Chromate passivation as described in this specification will normally be applied to cadmium and zinc e!ectroplating and to zinc base alloy die-castings unless the Design Authority stipulates otherwise.
It reduces corrosion ("white rusting") of cadmium and zinc, in particular under humid conditions, and helps to reduce the attack on both metals by certain vapours from organic substances (see Defence Guide DG-3). Passivation improves the performances of subsequently applied schemes based on conventional primers. but may not be necessary prior to the use of a pretreatment primer.
Alternative processes are not covered by this specification. In particular. some alternative processes operate at higher effective acidity that may cause attack and loss of thickness of zinc and cadmium plating which may be unacceptable on the restricted thicknesses of coating on threaded parts.