Selection of coating

This section contains the extracts from GOST 9.303-84 - advice on selection of coating.

The standard establishes general requirements for selecting of metallic and nonmetallic inorganic coatings of parts and assembly units (parts), which are applied by chemical, electrochemical and hot (tin and its alloys) ways.

The standard does not apply to coating that is used as technological, coating of details of watches and jewelry, exept of the requirements for the maximum thickness of the coating.

It schould be considered when selecting coatings:

  • Function of detail;

  • Function of coating;

  • Operating conditions of covered parts according to GOST 15150-69;

  • Material of detail;

  • Properties of the coating and its effect on the mechanical and other properties of the material of detail;

  • Method of coating and its effect on the mechanical and other properties of the material of detail;

  • Environmentally friendly metal of coating and it’s application process;

  • Admissibility of contact of metal and metal/ nonmetal coatings according to the GOST 9.005-72;

  • Economic feasibility.

MAIN CHARACTERISTICS OF COATINGS

Zinc coating

Zinc coating is anodic regarding ferrous metals and protects the steel against corrosion electrochemically at temperatures up to 70 °C, at higher temperatures - mechanically. Coating prevents contact corrosion of steel in combination with parts made from aluminum and its alloys.

To improve the corrosion resistance zinc coating is additionally coated with crome and phosphate. Chrome coating also improves decorative appearance of coating. Chromate pellicle is mechanically destroyable. Zinc coating with chrome loses its decorative appearance due to periodic mechanical impact: contact with instrument or hands. Without chrome and phosphate coating is used to conduct electricity and for molding by plastics at temperatures above 100 °C.

Electrochemical zinc galvanizing causes loss of plasticity of steels due to hydrogenation. Steels with tensile strength higher than 1380 MPa (140 kg / mm2) not to be galvanized.

Coating has strong adhesion with the base metal, low resistance to mechanical abrasion and increased fragility at temperatures above 250° C and below - 70 °C; matte finish endures bending. Coating has low resistance to chemical products that are released during the aging of organic materials.

Microhardness of coating that is applied by electrochemical method in average makes 490-1180 MPa (50-120 kg / mm2); resistivity at 18 °C makes 5,75⋅10-8 Om⋅m.

Nickel coating

Nickel coating is cathode regarding to steel, aluminum and zinc alloys. Coatings are used for protective, protective-decorative, decorative processing of details, for increasing of hardness of surface, wear resistance and conductivity.

To enhance the decorativeness of coating the chrome (thickness to 1 micron) is applied on a nickel substrate.

The increase of corrosion resistance is achieved by a combination of several layers of nickel coatings with different physical and chemical properties. At a thickness of 24 microns protective properties of two-layer coating (underlayer without copper) twice, and three-layer of filler - three times higher than the protective properties of shiny surfaces.

The resistivity at 18 °C - 7,23⋅10-8 Om⋅m; microhardness brilliant cover - 4420-4900 MPa (450-500 kgf / mm2) napivblyskuchoho - 2940-3930 MPa (300-400 kgf / mm2); coefficient of reflectance of shiny coating - 75%. The acceptable operating temperature - 650 °C.

Coating provides good spreading of solders and receiving of vacuum-tight connections at high-temperature of solder in different environments without the use of flux, as well as arc welding (without copper sublayer). Nickel coating thickness up to 6 mm can be subjected to spot welding.

Coating serves as the barrier layer under the cover of gold, silver, tin-lead alloy and other metals, preventing diffusion of copper, zinc, iron and other metals.

Chrome coating

Chrome coating is cathode regarding steel, aluminum and zinc alloys, provides protection against corrosion and improves decorative appearance.

Protective and decorative coating is applied to the nickel substrate with a thin mirror-shiny layer up to 1 micron. Coating with thickness to 0.5 microns is porous, with increasing of thickness the net of cracks is created.

Applying of chrome coatings on complex profile details is difficult because of the low scattering ability of chrome electrolytes.

To improve the corrosion resistance the details with chrome coating can be additionaly processed (soaking, etc.). When operating in conditions of direct effects of sea water it is recommended periodic oil recovery for additional protection of chrome details.

Microhardness of solid chrome coating - 7350-10780 MPa (750-1100 kgf / mm2).

Copper coating

Copper is the cathode coating to steel, aluminum, magnesium and zinc alloys. Coating is used as a technological layer to reduce porosity and improve adhesion of other coatings. For protection against corrosion copper coating as an independent coating is not recommended due to the low corrosion resistance.

Copper coating has high electrical and thermal conductivity, plasticity, maintains deep drawing, well polished, facilitates attaching, grinding and screwing. With low-temperature solder it forms intermetallic compounds wich worsen solder and strength of solder joints.

Admissible operating temperature for covering - 300 °C; microhardness of covering - 590-1470 MPa (60-150 kg / mm2); resistivity at 18 °C - 1,68⋅10-8 Om⋅m.

Read GOST 9.303-84 all