The Effect of Corrosion Inhibitors on the Corrosion Fatigue of Aircraft Aluminum Alloys

The Effect of Corrosion Inhibitors on the Corrosion Fatigue of Aircraft Aluminum Alloys

Sarah E. Galyon Dorman

SAFE, Inc

3290 Hamal Cir

Monument, CO 80132

United States

Scott A. Fawaz

SAFE, Inc

3290 Hamal Cir

Monument, CO 80132

United States

ABSTRACT

Corrosion fatigue is an area of concern for the United States Air Force (USAF) and other Department of Defense organizations. Often the USAF corrosion prevention systems include chromate containing coatings, typically in the form of chromate conversion coatings and primers. Chromate has been used successfully for many years on USAF aircraft to prevent corrosion damage. However the environmental and personnel risks associated with chromate coatings have caused the USAF to pursue non-chromate containing corrosion prevention coatings1. To fully quantify chromate replacement coatings an understanding of the effects that chromate has on corrosion fatigue must be fully documented and understood. Some researchers have shown that high levels of inhibitors (chromate; molybdate) added to full immersion corrosion fatigue tests on 7xxx series aluminum alloys slow the fatigue crack growth rate substantially 2-5. The limitation of this research was that the amount of inhibitor present in the environment was not related to leach rates of chromate from polymeric coatings.

The majority of USAF aircraft are protected from corrosion by polymer coatings loaded with corrosion inhibitors; for these inhibitors to slow fatigue crack propagation the corrosion inhibitors must become mobile from hydration of the polymer coating matrix. Based on this mechanism of corrosion inhibitor release, the examination of atmospheric corrosion fatigue becomes important to help understand how inhibitors work in real world situations with hydrated salt layers rather than fully immersed solutions.

Work has been completed by SAFE Inc., at the USAF Academy’s Center for Aircraft Structural Life Extension (CAStLE) examining the ability of corrosion inhibitors leached from coatings to protect against corrosion fatigue damage. Based on a literature review of inhibitor leach rates and leaching tests, three test protocols were designed using a single edge notch (SEN) fatigue sample to mimic the geometries present in aircraft. These test methodologies include a bulk solution test (500 mL), which would represent areas in an aircraft where large amounts of fluid accumulate, an occluded geometry test cell (0.1 mL) to mimic areas like lap-joints and other small spaces that can collect electrolyte, and a

thin film test method using a relative humidity controlled salt layer that would simulate the boldly exposed surface. Once this baseline database using the SEN samples has been produced, the three test geometries will be transitioned to the examination of the effect of chromate and other inhibitors on the pit-to-crack transition.

Key words: chromate, corrosion fatigue, AA7075-T651, environmentally induced cracking, fatigue inhibition, molybdate

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