Examination and prediction of corrosion fatigue damage and inhibition

The United States Department of Defense (DoD) estimated that the annual cost of corrosion, the unintended material degradation due to the environment, to weapon systems and infrastructure in 2010 exceeded $21 billion, and that the number was likely to continue to rise. Unfortunately, as the warfighters demand more from their systems, corrosion prevention and control is frequently traded during the acquisition cycle for weapon system performance. As a result, the DoD remains entrenched in a find-and-fix corrosion management philosophy which is expensive and unsustainable. Better standardized fatigue laboratory procedures are need to help the DoD develop (1) a fundamental understanding of corrosion damage, (2) material performance data relevant to corrosion damage, (3) prediction methodologies to help mitigate the effects of corrosion nucleated fatigue damage and (4) to develop an understanding of how corrosion preventative coatings can slow fatigue crack propagation. All of these research area lead to developing better damage prediction methods which can account for the effect of environment on fatigue crack propagation. Aircraft structural tear-down analysis provides substantial evidence of the importance of corrosion in nucleating fatigue cracks, as such the ability to protect against such damage would be of great benefit to the DoD. While chromate and other inhibitors have been shown to slow fatigue crack growth rates when added to a bulk solution of a fully immersed sample, research has not been completed showing how these inhibitors effect crack nucleation from corrosion damage. 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 being completed to develop a better understanding how to better predict and prevent environmental effects on fatigue crack propagation. While chromate has been used successfully for many years on United States Air Force (USAF) and Navy (USN) aircraft to prevent corrosion damage, the environmental and personnel risks associated with chromate have caused the DoD to pursue non-chromate containing corrosion prevention systems. 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 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. For these inhibitors to slow fatigue crack propagation the inhibitors must become mobile from the polymer coating matrix. Based on this mechanism of corrosion inhibitor release, the examination of atmospheric corrosion fatigue, hydrated salt layers which better represent operating environments compared to full immersion testing, becomes important to help quantify how inhibitors affect fatigue damage in service. Another focus area of the ONR funded research is in quantifying the corrosion damage to fatigue crack transition. A standardized specimen and testing protocol to evaluate the relative influence of material, environment, inhibitors, and loading spectrum on the pit-to-crack transition was developed. The methodology uses a narrow plate specimen with a centrally located hole with a preferential pit (diameter approximately 150 µm) placed at the corner of the hole; current work is being completed on legacy aluminum alloy AA7075-T651. The plate thickness and hole diameter are consistent with commercial and military airframe applications. Current DoD engineering approaches to manage corrosion-fatigue, are incomplete in terms of the effect of service environment. Poor component life prediction results in an increased flight line inspection and maintenance burden along with premature component retirement. The overarching objective of the research is to improve and transition the results on the effect of environmentally assisted fatigue in high performance metallic alloys (crack growth rate data) to the DoD research and depot maintenance activities by integrating all data into the AFGROW fracture toughness and fatigue crack prediction software allowing for the inclusion of corrosion damage and environment effects on fatigue crack life predictions