Surface Engineering of Metals - Principles, Equipment and Technologies Part 17 pptx

Non-metal coatings, particularly paints, varnishes and enamels, are used to coat rather big objects which are exposed to only small mechanical haz- ard. Exceptions to this rule are ceramic, enamel and metal ceramics which can sustain high mechanical loads. 6.3.2 Classification of coatings by application From the point of view of application, coatings can be divided into four groups [1, 4]: protective, decorative, decorative-protective and technical. 6.3.2.1 Protective coatings Protective coatings are those coatings whose task is exclusively protection of the object from the harmful effect of the environment, mainly the atmosphere and chemicals, as well as from mechanical hazards. These coatings may also feature properties other than anti-corrosion, e.g., higher hardness, resistance to tribological wear, or attractive appearance. These, however, are properties of secondary importance. Since they protect the object from different types of corrosion, they are sometimes referred to as anti-corrosion coatings. From the point of view of service life, protective coatings may be classified as temporary and permanent protection coatings. Coatings for temporary protection serve to secure (mostly metals) against corrosion during transport or storage, as well as during interoperational periods before the product goes into service [1]. Most often these are coatings easily removed by stripping, grinding down or washing off. Their composition is based on e.g., oils, greases, asphalts, waxes, some varnishes and some plastics [26, 27, 44, 45]. Temporary protection is also assured by tight packaging with the use of solid, liquid or gas corrosion inhibitors [26, 27]. Coatings for permanent protection serve to secure the surfaces of products during service. These are the majority of metallic coatings and many paints, varnishes and enamels, metal ceramic, ceramic, rubber and latex. The protective function of coatings may consist either of only insulation of the coated object from the environment or by electrochemical in- teraction of the coating with the substrate. In both cases the coating fulfills its function only when it adheres tightly to the substrate, has no cracks, pores and delaminations, scratches and other flaws, besides being mechanically strong. The substrate may be manufactured from any type of material. Likewise, the coating may be of any coating material. In the second case, both the substrate and the coating must be metallic. Coatings reacting electrochemically with the substrate metal can be further subdivided from the point of view of protection mechanism as anodic and cathodic. Usually, these are coatings deposited by electroplating. Anodic coatings (Fig. 6.2a) are made of metal which in a given environment exhibits a potential lower than that of the substrate. In other words, they are made of a less noble metal than the protected substrate © 1999 by CRC Press LLC Tin, tin alloy, aluminum, aluminum alloy and lead coatings are also used on a fairly broad scale. Paint coatings effectively protect the metallic substrate only when they are multi-layered. Typical are 4-layered coatings: primer, surfacer, enamel, varnish. The anti-corrosion function is fulfilled mainly by the primers, pigmented by elements with rustproof action, e.g., lead minium, zinc yellow and zinc dust. Pigments with rustproof action passivate steel and cast iron and are used exclusively as components of primers. Because primers are not sufficiently resistant to the effects of the external environment, they have to be coated by paints and varnishes which may also contain pigments, but ones not exhibiting rustproof action. The surfacer smoothes the irregularities of the coated surface [46]. Good protective properties are also exhibited by non- metal ceramic, plastic, enamel and cement coatings, the last especially in strongly corrosive environments. Research has been continuing for the past 10 to 15 years into a new class of anti-corrosive paint materials - both solvent and powder - which constitute mixtures of resin vapors and certain solvents which cause self-stratification of the resins. This takes place mainly due to different surface energies. The solvents are a decisive factor in the direction of stratification and adhesion of both layers between themselves and between the bottom sublayer and the substrate. As a result, deposition of a resin mixture allows the obtaining of a double layer coating. The ongoing research has already yielded some application successes, i.e., painting of submarine hulls and other structures. Good and even very good anti-corrosive properties are exhibited by vacuum deposited ceramic coatings (by PVD and CVD techniques) and those obtained by the Sol-Gel technique. 6.3.2.2 Decorative coatings Decorative coatings, once called ornamental, serve predominantly to give the metal or non-metal object an aesthetic external appearance. This depends first of all on color, luster and resistance to tarnishing and also, perhaps, surface finish of the coating (hammer finish, webbling, crystal, crocodile skin), as well as shine properties (fluorescence, phosphores- cence, radioactivity). It is evident that decorative coatings, in many cases, make good protective coatings. Decorative coatings may be both metallic and non-metallic. Among metallic coatings, electroplated coatings have found broad- est use. Each metal or alloy has its own characteristic color. Luster is dependent on surface smoothness or it is obtained by the addition of so-called lustering substances to the electrolyte bath. An alternate method is the periodic reversal of current flowing through the electrolyte [4, 47]. Among metals used in the manufacture of decorative coatings, the only ones which do not tarnish are chromium, gold [47], rhodium, palladium and platinum. Silver and nickel coatings become tarnished in some environments, e.g., in an atmosphere contaminated by sulfur compounds; silver sur- © 1999 by CRC Press LLC faces are covered with a gray-black film of silver sulfide. Tarnishing of some coatings is counteracted by their passivation (giving the metal a more electropositive potential than the equilibrium potential, in order to render electrochemical corrosion impossible) or by depositing on them varnish coatings, e.g., a thin and invisible layer of varnish is used to top bronze or brass coatings. In some cases, tarnishing is prevented by the deposition of additional coatings of rhodium or palladium [4]. Among non-metallic coatings, paints are the most broadly used. Al- most all paint coatings have decorative functions. The remaining non- metallic coatings, with the exception of enamels, some coatings in the form of deposited nitrided layers and some plastic coatings, are, as a rule, not used for decorative purposes. In the case of some electroplated and paint coatings, used both for protective and decorative purposes, the latter are much thinner than the former. Usually, natural environment penetrates easier to the substrate metal through a thin layer (through pores, leaks and other flaws), but protection is not the main function of the decorative coating. Besides, such coatings are designed to be used in gentler conditions than their protective counterparts. For example, thicknesses of electroplated decorative coatings are small and range from 0.25 to 3 µm, while protective coatings often reach 60 µm [4]. Sometimes, decorative coatings are artificially aged. 6.3.2.3 Protective-decorative coatings Protective-decorative coatings, sometimes also called decorative-protective [4], serve both to protect the object against corrosion and light mechanical damage, and to give the surface an aesthetic appearance. Regarding electrolytic coatings, the protective-decorative role is fulfilled by nickel, chrome, and copper-nickel-chrome coatings on condition that they are sufficiently thick. It is accepted that their thickness should not be less than 25 µm [4]. Corrosion protection is assured by sandwiched layers of nickel or copper and nickel. Sometimes, sandwiched copper layers are replaced by brass but with the same thickness these offer poorer corrosion protection than copper. Nickel is used for the protection of steel, copper and zinc substrates, as well as those made of copper alloys or zinc alloys. The external chrome coating in multi-layer coatings is very thin - its thickness is in the range of 0.25 to 1.5 µm. It protects the underlying layers against tarnishing, enhances wear resistance and makes the coating tighter. Single chrome layers deposited on steel are usually not tight, on account of pores and cracks which occur in them. Only when their thickness approaches 50 µm do they protect well against corrosion [4]. Black chrome coatings of 5 to 8 µm thickness suffice to protect copper or its alloys against atmospheric corrosion [48]. A gold coating of 25 to 30 µm thickness insulates copper alloys well from aggressive atmospheric effects; such a coating is used on the elements of the © 1999 by CRC Press LLC clock face of the 17th century clock on the tower of the royal castle in Warsaw [48]. The overwhelming majority of non-metallic coatings functions as both protective and decorative, particularly those of plastic, enamel and paint [49, 50]. Oxide coatings synthetically obtained on metals not only offer good protection, but also give the object an attractive appearance, e.g., on copper or copper alloys, from a bright to a dark-brown color. Often, these coatings are fixed by organic coatings [48]. 6.3.2.4 Technical coatings Technical coatings serve to give the object certain physical properties: mechanical, electrical and thermal. Coatings which enhance tribological properties. In the majority of cases better tribological properties are exhibited by harder than by softer metal coatings. The hardness of electroplated coatings is varied and ranges from the hardness of lead to that of rhodium. Moreover, hardness may also differ for coatings of the same material, depending on both composition and conditions of deposition [4]. Most frequently, as resistance coatings wear, hard chrome-plated layers of 10 to 30 µm thickness are used. For high sliding velocities and high unit loads, electroplated silver or indium coatings of 500 to 1500 µm, as well as porous chrome coatings, are used [4]. Among non-metallic coatings, very high hardness and excellent tribological properties are exhibited by nitride, oxide, carbide and boride coatings, deposited in vacuum by PVD and CVD techniques. Coatings which enhance electrical properties. These coatings serve first and foremost to enhance electrical conductivity of terminals and are used in electrical and electronics applications. Since very good electrical conductivity is exhibited by silver, very often silver coatings are deposited on copper, brass and bronze substrates. In conditions of normal use of terminals made of these metals, the thickness of silver coatings applied is 12 µm, while in the presence of moisture and condensing water vapor this thickness is doubled and in the case of sliding contacts it is even greater. Protection against tarnishing of silver coatings insulates them electrically; additional very thin (0.2 to 1 µm) electrodeposited layers of gold, rhodium or indium are used, with chemical or electrochemical passivation as an alternate method [4]. Technological coatings. These coatings serve to enhance different properties of semi-finished products during the execution of the technological process of manufacture or to protect the product against diffusion by undesired elements. Coatings which enhance solderability of joined surfaces are deposited by electroplating. These are tin and copper coatings, as well as alloys: tin- zinc or zinc-lead, less often cadmium and twin-layered cadmium-tin and copper-tin, deposited on brass and steel components. © 1999 by CRC Press LLC Copper coatings of 2.5 to 7 µm, often protected by a layer of varnish against tarnishing, are applied to components immediately before soldering. The thickness of tin coatings, which insignificantly oxidize and for that reason require the use of fluxes, is 5 to 15 µm. Alloy coatings composed of 70% tin and 30% zinc are well suited for soldering and for service in tropical climates. The best proportion of tin to lead in tin-lead coatings is 40:60 [4]. Coatings protecting against diffusion, particularly of carbon, nitrogen, the two elements together and less often of other elements in thermo- chemical treatment operations, are applied as electroplated or in the form of pastes. They fulfill the role of a blockade, stopping the passage of a given element to a given (coated) fragment of the component which is subjected to thermo-chemical treatment. Electroplated coatings are usually copper or tin, as well as alloys of copper and tin with a thickness of up to 25 µm [4]. Paste coatings with varied chemical composition may reach thicknesses of over 1 mm. Corrective coatings. Such coatings are used to re-create the initial dimensions or shape of a component partially worn in the course of service. In the case of re-creation of dimensions, especially when close tolerances are called, the coatings applied are electroplated: iron, chrome and nickel for the regeneration of steel components and copper - for the regeneration of components made of copper and its alloys. Shape correction (always) and dimensional correction (quite often) are carried out with the aid of various metal and alloy coatings, thermally sprayed or applied by welding and laser beam techniques. Their thickness may reach several mm. Catalytic coatings. These coatings serve to change the rate of reaction in the gaseous environment with which it is in contact, as well as to raise or lower the temperature at which the reactions occur. Since a large area of contact between the coating and the surrounding gas is required, the real surface should be developed as much as possible. This condition is best met by thermally sprayed coatings. Their real surface may in practice be even 5 times that of the nominal value. The coating material may be metallic or metal ceramic (mixtures of oxides of cerium, copper, manganese, aluminum, nickel, cobalt, lantha- num, neodymium, etc.). These coatings serve various purposes, among others, to reduce the content of carbon monoxide or nitrides (NO x -es) in gas and carbonaceous exhausts emitted to the atmosphere. They are also usually resistant to erosion from exhaust gases [51]. Their thickness does not, as a rule, exceed 1 mm. Coatings enhancing selected thermophysical properties. Most often, these coatings are applied in order to enhance resistance to the effect of elevated temperatures, as well as emissivity and thermal conductivity properties, themselves dependent on temperature. In most cases, these are metal ceramic or non-metal coatings, thermally sprayed on elements of steel mill equipment, heat exchangers, radiators, recuperators, walls of industrial furnaces and © 1999 by CRC Press LLC hearths. They are also applied to ballistic missile heads and to spacecraft components. Coatings which enhance emissivity may be monolayer, but those which retard or enhance thermal conductivity, as well as heat-resistant coatings, are all multi-layered, most often comprising three layers, in order to im- prove adhesion to the substrate and thus to prevent cracking of the external layer due to excessive residual stresses which rise with a change in temperature. Metal ceramic and ceramic coatings are characterized by high emissivity, within the range of 0.6 to 0.95. Coatings which retard thermal conductivity constitute a barrier for the flow of heat and change the heat flux by several to more than ten percent. Ablation coatings 1 . These constitute one type of coating with special thermophysical properties. They are most often produced by thermal spraying of ceramic refractory materials (the main constituents of which are Al 2 O 3 and ZrO 2 as well as silicides ZrSiO 4 and MoSiO 4 ) onto metallic or non-metallic surfaces with good thermal insulation, in order to protect them against the effect of elevated temperatures at which the surface may melt. Under the influence of heat they undergo ablation, thereby protecting the substrate material. Such coatings are deposited on gas turbine blades, components of high temperature equipment and, primarily, on short and long range ballistic rocket heads, as well as on external surfaces of space vehicles. In the latter case, they enable the vehicle to over- come the heat barrier during re-entry into the dense layers of the earth’s atmosphere. As an example, at a speed of 6000 km per hour, at an altitude of several kilometers, the temperature of the vehicle surface, due to friction from atmosphere particles, reaches approximately 1600 K. Optical coatings. These coatings may have different tasks. Steel and brass elements are coated with electroplated silver, chrome and rhodium, nickel- chrome and copper-nickel-chrome layers to enhance surface luster [4]. Thin multi-layered anti-reflection coatings are applied by PVD techniques to surfaces of glass and plastics. These coatings may absorb or reflect selected bands of thermal radiation, especially in the visible range; they may transmit radiation in one direction and they may counteract the accumulation of dust, gases, vapours, etc. 6.3.3 Classification of coatings by manufacturing methods This classification is not totally strict but takes into account traditional names and applied distinctions. Thus, in this chapter we will present only general information regarding significant processes of coating manufacture. A more detailed discussion regarding coating and coated materials is given in Part II, Chapter 1 and following chapters of this book. 1) Ablation (from the Latin: ablatio - take away) - the taking away of heat by evaporation or sublimation of the heated material. © 1999 by CRC Press LLC Fig. 6.3 Classification of coatings by method of production. © 1999 by CRC Press LLC From the point of view of coating manufacture we can distinguish five groups (Fig. 6.3): electroplated, immersion, spray, cladded and crystallization. 6.3.3.1 Galvanizing The manufacture of galvanized coatings is in the realm of galvanostegy, which is the most important branch of electroplating [55]. Coatings of this type are deposited directly from a galvanic bath or with the application of an external source of electric current [2]. The thickness of mono- or multi-layered coatings may even exceed 50 µm. The following groups of coatings belong to this category [2]. Electroplated (electrolitic) coatings. The electroplated coating is a metal coating applied in a process of electrolysis, with the application of an external source of electric current [1, 2]. It is usually direct current. In some of the most recent applications, pulsed alternating current may be used (Electropulse plating), which allows the obtaining of superior technological results, e.g., higher cathode current density, better penetration and good refinement of the crystallized coating grains. These coatings can be further subdivided into: – bath - deposited on objects immersed in an electrolyte tank; plated objects connected with the negative pole of the current source constitute the cathode, while the anode is formed by plates of the throwing metal which replenishes the loss of that metal in the electrolyte, or by non- soluble materials (lead, graphite). In such cases, loss of metal in the electrolyte is replenished by the addition of appropriate salts [5]. The vast majority of coatings is manufactured by this method; most frequently zinc, nickel and chromium. – tampon - applied locally on selected fragments of a fixed or moving object which forms a single electrode, with the aid of a tampon saturated by a warm or solid electrolyte or by brushes in contact with the other electrode by means of the electrolyte [54]. The quality of electroplated coatings (tightness, grain size, hardness), as well as rate of deposition depend on the type of electrolyte, its concen- tration, temperature and current density. Chemical (electroless) coatings. These are metal coatings obtained by way of chemical reaction, without the participation of externally supplied electric current. Depending on the mechanism of reaction, they may be obtained by: – exchange: the less noble metal (more electronegative) displaces the nobler metals (more electropositive) from the solution. For example, a steel object immersed in a solution of copper sulfate is covered with copper, the process continuing only until the moment of covering the entire object (i.e., moment when contact between bath and substrate ends). As a result, only very thin coatings (0.02 to 0.5 µm) are obtained. These are mainly used as decorative but sometimes as technical coatings [43]; © 1999 by CRC Press LLC – contact: the more noble metal (anode) is deposited on the surface of the less noble metal (cathode) in solution, as the result of contact between the coated object and the contact metal [2]. Contact coatings are significantly thicker than those obtained by exchange - their thickness is 1 to 2 µm - and they are well suited to be deposited on small objects [43]; – chemical reduction: a metallic element undergoes transition from the ion state (in solution) to the free state (metallic) as the result of attach- ing to the ion of an appropriate number of electrons from a substance which is capable of donating electrons, called a reducer. Usually, this is an organic compound. Simple salts are less frequently present in the solution. More often, the solution contains complex compounds of the metal used to coat the substrate surface. Reduction begins at the moment of addition of the reducer to the bath and occurs in the entire mass of the solution, of which only a small proportion is deposited on the substrate surface in the form of a coating. For this reason, the method is not very economical. It is used mostly in cases when other methods, notably electroplating, fails. More often it is applied by a spray gun, rather than by immersion. A modification of chemical reduction is catalytic coating, oc- curring when the bath (prepared from a complex compound and reducer) comes into contact with the catalyst which may be (but does not have to be) constituted by the metal substrate of the coated metal. The reducing reaction must be catalyzed by the metal being deposited. In this case auto- catalysis occurs and deposition of the coating proceeds until it reaches the required thickness. The reducing reaction takes place only on the catalyst surface [43]. Conversion coatings. These are non-metallic coatings, obtained on metal surfaces in the form of compounds of the substrate metals, e.g., chromate coatings on zinc, cadmium and silver or oxide layers on steel and aluminum [1,2]. These may be divided into – chemical - when the coating is formed by simple immersion of the metal in the solution (so-called oxidizing), e.g., oxide coatings on aluminum and its alloys, on steel (by the application of black oxide) and on copper and its alloys; – electrochemical (anodic) - when the process is controlled by electric current flowing through the electroplating tank, while the coated metal is the anode (so-called anodic oxidizing or anodizing), e.g., oxide coatings on aluminum and its alloys, copper and its alloys, zinc, cadmium and steel. Many conversion coatings may be obtained by chemical and electrochemical means. Conversion coatings are predominantly protective, also used as underlayers, less often as technical coatings (mainly technological) and decorative by the obtaining of color effects. This is especially true of aluminum coloring or electrolytic patination of copper. Most frequently applied conversion coating thicknesses are several to several tens of mi- crometers [5, 33, 43]. © 1999 by CRC Press LLC 6.3.3.2 Immersion coatings Immersion coatings are obtained by immersion of the entire object or of its portion in a bath of the coating material. Depending on the type of coating material, its physical state may be solid or liquid. In the first case, before depositing the coating, the material must, obviously, be melted. After removing from the bath, the coating material dries on the object, or solidifies, forming the coating. Immersion coatings are divided into three major groups: hot dip, gel and paint: Hot dip coatings. Coating materials in the form of solids are melted in a pot or a tank furnace, using the energy of gas burned in burners, or alternately, by electric resistance heating. Since coating materials are predominantly metals and the melting point of used coating materials is usually several hundred degrees Celsius (not exceeding 1000ºC), coatings made from them are traditionally called hot dip. Properties of hot dip coatings are decided by surface preparation, chemical composition of the metallic material (purity and composition of alloy) and its temperature, time of soaking of the objects in the bath and by the substrate material. Before dipping into the bath, the object is degreased, sand blasted and covered with fluxes by immersion or spraying. The fluxes (most often mixtures of zinc chloride and ammonium chloride with foaming agents - carbohydrates, glycerine, tallow) may also be added to the bath. Metallic materials are hot dip coated by tin, zinc, aluminum, lead and their alloys. Gel coatings. These are manufactured in colloidal systems in the form of sol, transformed into gel (e.g., due to polycondensation) and next into a coating through e.g., firing. Gel coatings are most often deposited by immersion but they may also be deposited by other methods, i.e., by centriguge, spraying or simply by paintbrush. Paint coatings. To manufacture paint coatings, the material is used is liquid (e.g., paints, varnishes) or pseudo-liquid (fluidized beds, suspensions of powdered materials, e.g., plastics in fluidizing gas [56]). The painted coating has the form of a coherent, more or less hard, smooth, shiny substance, adhering to the substrate after drying of the layer of filmogenic material. Only some coatings may be applied as single layers; in most cases, these are multi-layer compositions. Paint coatings are applied by painting, i.e. depositing the coating material onto the substrate (metallic, ceramic, wooden, etc.) by optional methods. The method selected affects the quality of the coating, as well as the effectiveness and economy of the operation. Some methods require special adjustments of the coating material. Two basic groups of methods of application are distinguished, i.e., immersion and spraying (see Section 6.3.3.3). Paint coatings applied by immersion may be divided into common and phoretic. Common immersion coatings are applied by very simple means and at low cost. High coating quality is achieved only with appropriate shapes of © 1999 by CRC Press LLC [...]...objects, e.g., castings, tooling, components of agricultural machines and tractors, auto chassis and small mass-produced elements The quality of the coating depends on the rate of immersion, viscosity of the material and conditions of drip-drying [46] Phoretic immersion coatings are applied with the utilization of phoresis, i.e movement of particles of a dispersed suspension or a colloidal system... (silver, palladium and gold, with a thickness of 0.5 to 2 µm) [55] 6.4.3.8 Magnetic properties Magnetic properties are exhibited by some metallic coatings containing ferromagnetics, mainly iron, cobalt, nickel and their alloys with phosphorus Binary alloys: Co-P, Ni-Fe, Ni-P and ternary: Ni-Co-P, Ni-Fe-P are characterized by high permeability and small coercion intensity Alloy coatings like Ni-Fe containing... result of dividing the mass of the deposited coating by the surface area of the coated object, and expressed in mg/cm 2 , – minimum - measured at a site where the thinnest coating is expected, or the least value obtained in a series of readings Coating thickness varies over a very broad range - from hundredths of a micrometer to several millimeters 6.4.1.2 Three-dimensional structure of the surface. .. orientation accompanying this often corresponds to perpendicular orientation of crystallographic axes Columnar structures are also formed in metals crystallizing out of the gas phase; – form fine-grained structure (Fig 6.5c) With strong polarization and strong inhibition, the rate of formation of new nuclei exceeds the rate of growth of nuclei already formed Very soon, crystallites of random orientation are... into the bath in the form of wetting agents, brighteners or reducers of residual stresses in the coating) [55] Non-equilibrium structures, characteristic of alloy coatings (multicomponent), are formed as the result of new limits of stability of the particular phase which differ from those in conditions of equilibrium In most cases there occurs the extension of ranges of existence of solid solutions, i.e... in need conditions (in electro-chemical processes and in PVD); - adhesive - consisting of the utilization of adhesion of the coating to a well-cleaned substrate (e.g., in paint, electroplated and crystallization coatings); adhesion may be utilized in cold or hot bonding of coatings to substrates, combined with significant deformations; often adhesion follows the formation of a proadhesive compound (intermediate... for example, such a compound may be FeTiO3; - diffusion - consisting of mutual displacement, through diffusion of components of the coating and the substrate This occurs very seldom Examples are some immersion and crystallizing coatings; - mechanical - consisting of creation of conditions in the substrate (e.g., by dovetail knurling) for mechanical anchoring of coating material This version occurs in... combinations of the above types of bonding are also possible, e.g., adhesion-diffusion or mechanical-adhesive In all cases, the condition essential for good adherence of coatings to their substrates is high purity of the substrate surface prior to deposition of coating Surface purity is understood here not only as the absence of greases, dust and other contaminants, but also as the removal from the substrate of. .. Ranges of thickness and hardness of some coatings (orientation) The following categories of coating thicknesses are distinguished: – point - measured only once in one point location, © 1999 by CRC Press LLC – local - measured usually as an average of several readings at a site of small dimensions (several to several tens of millimeters), – mean - which is the arithmetical average of results of point... mainly surface energy, – relating to radiation, mainly reflection and emissivity, and significantly less often (only for selected types of coatings): radiation transmittance, – catalytic (dependent on degree of surface development and coating components which accelerate or retard chemical reactions), – thermophysical, mainly thermal conductivity and solderability 6.4.3 Physico-chemical parameters of coatings . into a new class of anti-corrosive paint materials - both solvent and powder - which constitute mixtures of resin vapors and certain solvents which cause self-stratification of the resins solderability of joined surfaces are deposited by electroplating. These are tin and copper coatings, as well as alloys: tin- zinc or zinc-lead, less often cadmium and twin-layered cadmium-tin and copper-tin,. the rate of immersion, viscosity of the material and conditions of drip-drying [46]. Phoretic immersion coatings are applied with the utilization of phoresis, i.e. movement of particles of a dispersed