Coating Deterioration - ASM International

Coating DeteriorationKenneth B. Tator, KTA-Tator, Inc.

PAINTS AND COATINGS of all types arewidely used to provide color and pleasing aes-thetics, and to prevent deterioration of theunderlying substrate when exposed to variousenvironments. Besides protection and beauty,however, coatings provide light reflectivity,camouflage surfaces, reflect and absorb heat,and provide a variety of other functions.However, in order to provide these functions,

the protective coating must remain intact andadherent on the surface to which it has beenapplied. The vast majority of all protectivecoatings perform admirably until an old age,at which time natural deterioration and degra-dation occur. However, a coating can failprematurely, preventing its aforementionedfunctions from being realized.The major reasons for the occurrence of cor-

rosion usually are poor or deficient surfacepreparation, or insufficient coating thickness.There are, of course, many other reasons whycoatings deteriorate and corrosion occurs:

� A paint or coating is incorrectly formulatedor manufactured by the coating supplier.

� An unsuitable coating is specified for agiven environment.

� Environmental conditions are different thanthat understood by the specifier.

� There is improper, or insufficient, mixing ofthe coating at the time of application.

� There are adverse ambient conditions whenthe coating system is applied.

� The drying and/or curing of the coating afterapplication is impaired.

� There is chemical, physical, and/or mechan-ical damage to the coating system duringexposure.

These causes of failure are relevant only whena premature coating failure occurs. As men-tioned, however, premature coating failure isextremely rare: of the hundreds ofmillions of gal-lons of paint manufactured and applied each yearin theUnited States alone, it is estimated that onlya small fraction—less than one one-hundredth ofone percent—of these coatings ever fail prema-turely. Instead, most protective coatings are suc-cessfully specified and applied to a properlyprepared surface to the appropriate thickness.These coatings perform as intended, but over time

deteriorate and lose their protective or aestheticfunction as a result of old age, combined withexposure to aggressive environments.In this article, coating failures due to specifi-

cation errors, poor surface preparation or appli-cation, deficient film thickness, or anotherabnormality during application are not dis-cussed, even though, to a greater or lesserextent, all of them affect deterioration andresulting substrate corrosion during the normalservice life of any coating.Rather, the deteriorating effects of exposure

environments and their interaction with thepaint or coating are discussed. This discussionprovides an introduction to the mechanism ofpremature corrosion of a metallic substratewhen that substrate has been properly coatedwith a suitably resistant coating system in agiven environment.This article discusses some of the environ-

mental influences on a protective coating filmthat can result in deterioration:

� Energy: solar, heat� Permeation: moisture, solvent, chemical,

and gas� Stress: drying and curing-internal stress;

vibration- external stress; impact and abrasion� Biological influences: microbiological, mil-

dew, and marine fouling

These generalized categories of environmentalinfluences unfortunately do not act singly, butin combination, sometimes with unpredictablecatastrophic results.

Variability within a ProperlyApplied Coating Layer

Coating materials—even when thoroughlymixed, applied, dried, and cured properly—have, from a molecular point of view, greatvariability in their compositional makeup.The articles “Elemental Chemistry Introduc-

tion” and “Composition of a Paint Coating” inthis Volume describe coating resins, the wayatoms form molecules, and how the moleculesreact with other molecules to form a coating.Various ingredients such as pigments, fillers,co-reactants, and surfactants are included in

the formulation to enhance application and per-formance properties. These diverse ingredients,along with the ways molecules react with eachother and with the substituent ingredients, pro-vide the variability in the molecular structureof a coating.When a molecule crosslinks with another

molecule, the reactive sites of each of the react-ing molecules must align and come within veryclose proximity to each other (generally within3 to 5 angstroms (A = 1 � 10–10 m) for thechemical crosslinking reaction to occur (Ref 1).For example, in an epoxy resin that is cross-

linked with a polyamide copolymer, the molec-ular sizes of each co-reactant material arerelatively large, and the reactive functionalgroups are interspersed along the ends or mid-chain of the molecule. Stoichiometric (com-plete theoretical crosslinking) reactions arerare, and quite often the reacting groups donot come into sufficient proximity to react. Thisis because the coating resin is dispersed in asolvent that evaporates, reducing mobility ofthe molecules of the reactants. Additionally,low reactant temperatures reduce molecularmobility. The presence of pigments and otheringredients also separate the reacting molecularchains. Because there are billions of reactivesites, and because formulators add excesses ofreactivemoieties as appropriate to ensure suitablereactions do occur at room temperature (or what-ever the design reactive temperature is), suitablecrosslinking generally occurs. However, therecan be tens of millions of unreacted moietiesremaining in the crosslinked coating resin. Also,resin molecular reactivity often initiates at dis-crete localized areas and progresses from theseareas in a manner similar to the formation of froston a window. The intersection of one reactionarea with another results in an interstitial bound-ary with different properties than that of thereacted area. Similarly, the resin reactions aroundpigment particles and other paint constituentsalso have a different crosslinking density thanthat of the pure resin reaction.Solvents in solvent-borne coatings, and water

in latex or waterborne coatings, evaporate afterapplication, leaving micropores, microcracks,or capillaries within the coating. If evaporationis impeded, due to low temperature or otherreasons, the solvent or water can accumulate

ASM Handbook, Volume 5B, Protective Organic CoatingsK.B. Tator, editor

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and cause a void within the coating cross sec-tion. The inner or outer surface of the void canprovide a means of moisture penetration into thedried film. Similarly, pigment agglomerations,which are pigment particles in contact with eachother, can impede resin wetting, leaving a micro-void or discontinuity in the crosslinked coating.All of these result in the apparent presenceof inhomogeneities and phase separations in acrosslinked coating film. Even if the film is ther-moplastic, and not crosslinked, such inhomo-geneities and phase separations still are present,and for the same reasons.The presence of low-molecular-weight regions

in coating films has been demonstrated by elec-tron and light microscopy studies. Films madeof epoxy, phenolic, and phthalate resins wereobserved to consist of micelles or granules ofhigh-density segments separated by narrowboundary regions of low-molecular-weight mate-rial. At the film-substrate interface, the low-molecular-weight material exists as a thin contin-uous film or as channels between micelles,thereby providing pathways for easy entry ofwater to the interface (Ref 2).Accordingly, there is great variability in the

crosslinking density of a coating, even if it isformulated, mixed, applied, and cured properly.Figure 1 illustrates some of this variability.Relative to the properly pigmented, dried,

and cured portions of the cross section of acoating layer, the areas of the deficiencies men-tioned are quite small, both in area and in cross-sectional dimension. Thus, multiple layers of acoating system is not likely to provide an over-lap of deficient areas, and a porosity or area ofmoisture penetration in one layer almost cer-tainly will not coincide with that in anotherlayer, even though the weak areas for moisturepenetration remain. A three-coat system usuallyis better than a two-coat system, and both arebetter than a one-coat system, even if that layeris relatively thick.

Environmental Effects Resultingin Coating Deterioration

Characterizing an environment is a daunting,almost impossible task. The environment at oneend of a bridge can be different than that atanother end, and both may be different that thanof a center span that is suspended over water orhigh in the air. Similarly, an exterior environ-ment at the top of building can be different thanthat near the bottom of the building relative tosunlight, wind intensity and direction, and eventemperature. Ships hauling cargos have differ-ent environments, not only within the cargotanks, but above and below the waterline.The International Standards Organization

Standard has attempted to define principleenvironments for coatings (Ref 3). The environ-mental categories and descriptions are pre-sented in Table 1. In addition, the standarddiscusses metal loss/year for each categoryand time of wetness; special conditions such

as corrosion inside buildings, corrosion in boxgirders, and various stresses such as chemical,mechanical, condensation, temperature, andstress combinations.The Society for Protective Coatings (SSPC)

has also defined environmental zones for coat-ing systems (Ref 4):

� 0: Dry interiors where structural steel isembedded in concrete, encased in masonry,or protected by membrane or noncorrosivecontact type of fireproofing

� 1A: Interior, normally dry (or temporaryprotection). Very mild (oil-base paints donot last six years or more)

� 1B: Exteriors, normally dry (includes mostareas where oil-based paints last six yearsor more)

� 2A: Frequently wet with freshwater.Involves condensation, splash, spray, or fre-quent immersion. (Oil-based paints now lastfive years or less.)

� 2B: Frequently wet by saltwater. Involvescondensation, spray, or frequent immersion.(Oil-based paints now last three years orless.)

� 2C: Freshwater immersion� 2D: Saltwater immersion� 3A: Chemical atmospheric exposure, acidic

(pH 2.0 to 5.0)

� 3B: Chemical atmospheric exposure, neutral(pH 5.0 to 10.0)

� 3C: Chemical atmospheric exposure, alka-line (pH 10.0. to 12.0)

� 3D: Chemical atmospheric exposure, pres-ence of mild solvents, intermittent contactwith aliphatic hydrocarbons and their deriva-tives (mineral spirits, lower alcohols, gly-cols, etc.)

� 3E: Chemical atmospheric exposure, severe.Including oxidizing chemicals, strong sol-vents, extreme pHs, or combinations of thesewith high temperatures

For the most part, these environmentaldescriptions are somewhat similar to the extentthat they progress from a relatively mild, non-corrosive environment to a relatively aggressiveenvironment. The more benign mild environ-ments are generally warmer, dryer, and lesspolluted. The more severe environments gener-ally have more moisture, or are in immersion,and have salts or chemical constituents.Moisture, salts, and chemicals are primary

influences in the corrosion process on steel andmost metals and other materials. These influ-ences, and the degrading influences of other envi-ronmental effects can loosely be categorized asenergy related (solar, heat/cold, and nuclear radi-ation); permeation related (moisture, solvents,

Water droplet

Permeating water

Substrate

Penetrating moisture

Crosslinked resin

Pigment particles A B C D

E

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

through a pigment agglomeration. (D) Penetration through a microcrack or capillary. (E) Penetration through area oflow crosslink density

Table 1 Atmosphere corrosivity categories and examples of typical environments

Corrosivitycategory Exterior environment Interior environment

C1 very low . . . Heated buildings with clean atmospheres, e.g., offices,shops, schools, hotels

C2 low Atmospheres with low levels of pollution. Mostlyrural areas

Unheated buildings where condensation may occur, e.g.,depots, sports halls

C3 medium Urban and industrial atmospheres, moderate sulfurdioxide pollution. Coastal areas with low salinity

Production rooms with high humidity and some airpollution, e.g., food processing plants, laundries,breweries, dairies

C4 high Industrial areas and coastal areas with moderatesalinity

Chemical plants, swimming pools, coastal ship- andboatyards

C5-I veryhigh(industrial)

Industrial areas with high humidity and aggressiveatmosphere

Buildings or areas with almost permanent condensationand with high pollution

Categories for water and soil

Im 1 Fresh water River installations, hydroelectric plantsIm 2 Sea or brackish water Harbor areas with structures such as sluice gates, locks,

jetties; offshore structuresIm 3 Soil Buried tanks, steel piles, steel pipes

Coating Deterioration / 463

chemicals, and gases); mechanically related(internal crosslinking and curing stresses, exter-nal vibration and flexibility stresses, and impact/abrasion); and biological (microbiological andmacrobiological—mildew and fungus).Unfortunately, these categories are not all

inclusive, and most importantly, are not mutu-ally exclusive. In any environment, most if notall of the environmental influences are presentto varying degrees, along with perhaps otherinfluences not mentioned here. It is the synergis-tic effect of the combinations of these and otherenvironmental influences that degrade the coat-ing, or for that matter any material, resulting inloss of suitability for its intended purpose. Eachof these environmental categories is discussedsubsequently.

Energy Related Degradation

Energy acting on a coating (or material) candegrade a material by breaking or interferingwith the chemical bonds holding the resin (ora molecule) together and to a substrate. The influ-ence of energy in virtually every case makes anorganic molecule more susceptible to degradationby other environmental influences (i.e., perme-ation, mechanical, and biological). The primaryenergy influences are solar radiation, heat (andcold), and to a much lesser extent, nuclearradiation.Solar Energy. The sun was formed 4.5 bil-

lion years ago and is composed of 91.2% Hand 7.8% He gas. The remaining 1% is com-prised of oxygen, carbon, silicon, iron, magne-sium, neon, sulfur, and calcium. Each elementis important because its presence contributesto the solar spectrum as it is received on earth.The sun emits energy created by the thermonu-clear fusion of hydrogen into helium. Fourhydrogen nuclei have more mass than onehelium nucleus and as each helium atom isformed, the excess mass is converted intoenergy that powers the sun. The core of thesun contains more helium (65%) than hydro-gen. Hydrogen has been brought to this lowerlevel because of its conversion in the thermonu-clear reaction. It is estimated that the remaininghydrogen should last another 4 billion years atits rate of consumption. Variations in the activ-ity of the sun affect the wavelength of emittedradiation. Changes in ultraviolet light (UV)radiation are more pronounced than those ofother ranges of radiation. The distribution ofemitted energy is such that 9% is in the UVregion, 45% is in the visible range, and theremaining 46% is in the infrared range(Ref 5). However, the emitted energy by thesun is not necessarily of the same wavelengthor intensity as that absorbed by the earth. Theatmosphere of the earth and variability withinthat atmosphere and, in particular, ozoneabsorption and scattering of solar radiation byclouds, moisture, and other small molecules,all change the incidence of radiation on the sur-face of the earth. Figure 2 depicts the solar

spectrum as emitted and as absorbed on the sur-face of the earth (Ref 5).Electromagnetic radiation with the shortest

wavelengths has the greatest energy. However,the shorter wavelengths are more readilyabsorbed and have less penetrating effects thanlonger wavelengths (Ref 6), as can be seen inFig. 3. Radio waves can be transmitted overlong distances compared with shorter wave-length television and radar, which allow trans-mission generally along a line of sight. Veryshort radiation types such as cosmic rays, anda and b nuclear radiation, although highenergy, cannot penetrate even the thickness ofa sheet of paper. X-ray and g radiation are notfound in solar radiation but are man-made bybombardment of certain elements with elec-trons, or concentration of certain naturallyradioactive elements (such as uranium). Thesehigh-energy shortwave radiations are powerfulenough to ionize gases, readily cleave chemicalbonds, and induce potentially deadly chemicalchanges in human and animal tissues.Ultraviolet light falls within the wavelength

from 10 to 400 nm (1 nm is one-billionth of a

meter). This naturally occurring energy fromthe sun has a shorter wavelength than visiblelight (400 to 780 nm) and accordingly is moreenergetic. Ultraviolet light has sufficient energyto disrupt and break covalent bonds of organicmolecules. The UV light range is from approx-imately 10 to 400 nm and is divided into threesubcategories: UV A, 320 to 400 nm; UV B,280 to 320 nm; UV C, 10 to 280 nm. The detri-mental effects of UV radiation to paints wasbelieved, approximately ten years ago, to startat 295 nm and extend to approximately 400 nm.Recently, however, experience has shown thatthere is sufficient radiation and penetration of UVlight as low as 280 nm to cause deterioration ofpaint. Ultraviolet radiations below this wavelengthare not considered detrimental, because they aregenerally absorbed by moisture and other smallmolecules in theatmosphereand thereforeareof lit-tle consequence. Moreover, they have little abilityto penetrate into the surface of an organic material.The frequencies of radiation that aremost harm-

ful to polymeric systems are those from the bluepart of the visible light spectrum and the near-UVlight spectrum. The longer wave lengths are notenergetic enough to harm molecules, and most ofthe other potentially harmful high-frequency raysare screened out by the atmosphere of the earth.The breaking of molecular bonds and the for-

mation of free radicals by UV energy results ina shortening of the molecular chain group of anatom and, accordingly, a reduction in its molec-ular weight.Glass allows visible light to pass through it

without any absorption but is opaque to theshorter wavelengths of UV light and reduces thetransmission of UV light of longer wavelengths.Accordingly, materials exposed behind glassretain their color and last longer than thoseexposed in an exterior solar environment. How-ever, fading and embrittlement of plastics andother materials upon long-term interior exposurestill occurs in indoor environments exposed tosunlight. Even though window glass filters outmost UV light, the energy that transmits throughthe glass is still sufficient to degrade and fademost materials, including coatings over time. Itis generally accepted that radiation in the visible

Fig. 3 Electromagnetic spectrum of radiation types. Source: Ref 6

30

2043

2 1

5

Rel

ativ

e irr

andi

ance

, a.u

.

10

0200 600 1000

Wavelength (λ), nm1400

Fig. 2 Solar spectrum as emitted and after absorptionand scattering. 1. sun spectrum before entering

stratosphere (extraterrestial radiation), 2. spectrummodifiedby ozone absorption (in stratosphere and troposphere),3. spectrum after Rayleigh scattering (by small molecules),4. spectrum modified by aerosol (clouds) scattering andabsorption (excluding influence of water), 5. spectrum aftermoisture related scattering and absorption. a.u. – arbitraryunits, a ratio of solar irradiation intensity to a referencemeasurement. Source: Ref 5

464 / Coating Analysis and Evaluation

range and higher is nondetrimental to most paints,organic materials, and plant or animal tissues.Radiant solar energy in the form of light

photons excites certain electrons in the mole-cules of a resin. Depending on the wavelengthand frequency of the radiation, only certain elec-trons are affected, while other electrons remainunaffected. Excess electron energy as a result ofUV photon excitation is dissipated by fluores-cence, phosphorescence, and most importantly,a cascading down of the electronic energy intovibrational and rotational energy of a molecularelectrical bond. If sufficient energy is absorbedby the bond, it may break. Molecular groups withdouble bonds such as carbon to carbon (C––C),carbon to nitrogen (C––N), and carbon to oxygen(C––O) absorb UV energy, and their electronsare lifted into higher-energy levels. When theseelectrons decay to lower-energy states, energyis released in the form of vibrations that cancause a bond to break and create free radicals.Free radicals results when a chemical bond isbroken. A covalent bond can break in either oftwo ways: the atoms previously joined by thebond share the electrons (homolytic dissocia-tion), or the more electronegative of the atomsretains the electrons (heterolytic dissociation)(Ref 6). These two types of dissociations are:

Homolytic dissociationA : B ! Aþ B

Heterolytic dissociation A : B ! Aþ þ B�

Heterolytic dissociation produces ions. An ionis an electrically charged atom or molecule.A negatively charged atom has more electronsthan protons and a positively charged atom hasmore protons than electrons. Such electricallycharged atoms or molecules are polar and candissociate from one another when placed inwater solution. The high dielectric constant orinsulting property of pure water enables thepolar molecules to separate and exist separatelyin solution. Water itself is very weakly disso-ciated and forms hydrogen and hydroxide ions.

H2Oð Þ HOH Hþ þ OH�

Homolytic dissociation likely occurs if thetwo fragments are equally electronegative. Thisproduces neutral atoms or groups, each with anunsatisfied valency or unpaired valency elec-tron. Such groups are known as free radicals.Most free radicals are highly reactive andrecombine either with each other or other freeradicals to form chemical bonds.If the free radicals are so reactive, why don’t

they recombine? If they are in a fairly rigidstructure, it is likely that they will combine. Ifafter separation the free radicals are held in arelatively confined area and maintain closeproximity to each other, recombination islikely. If the molecular structure is crystallineand has relatively tight rigid chains in closeproximity, or if it is in the cyclical aromaticring or a very tightly and closely crosslinkedmolecular structure, it will be difficult for the

free radical ends of the molecule to separatesufficiently after the break in the bond occurs.Accordingly, the free radicals will remain inclose proximity and will likely recombine.However, if the molecular chains are somewhatflexible, and the temperature is sufficiently highthat there is vibrational and rotational move-ment of molecules of the polymer, the free radi-cals on opposite ends of the broken bond canbecome so separated that they will not recom-bine. A radical might pick up a hydrogen atomfrom an adjacent chain upon the breaking of thatbond and therefore transfer the radical to anotherportion of the chain. If the free radicals are on a flex-ible molecule that is moving around quite rapidly,then there is a high probability that the radical willpick off a hydrogen atom of its own chain five orseven carbon atoms back along the chain. Thenthere will be a transfer of the radical to a positionaway from the chain and a termination of activityat the chain end. The newly formed free radicalmight pick up another hydrogen atom somewhereelse, or react with a monomer, or react elsewhereto continue growth. Free radical reactions resultin chain scission (breaking of the molecularchain); depolymerization (reducing a polymericchain to its monomer units); branching (a shortgrowth at a free radical site); self cyclicization(forming a circular molecule by joining withanother portion of a backbone of a molecule);and the formation of double bonds.All of these free radical reactions, when they

occur billions of times in a molecule exposed toultraviolet light, shorten the molecular chains,reduce their flexibility, and increase permeabil-ity of the molecule and resin, thus degrading it.Certain resins, such as an epoxy, and particu-larly an amine crosslinked epoxy, are very sus-ceptible to UV degradation. Exposure to evenrelatively low amounts of sunlight is sufficient,in many cases, to cause a chalking deteriorationof the surface of the resin or paint. This chalk iscomposed of pigment particles and broken seg-ments of the colorless molecular resin thatrefract light to give a white appearance.However, certain other resins, notably the

acrylic and polyurethane, are mostly transpar-ent to UV light and allow UV energy to passthrough them with no molecular absorption.Accordingly, there is little deterioration to theseresins when exposed to UV light.Heat Energy. The addition of heat to a mate-

rial increases the vibrations of atoms, and whenuniformly applied, all atomic vibrations areuniform throughout the molecule. This is incontrast with radiation. For instance, both UVand nuclear radiation affect only certain elec-trons in the atoms of the molecule. Other elec-trons on other atoms remain unaffected.If the heat applied is of sufficient intensity, the

molecular vibrations increase to such a degreethat a bond can break. When that happens, freeradicals are formed and they react as previouslydescribed. Again, the end result is:

� A decrease of molecular weight of thechains comprising the resin of the coating

� A reduction of the tensile strength, modulusof elasticity, and toughness

� Potential introduction or formation of reac-tive polar groups that can cause changes incompatibility and electrical and opticalbehavior of the polymer

� Introduction of light absorbing groups thatcan cause discoloration and internal cycliza-tion of the chains, resulting in hardening anda decrease in toughness

Free radical initiation can also include addi-tional crosslinking between hitherto indepen-dent macromolecules, which, in excess, mayreduce impact strength and create brittleness.Energy in the form of UV light can passthrough some resins with little or no effector be absorbed in other molecular combina-tions without breaking bonds. In the lattercase, vibrational and rotational movementbetween atoms is increased and the energy isdissipated as heat, which is generally harmlessto the molecular structure. However, whereabsorbed heat energy is high enough, bondscan break and free radicals can form. In arigid, dense, closely packed, immovable, solidresin or structure, the free radicals mayrecombine with little effect on the molecule.However, in most cases, particularly in paintsand most plastics, the structure is not rigidenough to allow immediate recombination offree radicals, and a variety of unanticipatedsecondary and tertiary reactions often occur,resulting in a shortening of the molecularweight of the resin molecule and other detri-mental side effects, all resulting in deteriora-tion and loss of properties.

Permeation Effects

Permeation of a coating by materials in a ser-vice environment is a major factor in the deteri-oration of the coating. Coatings are specificallyformulated and tested to resist certain environ-ments in immersion or in the atmosphere. Pig-ments and resins must be carefully chosen fortheir resistances to a given set of environmentalconditions, and they must also be compatible.Even with utmost care, coating systems are stillvulnerable to permeation and the ultimatedestruction of protective capability. The follow-ing permeating species and mechanisms arediscussed subsequently: moisture, solvents,chemicals, gases, and ions.Moisture Permeation. The water molecule,

H2O, is a very small molecule consisting ofone oxygen and two hydrogen atoms. Both theweight and size of this molecule is small rela-tive to virtually all other molecules commonlyencountered in an environment. Water in liquidform comprises oceans, lakes, and rivers, con-denses from the atmosphere as dew, and fallsas precipitation in the form of rain or snow.Water in vapor form is always in the air tosome degree, as humidity. Any material usedin exterior environments, including coatings,must be resistant to the effects of water.


Water, in addition to being small in molecu-lar size, also is polar, because oxygen has ahigh electronegative attraction to other polarmolecules, including itself. Accordingly, watercan readily penetrate into microscopic pores,holidays, cracks, and defects inherent in almostany coating system. Water vapor, carried by air,can move in and out of porous materials withease, as long as there is a driving force causingits movement.What are driving forces causing water move-

ment? Simply placing the material in waterimmersion provides sufficient pressure fromthe pressure head (depth) of the water, eventhough the immersion is relatively shallow toprovide a driving force for water movement intoa material. Water molecules, because of theirrelatively small size, can pack quite tightly,and accordingly, a mass of water (measured asspecific gravity, pound per gallon, or unit mass)is quite dense compared to many liquids andall gases. In immersion, there is sufficientwater head pressure to cause water moleculesto migrate into cracks, crevices, pinholes, andmicroscopic fissures inherent in any coating sys-tem. Water, in permeating a coating, fills any“free” space left by solvents and other materialsthat have migrated from the coating during appli-cation and curing. Additionally, due to slightpolarity of the water molecule, water can bedrawn into the coating if there are any polar sol-vents, polar groups, or polar materials retainedor comprising the dry film. Thus, the presenceof ester groups, ether linkages, carboxyl groups,and other polar groups within a coating resincan draw water into the paint. An electric chargeapplied across the coating film, such as withcathodic protection or resulting from a corrosioncell, can induce or accelerate the permeation ofwater into a coating. This phenomenon is calledelectroendosmosis. Additionally, corrosion inhi-biting pigments (chromates, borates, molyb-dates), due to their water solubility, may drawwater through the coating film in an osmotic pro-cess. These pigments require water to partiallydissolve the oxygenated metal inhibitor that thencan wet and passivate the underlying steel or alu-minum metal substrate.Finally, rust deposits, dirts, salts, and other

contaminants remaining on a surface can bothprevent bonding of the paint and establishosmotic driving forces further promoting waterpermeation. At areas where paint adhesion isrelatively poor, crosslinking is less dense, orthere are agglomerations of pigments notcompletely wet out by the organic binder, watercan collect and “pool,” causing a swelling ofthe film and additional water penetration.Water-soluble salts, including sodium chloride,calcium chloride, and other chlorides (foundin marine environments and in deicing salts),and sulfates (from acid rains) are notorious forcausing osmotic blistering of coatings inimmersion service and/or accelerated rates ofcorrosion in atmospheric service if they areallowed to remain on a substrate before paint-ing or between coats of paint.

Once water enters the paint film, the smallwater molecules have the ability to penetratebetween and within molecular chains compris-ing the organic resin, and the intersticesbetween the resin and pigment, if the pigmentis not completely wetted by the resin. As thewater molecule penetrates, it separates loosebonds holding the resin particles together, suchas polar bonds, and becomes attracted to andswells the molecule at sites of covalent bondsthat are polar. This swelling forces the bondseven further apart, diminishing their tightnessand close packing. The volume of the coatingexpands due to the increased presence of thewater intrusion. Some films increase 20 to50% or more in volume when in contact withwater (Ref 8). The swelling caused by the coat-ing film can separate polar bonds and otherweak forces holding the molecule together andto the substrate such that polar attractions, sonecessary to coating film adhesion/cohesion,no longer occur. Additionally, the oxygen ofthe water molecule can be attracted to andreplace what otherwise would have been a polarattraction between two long-chain resin mole-cules. When this happens, the charge betweenthe resin molecules is terminated, and attractionto water molecules by each chain end occursinstead. The dried coating film, when wettedand saturated with water, becomes plasticizedand swollen. Wet adhesion of most coatings issubstantially less than dry adhesion. When thefilm dries out, often dry adhesion reestablishes,but usually not to the same extent as is wasbefore moisture saturation.This phenomena of swelling by moisture

penetration into a coating film occurs with vir-tually all coating materials except those thatare extremely tightly crosslinked with a highcrosslink density (such as some phenolicepoxies or phenolic coatings formulated forwater resistance) or some highly crystallinecoating materials (such as the fluoropolymers).These materials are relatively impervious towater permeation, penetration, and swellingdue to their dense molecular crosslinking orthe tight polar bonding between molecularchains.The effect of heating (either using hot water

or a hot or warm environment) increases molec-ular movement, enabling more rapid waterpenetration. Conversely, cooling, particularlybelow the glass transition temperature (Tg),reduces molecular movement and retards waterpermeation.Solvent Permeation. Solvents are not usually

found in most environments, and the presence ofa solvent in a paint film occurs primarily as aresult of solvent addition to the resin whenmanufacturing the paint. Solvents are added inorder to reduce the viscosity of a resin, thinningit for application purposes. Upon drying and cur-ing, the solvent must volatilize from the coatinginto the atmosphere in a timely manner. If suffi-cient solvent volatilization does not occur, andsolvent is retained in the film, the coating canremain soft and plasticized, because the relatively

large solvent molecules separate resin moleculesfrom adjacent resin molecules. Bonding thatcould otherwise occur cannot be done becausethe bonding moieties are not close enough forattraction to occur. Moreover, many solvents aresomewhat polar, particularly the oxygenated sol-vents (including the ketones, esters, and alco-hols). These solvents are generally used todissolve polar or somewhat polar resins and toprovide hydrogen bonding to other polar groupsof the resin, or to keep or slow the solvents fromcompletely evaporating from the resin. Hydrogenbonding is the attraction of the oxygen atom in amolecule to nearby hydrogen atoms in othermolecules. Retained polar solvents may drawwater into the resin. This is a particular problemwith slowly evaporating alcoholic solvents suchas the glycol ethers. Coatings used in immersionservice, particularly for the interior of deionizedwater or freshwater storage tanks, often blisterdue to retained solvents. This is a problem onthe tank bottoms because the tank bottom iscooled by the earth (acting as a heat sink), whilethe tank sidewalls are warmed by the sun and airconvection. The warmer temperatures assist involatilization of solvents, while the cooler tem-peratures of the tank bottom result in a slowerevaporation of retained solvents in the paint film.The entrapped solvents can draw water into thecoating at the bottom of the tank, causing osmoticblistering. This can be a problem not only on tankbottoms, but anywhere a heat sink might occur,such as exterior steel supports, cradles, or brac-ing. Baking, or heating of tank interior coatings,is often done both to ensure solvent evaporationfrom the paint film and to elevate the temperatureabove the Tg to attain a higher crosslinking den-sity of the chemically reacted coating resin.Chemical Permeation. Coatings are widely

used to protect against chemical attack on avariety of different substrates and in a varietyof different chemical environments. The wide-spread use of coatings for such protectionattests not only to the diversity of coating for-mulations, but also to the inherent capabilityof resin and pigment technologies.In the simplest sense, chemical attack can be

categorized as that by acids and bases. Chemicalattack does not occur at neutrality (pH of 7).However, water, salts, and solvents, all of neu-tral pH, can dramatically affect and degrade acoating. This type of “neutral” degradation isdescribed elsewhere in this article.Acids and bases, and the strength of the acid

or base, are a simple function of the degree ofdisassociation of the chemical into hydrogenions, H+ (acids), or hydroxyl ions, OH– (alkalisor bases). Acidic or alkaline strength ismeasured on the logarithmic pH scale, a scalewith each number being ten times greater thanthe preceding number. A pH of 3 is ten timesmore acidic than a pH of 4, for example.A pH of 7 is exactly neutral, while 1 is stronglyacidic and 13 is strongly basic or alkaline.For all practical purposes, the medium in

which the acid or alkaline disassociation occursis water. Even very small amounts of water are


sufficient to dissolve and disassociate mostchemical compounds into their acidic or basicconstituents. At the molecular level, when suchdisassociation occurs and the acid or basecomes in contact with a coating material, chem-ical attack can take place.However, many chemicals are hydroscopic:

they attract and react with water. Examplesare most sulfur chemicals, including sulfuricacid, sulfamic acid, and sodium sulfide; sodiumand potassium hydroxides; sodium carbonate;zinc chloride; most salts, such as sodium, potas-sium, and zinc chlorides; and many solvents, inparticular the alcohols and glycols.Acid Attack. Acids consist of inorganic min-

eral acids such as hydrochloric, sulfuric, andnitric acids, which disassociate completely inwater. Organic acids such as carboxylic acids,including formic acid and butyric acid, do notcompletely disassociate and as a consequenceare considered weaker acids. However, eventhese acids can aggressively attack most coat-ing systems.Acid gases such as sulfur dioxide (SO2), sul-

fur trioxide (SO3), hydrogen sulfide (H2S), andnitrogen oxide (NOX) react with moisture inthe air in the form of precipitation or condensa-tion to form sulfuric and nitric acids. Even car-bon dioxide (CO2) as a normal constituent ofthe atmosphere reacts with moisture to form aweak carbonic acid (H2CO3).Chemical attack by condensation on a coat-

ing is more aggressive than that deposited byprecipitation such as acid rain, because mois-ture condensing on a surface containing acidicconstituents usually evaporates as the substratewarms during the day. As the moisture evapo-rates, the acids within the condensation dropletconcentrate and more aggressively attack thesubstrate on which the condensation resides.Acid rain, on the other hand, is diluted by suc-cessive rainfall, and the chemical contaminantcan be diluted or even washed from the surface.The chemicals thus deposited, however,

attack and cleave chemical bonds that are sus-ceptible to deterioration. Chemical groupsspecifically vulnerable to acidic attack andcleavage are ether, urea, and urethane linkages,where cleavage occurs by a reaction of thehydrogen ion (the susceptible portion of thelinkage).Alkaline Attack. Similarly, strong alkalis

such as sodium, potassium, and calcium hydro-xides attack susceptible chemical groups incoatings. Perhaps the most widespread type ofalkaline attack is saponification, the alkalineattack of the ester linkage of drying oils usedin most oil-base coatings and alkyds. The attackcan occur when oil-containing coatings oralkyds are applied over concrete, which con-tains alkali salts, which, when combined withwater, form caustic alkalis. In a similar fashion,application of oil-based alkyds over zinc-richcoatings can also result in saponificationbecause zinc reacts with moisture to form alka-line zinc hydroxides. The hydroxyls (OH–)cleave (break) the ester linkage in the drying

oil to form an organic acid and alcohol. Thebond breaking reduces molecular flexibilityand embrittles the film; this ultimately leads toresin deterioration and the formation of a stickysoft coating under damp conditions, or a brittlepowdery coating when dried. All coating resinscontaining ester groups are susceptible to suchattack. However, some of those resins, such asthe polyesters and vinyl esters, are much morehighly crosslinked and formulated with epoxyresins and other materials to sterically hinderthe ester group, protecting it from alkali attack.Saponification (reaction with an alkali) andhydrolysis (reaction with water) are similar,but the saponification reaction is much fasterand more debilitating. An illustration of alka-line and hydrolytic (water) saponification isshown in Fig. 4.Figure 5 illustrates the vulnerability of vari-

ous organic linkages to hydrolysis (reactionwith water) and saponification (reaction withalkali) (Ref 9).Acids and alkalis not only cleave covalent

bonds of organic resins but can also attack acidor alkaline susceptible pigments in the paint.Where there are pinholes and permeabilitythrough the coating, chemical species can

penetrate, concentrate, and aggressively attackboth the pigment and binder. Pigment agglom-erations not completely wetted out by the resinof the coating are susceptible to this, particu-larly if there are pinholes and voids in the coat-ing. However, ionic permeation is much slowerthan moisture permeation, and unless the pig-ment is exposed on the surface (by chalkingor surface resin deterioration) or the chemicalenvironment has access through pinholes,voids, or other discontinuities in the paint film,chemical attack to pigments is not usually amajor problem.Table 2 shows sensitivity of some of the

more common pigment types to chemical attack(Ref 9).Oxygen and Other Gas Permeation. Oxy-

gen permeation at the cathode in a metallic cor-rosion cell is usually the rate determining factorin the corrosion reaction. The common anodicand cathodic reactions of metallic corrosion are:

Anodic reaction: M ! Mþ þ ne�

where M = metal, n = number (of valency elec-trons), and e = electrons.Cathodic reactions:

� In near-neutral and alkaline environments

1/2O2 þ H2Oþ 2e� ! 2OH�

� In acidic environments

O2 þ 4H� þ 8e� ! 4OH�ðin the presence of oxygenÞ

2Hþ þ 2e� ! H2 gasð Þ ðin highly acidic solution

and=or absence of oxygenÞ

Thus, permeation of molecular oxygen is neces-sary for metallic corrosion in near-neutral, alka-line, and mildly acidic environments, and inmany instances it determines the rate of corro-sion. Corrosion is an expansive process, andundercutting corrosion beneath a well applied

Fig. 4 (a) Saponification and (b) hydrolysis of an esterlinkage

Fig. 5 Vulnerability of organic linkages to saponification and hydrolysis. Source: Ref 9


coating system often causes the coating to crackand spall from the substrate, exposing theunderlying surface to the environment and fur-ther corrosion attack. A means of corrosion pro-tection in the oil industry is to remove oxygenfrom well injection water, and in the nuclear,chemical processing, and other industries, toinert the vapor space in a tank or vessel by add-ing nitrogen, carbon dioxide, combustion gases,or other gases to displace oxygen, therebyreducing or eliminating metallic corrosion.Oxygen species (nascent atomic elemental

oxygen, O; molecular oxygen, O2; ozone, O3)are very influential in the degradation oforganic materials by ultraviolet light and solarradiation. Molecular oxygen absorbs solar radi-ation in the range of 176 to 210 nm. Uponabsorption of UV radiation, the molecular oxy-gen is transformed into singlet oxygen, which

dissociates, forming two oxygen atoms. Theoxygen atoms recombine with molecular oxy-gen to form ozone. Ozone absorbs UV between200 to 320 nm and absorbs in the visible rangeat 420 and 700 nm. In the UV range, strato-spheric ozone dissipates energy as heat. This isbeneficial, because less high-energy UV, whichis detrimental to life on earth (sunburn for exam-ple), is absorbed. Ozone in the stratosphere doesnot influence the formation of ozone near the sur-face of the earth. On the earth’s surface, ozone isproduced by industrial combustion. Ozone isformed indirectly from nitrous oxide, whichabsorbs UV, forming molecular oxygen, whichfurther reacts with more molecular oxygen toform ozone. Ozone is a strong oxidizer that reactswith most organic materials, including coatings,to form free radicals and ultimately photochemi-cal embrittlement degradation.

In most cases, heat or radiation deteriorationdoes not act alone but acts in conjunction withoxygen. Oxygen is a very reactive molecule,and if a free radical forms in its presence, thenthe oxygen can combine immediately with itto form a different radical. This radical can thenabstract a hydrogen atom and form a hydroper-oxide. A hydroperoxide is unstable and decom-poses into two radicals. From the initial tworadicals, a total of six possible radicals canform. This explains the danger in chain scissionin the presence of oxygen leading to a chainreaction (Ref 10).The oxygen free radicals thus formed can fur-

ther react with molecules in a coating or organicmaterial in the same manner as described previ-ously, causing chain scission, depolymerization,and fragmentation of the molecule, reducing itsflexibility and resistance to permeation.Permeation of Water, Oxygen, and Ions

through Weak Areas of Crosslink Densityin the Coating. Water is in virtually everyenvironment around the world to some degree.Water in freshwater lakes, saltwater oceans,and in ponding rainwater results in a waterimmersion environment for materials exposedto these environments, and in relatively rapidwater permeation through a coating.In non-immersion atmospheric environments,

water is present as humidity, condensation, andprecipitation. In these environments, moisturepermeation into a coating is much slower,dependent principally on the duration of timethe coating is wet. There is little penetrationdriving force if water is present only as a gas,such as humidity, but if it is present as conden-sation or precipitation (rain, dew, fog droplets,and melted sleet or snow), the water canpenetrate the coating system. Furthermore, inatmospheric environments, oxygen is present,and the water has absorbed or entrained oxy-gen. Oxygen, as a gas in the atmosphere, haslittle or no driving force to penetrate a coatingfilm, but it is almost omnipresent, at least atthe initiation of corrosion. Accordingly, thepenetration by water of a coating in an immer-sion or semi-immersion environment is theprinciple cause of corrosion of the underlyingsubstrate.Virtually all organic coating materials are

permeable to water to some degree. Thick, highlycrosslinked coatings aremuchmore impermeableto water penetration than coatings with a lessercrosslinked density. However, even with rela-tively thick, highly crosslinked coatings, asdescribed previously, there are areas of variabil-ity resulting in lesser crosslink density. It is coat-ings in these areas that water penetrates.Numerous studies conclude that coating films

contain microscopic regions that absorb largeamounts of water and have low ion resistivity.That water does not defuse into the film uni-formly, but in a dense layer along boundariesin the polymer structure, followed by penetra-tion of the structure itself. Further, corrosionspots on the substrate have been found to bedirectly related to these regions (Ref 11, 12).

Table 2 Chemical sensitivity of selected organic and inorganic pigment families

Pigment type Examples

Sensitivity

Alkali Acid

Inorganic

Titanium dioxide . . . Excellent ExcellentZinc oxide . . . Moderate to good PoorAntimony oxide . . . Poor PoorRed iron oxide Synthetic red oxide; Spanish, Indian, or

Persian Gulf redExcellent Excellent

Cadmium red . . . Excellent PoorMolybdate orange . . . Poor to fair PoorLead Minium, mineral orange Good PoorYellow iron oxide Ferrite yellow, sienna, ochre, umber Excellent FairChrorme yellow . . . Poor to fair FairZinc yellow Zinc potassium chromate Fair PoorCadmium yellow . . . Excellent PoorNickel titanate yellow . . . Excellent ExcellentBismuth vanadate . . . Excellent FairZinc ferrite . . . Excellent GoodChrome green Brunswick green Poor PoorChromium green oxide . . . Excellent ExcellentIron blue Prussian blue, Midori blue, Chinese blue,

mineral bluePoor Very good

Ultramarine blue . . . Very good PoorCarbon black . . . Excellent ExcellentBlack iron oxide . . . Excellent FairMicaceous iron oxide . . . Excellent ExcellentZinc dust . . . Poor PoorAluminum . . . Poor PoorStainless steel flake . . . Excellent Very good to

Excellent

Organic

Metallized azo reds Lithols, permanents, rubines Poor PoorNonmetallized azo reds Toluidines, paras, naphthols Very good Very goodAzo-based benzimidazolone reds . . . Very good Very goodQuinacridones . . . Excellent ExcellentVat reds Dibromanthrone, anthraquinone, brominated

pyranthrone, perylenesExcellent Excellent

Azo-based oranges Dinitroaniline, pyrazolone, tolyl Good GoodNaphthol orange Very good Very good

Azo-based benzimidazolone oranges . . . Very good Very goodMetallized azo oranges Clarion red Poor ModerateMonoarylide yellows Hansa yellows Very good Very goodDiarylide yellows Benzidine yellows Very good Very goodAzo-based benzimidazolone yellows . . . Excellent ExcellentHeterocyclic yellows Isoindoline, quinophthalone, azomethine,

tetrachloroisoindolinone, triazinylExcellent Excellent

Phthalocyanine greens . . . Excellent ExcellentPhthalocyanine blues . . . Excellent ExcellentCarbazole violets . . . Very good Very good

Source: Ref 9


Initially, the water can penetrate the coatingonly partially to random depths at numeroussites. In atmospheric environments where thereis drying by the sun, or increasing daily tem-peratures, the penetrating water may diffuseout and evaporate. However, the water penetra-tion even in these areas can swell the coatingand dissolve any water-soluble constituents.Thus, subsequent water penetration may beeven easier at these same sites and penetratefurther, and penetration can potentially initiateat other new adjacent sites. One study estimatedthe apparent area of the pores increased from aninitial 0.6 to 6700 mm2 per cm2 of coatings after100 days of exposure to 0.6 mol/L sodium chlo-ride solution (Ref 13).With sufficient time, and duration of wetness,

water permeation through the coating cross sec-tion down to the underlying substrate ultimatelyoccurs.Oxygen and Ionic Permeation along with

Water into the Coating. Water precipitation(most often as rain or melted snow and ice) isnot a pure liquid. Even pure distilled water reactswith carbon dioxide in the air to form a weakcarbonic acid (H2CO3), giving it a pH of appro-ximately 5.6. Air pollutants can contribute furtherto ionic contaminants in rainwater. Acid rainconsists of sulfur and nitrogen compounds—principally in the form of sulfur dioxide and nitro-gen dioxide—that hydrolyze with water to formsulfuric acid (H2SO4) and nitric acid (HNO3).These rainfall pollutants principally result fromelectrical power generation and the burning offossil fuels, gasoline combustion in motor vehi-cles, and industrial smokestack output. Naturalsources consist of volcanic emissions contribut-ing sulfur dioxide, biological decay contributingdimethyl sulfide, and lightning contributing nitricoxide.Rainwater dissolves particulate materials in

the atmosphere when droplets of water formon atmospheric particles. Additionally, rainwa-ter dissolves atmospheric gases including pollu-tants and oxygen. Oxygen is ubiquitous, isdissolved in water, and permeates with waterinto a coating.In coastal areas rainwater has a salt content

essentially like that of seawater, but much moredilute. Generally within a mile or so of the sea-shore, and often much further depending onwind velocity and direction, wind-borne saltspray deposits upon and contaminates most sur-faces and structures, and concentrates due to theevaporation of water.Predictably, the composition of rainwater

varies geographically because atmospheric con-taminants also vary from place to place.Besides the previously mentioned inorganiccontaminants, organic contaminants also havebeen found in rainwater (Ref 14).The outcome of all of this is that water pre-

cipitation is not pure H2O, because there aremany other materials dissolved in or combinedwith the water droplet, including oxygen.Water, as precipitation, when in contact with acoating for any considerable amount of time,

permeates the coating through weak areas ofcrosslink density, micropores and cracks, voids,and pathways, ultimately to the underlying sub-strate. That permeating water carries with itoxygen and other materials, depending on theatmospheric environment.Of particular interest for corrosion purposes

is the prevalence of ionic contaminants—acidsand salts, notably anions, chlorides, sulfates,and nitrates, and their cations—that dissociatein the permeating water, which increases itsconductivity, and the rapidity of corrosionwhen these ions access a metallic substratethrough the coating. However, ionic permeationthrough a coating film is so extremely low thatunder-film corrosion may not be caused byionic permeation, but by surface contaminationprior to coating. Also, the observation thatcathodic blisters are highly alkaline providesstrong evidence that paint films also are imper-meable to hydroxyl ions (Ref 15).Under the influence of electric fields (such as

cathodic protection), functional groups asso-ciated with pores can become ionized andexchange ions with an exterior electrolyte solu-tion. This, combined with the plasticizing effectof water on some polymers, can, in certain coat-ings, result in inflated pores, which mayincrease ionic penetration to the substrate.Increasing temperature also has a profoundeffect by increasing the rates of moisture per-meation and ionic exchange through a coatingfilm (Ref 16).However, it is the general consensus of virtu-

ally every research document on the subjectthat ionic permeation through a paint film isfar slower, if it occurs at all, than permeationby moisture and oxygen. Moreover, moisture(H2O), as a result of a smaller molecular size,permeates much more rapidly than oxygen(O2).Steel Substrate Reactions Due to Permeat-

ing Water. After water permeates through acoating, it ultimately comes into contact withthe underlying substrate and can react with thatsubstrate. The substrate reaction, if any, withthe permeating water depends on the nature ofthe substrate (wood, concrete, masonry, galva-nized metal, plastic, aluminum, titanium, stain-less steel, or other metal, etc.). Furthermore,the constituents within the permeating waterstrongly influence any reactions that mightoccur with the underlying substrate. Accord-ingly, the discussion of such substrate reactionswith permeating water is beyond the scope ofthis article. However, because coatings arewidely applied to structural steel for corrosionprotection, the reactions of permeating waterwith a steel substrate are discussed.The reactions that occur on a steel substrate

(or, for that matter, on any other substrate)essentially are similar to those that occur at ascratch, mechanical damage, or other large-scale visible defect in a coating layer. Themajor difference is that the corrosion reactionsthat occur with these large-scale defects occurmuch more rapidly due to the ready access to

moisture, oxygen, and conductive ion contami-nation to the substrate. At these large-scaledefects, permeation through a coating film isnot necessary, because the coating film hasbeen damaged and the metal substrate isexposed. Corrosion is defined as the deteriora-tion of a material due to exposure to an envi-ronment. In every case with a metallicsubstrate, when corrosion occurs, it is an elec-trochemical reaction with the formation of ananode and a cathode. The anode forms at areaswhere there is more energy in the steel, forexample at scratches, impacts, and damagedareas; at areas of higher temperature; at grainboundaries within the steel alloy; and for manyother reasons. The cathodic areas form adjacentto the anodic areas.For steel, those reactions are detailed previ-

ously and summarized herein:

Fe iron=steelð Þ ! Feþþ þ 2e�

(The iron in the steel dissolves in the moisturesolution into positively charged ferrous ions,liberating two negatively charged electrons.)The corrosion continues as a depolarizer

removes (reduces) the electrons from the solu-tion at the cathode. Accordingly, in a neutralor near-neutral pH environment, the cathodicreaction with the most common reduction depo-larizer, oxygen, is:

O2 þ 2H2Oþ 4e� ! 4OH�

(Oxygen in the water/air and the water itselfreacts with the liberated electrons from the ironto form hydroxyl ions.)The hydroxyl ions formed at the cathode

react with sodium, potassium, and other posi-tively charged cations to form an alkalinesolution, commonly NaOH, a strong alkali.This alkali has a very high pH (often around11 to 13), and the alkalinity disbonds the coat-ing at the cathodic metal interface. Three possi-ble mechanisms have advocated for thecathodic delamination of a coating: dissolutionof an oxide layer on the substrate surface, alka-line hydrolysis of the coating polymer, andinterfacial failure due to the high alkalinity atthe cathode. It is likely that some or all of theseoccur in combination, simultaneously or instages. However, irrespective of mechanism,high alkalinity at the cathode is responsiblefor cathodic disbonding. Additionally, the accu-mulation of hydroxyl groups (OH–) attractsmore water due to hydrogen bonding, resultingin cathodic blistering at corrosion sites inimmersion or even in severe atmospheric expo-sures. If cathodic protection is used, either inthe form of an impressed current or sacrificialanodes, disbondment and blistering at the cath-ode may be substantially increased. Figure 6depicts cathodic blistering around a scribe ona test panel.The positively charged ferrous ions migrate

to the cathode, attracted by the negativehydroxide ions, and react with them, forming


an oxidized ferrous hydroxide brown coloredrust product. These ferrous ions going into solu-tion at the anode result in a metal loss thatforms pits in the steel substrate. This is depictedin Fig. 7.Subsequent oxidation and hydrolysis result

in a decrease of the pH and the formation ofa complex mixture of hydrated iron oxides(rust):

xFeþþ þ yO2 þ zH2O ! FexOy � zH2O

Initially, a lesser oxidized rust product isformed: Fe3O4. With further oxidation, a fullyoxidized rust product results: Fe2O3. These rustproducts are almost always formed withhydrated water (�H2O) attached.In coastal and chemical areas where chloride

ions are present, those negatively charged ionscan enter into the corrosion reaction to helppresent charge neutrality. They react withexcess ferrous ions to form hydrated ferrouschloride in a reversible reaction that can movein both directions, because ferrous chloride iswater soluble:

Feþþ þ 2Cl� þ H2O $ FeCl2 � H2O

Thus, the products in the corrosion solutionat the anodes beneath the rust layers at the bot-tom of a large-scale mechanically damagedarea of a coating, and also at a corroded areawhere moisture has permeated through an oth-erwise intact coating, most likely are hydratedFe3O4 and FeCl2. Oxidized corrosion productsare anion-selective (Ref 18), allowing the chlo-ride ions to continuously permeate through therust layers and reach the steel surface. A modelfor the degradation of steel in a neutral NaClsolution environment is presented in Fig. 8.In summary, variability within a properly

dried and cured coating film in the form ofmicroscopic cracks, porosities, capillaries, pig-ment agglomerations, and areas of low cross-linking will, in time, enable penetratingmoisture access to an underlying substrate. Vir-tually all organic coating materials are waterpermeable to some degree. On a metallic sub-strate this permeating moisture, along with dis-solved oxygen, enables the formation of anodicand cathodic areas, resulting in corrosion of the

substrate. The corrosion mechanism beneath acoating is essentially similar to that occurringat a scratch, mechanical damage, or any large-scale coating defect. The corrosion reactionsdiffer depending on the metal substrate; thosefor steel are illustrated previously.Soluble salts—such as those found in coastal

environments (notably NaCl) or where deicingsalts are used, in chemical environments, orwhere acid rain might deposit, carrying nitro-gen and sulfur oxides (NOx and SOx)—can bepresent in pits or on a substrate prior to paintingand can also permeate with water, increasingthe water conductivity and thus the rate and

severity of corrosion (Ref 20). In the priorexamples, the chloride ion (Cl–) has been usedas the anionic species salt example, but theeffect of nitrates, sulfates, and other anions issimilar.

Stress Influences

Internal coating stresses build up during dry-ing and curing and on aging. These internalstresses are considered by some to be a primarycause of premature coating failure. Addition-ally, external stresses to a coating system areapplied by movement of the substrate and by

Iron hydroxideforms andprecipitates.

The hydroxidequickly oxidizesto form rust.

Electronflow

Iron Cathode actionreduces oxygenfrom air, forminghydroxide ions.

Anode actioncauses pittingof the iron.

Electrochemicalcell action drivenby the energy ofoxidation continuesthe corrosionprocess.

OH–OH–

Fe2+ Fe2+

Waterdroplet

O2

e– e–e– e–

O2

Fig. 7 Positive ferrous ions react with negative hydroxide ions, forming rust and resulting in pits. Source: Ref 17

BlisteringH2O, O2

H2O, O2, NaCl

H2O

Coating

CathodeNa+OH–

Na+OH–Cathode

Anode

AnodeFe++Anode

Anode Anode

Anode

Blister

Cathode(Blister initiation site)

Cathode CathodeCathode

Corrosion product

Conductivepathways

Corrosion productCorrosion product

Scribe

(b)(a)e–

e–e–

e–

e–e–

e–Steel

Coating

Steel

Coating

Steel

Na+OHCathode

Na+OH–Delamination

Delamination

Fe++Fe++

Fe++

Fe++

Fe++

CathodeNa+OH–

CathodeNa+OH–

OH–OH–

H2O, O2

H2O, O2

H2O, O2

H2O

NaCl

NaCl

Na+

Na+

Na+

Fe3O

4,FeCl

2

NaCl

NaCl

NaCl

Correctionproduct

O2O2

Fig. 8 Conceptual model for the degradation of an organic coating on steel in a neutral NaCl solution. (a) With alarge-scale scribe or mechanically damaged area. (b) Without apparent defects but with moisturepermeation. Source: Ref 19

Fig. 6 Cathodic blisters around a scribe. The scribe isthe anode; the area immediately adjacent to itis the cathode. Courtesy: KTA-Tator, Inc.


mechanical damage. The role of stresses in thedeterioration of a coating system are the subjectof this section.Coating, Curing, and Drying Internal

Stresses. When one chooses to protect anobject from corrosion, to “spruce it up,” changeits color, provide an antigraffiti or sanitary sur-face, or to provide for a myriad of other func-tions, that individual usually turns to paints orcoatings. Why? Because paints and coatingscan be formulated to provide all of these attri-butes and are applied in liquid form, allowingthem to cover and coat all surfaces of the objectin a relatively fast, convenient, and inexpensivemeans of application. The reason this can bedone is that the coatings are spray applied inliquid form and convert by drying and/or curingto a solid coating. The fact that a paint can beapplied in liquid form gives it the ability towet, penetrate, seal, cover, and adhere to mostsubstrates, even those with complex shapes.Moreover, application of the liquid by brush,roller, or spray is fast and inexpensive and rela-tively convenient. However, the conversion of apaint from liquid to solid provides some inher-ent stress to the coating and, in extreme condi-tions or if adhesion is not adequate, can causefailure of the coating.If a coating is not 100% solids and contains a

volatile material such as water or solvents, dry-ing of the coating and its conversion from a liq-uid to a solid result in a volume decrease as thewater or solvent evaporates into the atmo-sphere. Most coating materials initially gelwithin seconds to hours after application asmost of the volatiles leave and the paint drieson the surface. The shrinkage that occurs withthe loss of volatile materials provides some ini-tial stress to the coating and to the adhesion ofthe coating. However, this stress is minimal,because the paint has not sufficiently solidified,is still deformable, and can internally dissipatethese initial stresses. However, as the paintdries further, and particularly if chemicalcrosslinking occurs, further stresses are appliedto the now dry coating. Sometimes low-molecular-weight plasticizers such as pthalates,phosphates, adipates, and chlorinated biphenylscan migrate to the surface of the paint andvolatilize, or at least collect onto the surface.When this happens, the molecular volume ofthe resin decreases. This diminishment involume applies a tensile stress to the cross sec-tion of the paint film, and in extreme cases canresult in cracking, peeling, or loss of adhesion.In a similar fashion, progressive crosslinking—either by reaction with oxygen from the air (autooxidation) or covalent bonding between reactivemoieties of part A and part B components of acoating—provides a further hardening, increas-ing brittleness, and a tensile stress to the coat-ing. In many coatings these latent crosslinkingreactions occur slowly over time, often years,such that the coating slowly hardens and embrit-tles with age. Where coating has been appliedthickly, stresses are greater than areas wherecoating is applied thinly. Variation in thickness

causes an uneven distribution of stress on thecoating, further aggravating the potential forcracking or peeling.Highly crosslinked coatings, particularly

100% solid materials formulated with low-viscosity co-reactants, are particularly suscepti-ble to internal crosslinking stresses. Polyesters,vinyl esters, and other thick-film highly cross-linked coatings must be properly formulatedand pigmented to satisfactorily reduce theseinternal stresses. The low-molecular-weightepoxies, including bisphenol F and novolacepoxies, must also be properly plasticized andpigmented to dissipate internal curing stresses.How does one measure the extent of internal

stress/curing stress on a coating? Until recently,the best means of assessing such stresses was toapply the coating system to a test panel or a testpatch in the area of intended service, wait forcuring and drying to occur, and then evaluatethe coating over time to see if stress cracking,peeling, disbonding, or other evidence of dis-tress occurred. Recently, however, there havebeen some tests developed (but not yet standar-dized) whereby a coating material is applied toone side of a thin foil strip that is held in posi-tion at one end and allowed to deflect at theother. The amount of deflection at the free endis indicative of the internal shrinkage stress asthe coating dries and cures.External Stress—Vibration, Flexibility,

Stress-Strain. External stresses on a coating aremany and varied and usually affect an appliedcoating to a greater extent than internal stresses.When a person walks across a bridge while eithera heavy truck or train is also crossing the bridge,the movement, flexing, and vibration of thatbridge is readily evident. In a similar fashion,any time a municipality fills or empties a waterstorage tank, flexing and bowing of the sidewallsand tank bottom occur. Wind, snow loads, pond-ing water, and other forces can deflect a metalsurface, stressing the coating applied to it. Cycli-cal stresses resulting from a vibration and flexingare most detrimental and can readily degradeboth the metal substrate and any coating appliedto that substrate.Solar heating by day and cooling by night

cause expansion and contraction of all materials.Stress resulting from such thermal expansionsand contractions can be aggravated under winterconditions in cold weather climates where acoating becomes somewhat embrittled due tocold temperatures. Relatively rapid heating andcooling during daily temperature fluctuations isoften more of a problem in the winter than inthe summer. Some accelerated tests performedby laboratories have used a freeze-thaw cycleto provide additional stress on a coating toassess its potential for a service environment,even in a warm climate where no freezing isexpected. This is because if the coating can sur-vive the freeze-thaw cycling, it is quite likelythat it will be able to resist any thermal cyclingencountered in actual service. The externalstresses resulting from vibration, flexibility,and cyclical stress-strain from thermal expansion

and contraction are major detriments to coatingsystems, particularly those that have high inter-nal stresses or have been applied to excessivethicknesses.Impact and Abrasion. While coating, cur-

ing, and drying stresses, as well as vibrationflexing and stress-strain (as discussed previ-ously) are relatively slow transient influences,impact and abrasion to a coating, in contrast, isusually sudden, localized, and abrupt. Mechan-ical damage from dropped tools or stones, orother types of mechanical damage where thecoating surface is impacted either directly orreversely (from the side the coating is on orfrom the opposite side), can cause the coatingto crack and/or spall. Impact damage is greatestwhen a coating has high internal stress and isvery brittle, or is at or below its glass transitiontemperature (Tg).Abrasion occurs as a result of scraping,

scuffing, or erosion due to contact with smallmoving particulate matter such as sand or slur-ries. As a general rule, harder more brittle coatsare more susceptible to abrasion damage thanrubbery softer coatings. However, specific resis-tances are dependent on the formulation ofthe coating, because many hard coatings haveabrasion-resistant pigments—such as aluminumoxide, quartz, silica (sand), garnet, and otherhard materials—embedded in them to resistabrasion and erosion wear. Rubbery elastomericcoatings have the ability to deform under abra-sion, up to a critical point, after which theyrecover their original form. Energy absorbedduring the abrasion and/or impact is absorbedby the elastomeric resin and dissipated as heatwithin the flexible molecular structure.However, hard brittle coatings and coatings

in cold temperatures, or below or close to theirTg, may not have the flexibility to resist abra-sion or erosion wear. In these instances, thecoating may tear on the surface or be scrapedor ablated away, resulting in a thinning of thecoating at the areas of abrasion/erosion.At areas where abrasion/erosion are expected,

natural or synthetic rubbers should be used. Anti-abrasion pigments should be added to the resin orcast and embedded into the top surface of thecoating to make it more abrasion resistant. Insome cases a softer more elastic thick film coat-ing can also be used to resist scuffing andabrasion.

Biological Influences

Microorganisms (nonvisible to the unaidedeye) are the earliest and most numerous lifeforms on earth. They are ubiquitous, occurringin virtually all natural environments, includingthose considered until recently to be inhospita-ble to life: undersea volcanic vents with highconcentrations of sulfur, hot springs, extremelyacidic and alkaline chemical environments,anaerobic (no oxygen) environments, and inlocations devoid of sunlight. Degradation ofcoatings and other materials results from


electrochemical and biological processes result-ing from the presence of microbes.Similarly, macroorganisms (visible) such as

mildew and marine flora and fauna can causeconsiderable damage to coatings and dramati-cally affect the properties and functions of coat-ing systems, negating their purpose.Microbiologically Induced Corrosion.

Microbial adhesion, establishment, and growthinto colonies are prerequisites for deteriorationof organic materials. Much, but not all, microbialgrowth occurs in a biofilm—a gel consistingprincipally of polysaccharides that protects themicrobe and enables it to form an environmentthat is conducive to its reproduction and colonygrowth (Ref 21). The process by which a materialis decomposed or otherwise altered by a microbecolony results in degradation products of thepolymer, which the microbe uses as a source ofenergy, notably, carbon and electrons. This canresult in the depolymerization of the organic mol-ecule, breaking it up into smaller units that can beassimilated by the organism.Mildew. In warm moist environments, and

particularly in the southeastern part of theUnited States and in tropical countries, the pres-ence of molds and mildews on paint (and manyother materials) are of concern. Molds and mil-dews are both forms of fungi whose spores areubiquitous in the environment. The sporesrequire moisture and heat in order to germinateand grow. Generally, humidity of 70% is neces-sary for mold and mildew grow, with tempera-tures between approximately 5 and 50 �C(40 and 120 �F). Slightly acidic pHs (in therange from 4.5 to 6.5) are preferable, althoughalkaline conditions above a pH of approximately8.5 are not conducive to most fungal growth.In order for fungal growth to occur, the spore

must remain in contact with the surface andhave a source of food. Accordingly, a roughtextured surface that collects dirt and holdsmoisture provides a good surface for fungalgrowth. Food sources for the spore can be dustand dirt picked up from the atmosphere, wind-blown contamination on the painted surface,or from the paint itself. Oil-based coatings,alkyds, and polyamide-cured epoxies are sus-ceptible to fungal attack due to the fatty acidsin the oils or crosslinking copolymers. Phthal-ate plasticizers can also be a food source forfungi. Latex paint systems, particularly thosecontaining oil or alkyd modifications for adhe-sion to chalky surfaces, are also very suscepti-ble to mildew or mold growth. The fungalspores can either feed directly on the fatty acidconstituents in the paint or secrete enzymes thatbreak down portions of the paint binder intocomponents which then can be used as food.When a mold or mildew grows on the surface

of a paint, the growth can collect dirt and dustfrom the atmosphere and hold moisture, perpe-tuating its growth. As the fungus feeds on thepaint, the resinous binder is degraded by scis-sion of ester linkages, oxidation of oil fattyacids, and accumulation of metabolic acidic

products. Not only is the paint degraded, but itis also covered with an unsightly black growth.Fungicides can be added to paint to reduce or

eliminate mildew and mold growth. Basic zincoxide added to oil-base and latex paints inamounts of one to three pounds per gallon, per-haps in combination with some organic fungi-cides, can substantially reduce or eliminatefungal growth.Molds and mildews can be killed by a dilute

hypochlorite solution (bleach). Bleach kills thefungus, turning it white. If dirt is present, it willnot be bleached and will remain a dark color.After the fungus is killed, it should be removedfrom the paint surface by scraping and/or scrub-bing prior to painting. However, if heat, mois-ture, and a food source are still present afterrepainting, it is likely that spores will againreattach and grow. Mildews and mold are rela-tively easy to kill by bleaching, but their sporescan often survive a bleach treatment and com-mence growth if conditions are right.Marine fouling consists of attachment of

plant or animal life to an immersed structure. Vir-tually all underwater surfaces have some type ofmarine attachment, including ships, piers, pilings,and even whales, large fish, crabs, and othercrustaceans.Animal fouling species are most commonly

barnacles, muscles, or tubeworms. Fouling veg-etation are algae, or seaweeds (Ectocarpus,Enteromorpha, or Laminaria). Both animaland plant fouling requires contact with the sub-strate for 24 hours or more in order to attach.Consequently, if water is fast moving (overapproximately ten knots) or if the surface issmooth with no fissures or crevices, marinefouling is minimized or eliminated.Marine foulants generally do not attack the

paint and degrade it, but by virtue of their adhe-sion, roughen the surface to which they areattached, providing considerable friction to thesmooth flow of water around the fouled object.If that object is a ship, even minimal marinefouling can reduce its speed and increase fuelconsumption by 10% or more.Antifouling coatings are used to kill or pre-

vent attachment of marine organisms. Tributyltin antifoulants have been used very success-fully in the past, but because they are toxic tomost marine life, have been banned from useon most U.S. flagships. Currently antifoulantsbased on copper oxides are most commonlyused, but these too are toxins, and there is con-cern that ultimately they could also be prohib-ited. Nonstick fluorinated hydrocarbon resinsand silicone-based binders are being used withonly limited success at present. These materialsfunction by providing a surface to which themarine fouling organism cannot attach tightly.When a ship is underway, the friction fromflowing water is sufficient to wash growth fromthe nonstick antifoulant paint surface. Evenwhen marine fouling does attach, it can be read-ily removed by scraping or hosing down withhigh-pressure water washing.

Conclusion

There are a variety of environmental stressesthat combine to degrade coatings exposed in aservice environment. Some of the individualmajor influences are discussed in this article.However, this discussion does not include thestress from combinations of influences, whichalways happens in nature. Accordingly, failureanalysis of coatings is as much an art as a sci-ence, although science always underlies anyattempt to explain what has happened when acoating system fails.

REFERENCES

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3rd ed., ChemTec Publishing, Toronto,Ontario, Canada, 2003, p 58

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7. G.P.A. Turner, Paint Chemistry, 3rd ed.,Chapman & Hall, London, 1993, p 27

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11. M.I. Karyakina and A.E. Kuzmak, Prog.Org. Coat., Vol 18, 1990, p 325

12. H. Corti, R. Fernandez-Prini, and D. Gomez,Prog. Org. Coat., Vol 10, 1982, p 5

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Films by Permeability Data, J. Oil ColourChem. As., Vol 58, 1975, p 359–364

16. A.K. Van Dyk, “Diffusion and Uptake ofMoisture through Paint Films Leadingto Corrosion of Metal Substrates: ADiffusion—Adsorption Model with Reac-tion,” Ph.D. thesis, Massey University,University of New Zealand, 1996

17. HyperPhysics, “Corrosion as an Electro-chemical Process,” Dept. of Physics andAstronomy, Georgia State University,http://hyperphysics.phy-astr.gsu.edu/hbase/

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20. “Surface Preparation of Soluble SaltContaminated Steel Substrates Prior toCoating,” NACE Technical CommitteeReport 6G186, NACE International,March 2010

21. K.B. Tator, Preventing Hydrogen Sulfideand Microbiologically Influenced Corro-sion for Wastewater Facilities, Mater. Per-formance, July 2003, p 33

SELECTED REFERENCE

� K.B. Tator, Organic Coatings and Linings,Corrosion: Fundamentals, Testing, and Pro-tection,Vol 13A, ASMHandbook,ASM Inter-national, Materials Park, OH, 2003, p 826


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