ucts, although in terms of pure sulfur, annual precipitations reach several tons per km 2 , , while the emission of NO x is on the increase due to ever-increas- ing traffic [63]. Most frequently, from a statistical point of view, metallic materials are subjected 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 biological corrosion, caused by living organisms, intercrystalline corrosion (occur- ring along grain boundaries), stress corrosion (occurring as the result of simultaneous action of the environment and residual stresses), and fa- tigue corrosion (occurring as the result of simultaneous action of the environment and rapidly variable stresses induced by extraneous loads). From a qualitative point of view, we distinguish the following types of corrosion [62]: – Chemical - occurring as the result of direct action on metallic mate- rials of dry gases, especially at elevated temperatures, or of liquid envi- ronments which do not conduct electricity; – Electrochemical - caused by the action of short-circuited local cor- rosion sources, formed upon contact of metallic phases with an electro- lyte. Once initiated on the surface of a metal or alloy (surface layer or coat- ing), chemical or electrochemical corrosion at first causes the creation of a thin layer of corrosion products (most frequently oxides or sulfides, less frequently nitrides, carbides, etc.) which with time increases in thickness and is often aided by other types of corrosion. This may lead to – a total inhibition of further corrosion if the corroded layer covers the metal completely, does not dissolve in the surrounding environment, ad- heres tightly to the metal substrate and has a coefficient of expansion similar to that of the substrate. Such a mechanism occurs very seldom and 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 the conditions quoted above. This is the case in the overwhelming majority of metals and alloys. Of the typically used metals, like lead, copper, nickel, zinc, iron and alloys like brass, bronze and steel, the least resistant to corrosion is the one used in most applications, on account of its strength and wear resistance, i.e., steel, primarily non-alloyed and low carbon. Its corrosion 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 resis- tance we understand it to be the ability of coatings to withstand the effects of different types of corrosion. Taking into account that besides chemical corrosion there also occurs electrochemical corrosion, the corrosion resistance of coatings should be considered jointly with the substrate, with respect to which the coating may © 1999 by CRC Press LLC be either anodic or cathodic (see Section 6.3.2.1). Corrosion resistance of met- als alone can only be considered when the bulk metals are thick and tight and have no surface defects which disturb their cohesiveness. When those conditions are met, in most cases the corrosion resistance of the coating is the same or better than that of the bulk metal [55]. The same object, coated with the same type of coating, exhibits differ- 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. In general, thick coatings offer better protection than thin ones. More- over, the coatings should be tight and should ensure anodic or cathodic protection (depending on the material of substrate and coating and on the corrosive environment). Corrosion resistance of coatings is determined experimentally by cor- rosion testing. All methods of corrosion resistance testing on specimens in laboratory (including accelerated testing methods) and natural condi- tions allow only an introductory evaluation of the behavior of coatings on real components in real service conditions, in a similar way as testing of the effect of the surface layer on the fatigue strength of specimens. An absolute indicator of corrosion resistance of a coating is the life of that coating on an object used in service in given conditions of external chemi- cal, electrical, mechanical and other loads. The corrosion resistance of a coating which is not subjected to any loads may be good, but in given service conditions it may be subjected to constant, variable or impact-type loads, often in the presence of electrical or magnetic fields which signifi- cantly change the value of residual stresses and, in consequence, service life. To a certain extent, the corrosion resistance of paint coatings depends on their tightness, permeability and resistance to swelling. 6.5.1.3 Porosity Porosity is a characteristic of coatings, manifest by the existence in them of pores. It is usually determined by the ratio of joint volume of pores to the total volume of the coating. Pores are understood as recesses in the coating in the form of narrow channels 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, with the 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]. © 1999 by CRC Press LLC From the point of view of shape, the following types of pores are distin- guished: – specific: penetrating the coating from the substrate to the surface, where the pores may be perpendicular, inclined or curved relative to the coating surface; – masked (blind): running in the coating from the substrate surface, narrowing down and closed or covered with the next coating layer; in particularly 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 tightness 1 of coatings, substantially reducing their corrosion resistance. This is true especially of coatings which are cathodic relative to the substrate metal and does not apply almost at all to anodic coatings. Porous coatings which are not tight do not assure total insulation from the surrounding corrosive environment, and do not totally inhibit the diffusion of aggressive agents through the coating which leads to the forma- tion of local corrosion sources, sub-coating corrosion of the substrate and blistering of the coating [4, 43]. The size and extent of porosity change during service. Only in excep- tional situations do they close. In most cases they grow in a way which depends on the type of corrosive medium which enters the pores and cause accelerated bulging of paint coatings, as well their premature aging and loss of protective properties. Thicker coatings ensure better tightness because they contain proportionally more masked pores than thin coat- ings. The most frequent causes of formation of pores in metallic coatings are defects of the metallic substrate, insufficiently clean substrate surface (con- taminations in the form of oxides, sulfides, greases, oils, sand, dust, adsorbed gases, salts and polishing pastes), inappropriate technological processing during 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 of evaporated solvent and on the size of solvent particles. Coating materials with good fluidity exhibit a lesser tendency to formation of pores in the dried 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]. © 1999 by CRC Press LLC 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 atmospheric oxygen and solar radiation. The porosity of paint coatings is the main cause of coating deterioration. All ionic reactions which cause corrosion of the metallic substrate occur as the result of the existence of pores which form a passage for the corrosive medium (including water) from the outside to the substrate (Fig. 6.8). Water coming in contact with an anodic metal substrate, e.g., iron, causes the transition of the iron to the solution which initiates corrosion. In the presence of moisture, the rust formed becomes a cathode relative to the iron and sub-coating corrosion progresses continu- ously, making the substrate non-homogenous. This favors the detachment of the 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 of liquids, most frequently of water. It will depend on the surface tension and the dielectric constant of the bulging liquid if dissolution, bulg- ing, or solvation 1 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 be subjected 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 hydra- tion. The amount of solvation depends on the charges and size of the ion or 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. © 1999 by CRC Press LLC 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 in the coating. It may transform to intramicellar bulging (Fig. 6.9) [46]. The absorbed 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 coating dries, its shrinkage occurs, but it will be smaller than the former volume increment caused by bulging. This absorption and expulsion of water (de- sorption) cause structural changes in the coating and its aging [46]. 6.5.1.5 Permeability Permeability is the ability to allow fluids to pass through a paint coating, due to porosity. It is closely connected with the ability of an organic coating to bulge. In the case of paint coatings, the focus is mainly on permeability of water vapour. Permeability of a coating by water vapor depends on air humidity and temperature and rises with their increase. The rate of diffu- sion rises by approximately 10%, with a rise of air temperature by 1 K, which causes that the humid tropical climate particularly favors perme- ation of water vapour into the coating. A greater permeability of the paint © 1999 by CRC Press LLC 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 electroplated coating) 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 condition that all volatile components have been allowed to evaporate. On the other hand, 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 con- tent in the coating (Fig. 6.10) and on the appropriate quantitative and qualitative selection of the binder. Pigments act very favorably, limiting bulging of coatings exposed to moisture by making access of water vapour into the organic substance difficult (aluminum bronze, lead minium, lead © 1999 by CRC Press LLC litharge). Moreover, they passivate the metal surface (lead minium and chromate pigments), ensure electrochemical protection (zinc dust), neu- tralize acids permeating into the coating from the exterior or those formed within 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 almost only pigments would have high permeability, while a coating with almost only binder - very low permeability (Fig. 6.11). The binder, however, does not exhibit protective properties or ones that color the coating. Pigments are bonded in a stable manner to particles of the binder by Van der Waals forces. The remaining part of the binder fills free space between the par- ticular particles of the pigments or their agglomerates in the case of close packing (so-called interparticle binder). The optimum volume proportion of pigments to binder is below the critical volume concentration of pig- ments, i.e., below the bend of the curve in Fig. 6.10. 6.5.2. Decorative properties 6.5.2.1. External appearance All coatings, to a greater or lesser extent, feature decorative values, but special decorative properties are required of decorative coatings, as well as protective-decorative paint, electroplated and vacuum deposited coat- ings. The basic criterion of a coating’s decorative value is its external ap- pearance. Of all the properties of coatings, external appearance is the easiest to evaluate because it can be done immediately, by visual means. Visual methods may also be used to evaluate not only the external ap- pearance but also the quality of the coating and - before their deposition - the quality of the substrate or the particular layers (primers, intermedi- ate layers, etc.) Eyesight, while presenting the observer with aesthetic sensations, simi- larly to any organoleptic method of quality assessment, is a subjective factor. A subjective evaluation depends on visual acuity, absence of sight defects (in particular color-blindness), the effect of external factors (color and lighting intensity, presence of dust or smoke in the air). The external appearance of almost all coatings deteriorates with time of service, causing coating aging. Exceptions to this rule are coatings made to look 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 the external appearance of coatings are color, luster, smoothness (opposite of roughness) and the ability to cover the substrate. These properties can be not only evaluated visually but also measured in an objective way, similarly to the resistance of coatings to intense ultraviolet and infrared radiation, as well as to tarnishing. © 1999 by CRC Press LLC 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 elec- tromagnetic radiation in the visible range (of 0.36 to 0.76 µm), – that of a psychological property of a visual sensation which allows the observer to distinguish differences in light stimuli caused by differences in the spectral distribution of the stimulus; visible radiation reflected by the coating 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 radia- tion is composed of 6 basic colors (violet, blue, green, yellow, orange and red) which may form the so-called color wheel by adding intermediate colors. This color wheel is composed of 12 or 23 chromatic colors. Besides these colors there are also achromatic colors like white, black and gray with various shades [46]. 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 radia- tion monochromatically (as one color); – 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 or overlaying. In all these case the influence on color is small. Only some metallic coatings may be colored. Also, different types of coatings may have the same color, e.g., both gold and titanium nitrided coatings are yellow; – pigmentation of paint materials or ceramic enamels, i.e., introduction of pigments into the coating composition. Pigments may be organic or inor- ganic coloring substances, practically insoluble in water and exhibiting the ability 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 affect- ing the human psyche, as well as the physiological and physical changes which take place in the human organism. The force of color action is called color dynamics. For example, the application of cold colors in hot industrial production rooms and warm colors in cold rooms affects the sensing of tem- perature by the organism. Red color surrounding man from every side pro- © 1999 by CRC Press LLC duces excitation and nervousness, yellow - brings on a happy mood, green may 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 an unfavorable effect on people who easily succumb to psychological depres- sion [46]. For those reasons, as well as visibility, bodies and fixed components of machines are painted with such colors which attract least visual attention (light gray, light green, light green-gray). Moving parts are painted with colors which easily attract attention even in bad lighting conditions (yel- low, canary and light orange). Stamps, levers and valves are usually painted with 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 that clear 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 more equal is the angle of incidence to the angle of reflection. The more luster the coating has, the more mirrorlike it is [68]. The degree of luster is described by the ratio of the coefficient of ori- ented reflection of the observed surface to the coefficient of total reflec- tion. The numerical value of this degree of luster varies from zero (ideally dispersive surface, practically non-existent with approximate properties exhibited 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 smooth surface [69]. The degree of luster of coatings decreases with time of service, as the result of aging and absorption of particles from the environment. Moreover, in subjective observation it depends on lighting conditions, angle of view- ing, acuity of contrast of a visible object, seen as a reflection by the surface. Highest luster is exhibited by metallic electroplated coatings, mechanically and chemically polished, some vacuum deposited coatings, as well as by paint coatings. In the case of paint coatings it depends on the type and amount of pigment in the coating and on the degree and uniformity of its dispersion in the coating material. The type and intensity of luster affect the psychological sensations caused by colors. They may be enhanced or weakened (Table 6.2). Luster deepens the 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]. © 1999 by CRC Press LLC – ability to form dimples on the top surface of the coating - characteristic of a hammer finish (from fast-drying nitrocellulose or synthetic varnishes) or mosaic 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 a high pigment content; – ability to reflect in preferred orientations - characteristic of reflective coatings, containing glass pellets with diameters up to several tens of mi- crometers; – fluorescence, phosphorescence, radioactivity - characteristic of coatings which feature fluorescent, phosphorescent or radioactive shine in which, in order 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 the above-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 mate- rial against various types of corrosion. They may also fulfill a decorative role as a sideline. They are only seldom applied for solely decorative purposes (if so, mainly in building construction). More often, they are used for technical purposes, mainly for enhancement of tribological properties and for repairs. The significance of coatings in technology is derived mainly from their anti-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 protec- tion which, however, does not act infinitely. These losses comprise cost of components or objects physically destroyed, costs of repairing failures, over- hauls and costs stemming from shortened life of components, devices and objects; – 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 ma- terials, labor and machinery, cost of material stock to accommodate corro- sion, cost of maintenance of already applied corrosion protection, as well as costs of research and development of corrosion protection. © 1999 by CRC Press LLC [...]... between atoms and ions of the coating material and atoms of the reactive gas during their movement in the direction of the substrate or on the substrate, – collisions of particles of coating material with particles of gas, leading to so-called gas dispersion and to a rise of the energy of particles of the coating material, – condensation of particles of the deposited material to the form of crystallization... (effluents and used technological solutions), solid (post-neutralization deposits, metals, etc.) and gaseous (different gaseous compounds) - it means purification and neutralization of liquid effluents, especially those containing cyanide and toxic heavy metals from electroplating and pickling, of used oils and emulsions from washing, degreasing, machining and heat treatment; paint wastes and hy- © 1999... positions, surface defects are formed, centers of nucleation (condensation) of high density are formed Increased surface atom mobility and surface chemical activity occur All these effects lead to obtaining coatings with good physical properties and good adhesion to substrate even at low temperature [28] Further rise of ion energy leads to knocking out of particles of deposited coating and substrate (surface. .. on Techniques of producing surface layers on metals, Rzeszow, Poland, June 1988, pp 13 5-1 43 32 Burakowski, T., Miernik, K., and Walkowicz, J.: Manufacturing techniques of thin tribological coatings with utilization of plasma (in Polish) Metaloznawstwo, Obróbka Cieplna, In¿ynieria Powierzchni (Metallurgy, Heat Treatment, Surface Engineering) , No 12 4-1 26, 1993, pp 1 6-2 5 33 Biestek, T., and Weber, J.:... dusts of industrial, communal or household origin, and their utilization, 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, desulfurize exhaust gases and remove from them other components, and the application of catalytic... remaining 50% of the solvent evaporates and pollutes the atmosphere, and the utilization of the coating material is only 25% [2]; – rise in production and application of aluminum sheet, anodized and varnished in a continuous operation; – increase and greater diversity in applications of coatings offering temporary protection against corrosion, especially in the case of different means of transportation,... plasma and its localization in the direct neighborhood of the magnetron; 3 Situation of zone of obtaining of substrate vapours through evaporation which may be – simultaneous from the entire surface of the molten substrate in the evaporator, – local from consecutive fragments of substrate surface in the solid state; 4 Technique of depositing of metal vapour on the substrate [13]: – Evaporation - E Deposition... addition of 0.5 to 1% Co and Sn to coat metallic components of firearms, as substitute of the commonly used cadmium coatings Cadmium is suspected of having a carcinogenic effect! Finally, there should also be a rise in the application of lead-base coatings, including lead alloys, primarily lead-tin, as well as the replacement of white tin-coated sheet by other coatings [70] The application of double and. .. technical and technological point of view, we should note the current development in the direction of lowering CVD process temperatures due to the application of auxiliary heating, mainly by glow discharge, and raising of PVD process temperatures in order to achieve better binding of coating to substrate [63] Table 6.3 lists the percentage share of costs of different methods of obtaining surface layers,... trends of development [70]: – reduction of the percent share of paint materials based on traditional binders, i.e., organic solvents, particularly of costly and harmful aromatic hydrocarbons (xylene and toluene) and an increase in the percent share of © 1999 by CRC Press LLC paint materials based on synthetic resins (including water-soluble dispersive paints) [2]; – a rise in the application of paints . 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. 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. In. bulg- ing, or solvation 1 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