Hot Diffusion Processes

There are a number of processes in use on a limited scale in which steel articles are coated with another metal by being heated in contact with the metal or one of its compound in the absence of air.

1. Colorizing: This process is a means of obtaining on aluminum coating on heavy steel vessels and boxes, furnace parts and other pieces which are used in heat treatment or firing of ceramic goods, or exposed in some other way continually to high temperatures in air. The aluminum coating prevents the steel scaling and wasting away and thus materially prolongs its life. The process of colorizing is carried out by prolonged heating of the steel articles at about 500^oC in a box with a mixture of aluminum powder, alumina and ammonium chloride, air being excluded. If probably functions via the formation of aluminum chloride vapor and the reaction of this with the iron in the surface layers.
2. Shrerardizing: This is a means of zinc coating small steel articles such as nuts and bolts. The coating is extremely uniform and thus, for example, the functioning of screw thread is not impaired. The articles are tumbled of some hours in zinc dust at a temperature of about 370^oC, air being excluded. The coating contains a high proportion of iron-zinc alloy; it is dark-blue in color and brittle. A normal thickness is 0.5 0z/ft² (0.0005 in).
3. Chromizing: The chlorides of a number of metals (and some non-metals) are volatile and will react with steel at elevated temperatures to form iron chloride and deposit the metal in the place of the iron removed. Thus steel articles can be chromized by heating in chromic chloride vapor. An iron-chromium alloy is formed on the surface, which has corrosion-resisting properties similar to those of stainless steel; if carbon is present, the process may be used to effect a surface hardening. The process can also be used to confer resistance to heat oxidation onto steel articles.