Conversion Coating for Aluminum and Magnesium

摘要

A conversion coating refers to the chemical treatment of a metal surface where the metal is converted into a non-metallic form at the surface, forming a film. Conversion coatings have an important role in the metal finishing industry because of their enhanced corrosion protection and decorative appeal. Conversion coatings have been around for many years. Coatings for iron and iron alloys such as black, and bluing, for copper and brass, such as patina Verdi green and many others are well known. Chromate coating was used for lead from 1872. Chromate solutions were used to treat magnesium as early as 1924. Chromate conversion coatings were developed for zinc and cadmium from 1924 to 1936.' Chromates for copper, silver and aluminum followed. The first true chromate for aluminum was introduced by Stricklen in 1952. Later, improved chromate conversion coatings for aluminum were introduced. Chromated aluminum became very important for reasons of corrosion protection, low electrical resistance, improved paint and adhesive bonding. Further, they possess self-healing properties that protect scratches by the inhibiting characteristics of hexavalent and trivalent chromium that migrate into the scratch and maintain the protection. Chromates also have low emis-sivity coefficients, less than those of anodized or painted surfaces. This is important in preventing parts from heating up during exposure to infrared radiation. However for other applications, emissivity can be increased by applying a thicker film. Recently, non-chromium containing conversion coatings were introduced that are improvements over earlier such processes and may rival chromate coatings in some respects.