Ian, it would be BZP (bright zinc plate) and passivate (clear or yellow to look like cadmium plate) and the zinc is electro plate if it is big and complex shape or mechanical if it is suitable – the mechanical being the preference for stressed small parts.
Mark
Also, black zinc has become a popular in the auto industry for decorative finish and the cold alternative to sheradise finish is phosphate (dry or with oil) which can also be done on a small scale if you can get the chemistry.
Edited By Mark C on 21/05/2014 14:58:41