Thermodiffusion zinc coating is anodic in relation to ferrous metals and protects steel from corrosion electrochemically. The main advantage of these coatings compared to electroplating is the absence of hydrogen embrittlement.
Thermodiffusion zinc coating is obtained by heating in a container with a diffusion mixture consisting of zinc powder and an inert diluent. The operating temperatures of the coating are 400-700 °C.
Since the coating process is quite long in time, thermodiffusion galvanizing is subject to parts made of materials that do not change their properties at these temperatures. The process temperature can be reduced by using a shallow vacuum ("1.33 Pa).
The resulting coating exactly follows the contours of the product, it is uniform in thickness on the entire part, including products of complex shape (including threaded connections). The coating has a strong adhesion to the base metal due to the mutual diffusion of zinc and iron.
The corrosion resistance of thermodiffusion zinc coating is 3-5 times higher than that of electroplating and 1.5-2 times higher than hot zinc coating when operating in an industrial and marine atmosphere.Data on the corrosion rate of the coating under operating conditions according to GOST 9.303 are given in the table:
according to GOST 9.303
Due to the presence of iron in the coating, when exposed to high humidity or condensation, a brown coating may appear on the surface of the galvanized product. This is due to the release of iron ions from the coating, which are easily washed off by water or rain. The corrosion products of the coating have a brown color.
If it is impossible to distinguish between the corrosion products of the coating and the base metal, the presence of the coating is determined metallographically.