Current Progress in the Development of a Complete Cr-Free System

There are currently no chromium-free replacements for traditional hexavalent chromium pretreatments that perform as well as chrome unpainted and that are commercially available for the corrosion protection of 2000 series aluminum alloys used in aerospace.

Two approaches have been used to provide the military with a chrome (VI) alternative.  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 the corrosion protection mechanisms learned from Deft’s 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.  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.  The proposed mechanism of the new conversion coatings is reviewed using 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.