This chromate conversion coating is a type of coating used to passivate aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys.  It is primarily used as a corrosion inhibitor, primer, decorative finish and it can retain electrical conductivity.  The process is named after the chromate found in the chromic acid used in the bath, more commonly known as trivalent chromium.  Trivalent chromate is an iridescent conversion that can exhibit different hues. This is considered normal, when applying iridescents.