Effect of chromate primer on corrosion fatigue in aluminum alloy 7075

Abstract

The Center for Aircraft Structural Life Extension (CAStLE) at the United States Air Force (USAF) Academy has undertaken work to produce a standardized test method for determining the effect of a corrosion inhibitor containing chromate on small scale fatigue damage. CAStLE is specifically focusing their research on the effect of chromate primers on the pit-to-crack transition and crack growth under the USAF damage tolerant flaw size (1.5 mm). Quantifying the chromate effect on the small damage scale is necessary to provide a baseline to compare the efficacy of new and safer coatings on fatigue life. To correctly account for these factors, bare and chromate primered test specimens of AA7075-T651 were produced with a center hole and a corrosion pit at the edge of the hole, and subsequently fatigued using representative environments and loading schemes until a thumbnail crack of approximately 1 mm radius was produced. How chromate coatings effect fatigue crack growth is dependent on enough chromate leaching from a coating to reduce the fatigue crack growth rate (FCGR) in the time it takes to grow from a 300 μm pit to a 1 mm fatigue crack. The current results suggest that the amount of chromate required to leach from the primer to affect FCGR may be highly dependent on the amount of chloride present.