Two approaches have been used to provide the military with chromium (VI) alternatives.  One approach is the use of a chromium (III) based pretreatment, however, the process still contains chromium, and toxic forms of cobalt are sometimes used and present in waste streams. The other option is a system’s approach where the corrosion resistance of the conversion coatings are evaluated not as stand alone coatings, but rather as a complete paint coating system.  One of several issues arising from this approach is that most of the commercially viable pretreatments are not conversion coatings, but rather adhesion promoters that do not provide inherent corrosion protection if the outer paint coatings are breached.

Based on current corrosion protection mechanisms learned from successful rare earth-based primers, a pretreatment chemistry is being developed that is designed to provide inherent corrosion protection for Al 2024-T3 and have improved interaction with the chrome-free primers to provide a higher state of the art chrome-free pretreatment and system.  The design includes conversion coating composition, which in turn is based on similar chemistries as the chrome-free primer.  No chromium is present in the proposed pretreatment or primers used in the coating system.   

Salt spray performance data of the conversion coating by itself and in conjunction with a chrome-free primer will be illustrated.  Some of the key developmental screening methods are reviewed using simple wet chemistry experiments, as well as SEM characterization of the conversion coating’s surface morphology and interpretation of salt spray corrosion resistance.  An update of the current chrome-free primer technology and continued implementation is also provided.