MECHANISMS OF INHIBITIVE CHROMATE PRIMERS.

This program continued the studies of chromate primer corrosion inhibition mechanisms and investigated other materials as inhibitive pigments. It was determined that failure occurred by destruction of the protective aluminum oxide layer at cathodic areas by hydrogen evolution. Chromate primers prevent hydrogen evolution by precipitation of trivalent chromium compounds. Chromates do not inhibit corrosion at anodic sites. Damaging effects of hydrogen evolution might be avoided by using an electronically conductive paint. Buffering action of zinc and cadmium salts may be beneficial to alkali-sensitive alkyds. Epoxy primers were found to be less prone to embrittlement and cracking than alkyds upon exposure to oxidative degradation. A number of materials were investigated for use in protective primers. A cadmium phosphate-chromate gave better protection at crevice areas than did standard chromates. Tungstate pigments were superior to molybdates, but were inferior to chromates. Conductive graphite pigments were superior to aluminum or stainless steel, but are not suitable for use with conventional paint systems. Author