Sherardizing

Sherardizing is a diffusion coating process that protects iron or steel parts from corrosion by tumbling them in a sealed, rotating drum together with zinc dust and an inert filler at a temperature high enough for zinc vapor to diffuse into the metal's surface, forming a tightly bonded layer of zinc-iron alloy rather than the separate coating of pure zinc that hot-dip galvanizing produces. The English metallurgist Sherard Cowper-Coles developed and patented the process around 1900, naming it after himself, as an alternative to hot-dip galvanizing particularly suited to small, intricately shaped parts such as nuts, bolts and fittings that a molten zinc bath could coat unevenly. Because the resulting coating has no minimum thickness limit imposed by surface tension the way a dipped coating does, sherardizing remains used today wherever small fasteners and hardware need corrosion protection without altering their close dimensional tolerances.