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Sherardising or Zinc thermal diffusion is a process of galvanization of ferrous metal surfaces, also called vapour galvanising and dry galvanizing. The process is named after British metallurgist Sherard Osborn Cowper-Coles (son of naval inventor Cowper Phipps Coles) who invented and patented the method c. 1900. (2014). 9780857096524, Elsevier Science. ISBN 9780857096524 This process involves heating the steel parts up to 500 °C in a closed rotating drum that contains metallic zinc dust and possibly an inert filler, such as sand.H. G. Arlt, "Finishes on the Metal Parts of Telephone Apparatus", Bell Laboratories Record, Volume 9(4), 175 (December 1932) At temperatures above 300 °C, zinc evaporates and diffuses into the steel substrate forming diffusion bonded Zn-Fe-phases.
Sherardising is ideal for small parts and parts that require coating of inner surfaces, such as batches of small items. Part size is limited by drum size. It is reported that pipes up to 6 m in length for the oil industry are sherardised. If the metal surface is free of scale or oxides, no pretreatment is needed. The process is hydrogen-free, hence hydrogen embrittlement is prevented.
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