Observations on the Testing of Mg-Rich Primers for the Totally Chromate-free Corrosion Protection of Aerospace Alloys

The qualification testing of the Mg-rich primer (MgRP) technology, developed and patented at NDSU and licensed by ANAC for use in aircraft is reviewed and described in detail.  From its early stages of development through to the present, testing of this technology has always indicated that MgRPs when topcoated with a standard aerospace topcoat give equal or superior corrosion protection to an equivalent chromate-based primer.  An additional strong feature of this technology was that it could be successfully used without any  chromate pretreatment (eg. Alodine™ 1200 or equivalent) so that it is uniquely a totally Cr-free corrosion prevention technology,  It was noted early in the course of the development of this technology that MgRPs cannot be handled and interpreted in the same manner as a the chromate primer it was designed to replace.  These two dissimilar primer technologies provide corrosion protection by entirely different modes, and must thus their performances must be judged differently and in the context of the composition and mode of corrosion protection of the final coating coating systems. The initial mode of corrosion prevention by MgRPs is cathodic protection as documented extensively by work at NDSU.   The later stages of the exposure lifetimes of MgRP’s in various accelerated exposure environments vary, but it appears that formation, dissolution and regrowth of possible thin films of Mg oxidation products in the specific exposure environments used in the testing also contributes to the corrosion protection provided by the MgRP’s. The unique aspects of MgRP protection and the behavior that one can expect a priori from such coating systems are discussed.  Observations from the authors’ laboratories are presented as well as a review of the testing of this technology by other laboratories.