10 Glass allows visible light to pass through it and 7.8% He gas. The remaining 1% is com- without any absorption but is opaque to the prised of oxygen, carbon, silicon, iron, magne- Relative irrandiance, a.u. shorter wavelengths of UV light and reduces the sium, neon, sulfur, and calcium. Each element 0 transmission of UV light of longer wavelengths. is important because its presence contributes 200 600 1000 1400 Accordingly, materials exposed behind glass to the solar spectrum as it is received on earth. Wavelength (λ), nm retain their color and last longer than those The sun emits energy created by the thermonu- exposed in an exterior solar environment. How- clear fusion of hydrogen into helium. Four Fig. 2 Solar spectrum as emitted and after absorption ever, fading and embrittlement of plastics and hydrogen nuclei have more mass than one and scattering. 1. sun spectrum before entering other materials upon long-term interior exposure stratosphere (extraterrestial radiation), 2. spectrum modified helium nucleus and as each helium atom is by ozone absorption (in stratosphere and troposphere), still occurs in indoor environments exposed to formed, the excess mass is converted into 3. spectrum after Rayleigh scattering (by small molecules), sunlight. Even though window glass filters out energy that powers the sun. The core of the 4. spectrum modified by aerosol (clouds) scattering and most UV light, the energy that transmits through sun contains more helium (65%) than hydro- absorption (excluding influence of water), 5. spectrum after the glass is still sufficient to degrade and fade moisture related scattering and absorption. a.u. – arbitrary gen. Hydrogen has been brought to this lower units, a ratio of solar irradiation intensity to a reference most materials, including coatings over time. It level because of its conversion in the thermonu- measurement. Source: Ref 5 is generally accepted that radiation in the visible clear reaction. It is estimated that the remaining hydrogen should last another 4 billion years at its rate of consumption. Variations in the activ- ity of the sun affect the wavelength of emitted radiation. Changes in ultraviolet light (UV) radiation are more pronounced than those of other ranges of radiation. The distribution of emitted energy is such that 9% is in the UV region, 45% is in the visible range, and the remaining 46% is in the infrared range (Ref 5). However, the emitted energy by the sun is not necessarily of the same wavelength or intensity as that absorbed by the earth. The atmosphere of the earth and variability within that atmosphere and, in particular, ozone absorption and scattering of solar radiation by clouds, moisture, and other small molecules, all change the incidence of radiation on the sur- face of the earth. Figure 2 depicts the solar Fig. 3 Electromagnetic spectrum of radiation types. Source: Ref 6 Coating Deterioration / 465 range and higher is nondetrimental to most paints, free radical ends of the molecule to separate A reduction of the tensile strength, modulus organic materials, and plantoranimaltissues. sufficiently after the break in the bond occurs. of elasticity, and toughness Radiant solar energy in the form of light Accordingly, the free radicals will remain in Potential introduction or formation of reac- photons excites certain electrons in the mole- close proximity and will likely recombine. tive polar groups that can cause changes in cules of a resin. Depending on the wavelength However, if the molecular chains are somewhat compatibility and electrical and optical and frequency of the radiation, only certain elec- flexible, and the temperature is sufficiently high behavior of the polymer trons are affected, while other electrons remain that there is vibrational and rotational move- Introduction of light absorbing groups that unaffected. Excess electron energy as a result of ment of molecules of the polymer, the free radi- can cause discoloration and internal cycliza- UV photon excitation is dissipated by fluores- cals on opposite ends of the broken bond can tion of the chains, resulting in hardening and cence, phosphorescence, and most importantly, become so separated that they will not recom- a decrease in toughness a cascading down of the electronic energy into bine. A radical might pick up a hydrogen atom vibrational and rotational energy of a molecular from an adjacent chain upon the breaking of that Free radical initiation can also include addi- electrical bond. If sufficient energy is absorbed bond and therefore transfer the radical to another tional crosslinking between hitherto indepen- by the bond, it may break. Molecular groups with portion of the chain. If the free radicals are on a flex- dent macromolecules, which, in excess, may double bonds such as carbon to carbon (C–C),– ible molecule that is moving around quite rapidly, reduce impact strength and create brittleness. carbon to nitrogen (C–N),– and carbon to oxygen then there is a high probability that the radical will Energy in the form of UV light can pass (C–O)– absorb UV energy, and their electrons pick off a hydrogen atom of its own chain five or through some resins with little or no effect are lifted into higher-energy levels. When these seven carbon atoms back along the chain. Then or be absorbed in other molecular combina- electrons decay to lower-energy states, energy there will be a transfer of the radical to a position tions without breaking bonds. In the latter is released in the form of vibrations that can away from the chain and a termination of activity case, vibrational and rotational movement cause a bond to break and create free radicals. at the chain end. The newly formed free radical between atoms is increased and the energy is Free radicals results when a chemical bond is might pick up another hydrogen atom somewhere dissipated as heat, which is generally harmless broken. A covalent bond can break in either of else, or react with a monomer, or react elsewhere to the molecular structure. However, where two ways: the atoms previously joined by the to continue growth. Free radical reactions result absorbed heat energy is high enough, bonds bond share the electrons (homolytic dissocia- in chain scission (breaking of the molecular can break and free radicals can form. In a tion), or the more electronegative of the atoms chain); depolymerization (reducing a polymeric rigid, dense, closely packed, immovable, solid retains the electrons (heterolytic dissociation) chain to its monomer units); branching (a short resin or structure, the free radicals may (Ref 6). These two types of dissociations are: growth at a free radical site); self cyclicization recombine with little effect on the molecule. (forming a circular molecule by joining with However, in most cases, particularly in paints Homolytic dissociation A : B ! A þ B another portion of a backbone of a molecule); and most plastics, the structure is not rigid and the formation of double bonds. enough to allow immediate recombination of þ Heterolytic dissociation A : B ! A þ B All of these free radical reactions, when they free radicals, and a variety of unanticipated occur billions of times in a molecule exposed to secondary and tertiary reactions often occur, Heterolytic dissociation produces ions. An ion ultraviolet light, shorten the molecular chains, resulting in a shortening of the molecular is an electrically charged atom or molecule. reduce their flexibility, and increase permeabil- weight of the resin molecule and other detri- A negatively charged atom has more electrons ity of the molecule and resin, thus degrading it. mental side effects, all resulting in deteriora- than protons and a positively charged atom has Certain resins, such as an epoxy, and particu- tion and loss of properties. more protons than electrons. Such electrically larly an amine crosslinked epoxy, are very sus- charged atoms or molecules are polar and can ceptible to UV degradation. Exposure to even Permeation Effects dissociate from one another when placed in relatively low amounts of sunlight is sufficient, water solution. The high dielectric constant or in many cases, to cause a chalking deterioration Permeation of a coating by materials in a ser- insulting property of pure water enables the of the surface of the resin or paint. This chalk is vice environment is a major factor in the deteri- polar molecules to separate and exist separately composed of pigment particles and broken seg- oration of the coating. Coatings are specifically in solution. Water itself is very weakly disso- ments of the colorless molecular resin that formulated and tested to resist certain environ- ciated and forms hydrogen and hydroxide ions. refract light to give a white appearance. ments in immersion or in the atmosphere. Pig- However, certain other resins, notably the ments and resins must be carefully chosen for þ ðÞH2O HOH H þ OH acrylic and polyurethane, are mostly transpar- their resistances to a given set of environmental ent to UV light and allow UV energy to pass conditions, and they must also be compatible. Homolytic dissociation likely occurs if the through them with no molecular absorption. Even with utmost care, coating systems are still two fragments are equally electronegative. This Accordingly, there is little deterioration to these vulnerable to permeation and the ultimate produces neutral atoms or groups, each with an resins when exposed to UV light. destruction of protective capability. The follow- unsatisfied valency or unpaired valency elec- Heat Energy. The addition of heat to a mate- ing permeating species and mechanisms are tron. Such groups are known as free radicals. rial increases the vibrations of atoms, and when discussed subsequently: moisture, solvents, Most free radicals are highly reactive and uniformly applied, all atomic vibrations are chemicals, gases, and ions. recombine either with each other or other free uniform throughout the molecule. This is in Moisture Permeation. The water molecule, radicals to form chemical bonds. contrast with radiation. For instance, both UV H2O, is a very small molecule consisting of If the free radicals are so reactive, why don’t and nuclear radiation affect only certain elec- one oxygen and two hydrogen atoms. Both the they recombine? If they are in a fairly rigid trons in the atoms of the molecule. Other elec- weight and size of this molecule is small rela- structure, it is likely that they will combine. If trons on other atoms remain unaffected. tive to virtually all other molecules commonly after separation the free radicals are held in a If the heat applied is of sufficient intensity, the encountered in an environment. Water in liquid relatively confined area and maintain close molecular vibrations increase to such a degree form comprises oceans, lakes, and rivers, con- proximity to each other, recombination is that a bond can break. When that happens, free denses from the atmosphere as dew, and falls likely. If the molecular structure is crystalline radicals are formed and they react as previously as precipitation in the form of rain or snow. and has relatively tight rigid chains in close described. Again, the end result is: Water in vapor form is always in the air to proximity, or if it is in the cyclical aromatic some degree, as humidity. Any material used ring or a very tightly and closely crosslinked A decrease of molecular weight of the in exterior environments, including coatings, molecular structure, it will be difficult for the chains comprising the resin of the coating must be resistant to the effects of water. 466 / Coating Analysis and Evaluation

Water, in addition to being small in molecu- Once water enters the paint film, the small large solvent molecules separate resin molecules lar size, also is polar, because oxygen has a water molecules have the ability to penetrate from adjacent resin molecules. Bonding that high electronegative attraction to other polar between and within molecular chains compris- could otherwise occur cannot be done because molecules, including itself. Accordingly, water ing the organic resin, and the interstices the bonding moieties are not close enough for can readily penetrate into microscopic pores, between the resin and pigment, if the pigment attraction to occur. Moreover, many solvents are holidays, cracks, and defects inherent in almost is not completely wetted by the resin. As the somewhat polar, particularly the oxygenated sol- any coating system. Water vapor, carried by air, water molecule penetrates, it separates loose vents (including the ketones, esters, and alco- can move in and out of porous materials with bonds holding the resin particles together, such hols). These solvents are generally used to ease, as long as there is a driving force causing as polar bonds, and becomes attracted to and dissolve polar or somewhat polar resins and to its movement. swells the molecule at sites of covalent bonds provide hydrogen bonding to other polar groups What are driving forces causing water move- that are polar. This swelling forces the bonds of the resin, or to keep or slow the solvents from ment? Simply placing the material in water even further apart, diminishing their tightness completely evaporating from the resin. Hydrogen immersion provides sufficient pressure from and close packing. The volume of the coating bonding is the attraction of the oxygen atom in a the pressure head (depth) of the water, even expands due to the increased presence of the molecule to nearby hydrogen atoms in other though the immersion is relatively shallow to water intrusion. Some films increase 20 to molecules. Retained polar solvents may draw provide a driving force for water movement into 50% or more in volume when in contact with water into the resin. This is a particular problem a material. Water molecules, because of their water (Ref 8). The swelling caused by the coat- with slowly evaporating alcoholic solvents such relatively small size, can pack quite tightly, ing film can separate polar bonds and other as the glycol ethers. Coatings used in immersion and accordingly, a mass of water (measured as weak forces holding the molecule together and service, particularly for the interior of deionized specific gravity, pound per gallon, or unit mass) to the substrate such that polar attractions, so water or freshwater storage tanks, often blister is quite dense compared to many liquids and necessary to coating film adhesion/cohesion, due to retained solvents. This is a problem on all gases. In immersion, there is sufficient no longer occur. Additionally, the oxygen of the tank bottoms because the tank bottom is water head pressure to cause water molecules the water molecule can be attracted to and cooled by the earth (acting as a heat sink), while to migrate into cracks, crevices, pinholes, and replace what otherwise would have been a polar the tank sidewalls are warmed by the sun and air microscopic fissures inherent in any coating sys- attraction between two long-chain resin mole- convection. The warmer temperatures assist in tem. Water, in permeating a coating, fills any cules. When this happens, the charge between volatilization of solvents, while the cooler tem- “free” space left by solvents and other materials the resin molecules is terminated, and attraction peratures of the tank bottom result in a slower that have migrated from the coating during appli- to water molecules by each chain end occurs evaporation of retained solvents in the paint film. cation and curing. Additionally, due to slight instead. The dried coating film, when wetted The entrapped solvents can draw water into the polarity of the water molecule, water can be and saturated with water, becomes plasticized coating at the bottom of the tank, causing osmotic drawn into the coating if there are any polar sol- and swollen. Wet adhesion of most coatings is blistering. This can be a problem not only on tank vents, polar groups, or polar materials retained substantially less than dry adhesion. When the bottoms, but anywhere a heat sink might occur, or comprising the dry film. Thus, the presence film dries out, often dry adhesion reestablishes, such as exterior steel supports, cradles, or brac- of ester groups, ether linkages, carboxyl groups, but usually not to the same extent as is was ing. Baking, or heating of tank interior coatings, and other polar groups within a coating resin before moisture saturation. is often done both to ensure solvent evaporation can draw water into the paint. An electric charge This phenomena of swelling by moisture from the paint film and to elevate the temperature applied across the coating film, such as with penetration into a coating film occurs with vir- above the Tg to attain a higher crosslinking den- cathodic protection or resulting from a corrosion tually all coating materials except those that sity of the chemically reacted coating resin. cell, can induce or accelerate the permeation of are extremely tightly crosslinked with a high Chemical Permeation. Coatings are widely water into a coating. This phenomenon is called crosslink density (such as some phenolic used to protect against chemical attack on a electroendosmosis. Additionally, corrosion inhi- epoxies or phenolic coatings formulated for variety of different substrates and in a variety biting pigments (chromates, borates, molyb- water resistance) or some highly crystalline of different chemical environments. The wide- dates), due to their water solubility, may draw coating materials (such as the fluoropolymers). spread use of coatings for such protection water through the coating film in an osmotic pro- These materials are relatively impervious to attests not only to the diversity of coating for- cess. These pigments require water to partially water permeation, penetration, and swelling mulations, but also to the inherent capability dissolve the oxygenated metal inhibitor that then due to their dense molecular crosslinking or of resin and pigment technologies. can wet and passivate the underlying steel or alu- the tight polar bonding between molecular In the simplest sense, chemical attack can be minum metal substrate. chains. categorized as that by acids and bases. Chemical Finally, rust deposits, dirts, salts, and other The effect of heating (either using hot water attack does not occur at neutrality (pH of 7). contaminants remaining on a surface can both or a hot or warm environment) increases molec- However, water, salts, and solvents, all of neu- prevent bonding of the paint and establish ular movement, enabling more rapid water tral pH, can dramatically affect and degrade a osmotic driving forces further promoting water penetration. Conversely, cooling, particularly coating. This type of “neutral” degradation is permeation. At areas where paint adhesion is below the glass transition temperature (Tg), described elsewhere in this article. relatively poor, crosslinking is less dense, or reduces molecular movement and retards water Acids and bases, and the strength of the acid there are agglomerations of pigments not permeation. or base, are a simple function of the degree of completely wet out by the organic binder, water Solvent Permeation. Solvents are not usually disassociation of the chemical into hydrogen can collect and “pool,” causing a swelling of found in most environments, and the presence of ions, H+ (acids), or hydroxyl ions, OH– (alkalis the film and additional water penetration. a solvent in a paint film occurs primarily as a or bases). Acidic or alkaline strength is Water-soluble salts, including sodium chloride, result of solvent addition to the resin when measured on the logarithmic pH scale, a scale calcium chloride, and other chlorides (found manufacturing the paint. Solvents are added in with each number being ten times greater than in marine environments and in deicing salts), order to reduce the viscosity of a resin, thinning the preceding number. A pH of 3 is ten times and sulfates (from acid rains) are notorious for it for application purposes. Upon drying and cur- more acidic than a pH of 4, for example. causing osmotic blistering of coatings in ing, the solvent must volatilize from the coating A pH of 7 is exactly neutral, while 1 is strongly immersion service and/or accelerated rates of into the atmosphere in a timely manner. If suffi- acidic and 13 is strongly basic or alkaline. corrosion in atmospheric service if they are cient solvent volatilization does not occur, and For all practical purposes, the medium in allowed to remain on a substrate before paint- solvent is retained in the film, the coating can which the acid or alkaline disassociation occurs ing or between coats of paint. remain soft and plasticized, because the relatively is water. Even very small amounts of water are Coating Deterioration / 467 sufficient to dissolve and disassociate most oil to form an organic acid and alcohol. The penetrate, concentrate, and aggressively attack chemical compounds into their acidic or basic bond breaking reduces molecular flexibility both the pigment and binder. Pigment agglom- constituents. At the molecular level, when such and embrittles the film; this ultimately leads to erations not completely wetted out by the resin disassociation occurs and the acid or base resin deterioration and the formation of a sticky of the coating are susceptible to this, particu- comes in contact with a coating material, chem- soft coating under damp conditions, or a brittle larly if there are pinholes and voids in the coat- ical attack can take place. powdery coating when dried. All coating resins ing. However, ionic permeation is much slower However, many chemicals are hydroscopic: containing ester groups are susceptible to such than moisture permeation, and unless the pig- they attract and react with water. Examples attack. However, some of those resins, such as ment is exposed on the surface (by chalking are most sulfur chemicals, including sulfuric the polyesters and vinyl esters, are much more or surface resin deterioration) or the chemical acid, sulfamic acid, and sodium sulfide; sodium highly crosslinked and formulated with epoxy environment has access through pinholes, and potassium hydroxides; sodium carbonate; resins and other materials to sterically hinder voids, or other discontinuities in the paint film, zinc chloride; most salts, such as sodium, potas- the ester group, protecting it from alkali attack. chemical attack to pigments is not usually a sium, and zinc chlorides; and many solvents, in Saponification (reaction with an alkali) and major problem. particular the alcohols and glycols. hydrolysis (reaction with water) are similar, Table 2 shows sensitivity of some of the Acid Attack. Acids consist of inorganic min- but the saponification reaction is much faster more common pigment types to chemical attack eral acids such as hydrochloric, sulfuric, and and more debilitating. An illustration of alka- (Ref 9). nitric acids, which disassociate completely in line and hydrolytic (water) saponification is Oxygen and Other Gas Permeation. Oxy- water. Organic acids such as carboxylic acids, shown in Fig. 4. gen permeation at the cathode in a metallic cor- including formic acid and butyric acid, do not Figure 5 illustrates the vulnerability of vari- rosion cell is usually the rate determining factor completely disassociate and as a consequence ous organic linkages to hydrolysis (reaction in the corrosion reaction. The common anodic are considered weaker acids. However, even with water) and saponification (reaction with and cathodic reactions of metallic corrosion are: these acids can aggressively attack most coat- alkali) (Ref 9). ing systems. Acids and alkalis not only cleave covalent Anodic reaction: M ! Mþ þ ne Acid gases such as sulfur dioxide (SO2), sul- bonds of organic resins but can also attack acid fur trioxide (SO3), hydrogen sulfide (H2S), and or alkaline susceptible pigments in the paint. where M = metal, n = number (of valency elec- nitrogen oxide (NOX) react with moisture in Where there are pinholes and permeability trons), and e = electrons. the air in the form of precipitation or condensa- through the coating, chemical species can Cathodic reactions: tion to form sulfuric and nitric acids. Even car- In near-neutral and alkaline environments bon dioxide (CO2) as a normal constituent of the atmosphere reacts with moisture to form a 1/2O þ H O þ 2e ! 2OH weak carbonic acid (H2CO3). 2 2 Chemical attack by condensation on a coat- ing is more aggressive than that deposited by In acidic environments precipitation such as acid rain, because mois- ture condensing on a surface containing acidic þ þ ! ð Þ constituents usually evaporates as the substrate O2 4H 8e 4OH in the presence of oxygen warms during the day. As the moisture evapo- þ þ ! ðÞð rates, the acids within the condensation droplet 2H 2e H2 gas in highly acidic solution concentrate and more aggressively attack the and=or absence of oxygenÞ substrate on which the condensation resides. Acid rain, on the other hand, is diluted by suc- Thus, permeation of molecular oxygen is neces- cessive rainfall, and the chemical contaminant sary for metallic corrosion in near-neutral, alka- can be diluted or even washed from the surface. line, and mildly acidic environments, and in The chemicals thus deposited, however, many instances it determines the rate of corro- attack and cleave chemical bonds that are sus- Fig. 4 (a) Saponification and (b) hydrolysis of an ester sion. Corrosion is an expansive process, and ceptible to deterioration. Chemical groups linkage undercutting corrosion beneath a well applied specifically vulnerable to acidic attack and cleavage are ether, urea, and urethane linkages, where cleavage occurs by a reaction of the hydrogen ion (the susceptible portion of the linkage). Alkaline Attack. Similarly, strong alkalis such as sodium, potassium, and calcium hydro- xides attack susceptible chemical groups in coatings. Perhaps the most widespread type of alkaline attack is saponification, the alkaline attack of the ester linkage of drying oils used in most oil-base coatings and alkyds. The attack can occur when oil-containing coatings or alkyds are applied over concrete, which con- tains alkali salts, which, when combined with water, form caustic alkalis. In a similar fashion, application of oil-based alkyds over zinc-rich coatings can also result in saponification because zinc reacts with moisture to form alka- line zinc hydroxides. The hydroxyls (OH–) cleave (break) the ester linkage in the drying Fig. 5 Vulnerability of organic linkages to saponification and hydrolysis. Source: Ref 9 468 / Coating Analysis and Evaluation

Table 2 Chemical sensitivity of selected organic and inorganic pigment families In most cases, heat or radiation deterioration does not act alone but acts in conjunction with Sensitivity oxygen. Oxygen is a very reactive molecule, Pigment type Examples Alkali Acid and if a free radical forms in its presence, then Inorganic the oxygen can combine immediately with it ... Titanium dioxide Excellent Excellent to form a different radical. This radical can then ... Zinc oxide Moderate to good Poor ... abstract a hydrogen atom and form a hydroper- Antimony oxide Poor Poor oxide. A hydroperoxide is unstable and decom- Red iron oxide Synthetic red oxide; Spanish, Indian, or Excellent Excellent Persian Gulf red poses into two radicals. From the initial two ... Cadmium red Excellent Poor radicals, a total of six possible radicals can ... Molybdate orange Poor to fair Poor form. This explains the danger in chain scission Lead Minium, mineral orange Good Poor in the presence of oxygen leading to a chain Yellow iron oxide Ferrite yellow, sienna, ochre, umber Excellent Fair ... Chrorme yellow Poor to fair Fair reaction (Ref 10). Zinc yellow Zinc potassium chromate Fair Poor The oxygen free radicals thus formed can fur- ... Cadmium yellow Excellent Poor ... ther react with molecules in a coating or organic Nickel titanate yellow Excellent Excellent ... material in the same manner as described previ- Bismuth vanadate Excellent Fair ... Zinc ferrite Excellent Good ously, causing chain scission, depolymerization, Chrome green Brunswick green Poor Poor and fragmentation of the molecule, reducing its ... Chromium green oxide Excellent Excellent flexibility and resistance to permeation. Iron blue Prussian blue, Midori blue, Chinese blue, Poor Very good mineral blue Permeation of Water, Oxygen, and Ions ... Ultramarine blue Very good Poor through Weak Areas of Crosslink Density ... Carbon black Excellent Excellent in the Coating. Water is in virtually every ... Black iron oxide Excellent Fair ... environment around the world to some degree. Micaceous iron oxide Excellent Excellent ... Water in freshwater lakes, saltwater oceans, Zinc dust Poor Poor ... Aluminum Poor Poor and in ponding rainwater results in a water ... Stainless steel flake Excellent Very good to immersion environment for materials exposed Excellent to these environments, and in relatively rapid Organic water permeation through a coating. Metallized azo reds Lithols, permanents, rubines Poor Poor In non-immersion atmospheric environments, Nonmetallized azo reds Toluidines, paras, naphthols Very good Very good water is present as humidity, condensation, and ... Azo-based benzimidazolone reds Very good Very good ... precipitation. In these environments, moisture Quinacridones Excellent Excellent permeation into a coating is much slower, Vat reds Dibromanthrone, anthraquinone, brominated Excellent Excellent pyranthrone, perylenes dependent principally on the duration of time Azo-based oranges Dinitroaniline, pyrazolone, tolyl Good Good the coating is wet. There is little penetration Naphthol orange Very good Very good ... driving force if water is present only as a gas, Azo-based benzimidazolone oranges Very good Very good such as humidity, but if it is present as conden- Metallized azo oranges Clarion red Poor Moderate Monoarylide yellows Hansa yellows Very good Very good sation or precipitation (rain, dew, fog droplets, Diarylide yellows Benzidine yellows Very good Very good and melted sleet or snow), the water can ... Azo-based benzimidazolone yellows Excellent Excellent penetrate the coating system. Furthermore, in Heterocyclic yellows Isoindoline, quinophthalone, azomethine, Excellent Excellent atmospheric environments, oxygen is present, tetrachloroisoindolinone, triazinyl ... Phthalocyanine greens Excellent Excellent and the water has absorbed or entrained oxy- ... Phthalocyanine blues Excellent Excellent gen. Oxygen, as a gas in the atmosphere, has ... Carbazole violets Very good Very good little or no driving force to penetrate a coating Source: Ref 9 film, but it is almost omnipresent, at least at the initiation of corrosion. Accordingly, the penetration by water of a coating in an immer- sion or semi-immersion environment is the principle cause of corrosion of the underlying coating system often causes the coating to crack dissociates, forming two oxygen atoms. The substrate. and spall from the substrate, exposing the oxygen atoms recombine with molecular oxy- Virtually all organic coating materials are underlying surface to the environment and fur- gen to form ozone. Ozone absorbs UV between permeable to water to some degree. Thick, highly ther corrosion attack. A means of corrosion pro- 200 to 320 nm and absorbs in the visible range crosslinked coatings are much more impermeable tection in the oil industry is to remove oxygen at 420 and 700 nm. In the UV range, strato- to water penetration than coatings with a lesser from well injection water, and in the nuclear, spheric ozone dissipates energy as heat. This is crosslinked density. However, even with rela- chemical processing, and other industries, to beneficial, because less high-energy UV, which tively thick, highly crosslinked coatings, as inert the vapor space in a tank or vessel by add- is detrimental to life on earth (sunburn for exam- described previously, there are areas of variabil- ing nitrogen, carbon dioxide, combustion gases, ple), is absorbed. Ozone in the stratosphere does ity resulting in lesser crosslink density. It is coat- or other gases to displace oxygen, thereby not influence the formation of ozone near the sur- ings in these areas that water penetrates. reducing or eliminating metallic corrosion. face of the earth. On the earth’s surface, ozone is Numerous studies conclude that coating films Oxygen species (nascent atomic elemental produced by industrial combustion. Ozone is contain microscopic regions that absorb large oxygen, O; molecular oxygen, O2; ozone, O3) formed indirectly from nitrous oxide, which amounts of water and have low ion resistivity. are very influential in the degradation of absorbs UV, forming molecular oxygen, which That water does not defuse into the film uni- organic materials by ultraviolet light and solar further reacts with more molecular oxygen to formly, but in a dense layer along boundaries radiation. Molecular oxygen absorbs solar radi- form ozone. Ozone is a strong oxidizer that reacts in the polymer structure, followed by penetra- ation in the range of 176 to 210 nm. Upon with most organic materials, including coatings, tion of the structure itself. Further, corrosion absorption of UV radiation, the molecular oxy- to form free radicals and ultimately photochemi- spots on the substrate have been found to be gen is transformed into singlet oxygen, which cal embrittlement degradation. directly related to these regions (Ref 11, 12). Coating Deterioration / 469

Initially, the water can penetrate the coating permeates the coating through weak areas of moisture, oxygen, and conductive ion contami- only partially to random depths at numerous crosslink density, micropores and cracks, voids, nation to the substrate. At these large-scale sites. In atmospheric environments where there and pathways, ultimately to the underlying sub- defects, permeation through a coating film is is drying by the sun, or increasing daily tem- strate. That permeating water carries with it not necessary, because the coating film has peratures, the penetrating water may diffuse oxygen and other materials, depending on the been damaged and the metal substrate is out and evaporate. However, the water penetra- atmospheric environment. exposed. Corrosion is defined as the deteriora- tion even in these areas can swell the coating Of particular interest for corrosion purposes tion of a material due to exposure to an envi- and dissolve any water-soluble constituents. is the prevalence of ionic contaminants—acids ronment. In every case with a metallic Thus, subsequent water penetration may be and salts, notably anions, chlorides, sulfates, substrate, when corrosion occurs, it is an elec- even easier at these same sites and penetrate and nitrates, and their cations—that dissociate trochemical reaction with the formation of an further, and penetration can potentially initiate in the permeating water, which increases its anode and a cathode. The anode forms at areas at other new adjacent sites. One study estimated conductivity, and the rapidity of corrosion where there is more energy in the steel, for the apparent area of the pores increased from an when these ions access a metallic substrate example at scratches, impacts, and damaged initial 0.6 to 6700 mm2 per cm2 of coatings after through the coating. However, ionic permeation areas; at areas of higher temperature; at grain 100 days of exposure to 0.6 mol/L sodium chlo- through a coating film is so extremely low that boundaries within the steel alloy; and for many ride solution (Ref 13). under-film corrosion may not be caused by other reasons. The cathodic areas form adjacent With sufficient time, and duration of wetness, ionic permeation, but by surface contamination to the anodic areas. water permeation through the coating cross sec- prior to coating. Also, the observation that For steel, those reactions are detailed previ- tion down to the underlying substrate ultimately cathodic blisters are highly alkaline provides ously and summarized herein: occurs. strong evidence that paint films also are imper- Oxygen and Ionic Permeation along with meable to hydroxyl ions (Ref 15). FeðÞ! iron=steel Feþþ þ 2e Water into the Coating. Water precipitation Under the influence of electric fields (such as (most often as rain or melted snow and ice) is cathodic protection), functional groups asso- (The iron in the steel dissolves in the moisture not a pure liquid. Even pure distilled water reacts ciated with pores can become ionized and solution into positively charged ferrous ions, with carbon dioxide in the air to form a weak exchange ions with an exterior electrolyte solu- liberating two negatively charged electrons.) carbonic acid (H2CO3), giving it a pH of appro- tion. This, combined with the plasticizing effect The corrosion continues as a depolarizer ximately 5.6. Air pollutants can contribute further of water on some polymers, can, in certain coat- removes (reduces) the electrons from the solu- to ionic contaminants in rainwater. Acid rain ings, result in inflated pores, which may tion at the cathode. Accordingly, in a neutral consists of sulfur and nitrogen compounds— increase ionic penetration to the substrate. or near-neutral pH environment, the cathodic principally in the form of sulfur dioxide and nitro- Increasing temperature also has a profound reaction with the most common reduction depo- gen dioxide—that hydrolyze with water to form effect by increasing the rates of moisture per- larizer, oxygen, is: sulfuric acid (H2SO4) and nitric acid (HNO3). meation and ionic exchange through a coating These rainfall pollutants principally result from film (Ref 16). O2 þ 2H2O þ 4e ! 4OH electrical power generation and the burning of However, it is the general consensus of virtu- fossil fuels, gasoline combustion in motor vehi- ally every research document on the subject (Oxygen in the water/air and the water itself cles, and industrial smokestack output. Natural that ionic permeation through a paint film is reacts with the liberated electrons from the iron sources consist of volcanic emissions contribut- far slower, if it occurs at all, than permeation to form hydroxyl ions.) ing sulfur dioxide, biological decay contributing by moisture and oxygen. Moreover, moisture The hydroxyl ions formed at the cathode dimethyl sulfide, and lightning contributing nitric (H2O), as a result of a smaller molecular size, react with sodium, potassium, and other posi- oxide. permeates much more rapidly than oxygen tively charged cations to form an alkaline Rainwater dissolves particulate materials in (O2). solution, commonly NaOH, a strong alkali. the atmosphere when droplets of water form Steel Substrate Reactions Due to Permeat- This alkali has a very high pH (often around on atmospheric particles. Additionally, rainwa- ing Water. After water permeates through a 11 to 13), and the alkalinity disbonds the coat- ter dissolves atmospheric gases including pollu- coating, it ultimately comes into contact with ing at the cathodic metal interface. Three possi- tants and oxygen. Oxygen is ubiquitous, is the underlying substrate and can react with that ble mechanisms have advocated for the dissolved in water, and permeates with water substrate. The substrate reaction, if any, with cathodic delamination of a coating: dissolution into a coating. the permeating water depends on the nature of of an oxide layer on the substrate surface, alka- In coastal areas rainwater has a salt content the substrate (wood, concrete, masonry, galva- line hydrolysis of the coating polymer, and essentially like that of seawater, but much more nized metal, plastic, aluminum, titanium, stain- interfacial failure due to the high alkalinity at dilute. Generally within a mile or so of the sea- less steel, or other metal, etc.). Furthermore, the cathode. It is likely that some or all of these shore, and often much further depending on the constituents within the permeating water occur in combination, simultaneously or in wind velocity and direction, wind-borne salt strongly influence any reactions that might stages. However, irrespective of mechanism, spray deposits upon and contaminates most sur- occur with the underlying substrate. Accord- high alkalinity at the cathode is responsible faces and structures, and concentrates due to the ingly, the discussion of such substrate reactions for cathodic disbonding. Additionally, the accu- evaporation of water. with permeating water is beyond the scope of mulation of hydroxyl groups (OH–) attracts Predictably, the composition of rainwater this article. However, because coatings are more water due to hydrogen bonding, resulting varies geographically because atmospheric con- widely applied to structural steel for corrosion in cathodic blistering at corrosion sites in taminants also vary from place to place. protection, the reactions of permeating water immersion or even in severe atmospheric expo- Besides the previously mentioned inorganic with a steel substrate are discussed. sures. If cathodic protection is used, either in contaminants, organic contaminants also have The reactions that occur on a steel substrate the form of an impressed current or sacrificial been found in rainwater (Ref 14). (or, for that matter, on any other substrate) anodes, disbondment and blistering at the cath- The outcome of all of this is that water pre- essentially are similar to those that occur at a ode may be substantially increased. Figure 6 cipitation is not pure H2O, because there are scratch, mechanical damage, or other large- depicts cathodic blistering around a scribe on many other materials dissolved in or combined scale visible defect in a coating layer. The a test panel. with the water droplet, including oxygen. major difference is that the corrosion reactions The positively charged ferrous ions migrate Water, as precipitation, when in contact with a that occur with these large-scale defects occur to the cathode, attracted by the negative coating for any considerable amount of time, much more rapidly due to the ready access to hydroxide ions, and react with them, forming 470 / Coating Analysis and Evaluation an oxidized ferrous hydroxide brown colored substrate. The corrosion mechanism beneath a severity of corrosion (Ref 20). In the prior rust product. These ferrous ions going into solu- coating is essentially similar to that occurring examples, the chloride ion (Cl–) has been used tion at the anode result in a metal loss that at a scratch, mechanical damage, or any large- as the anionic species salt example, but the forms pits in the steel substrate. This is depicted scale coating defect. The corrosion reactions effect of nitrates, sulfates, and other anions is in Fig. 7. differ depending on the metal substrate; those similar. Subsequent oxidation and hydrolysis result for steel are illustrated previously. in a decrease of the pH and the formation of Soluble salts—such as those found in coastal Stress Influences a complex mixture of hydrated iron oxides environments (notably NaCl) or where deicing (rust): salts are used, in chemical environments, or Internal coating stresses build up during dry- where acid rain might deposit, carrying nitro- ing and curing and on aging. These internal þþ x Fe þ y O2 þ z H2O ! FexOy z H2O gen and sulfur oxides (NOx and SOx)—can be stresses are considered by some to be a primary present in pits or on a substrate prior to painting cause of premature coating failure. Addition- Initially, a lesser oxidized rust product is and can also permeate with water, increasing ally, external stresses to a coating system are applied by movement of the substrate and by formed: Fe3O4. With further oxidation, a fully the water conductivity and thus the rate and oxidized rust product results: Fe2O3. These rust products are almost always formed with hydrated water (H O) attached. 2 The hydroxide In coastal and chemical areas where chloride quickly oxidizes Water ions are present, those negatively charged ions Iron hydroxide to form rust. droplet can enter into the corrosion reaction to help forms and present charge neutrality. They react with precipitates. excess ferrous ions to form hydrated ferrous – 2+ OH O Electrochemical chloride in a reversible reaction that can move O – Fe Fe2+ 2 2 OH cell action driven in both directions, because ferrous chloride is by the energy of water soluble: oxidation continues the corrosion þþ þ þ $ Fe 2Cl H2O FeCl2 H2O e– – e– process. e e– Electron Thus, the products in the corrosion solution flow at the anodes beneath the rust layers at the bot- Anode action causes pitting tom of a large-scale mechanically damaged of the iron. area of a coating, and also at a corroded area Iron Cathode action reduces oxygen where moisture has permeated through an oth- from air, forming erwise intact coating, most likely are hydrated hydroxide ions. Fe3O4 and FeCl2. Oxidized corrosion products are anion-selective (Ref 18), allowing the chlo- ride ions to continuously permeate through the Fig. 7 Positive ferrous ions react with negative hydroxide ions, forming rust and resulting in pits. Source: Ref 17 rust layers and reach the steel surface. A model for the degradation of steel in a neutral NaCl solution environment is presented in Fig. 8. In summary, variability within a properly NaCl H O, O , NaCl Conductive dried and cured coating film in the form of 2 2 Scribe H2O, O2 pathways microscopic cracks, porosities, capillaries, pig- Coating ment agglomerations, and areas of low cross- Fe++ – – Cathode e e Cathode – linking will, in time, enable penetrating – Anode e Cathode Steel Anode OH – moisture access to an underlying substrate. Vir- Fe++ OH tually all organic coating materials are water permeable to some degree. On a metallic sub- H2O H O strate this permeating moisture, along with dis- O2 NaCl 2 O NaCl solved oxygen, enables the formation of anodic Corrosion product 2 Corrosion product and cathodic areas, resulting in corrosion of the Coating Na+ Na+OH– Delamination Fe++ Cathode – Cathode e Anode Cathode Steel (Blister initiation site) e– Anode ++ + – Fe Na+ Na OH

Corrosion product Blistering NaCl H2O, O2 Correction NaCl H O, O Blister H2O, O2 product 2 2

Fe O ,FeCl Coating + 3 4 2 Na Fe++ Fe++ + + – Na OH e– Na OH Cathode Cathode – + – + – Steel Cathode e Anode Delamination Na OH Anode Na OH (a) (b)

Fig. 6 Cathodic blisters around a scribe. The scribe is Fig. 8 Conceptual model for the degradation of an organic coating on steel in a neutral NaCl solution. (a) With a the anode; the area immediately adjacent to it large-scale scribe or mechanically damaged area. (b) Without apparent defects but with moisture is the cathode. Courtesy: KTA-Tator, Inc. permeation. Source: Ref 19 Coating Deterioration / 471 mechanical damage. The role of stresses in the causes an uneven distribution of stress on the and contraction are major detriments to coating deterioration of a coating system are the subject coating, further aggravating the potential for systems, particularly those that have high inter- of this section. cracking or peeling. nal stresses or have been applied to excessive Coating, Curing, and Drying Internal Highly crosslinked coatings, particularly thicknesses. Stresses. When one chooses to protect an 100% solid materials formulated with low- Impact and Abrasion. While coating, cur- object from corrosion, to “spruce it up,” change viscosity co-reactants, are particularly suscepti- ing, and drying stresses, as well as vibration its color, provide an antigraffiti or sanitary sur- ble to internal crosslinking stresses. Polyesters, flexing and stress-strain (as discussed previ- face, or to provide for a myriad of other func- vinyl esters, and other thick-film highly cross- ously) are relatively slow transient influences, tions, that individual usually turns to paints or linked coatings must be properly formulated impact and abrasion to a coating, in contrast, is coatings. Why? Because paints and coatings and pigmented to satisfactorily reduce these usually sudden, localized, and abrupt. Mechan- can be formulated to provide all of these attri- internal stresses. The low-molecular-weight ical damage from dropped tools or stones, or butes and are applied in liquid form, allowing epoxies, including bisphenol F and novolac other types of mechanical damage where the them to cover and coat all surfaces of the object epoxies, must also be properly plasticized and coating surface is impacted either directly or in a relatively fast, convenient, and inexpensive pigmented to dissipate internal curing stresses. reversely (from the side the coating is on or means of application. The reason this can be How does one measure the extent of internal from the opposite side), can cause the coating done is that the coatings are spray applied in stress/curing stress on a coating? Until recently, to crack and/or spall. Impact damage is greatest liquid form and convert by drying and/or curing the best means of assessing such stresses was to when a coating has high internal stress and is to a solid coating. The fact that a paint can be apply the coating system to a test panel or a test very brittle, or is at or below its glass transition applied in liquid form gives it the ability to patch in the area of intended service, wait for temperature (Tg). wet, penetrate, seal, cover, and adhere to most curing and drying to occur, and then evaluate Abrasion occurs as a result of scraping, substrates, even those with complex shapes. the coating over time to see if stress cracking, scuffing, or erosion due to contact with small Moreover, application of the liquid by brush, peeling, disbonding, or other evidence of dis- moving particulate matter such as sand or slur- roller, or spray is fast and inexpensive and rela- tress occurred. Recently, however, there have ries. As a general rule, harder more brittle coats tively convenient. However, the conversion of a been some tests developed (but not yet standar- are more susceptible to abrasion damage than paint from liquid to solid provides some inher- dized) whereby a coating material is applied to rubbery softer coatings. However, specific resis- ent stress to the coating and, in extreme condi- one side of a thin foil strip that is held in posi- tances are dependent on the formulation of tions or if adhesion is not adequate, can cause tion at one end and allowed to deflect at the the coating, because many hard coatings have failure of the coating. other. The amount of deflection at the free end abrasion-resistant pigments—such as aluminum If a coating is not 100% solids and contains a is indicative of the internal shrinkage stress as oxide, quartz, silica (sand), garnet, and other volatile material such as water or solvents, dry- the coating dries and cures. hard materials—embedded in them to resist ing of the coating and its conversion from a liq- External Stress—Vibration, Flexibility, abrasion and erosion wear. Rubbery elastomeric uid to a solid result in a volume decrease as the Stress-Strain. External stresses on a coating are coatings have the ability to deform under abra- water or solvent evaporates into the atmo- many and varied and usually affect an applied sion, up to a critical point, after which they sphere. Most coating materials initially gel coating to a greater extent than internal stresses. recover their original form. Energy absorbed within seconds to hours after application as When a person walks across a bridge while either during the abrasion and/or impact is absorbed most of the volatiles leave and the paint dries a heavy truck or train is also crossing the bridge, by the elastomeric resin and dissipated as heat on the surface. The shrinkage that occurs with the movement, flexing, and vibration of that within the flexible molecular structure. the loss of volatile materials provides some ini- bridge is readily evident. In a similar fashion, However, hard brittle coatings and coatings tial stress to the coating and to the adhesion of any time a municipality fills or empties a water in cold temperatures, or below or close to their the coating. However, this stress is minimal, storage tank, flexing and bowing of the sidewalls Tg, may not have the flexibility to resist abra- because the paint has not sufficiently solidified, and tank bottom occur. Wind, snow loads, pond- sion or erosion wear. In these instances, the is still deformable, and can internally dissipate ing water, and other forces can deflect a metal coating may tear on the surface or be scraped these initial stresses. However, as the paint surface, stressing the coating applied to it. Cycli- or ablated away, resulting in a thinning of the dries further, and particularly if chemical cal stresses resulting from a vibration and flexing coating at the areas of abrasion/erosion. crosslinking occurs, further stresses are applied are most detrimental and can readily degrade At areas where abrasion/erosion are expected, to the now dry coating. Sometimes low- both the metal substrate and any coating applied natural or synthetic rubbers should be used. Anti- molecular-weight plasticizers such as pthalates, to that substrate. abrasion pigments should be added to the resin or phosphates, adipates, and chlorinated biphenyls Solar heating by day and cooling by night cast and embedded into the top surface of the can migrate to the surface of the paint and cause expansion and contraction of all materials. coating to make it more abrasion resistant. In volatilize, or at least collect onto the surface. Stress resulting from such thermal expansions some cases a softer more elastic thick film coat- When this happens, the molecular volume of and contractions can be aggravated under winter ing can also be used to resist scuffing and the resin decreases. This diminishment in conditions in cold weather climates where a abrasion. volume applies a tensile stress to the cross sec- coating becomes somewhat embrittled due to tion of the paint film, and in extreme cases can cold temperatures. Relatively rapid heating and result in cracking, peeling, or loss of adhesion. cooling during daily temperature fluctuations is Biological Influences In a similar fashion, progressive crosslinking— often more of a problem in the winter than in either by reaction with oxygen from the air (auto the summer. Some accelerated tests performed Microorganisms (nonvisible to the unaided oxidation) or covalent bonding between reactive by laboratories have used a freeze-thaw cycle eye) are the earliest and most numerous life moieties of part A and part B components of a to provide additional stress on a coating to forms on earth. They are ubiquitous, occurring coating—provides a further hardening, increas- assess its potential for a service environment, in virtually all natural environments, including ing brittleness, and a tensile stress to the coat- even in a warm climate where no freezing is those considered until recently to be inhospita- ing. In many coatings these latent crosslinking expected. This is because if the coating can sur- ble to life: undersea volcanic vents with high reactions occur slowly over time, often years, vive the freeze-thaw cycling, it is quite likely concentrations of sulfur, hot springs, extremely such that the coating slowly hardens and embrit- that it will be able to resist any thermal cycling acidic and alkaline chemical environments, tles with age. Where coating has been applied encountered in actual service. The external anaerobic (no oxygen) environments, and in thickly, stresses are greater than areas where stresses resulting from vibration, flexibility, locations devoid of sunlight. Degradation of coating is applied thinly. Variation in thickness and cyclical stress-strain from thermal expansion coatings and other materials results from 472 / Coating Analysis and Evaluation electrochemical and biological processes result- products. Not only is the paint degraded, but it Conclusion ing from the presence of microbes. is also covered with an unsightly black growth. Similarly, macroorganisms (visible) such as Fungicides can be added to paint to reduce or There are a variety of environmental stresses mildew and marine flora and fauna can cause eliminate mildew and mold growth. Basic zinc that combine to degrade coatings exposed in a considerable damage to coatings and dramati- oxide added to oil-base and latex paints in service environment. Some of the individual cally affect the properties and functions of coat- amounts of one to three pounds per gallon, per- major influences are discussed in this article. ing systems, negating their purpose. haps in combination with some organic fungi- However, this discussion does not include the Microbiologically Induced Corrosion. cides, can substantially reduce or eliminate stress from combinations of influences, which Microbial adhesion, establishment, and growth fungal growth. always happens in nature. Accordingly, failure into colonies are prerequisites for deterioration Molds and mildews can be killed by a dilute analysis of coatings is as much an art as a sci- of organic materials. Much, but not all, microbial hypochlorite solution (bleach). Bleach kills the ence, although science always underlies any growth occurs in a biofilm—a gel consisting fungus, turning it white. If dirt is present, it will attempt to explain what has happened when a principally of polysaccharides that protects the not be bleached and will remain a dark color. coating system fails. microbe and enables it to form an environment After the fungus is killed, it should be removed that is conducive to its reproduction and colony from the paint surface by scraping and/or scrub- growth (Ref 21). The process by which a material bing prior to painting. However, if heat, mois- REFERENCES is decomposed or otherwise altered by a microbe ture, and a food source are still present after colony results in degradation products of the repainting, it is likely that spores will again 1. F.W. Billmeyer, Textbook of Polymer Sci- polymer, which the microbe uses as a source of reattach and grow. Mildews and mold are rela- ence, Interscience Publishers, March 1966, energy, notably, carbon and electrons. This can tively easy to kill by bleaching, but their spores p18 result in the depolymerization of the organic mol- can often survive a bleach treatment and com- 2. W.D. Bascom, J. Adhesion, Vol 2, 1970, ecule, breaking it up into smaller units that can be mence growth if conditions are right. p 168 assimilated by the organism. Marine fouling consists of attachment of 3. “Corrosion Protection of Steel Structures Mildew. In warm moist environments, and plant or animal life to an immersed structure. Vir- by Protective Paint Systems—Part 2: Clas- particularly in the southeastern part of the tually all underwater surfaces have some type of sification of Environments,” ISO 129444-2, United States and in tropical countries, the pres- marine attachment, including ships, piers, pilings, International Organization for Standardiza- ence of molds and mildews on paint (and many and even whales, large fish, crabs, and other tion, 1998 other materials) are of concern. Molds and mil- crustaceans. 4. Society for Protective Coatings dews are both forms of fungi whose spores are Animal fouling species are most commonly 5. G. Wypch, Handbook of Material Weathering, ubiquitous in the environment. The spores barnacles, muscles, or tubeworms. Fouling veg- 3rd ed., ChemTec Publishing, Toronto, require moisture and heat in order to germinate etation are algae, or seaweeds (Ectocarpus, Ontario, Canada, 2003, p 58 and grow. Generally, humidity of 70% is neces- Enteromorpha,orLaminaria). Both animal 6. C.H. Hare, Protective Coatings: Funda- sary for mold and mildew grow, with tempera- and plant fouling requires contact with the sub- mentals of Chemistry and Composition, tures between approximately 5 and 50 C strate for 24 hours or more in order to attach. SSPC 94-17, Technology Publishing Com- (40 and 120 F). Slightly acidic pHs (in the Consequently, if water is fast moving (over pany, Pittsburgh, PA, 1994, p 464 range from 4.5 to 6.5) are preferable, although approximately ten knots) or if the surface is 7. G.P.A. Turner, Paint Chemistry, 3rd ed., alkaline conditions above a pH of approximately smooth with no fissures or crevices, marine Chapman & Hall, London, 1993, p 27 8.5 are not conducive to most fungal growth. fouling is minimized or eliminated. 8. M. Hess, H.R. Hamburg, and W.M. Mor- In order for fungal growth to occur, the spore Marine foulants generally do not attack the gans, Hess’s Paint Film Defects, 3rd ed., must remain in contact with the surface and paint and degrade it, but by virtue of their adhe- Chapman & Hall, London, 1979, p 185 have a source of food. Accordingly, a rough sion, roughen the surface to which they are 9. C.H. Hare, Paint Film Degradation: textured surface that collects dirt and holds attached, providing considerable friction to the Mechanisms and Control, Society of Pro- moisture provides a good surface for fungal smooth flow of water around the fouled object. tective Coatings, Pittsburgh, PA, 2001, growth. Food sources for the spore can be dust If that object is a ship, even minimal marine p 311, 318–319 and dirt picked up from the atmosphere, wind- fouling can reduce its speed and increase fuel 10. D.P. Garner and G.A. Stahl, The Effect of blown contamination on the painted surface, consumption by 10% or more. Hostile Environments on Coatings and Plas- or from the paint itself. Oil-based coatings, Antifouling coatings are used to kill or pre- tics, ACS Symposium Series 229, American alkyds, and polyamide-cured epoxies are sus- vent attachment of marine organisms. Tributyl Chemical Society, Washington, DC, 1983, ceptible to fungal attack due to the fatty acids tin antifoulants have been used very success- p15 in the oils or crosslinking copolymers. Phthal- fully in the past, but because they are toxic to 11. M.I. Karyakina and A.E. Kuzmak, Prog. ate plasticizers can also be a food source for most marine life, have been banned from use Org. Coat., Vol 18, 1990, p 325 fungi. Latex paint systems, particularly those on most U.S. flagships. Currently antifoulants 12. H. Corti, R. Fernandez-Prini, and D. Gomez, containing oil or alkyd modifications for adhe- based on copper oxides are most commonly Prog. Org. Coat., Vol 10, 1982, p 5 sion to chalky surfaces, are also very suscepti- used, but these too are toxins, and there is con- 13. B.S. Skerry and D.A. Eden, Prog. Org. ble to mildew or mold growth. The fungal cern that ultimately they could also be prohib- Coat., Vol 15, 1981, p 269 spores can either feed directly on the fatty acid ited. Nonstick fluorinated hydrocarbon resins 14. D.L. Poster and J.E. Baker, Influence of Sub- constituents in the paint or secrete enzymes that and silicone-based binders are being used with micron Particles on Hydrophobic Organic break down portions of the paint binder into only limited success at present. These materials Contaminants in Precipitation, Part 1: components which then can be used as food. function by providing a surface to which the Concentrations and Distributions of Poly- When a mold or mildew grows on the surface marine fouling organism cannot attach tightly. cyclic Aromatic Hydrocarbons and Poly- of a paint, the growth can collect dirt and dust When a ship is underway, the friction from chlorinated Biphenyls in Rainwater; and from the atmosphere and hold moisture, perpe- flowing water is sufficient to wash growth from Part 2: Scavenging of Polycyclic Aromatic tuating its growth. As the fungus feeds on the the nonstick antifoulant paint surface. Even Hydrocarbons by Rain, Environ. Sci. Tech- paint, the resinous binder is degraded by scis- when marine fouling does attach, it can be read- nol., Vol 30 (No. 1), 1995, p 341–348 sion of ester linkages, oxidation of oil fatty ily removed by scraping or hosing down with 15. H. Haagan and W. Funke, Prediction of the acids, and accumulation of metabolic acidic high-pressure water washing. Corrosion Protective Properties of Paint Coating Deterioration / 473

Films by Permeability Data, J. Oil Colour chemical/corrosion.html (accessed Feb 8, 21. K.B. Tator, Preventing Hydrogen Sulfide Chem. As., Vol 58, 1975, p 359–364 2015) and Microbiologically Influenced Corro- 16. A.K. Van Dyk, “Diffusion and Uptake of 18. N. Sato, Corros. Sci., Vol 27, 1987, p 421 sion for Wastewater Facilities, Mater. Per- Moisture through Paint Films Leading 19. T. Nguyen, J.B. Hubbard, and J.M. Pommer- formance, July 2003, p 33 to Corrosion of Metal Substrates: A sheim, Unified Model for the Degradation of Diffusion—Adsorption Model with Reac- Organic Coatings on Steel in a Neutral Elec- tion,” Ph.D. thesis, Massey University, trolyte, J. Coating. Technol., 1995 SELECTED REFERENCE University of New Zealand, 1996 20. “Surface Preparation of Soluble Salt 17. HyperPhysics, “Corrosion as an Electro- Contaminated Steel Substrates Prior to K.B. Tator, Organic Coatings and Linings, chemical Process,” Dept. of Physics and Coating,” NACE Technical Committee Corrosion: Fundamentals, Testing, and Pro- Astronomy, Georgia State University, Report 6G186, NACE International, tection, Vol 13A, ASM Handbook, ASM Inter- http://hyperphysics.phy-astr.gsu.edu/hbase/ March 2010 national, Materials Park, OH, 2003, p 826