When coating fasteners (bolts, nuts, washers, studs, screws, etc.), metal and non-metallic types of coatings are used. The latter, in turn, are divided into single-layer and multi-layer. Multilayer coatings are more popular and outperform single-layer coatings in terms of protective and decorative qualities, as the degree of protection is characterized by lower coating porosity.
Methods of coating fasteners are:
- CHEMICAL – (metallic and non-metallic) – metal is immersed in a salt solution (phosphotation, chromatization, reduction,
nickel plating, etc.) - ELECTROCHEMICAL – this method is characterized by immersion of fasteners in dissolved salts with an electrolyte (anode,
electrochemical galvanizing, cadmium plating). As a result, the fasteners have a coating thickness of 6 to 16 microns.
On average, the coating is 9 microns thick. Chromatization is performed to improve the appearance. - In electroplating, the product to be coated is the cathode. This method is used for nickel plating and
chromatization. - Hot-dip galvanizing is performed by immersing the part in a hot zinc solution. Therefore, the thickness of the coating has
average value of 40-50 microns. The maximum thickness can reach 120 microns. By processing fasteners
in a centrifuge to remove the excess zinc layer.
This method requires a preliminary special rolling (cutting) of the thread with a tolerance field different from the standard
7N (for nuts) and 8G (for bolts and studs), which will help to maintain the threaded connection even after such a thick layer of zinc.
Hot-dip galvanizing is considered to be one of the most reliable, cost-effective and therefore widespread methods of protecting iron and steel from corrosion. - Thermal diffusion galvanizing is characterized by the exchange of the product material with the protective coating composition itself at the molecular level.