Corrosion Protection Mechanisms of Rare-Earth Based Coatings on Aluminum Alloys

Conversion coatings and primers containing rare-earth based corrosion inhibitors have proven to be viable alternatives to chromate coatings, which are toxic and carcinogenic.  Rare-earth based inhibitors in conversion coatings and primers have shown that they can meet or exceed the corrosion protection requirements in U.S. military specifications.  However, little is known about the mechanisms by which rare-earth based coatings provide corrosion protection.  To investigate the corrosion protection mechanisms of rare-earth based inhibitors, coatings were examined before and after testing in ASTM B117 salt spray.  Scanning electron microscopy and energy dispersive spectroscopy were used to characterize changes in the surface morphology and composition of the coating during salt spray testing.  To complement the surface studies, a focus ion beam system was used to produce cross sections of the coatings near features of interest such as cracks, corrosion pits, and other morphological features, which allowed for examination of changes through the thickness of the coatings.  In addition, electrochemical testing including polarization curves and electrochemical impedance spectroscopy were used to characterize the response of both pristine coatings and coatings with intentional defects to understand the coating response to corrosive environments.