Coatings, Cadmium, Tin-Cadmium and Zinc (Mechanically Deposited)
|Publication Date:||1 January 2018|
This specification covers the requirements for three types and three classes of mechanically deposited cadmium, tin-cadmium or zinc coatings on metallic surfaces by impacting (peen plating).
The coatings, cadmium, tin-cadmium and zinc shall be of the following types and classes, as specified (see 6.2.1).
Type I - As coated
Type II - With supplementary chromate treatment (see 3.2.9 and 6.1.2)
Type III - With supplementary phosphate treatment (see 3.2.10)
General usage: The mechanically deposited coatings of cadmium, tin-cadmium and zinc covered by this specification are intended for use as protective coatings on ferrous and other basis metal... View More
General usage: The mechanically deposited coatings of cadmium, tin-cadmium and zinc covered by this specification are intended for use as protective coatings on ferrous and other basis metal parts. Mechanically deposited coatings, often referred to as "peen platings", are formed without the use of electric current by impacting soft, ductile metal powders so that they will adhere onto the surface of parts.
Type II treatment: The prime purpose of chromate finishes (Type II) on mechanically deposited coatings is to retard or prevent the formation of white corrosion products on surfaces exposed to stagnant water, high humidity atmospheres, salt water, marine atmospheres or cyclic condensation and drying. Some types of chromate films have proved satisfactory as a base for paints. The surfaces of mechanically deposited coatings, particularly cadmium and zinc, are attacked by supplementary chromate treatment, thereby diminishing the amount of metallic coating present. With coatings of 0.0003 inch (0.3 mil) and greater, this reduction is insignificant, but it is significant with a 0.0002 inch (0.2 mil) thick coating. It is therefore recommended that supplementary chromate treatment not be applied to coatings having a nominal thickness of 0.0002 inch (0.2 mil) or less.
Type III treatment: The prime purpose of phosphate finishes (Type III) on mechanically deposited coatings is to form a paint base.
Application: The application of mechanically deposited coatings is a two part process: surface conditioning and coating. The surface conditioning step is designed to remove contaminations (minor soils), to produce a mildly etched surface (see 3.2.2), and to provide a flash coating which may be copper (see 3.2.4) to make the surface receptive to the subsequent coating operation. The coating operation is carried out in a tumbling barrel containing the part with a water-slurry of spherically shaped, fine metal powder or powders, glass beads as the impact media, and a chemical compound known as the "promoter". The temperature ranges from 18° to 27°C (65° to 80°F) and the coating is done in 15 to 60 minutes. Coating thickness is controlled by the amount of the metal powder materials used.View Less