Effect of Chromate on Corrosion Fatigue in Service Relevant Concentrations
Sarah E. Galyon Dorman1 Saravanan Arunachalam1 and Scott A. Fawaz1
Abstract: The corrosion protection chromate provides to metallic structures has been well documented. To fully quantify chromate replacement coatings an understanding of the effects that chromate has on corrosion fatigue must be fully documented and understood. Researchers have shown that high levels of inhibitors (chromate, molybdate) added to full immersion corrosion fatigue tests on aluminum alloys slow the fatigue crack growth rate substantially. 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 used on commercial and military aircraft. For these inhibitors to slow fatigue crack propagation the inhibitors must become mobile from the polymer coating matrix.
In research funded by the Office of Naval Research (ONR) and the Office of the Secretary of Defense’s Office of Corrosion Policy and Oversight (OSD-CPO) work is ongoing to develop a better understanding how to better predict and prevent environmental effects on fatigue crack propagation. In this work it has been shown that chromate in levels related to coating leaching effects (SrCrO4 = 0.05mM) can inhibit corrosion fatigue damage in full immersion sodium chloride (NaCl) solution at two different stress ratios (R), 0.65 and 0.02.
Investigations were also completed into manufacturing considerations for environmental fatigue samples and the effect of load introduction location to sample width for single edge notch samples.