In order to obtain protective decorative coating of an appropriate quality and good adhesion, one should clean the metal and base alloy surface precisely by removing oil, grease and corrosion products. The remaining grease, even small quantities of it cause exfoliation of the applied metal coating and corrosion products; not precisely removed may create corrosion focus under the applied coating. The basis surface procession before galvanic coatings application is based on the removal of mechanical impurities from it such as oil, greases, grease-like and derivative substances, corrosion products and other impurities gathered during the part production process.
Mechanical impurities are residues from mechanical treatment of an object (milling, grinding etc) and are mostly relatively easy to remove as their adhesion is conditioned by the grease viscosity, which they are usually moistened with. The exception is the presence of graphite from the drawing operation, which sticked in the cracks and crevices is difficult to remove both with the use of mechanical and chemical methods. During production process objects are stained intentionally with grease (oils and grease which enable to carry out certain technological operations, with preservation greases etc.) or unintentionally (through the contact with greased objects).
Corrosion products- surface layers being the result of the basis oxidation in the manufacturing process(scale), or the result of air or humidity interaction on it (corrosion, metal oxides, alkaline salts as a result of secondary reactions etc). Other impurities generally defined as other include: dust, dirt, sand, salts being the result of an influence on the perspiration base, residue of an abrasive from preceding processes and many others.
The above mentioned impurities are removed:/p>
- Mechanically(abrasion)
- With a physical method(dissolution, emulsification)
- With a chemical method(processing in molten salts, fat saponification, the dissolution of corrosion products in acids)
- With an electrochemical method and with a simultaneous mechanic interaction of gases emitting on the object's surface as a result of secondary reactions on electrolyte)
- With a physicochemical method( e.g. treatment in an appropriate environment with the use of ultrasounds of frequency above 20 kiloHertz (20 kHz).
Parts subjected to galvanic treatment should be completely finished with regard to mechanics, hence they must have adequate sizes within tolerance limits, required surface and edge smoothness degree as well as an adequate catch preventing accidental fall of the hanger or creation of a loose, variable contact through the hanger with the current source ( does not concern small fragments which are mass-processed)
chromium plating
Chromium is a silver-gray metal of a bluish tint. It does not change its colour in the atmosphere. Chromium is resistant to organic acids activity; neither nitric acid nor sulpuretted hydrogen have an impact on it. Only hot sulphuric acid and hydrochloric acid (even in a room temperature)dissolve the chromium coating. Hardness of chromium layers obtained in adequate conditions is bigger than the hardness of one of the hardest grades of hardened steel. Chromium decorative coatings are usually applied on the lower layer of the copper and/or nickel but not directly on steel as in galvanic cells, which are created in uncovered places(pores, leaks), iron compared to chromium is the anode. Methods of application of thin chromium layer (around l ľm) on the copper-nickel lower layer are generally used for example during decorative and protective coating of car, motorcycle, bike parts, in a railway and tram carriage industry, during the construction of measurement equipment(especially for chemical laboratories) and precision instruments (e.g. watches/clocks).
Implementation of chromium coatings
Wide implementation
Technical chromium plating is widely used. Thicker chromium layers, average of 0,005 - 0,2 mm are applied for this purpose and in some special cases to 1 mm directly on steel for the improvement of mechanic properties and the corrosion resistance of chromium parts. Chromium plating of pressure casting dies and moulds gives the best results especially in the rubber, plastic and leather objects manufacturing processes. Smoothness, chemical resistance and a small friction coefficient of chromium layers constitute a great advantage in this case, which together with the great hardness have an influence on the form durability and beautiful appearance of manufactured parts. Technical chromium plating has found application in machine construction, aviation and transport to protect the rubbing surfaces against mechanical destruction. Cylinders of combustion engines, piston rings, rods of hydraulic jacks etc are submitted to chromium plating, thereby yielding good results. Technical chromium plating has also found wide application in the tool industry. New tools destined predominantly for metal, non-iron alloys and soft steel treatment are plated with chromium of coating thickness, which exceeds slightly the tool wear tolerance. Other chromium coating properties are also beneficial, namely propensity for little adhesion and because of this adhesion of shavings to the operating surfaces of tools is lowered to minimum. Chromium plating has found application in calibration production as it increases their durability even a hundredfold in this way.
Decorative/protective chromium plating is widely used in practice. Thin layer of coating applied for a gloss nickel coating, remaining constant in normal atmospheric condition, maintains an aesthetic appearance of a chromium plated object for many years. Moreover, the great hardness of chromium coating protects this object against mechanical cracks.
It has been confirmed in recent years that chromium coating apart from mechanical protection also has a considerable influence on the corrosion resistance of the whole Zn-Ni-Cr triple-layer protective coating. Hence, the strong development of technology enabling application of chromium coatings of specific structure such as micro crack or micro-pore coatings has been marked in recent years. Thin chromium layer of 1,0-2,0 ľm thick is commonly used nowadays as the top layer of protective decorative coatings in many branches of industry.
zinc plating
On account on the zinc location in an electrochemical sequence, zinc coatings electrolytically-applied on cast iron and steel show anode coatings character. They protect the basis through its anode dissolution in corrosion cell Zn| water solution|Fe. At the same time fairly hermetic corrosion products layers and widely used thin layers produced on the zinc surface by chromates water solutions or Chromic acid constitute an additional protection.
According to Lyons, zinc coatings compared to other metal, show the best protective properties on iron and steel with regard to thickness as well as sedimentation costs. The coatings protective value is proportional to their thickness. In comparison to cadmium coatings, Zinc shows worse protective properties only in a typical seaside environment or of great air humidity.
Application of Zinc coatings
Galvanic zinc coatings are widely used for the protection of cast iron and steel against corrosion.
Particularly great amounts of zinc are used in an industry for sheet-metal covering as well as tapes, steel wires and small fragments. In adequate baths and with the use of an appropriate technological process, springy products can be galvanically zinc-plated with no fear of hydrogen embrittlement occurrence. However, zinc cannot be used for the protection of food containers on account of the toxicity of its salts. The more and more wide use of zinc coating with the glossiness in industry has been observed in recent years. Shiny zinc coatings covered additionally with a bluish chromate thin layer can in some applications, with regard to similar colour, replace decorative coatings Cu-Ni-Cr, especially when the gloss durability is not required.
tin plating
Despite wide application of tin plating with the fire-gilding method reaching back to the Roman Empire, electrolytic tin plating found industrial application only in last 40 years. In the course of time the tin process was completely mastered both from aicidic and alkaline solutions. During the World War II galvanic tin plating was gradually replacing the fire-gilding method satisfying around 50% of the general demand for tinned sheet. The advantage of an electrolytic method is based on a steady coating disposition and connected with it smaller usage of tin for 1 square meter of zinc coated metal sheet. Another advantage of an electrolytic method is the possibility of obtaining different thicknesses of the coating on both sides of the metal sheet. Galvanic coating is especially economical when tin plating of small parts having on its surface small threads and slots, which during the tin plating with the fire-gilding method are subjected to a leveling and pouring, at the same time preventing next mounting.
The creation of wiskers- hairy crystals is natural in not yet defined conditions. They may be created spontaneously on the tin-plated surface during storage. As these are single crystals they are a good conductor of electricity, may cause disruption in electronic circuits. The darkening of tin coatings during storage can also be noticed. This phenomenon occurs on tin coatings applied on brass parts. Diffusion of zinc atoms from the base to the tin coating is the reason of its darkening. Coatings are becoming grey, loose its aesthetic appearance and its ability to solder. In order to prevent zinc diffusion in case of brass parts tin plating, nickel or copper lower layers of around 2,5 žm thick should be applied.
Tin coatings application
Tin coatings have found wide application in food industry as a tinned sheet for cans as a coating of kitchen utensils. In electrotechnical industry tin plating of copper wires for the protection against sulfur activity during rubber coating processes and enabling the installation in radio and teletechnical industry. Tin plating similarly to copper plating can be used as a partial protection of the surface against nitriding. Galvanic tin plating on a copper or brass basis is also used as a protection of precision small-scale springs against corrosion without significant worsening of their mechanical properties. The thin tin coating or the lead-tin alloys applied on bearing bushings of lead-tin alloys improves the quality of bashings by obtaining lower friction coefficient, better wettability and the increase of corrosion resistance to greases in increased temperature. Very thin tin coatings up to ( l µm) are also used as a base under organic coatings.
cadmium plating
Cadmium coating application
Cadmium coatings are used mainly for the protection against corrosion of steel products. Aesthetic appearance of cadmium coatings also has decorative meaning. Approximated cadmium and iron potential values decide about high anticorrosion qualities of cadmium coatings applied on steel. Comparing protective properties of zinc and cadmium coatings on steel on the basis of the results of many years of corrosion studies in natural atmospheres of different types, it was stated that cadmium coatings have better corrosion resistance only in atmospheres, which are slightly polluted and humid, for e.g. in tropical and seaside zones. However, cadmium resistance to corrosion considerably lowers in industrial and municipal environments in which it clearly loses with zinc resistance. The above phenomenon is explained by the cadmium small resistance to sulphur compounds activity (particularly sulphur dioxide) which are created as a result of the burning of liquid and solid fuels. It is widely known, that the degree of atmosphere pollution with sulphur compounds is rapidly increasing in due measure of industrial growth and constantly developing motorization, particularly in large urban centers.
In some branches of industry, particularly in electrotechnics and electronics, application of cadmium coatings results from specific properties of processed elements. Thanks to great profundity of cyanide baths, cadmium plating is recommended for the coating of elements which have complex shapes such as rotors and stators of rotary condensers, embossed objects of large cavities etc. Good soldering abilities of cadmium enable in some cases replacement of silver coatings with cadmium ones for e.g. on solder terminals providing that the larger resistivity of cadmium than of silver will not worsen the quality of the equipment operation.
Cadmium is more often used for the protection of strength steel than zinc as galvanic cadmium plating causes smaller hydrogen charging of steel than zinc plating.
In some cases cadmium is applied on elements of copper alloys, stainless steel and on titanium to avoid contact corrosion occurring during contact of those metals with aluminium, aluminium alloys and magnesium alloys. Hence, the spread of cadmium process in aviation industry results from.
copper plating
Copper coatings application
Electrolytic copper coatings are applied both as a protection and decoration, mostly as one of the layers of multi-layered coating copper-nickel-chromium- as well as for technical purposes -as protective layers against carborizing and nitriding, for the production of printed circuits, as coatings of printing rollers in Poligraphic industry and in Galvanoplastics etc.
nickel plating
Nickel coatings application
Electrolytically applied nickel coatings have different destination- first of all they are used as decorative, decorative-protective and even sometimes as technical layers. Besides, electrochemical deposition of nickel is used in Galvanoplastics.
silver plating
Silver coatings application
Silver coatings found wide application as decorative protective coatings, mainly for jewellery products and table cutleries and as protective and technical coatings in electrotechnics and electronics.
Silver coatings application instead of silver in the production of electric plugs and a high-frequency current wires, considerably lowered silver usage.
Great ray-reflecting capacity from the silver surface was used for reflectors and mirrors coating and chemical and corrosion resistance in the construction of chemical equipment.
What is more, silver is used for technical purposes for e.g. for the application of very thick coatings on special types of bearings. |
