SAE AMS-QQ-C-320

Description / Abstract: Class 1 plating: Class 1 plating is applied as a decorative finish, usually over nickel, or copper and nickel, on basis metals such as iron and steel, copper and copper-base alloys, and zinc and zincbase diecasting where necessary to protect the basis metal from corrosion and wear and to provide a pleasing appearance. The function of the underlayers of nickel is to provide a pore-free continuous underplate for the chromium outer layer. Generally, the thicker the nickel layer, the better the corrosion resistance. The systems of an outer layer of chromium over the combined plated nickel and copper are generally used in a combined total thickness of 0.0001 to 0.002 inch (2.5 to 51 µm) depending upon service conditions and the basis metal.

Chromium platings 0.0005 inch (13 µm) or more in thickness are likely to crack nickel plating on brass basis metal. The minimum thickness of chromium should be obtained under conditions such that the maximum thicknesses are less than 0.00005 inch (1.3 µm).

Class 2 plating: Class 2 plating, also known as "industrial chromium" or "hard chromium", is used for wear resistance, abrasion resistance and such incidental corrosion protection of parts as the specified thickness of the plating may afford. Engineering chromium is usually applied directly to the basis metal and is finished by grinding to the specified dimensions. It lacks the brightness of class 1 plating. Additional corrosion resistance can be obtained by use of an undercoat of electrodeposited nickel in thickness of 0.001 to 0.002 inch (25 to 51 µm) on ferrous parts, the minimum thickness to be determined by service conditions. Heavy deposits of the class 2 plating may be used for buildup of worn or undersized parts, or for salvage purposes, and to provide protection against corrosive chemical environments. Final grinding of the chromium plating can increase the number of cracks in the deposit. For greater corrosion resistance, based upon equal thickness, unground deposits should be selected rather than ground deposits.