sherardizace

Sherardizace, known in English as sherardizing, is a metallurgical surface-treatment process used to apply a zinc-containing diffusion coating to ferrous metals. The coating provides corrosion resistance and improves paint adhesion, and it is particularly well suited for parts with complex geometries where other coating methods struggle to reach internal features.

The process is carried out in a closed rotating container (drum or retort) containing the metal parts, zinc dust, and a small amount of moisture. The container is heated to about 290–320 °C and rotated for several hours. Under these conditions zinc diffusion occurs between the surface of the base metal and the zinc, forming zinc-iron intermetallic layers and an adherent zinc coating. The resulting layer is relatively uniform and can cover intricate shapes, including internal threads and bores. After cooling, the parts may be passivated or painted as part of a finishing sequence.

Materials and coatings: sherardizing is most commonly applied to steel and cast iron. The resulting coating thickness is modest, typically a few to a few tens of micrometers, and is controlled by time and temperature. It is generally more economical for certain geometries and batch sizes than some liquid plating processes, and it eliminates the need for a molten zinc bath. However, it can be slower and may not achieve the same level of protection as hot-dip galvanizing in all environments.

Applications and history: the technique emerged in the late 19th to early 20th century and has been used for fasteners, gears, bearings, and precision components where uniform coverage is advantageous. It is often followed by passivation or painting for enhanced durability.