Surface Engineering of Metals - Principles, Equipment and Technologies Part 18 pot

resis-Taking into account that besides chemical corrosion there also occurselectrochemical corrosion, the corrosion resistance of coatings should be considered jointly with the substrate

ucts, although in terms of pure sulfur, annual precipitations reach severaltons per km2

,, while the emission of NOx is on the increase due to ing traffic [63]

ever-increas-Most frequently, from a statistical point of view, metallic materials aresubjected to atmospheric corrosion, less frequently, to water corrosion (in-cluding sea water) and to soil corrosion (including the effect of eddy cur-rents) During service, metallic materials are also subjected to biologicalcorrosion, caused by living organisms, intercrystalline corrosion (occur-ring along grain boundaries), stress corrosion (occurring as the result ofsimultaneous action of the environment and residual stresses), and fa-tigue corrosion (occurring as the result of simultaneous action of theenvironment and rapidly variable stresses induced by extraneous loads).From a qualitative point of view, we distinguish the following types ofcorrosion [62]:

– Chemical - occurring as the result of direct action on metallic

mate-rials of dry gases, especially at elevated temperatures, or of liquid ronments which do not conduct electricity;

envi-– Electrochemical - caused by the action of short-circuited local

cor-rosion sources, formed upon contact of metallic phases with an lyte

electro-Once initiated on the surface of a metal or alloy (surface layer or ing), chemical or electrochemical corrosion at first causes the creation of

coat-a thin lcoat-ayer of corrosion products (most frequently oxides or sulfides, lessfrequently nitrides, carbides, etc.) which with time increases in thicknessand is often aided by other types of corrosion This may lead to

– a total inhibition of further corrosion if the corroded layer covers themetal completely, does not dissolve in the surrounding environment, ad-heres tightly to the metal substrate and has a coefficient of expansionsimilar to that of the substrate Such a mechanism occurs very seldomand then only in some metals (e.g., oxides on the surface of aluminum, pro-tecting it from further oxidation);

– total destruction of the metal if the corroded layer does not meet theconditions quoted above This is the case in the overwhelming majority ofmetals and alloys Of the typically used metals, like lead, copper, nickel,zinc, iron and alloys like brass, bronze and steel, the least resistant tocorrosion is the one used in most applications, on account of its strengthand wear resistance, i.e., steel, primarily non-alloyed and low carbon Itscorrosion occurs in humid atmosphere

6.5.1.2 Corrosion resistance

Ensuring protection against corrosion, i.e imparting corrosion resistance,

is the fundamental function of the majority of coatings By corrosion tance we understand it to be the ability of coatings to withstand the effects ofdifferent types of corrosion

resis-Taking into account that besides chemical corrosion there also occurselectrochemical corrosion, the corrosion resistance of coatings should be

considered jointly with the substrate, with respect to which the coating may

be either anodic or cathodic (see Section 6.3.2.1) Corrosion resistance of als alone can only be considered when the bulk metals are thick and tightand have no surface defects which disturb their cohesiveness When thoseconditions are met, in most cases the corrosion resistance of the coating is thesame or better than that of the bulk metal [55].

met-The same object, coated with the same type of coating, exhibits ent corrosion resistance in different environments For that reason, a gener-alization of the problem of corrosion resistance is extremely difficult It de-pends most significantly on: chemical composition, structure of the coating,three-dimensional structure of coating surface, on defects, residual stresses,type and condition of the substrate, type and intensity (temperature and con-centration) of the corrosive medium and time of exposure

differ-In general, thick coatings offer better protection than thin ones over, the coatings should be tight and should ensure anodic or cathodicprotection (depending on the material of substrate and coating and on thecorrosive environment)

More-Corrosion resistance of coatings is determined experimentally by rosion testing All methods of corrosion resistance testing on specimens

cor-in laboratory (cor-includcor-ing accelerated testcor-ing methods) and natural tions allow only an introductory evaluation of the behavior of coatings onreal components in real service conditions, in a similar way as testing ofthe effect of the surface layer on the fatigue strength of specimens Anabsolute indicator of corrosion resistance of a coating is the life of thatcoating on an object used in service in given conditions of external chemi-cal, electrical, mechanical and other loads The corrosion resistance of acoating which is not subjected to any loads may be good, but in givenservice conditions it may be subjected to constant, variable or impact-typeloads, often in the presence of electrical or magnetic fields which signifi-cantly change the value of residual stresses and, in consequence, servicelife

condi-To a certain extent, the corrosion resistance of paint coatings depends ontheir tightness, permeability and resistance to swelling

6.5.1.3 Porosity

Porosity is a characteristic of coatings, manifest by the existence in them ofpores It is usually determined by the ratio of joint volume of pores to the totalvolume of the coating

Pores are understood as recesses in the coating in the form of narrowchannels of diverse shapes and cross-sections, filled with substances which

do not constitute coating material, like air and other gases, liquids, solids,etc In a broader sense, cracks and scratches are also treated as pores, withthe understanding that these are pores which are significantly extended,parallel to the surface

From the point of view of size, pores may be macroscopic (visible with the unaided eye), microscopic (visible under a minimum 10 x magnifica-

tion) and submicroscopic (invisible under an optical microscope) [4].

From the point of view of shape, the following types of pores are guished:

distin-– specific: penetrating the coating from the substrate to the surface, where

the pores may be perpendicular, inclined or curved relative to the coatingsurface;

– masked (blind): running in the coating from the substrate surface,

narrowing down and closed or covered with the next coating layer; inparticularly aggressive environments they can easily transform into spe-cific pores;

– superficial: forming from the external surface of the coating and

reach-ing inward but not as deep as the substrate

In some coatings there may also occur branched pores [4] with

irregu-lar and complicated shapes

Pores negatively affect tightness1 of coatings, substantially reducing theircorrosion resistance This is true especially of coatings which are cathodicrelative to the substrate metal and does not apply almost at all to anodiccoatings Porous coatings which are not tight do not assure total insulationfrom the surrounding corrosive environment, and do not totally inhibit thediffusion of aggressive agents through the coating which leads to the forma-tion of local corrosion sources, sub-coating corrosion of the substrate andblistering of the coating [4, 43]

The size and extent of porosity change during service Only in tional situations do they close In most cases they grow in a way whichdepends on the type of corrosive medium which enters the pores andcause accelerated bulging of paint coatings, as well their premature agingand loss of protective properties Thicker coatings ensure better tightnessbecause they contain proportionally more masked pores than thin coat-ings

excep-The most frequent causes of formation of pores in metallic coatings aredefects of the metallic substrate, insufficiently clean substrate surface (con-taminations in the form of oxides, sulfides, greases, oils, sand, dust, adsorbedgases, salts and polishing pastes), inappropriate technological processingduring coating deposition, and chemical and mechanical effects (e.g., scratches)during deposition and service

Some coatings, e.g., thermally sprayed, are porous, regardless of the method

of spraying and their porosity stems from the very nature of spraying

In paint coatings the number and size of pores depend on the amount ofevaporated solvent and on the size of solvent particles Coating materialswith good fluidity exhibit a lesser tendency to formation of pores in thedried coating Pores present at the moment of formation of the coating

1) Tightness of coatings is the resistance to penetration of liquids and gases A

measure of tightness of coatings is the number of pores penetrating the coating to the substrate, per unit area Coating tightness is a concept used mainly in electro- plating [43].

Fig 6.8 Schematic representation of porous paint coating and a system of possible

microcells caused by porosity at the surface of the metallic substrate (From [46] With permission.)

grow deeper and wider as the result of constant chemical decomposition

of organic components of the coatings, due to the action of atmosphericoxygen and solar radiation The porosity of paint coatings is the maincause of coating deterioration All ionic reactions which cause corrosion

of the metallic substrate occur as the result of the existence of pores whichform a passage for the corrosive medium (including water) from the outside

to the substrate (Fig 6.8) Water coming in contact with an anodic metalsubstrate, e.g., iron, causes the transition of the iron to the solution whichinitiates corrosion In the presence of moisture, the rust formed becomes acathode relative to the iron and sub-coating corrosion progresses continu-ously, making the substrate non-homogenous This favors the detachment ofthe coating If the water comes in contact with the cathodic space, it reacts as

an alkali and exerts a chemical effect on the paint coating [46]

6.5.1.4 Bulging

Bulging is the rise of volume of the paint coating due to absorption ofliquids, most frequently of water It will depend on the surface tensionand the dielectric constant of the bulging liquid if dissolution, bulg-ing, or solvation1 will take place or not Paint coatings constitute sys-tems of macro-particles, connected into micelles (compounds of macro-particles) which under the influence of water and atmospheric mois-ture may solvatize, i.e., surround themselves with water particles or besubjected to the next stage of destruction, i.e., bulging Water may pen-

1) Solvation - the process of reaction of an ion or particle with particles of the

solvent, resulting in the formation around the ion or particle of zones of loose groups of solvent particles with a smaller or greater degree of ordering These

zones are called solvates When the solvent is water, this effect is called tion The amount of solvation depends on the charges and size of the ion or

hydra-particle and on the type of solvent The degree of solvation of the ion affects numerous properties of ion solutions, e.g., electrical conductivity, coefficient of diffusion.

Fig 6.9 Schematic representation of the effect of water on particles of chain (I) and on

the microparticle or cluster (II): a) solvatation; b) externally micellar swelling; c)

inter-nally micellar swelling (From [46] With permission.)

etrate into the micelle and then the micelle swells (intramicellar bulging)

or concentrates on its surface (extramicellar or intermicellar bulging).Extramicellar bulging is the first stage of absorprtion of the liquid (water)

by the paint coating and is critical to the rate of diffusion of moisture inthe coating It may transform to intramicellar bulging (Fig 6.9) [46] Theabsorbed water may remain in the bulged coating causing its further deg-radation and creating an intermediate stage between solubility and non-solubility of the coating If the absorbed water evaporates and the coatingdries, its shrinkage occurs, but it will be smaller than the former volumeincrement caused by bulging This absorption and expulsion of water (de-sorption) cause structural changes in the coating and its aging [46]

Fig 6.10 Typical course of changes of properties of paint coatings, depending

on volume concentration of pigments: 1 - luster; 2 - blistering; 3 - rusting; 4 -

perme-ability (From [46] With permission.)

Fig 6.11 Typical variation curve for permeability of water vapour within typical ranges

of: I - adsorption of binder on pigment grains; II - spatial packing; III - free excess of binder (From [46] With permission.)

coating (which is opposite in concept to tightness of the electroplatedcoating) favors intensification of sub-coating corrosion

The permeability of coatings depends on the type of coating substance.Polyvinyl and chlorolatex coatings are almost impermeable, on conditionthat all volatile components have been allowed to evaporate On the otherhand, oil and oil-resin coatings allow permeation of water vapour, depend-ing on type of oil or resin

Permeability of paint coatings depends strongly on the pigment tent in the coating (Fig 6.10) and on the appropriate quantitative andqualitative selection of the binder Pigments act very favorably, limitingbulging of coatings exposed to moisture by making access of water vapourinto the organic substance difficult (aluminum bronze, lead minium, lead

con-litharge) Moreover, they passivate the metal surface (lead minium andchromate pigments), ensure electrochemical protection (zinc dust), neu-tralize acids permeating into the coating from the exterior or those formedwithin the coating, due to aging (alkaline pigments), and give the coat-ings their color And for that reason, the percent proportion of pigments

in the coating should be relatively high A coating composed of almostonly pigments would have high permeability, while a coating with almostonly binder - very low permeability (Fig 6.11) The binder, however, doesnot exhibit protective properties or ones that color the coating Pigmentsare bonded in a stable manner to particles of the binder by Van der Waalsforces The remaining part of the binder fills free space between the par-ticular particles of the pigments or their agglomerates in the case of closepacking (so-called interparticle binder) The optimum volume proportion

of pigments to binder is below the critical volume concentration of ments, i.e., below the bend of the curve in Fig 6.10

ap the quality of the substrate or the particular layers (primers, intermediap ate layers, etc.)

intermedi-Eyesight, while presenting the observer with aesthetic sensations, larly to any organoleptic method of quality assessment, is a subjective factor

simi-A subjective evaluation depends on visual acuity, absence of sight defects (inparticular color-blindness), the effect of external factors (color and lightingintensity, presence of dust or smoke in the air)

The external appearance of almost all coatings deteriorates with time ofservice, causing coating aging Exceptions to this rule are coatings made tolook like the patina The older they get, the more they resemble real old coat-ings covered by natural patina

The most important factors taken into consideration when evaluating theexternal appearance of coatings are color, luster, smoothness (opposite ofroughness) and the ability to cover the substrate These properties can be notonly evaluated visually but also measured in an objective way, similarly tothe resistance of coatings to intense ultraviolet and infrared radiation, aswell as to tarnishing

6.5.2.2 Color

The concept of color has two meanings [65, 66]:

– that of a physical property of light from a coating illuminated by tromagnetic radiation in the visible range (of 0.36 to 0.76 µm),

elec-– that of a psychological property of a visual sensation which allowsthe observer to distinguish differences in light stimuli caused by differences

in the spectral distribution of the stimulus; visible radiation reflected by thecoating surface (or its external layer) enters the eye and stimulates photo-sensitive elements of the macula, giving a sensation of color [67]

The color may be treated as a subjective experience of the eye, caused

by light radiation reflected by the coating The spectrum of visible tion is composed of 6 basic colors (violet, blue, green, yellow, orange andred) which may form the so-called color wheel by adding intermediatecolors This color wheel is composed of 12 or 23 chromatic colors Besidesthese colors there are also achromatic colors like white, black and graywith various shades [46]

radia-The following color characteristics are distinguished:

– the color itself - dependent on the wavelength of light radiation,

re-flected by the coating This is a qualitative characteristic, described by a name,e.g., green, red, etc

– saturation - dependent on the degree to which the color is closer to

white or black;

– purity - dependent on the width of spectral band, i.e., on additions of

other colors The purity of a color is highest when the coating reflects tion monochromatically (as one color);

radia-– brightness - dependent on the intensity of radiation reflected by the

coating

Coating colors stem from

– the nature of the components forming the metal or ceramic coating,

be it electroplated or deposited chemically, by immersion, spraying oroverlaying In all these case the influence on color is small Only somemetallic coatings may be colored Also, different types of coatings mayhave the same color, e.g., both gold and titanium nitrided coatings areyellow;

– pigmentation of paint materials or ceramic enamels, i.e., introduction ofpigments into the coating composition Pigments may be organic or inor-ganic coloring substances, practically insoluble in water and exhibiting theability to color paints and varnishes, as well as ceramic enamels in the un-dissolved condition The ability to color paint materials increases with pig-ment refinement

Besides offering aesthetic sensations, colors have their own way of ing the human psyche, as well as the physiological and physical changeswhich take place in the human organism The force of color action is called

affect-color dynamics For example, the application of cold affect-colors in hot industrial

production rooms and warm colors in cold rooms affects the sensing of perature by the organism Red color surrounding man from every side pro-

tem-duces excitation and nervousness, yellow - brings on a happy mood, greenmay act depressively on neurotics, some shades of brown may cause a feel-ing of sadness; white retards the functions of the brain while black has anunfavorable effect on people who easily succumb to psychological depres-sion [46].

For those reasons, as well as visibility, bodies and fixed components ofmachines are painted with such colors which attract least visual attention(light gray, light green, light green-gray) Moving parts are painted withcolors which easily attract attention even in bad lighting conditions (yel-low, canary and light orange) Stamps, levers and valves are usually paintedwith colors that strongly stand out and attract the eye (bright yellow, ver-milion, orange or turquoise) [46]

6.5.2.3 Luster

Luster is a property of the surface of a smooth coating (or surface layer)consisting of oriented reflection of radiation falling on it in such a way thatclear images of bright objects are formed in the field of vision of the observer.The smoother the surface, the more ordered is this reflection and the moreequal is the angle of incidence to the angle of reflection The more luster thecoating has, the more mirrorlike it is [68]

The degree of luster is described by the ratio of the coefficient of ented reflection of the observed surface to the coefficient of total reflec-tion The numerical value of this degree of luster varies from zero (ideallydispersive surface, practically non-existent with approximate propertiesexhibited by coarse, rough and matte surfaces, e.g., those obtained by ther-mal spraying) and unity (ideally reflecting surface, practically non-exis-tent, with approximate properties exhibited by very smooth, polished sur-faces, i.e., mirrorlike) An example of the latter is the surface of an electro-plated coating with addition of brighteners, deposited on an ideally smoothsurface [69]

ori-The degree of luster of coatings decreases with time of service, as theresult of aging and absorption of particles from the environment Moreover,

in subjective observation it depends on lighting conditions, angle of ing, acuity of contrast of a visible object, seen as a reflection by the surface.Highest luster is exhibited by metallic electroplated coatings, mechanicallyand chemically polished, some vacuum deposited coatings, as well as bypaint coatings In the case of paint coatings it depends on the type andamount of pigment in the coating and on the degree and uniformity of itsdispersion in the coating material

view-The type and intensity of luster affect the psychological sensations caused

by colors They may be enhanced or weakened (Table 6.2) Luster deepensthe vitality of colors of painted surfaces (particularly of the golden color),vitalizes gray tones, regarded as devoid of expression, attenuates the som-ber and depressing appearance of the black coating, gives green the feeling

of coolness and peace [43]

– ability to form dimples on the top surface of the coating - characteristic

of a hammer finish (from fast-drying nitrocellulose or synthetic varnishes) ormosaic finish surface;

– ability to form the “crocodile skin” effect - characteristic of coatings with

at least two layers: “rich” primer” (e.g., oil paint) and “lean” enamel with ahigh pigment content;

– ability to reflect in preferred orientations - characteristic of reflectivecoatings, containing glass pellets with diameters up to several tens of mi-crometers;

– fluorescence, phosphorescence, radioactivity - characteristic of coatingswhich feature fluorescent, phosphorescent or radioactive shine in which, inorder to initiate the effect not only light is utilized but also radioactive sub-stances, introduced into the coating composition;

– ability to dull (lose luster) - characteristic of matte coatings

It should be noted that in principle, decorative effects, especially theabove-mentioned specific ones, weaken protective properties of the coat-ing

6.6 Significance and directions of development

of coatings

Coatings primarily play a protective role - by protecting the substrate rial against various types of corrosion They may also fulfill a decorative role

mate-as a sideline They are only seldom applied for solely decorative purposes (if

so, mainly in building construction) More often, they are used for technicalpurposes, mainly for enhancement of tribological properties and for repairs.The significance of coatings in technology is derived mainly from theiranti-corrosion role Corrosion, by destroying materials, causes certain eco-nomic effects, classified as [62]:

1 Losses due to corrosion, including

– direct losses - stemming from a lack of protection, inappropriate or

insuf-ficient protection against corrosion, or those occurring despite good tion which, however, does not act infinitely These losses comprise cost ofcomponents or objects physically destroyed, costs of repairing failures, over-hauls and costs stemming from shortened life of components, devices andobjects;

protec-– indirect losses - stemming primarily from the need to remove the effects of

corrosions, e.g., down-time of production installations, public utility plants(e.g., water works), contaminations of products and of the environment, fines

to pay, etc

2 Investment costs for anti-corrosion protection, comprising costs of terials, labor and machinery, cost of material stock to accommodate corro-sion, cost of maintenance of already applied corrosion protection, as well ascosts of research and development of corrosion protection

ma-The distribution of costs varies from country to country On an average, inthe productive sector, effects of corrosion amount to approximately 70% inlosses and approximately 30% in expenses on anti-corrosion protection.Strict computation of the economic effects of corrosion is extremely diffi-cult, due to the very high cost of carrying out such research, the universalnature of corrosion, its many and varied effects, irrationality in the evalua-tion of corrosion damage, subjective assessments and practical impossibility

of accurate quantification of all negative effects of corrosion In some parts ofthe world, primarily in industrialized countries, such analyses have beenmade, even repeatedly More or less approaching reality, such analyses al-low the following conclusions:

– corrosion losses grow incessantly, especially dynamically in less trialized countries; a rational approach and economical possibilities of highlyindustrialized countries allow a limitation or retardation of the growth rate

indus-of these losses, at the cost indus-of a rise in expenditure on anti-corrosion tion,

protec-– economic losses due to corrosion may even reach 5% of national income– 15 to 35% of corrosion losses may be avoided by the application ofappropriate anti-corrosion protection, especially of steel products

Methods of counteracting corrosion are many and varied and, in general,comprise two areas of activity [63]:

– Indirect: involving creation of conditions for maximum reduction corrosion hazard for components, products and constructions used in

production These conditions may be reduced to the following groups ofproblems

1 Reduction of pollution of the natural environment by precipitations,wastes, smoke, dusts of industrial, communal or household origin, and theirutilization, often combined with recycling of components in short supply.For example, on an industrial scale in:

– steelmaking - it means reduction of pollution of the atmosphere by the

application of filters which absorb solid particles from smoke, desulfurizeexhaust gases and remove from them other components, and the application

of catalytic coatings in heating installations, for the purpose of reducing theemission of harmful NOx-es;

– electromachine industry, in which production processes used are

usually burdensome to the environment, e.g., forging, heat treatment andmachining, pickling, degreasing, washing, surface cleaning, abrasivetreatment and polishing, electrolytic and electroless deposition of met-als, production of conversion coatings, zinc plating, hot dip aluminiz-ing, metal spraying, explosive cladding; painting which produced toxicwastes, both liquid (effluents and used technological solutions), solid(post-neutralization deposits, metals, etc.) and gaseous (different gas-eous compounds) - it means purification and neutralization of liquideffluents, especially those containing cyanide and toxic heavy metalsfrom electroplating and pickling, of used oils and emulsions from wash-ing, degreasing, machining and heat treatment; paint wastes and hy-

drated deposits originating from neutralization of wastes from plating and pickling shops [62, 70].

electro-2 Replacement of energy-consuming technologies used in production

by technologies which are energy-efficient, including high energy electronbeam, glow, plasma and induction technologies, which, allowing a reduc-tion of consumption of primary fuels (coal, petroleum, natural gas), alsoalleviates environmental pollution, especially by sulfur compounds, as well

as reduces risks caused by acid rain A similar role is played by the ing utilization of natural sources of energy (solar, wind, geothermal andhydroenergy)

grow-3 Maximum degree of elimination of usage of these structural materialswhich are especially susceptible to corrosion (thus, naturally of steel), andtheir replacement by materials which are more resistant (e.g., aluminum, syn-thetic materials, various composites)

4 Use of structural materials amenable to low energy recycling after vice which implies preference of aluminum over steel

ser-5 Creation of artificial anti-corrosive atmospheres by tight packaging offinished products, coupled with the introduction of various corrosion in-hibitors between the protected object and the packaging, including objectsdestined for the tropical climate

– Direct: involving design of appropriately resistant materials or ing of components, products or structures by the deposition of protective

protect-surface layers, more resistant than the substrate material in the working ronment These can be summed up as

envi-1 Application of structural materials which are resistant to the ment in which they work, e.g use of austenitic stainless steel and specialmaterials in the chemical industry and in nuclear energy These materialsshould be so designed that the appropriate material exhibit maximum resis-tance to corrosion by sulfates, acids, pitting (chemical), welds, cavitation,fatigue, stresses, friction, contact or high energy, all this with retention ofappropriate strength This problem is not of a universal character and per-tains only to special applications Nevertheless, it is of great significancefrom a practical aspect These are so-called tailor-made materials, customdesigned for the user’s needs [62, 63]

environ-2 Development on high strength materials (usually steel) which are alsonot highly corrosion resistant, of surface layers of high corrosion resistance

or coating them with corrosion-resistant coatings

Organic coatings, primarily paint, play by far the most important role in

corrosion protection Of the approximately 95% of surfaces of steel structuresprotected against corrosion by protective coatings, as much as 90% are pro-tected by paint The life of these coatings ranges from several months tobetween ten and twenty years In this area we can distinguish the followingtrends of development [70]:

– reduction of the percent share of paint materials based on traditionalbinders, i.e., organic solvents, particularly of costly and harmful aromatichydrocarbons (xylene and toluene) and an increase in the percent share of