As new generation aluminum alloys become available, improved accelerated corrosion tests must be designed to accurately predict the corrosion response under atmospheric conditions in a short time. Many different approaches can be used to accelerate corrosion (high testing temperature, high chloride content in the testing solution, added oxidizing agents, very acidic or very alkaline testing solution etc.), and each of these parameters alters the corrosion response by changing electrochemical kinetics. Improved accelerated test protocol requires a better understanding of the relationship between testing parameters, electrochemical kinetics, and corrosion morphology.
The purpose of the current study is to develop a framework for new accelerated corrosion test design by connecting attack morphology to electrochemical kinetics. Both the exfoliation susceptible under-aged (T36) temper and the exfoliation resistant near peak-aged (T86) temper of Al-Cu-Li alloy 2060 were considered in this work. ASTM G343, ANCIT2, and ASTM G85-A25 were used to study the impact of various testing parameters on the corrosion response of AA2060. Electrochemical measurements were used to determine corrosion potential (Ecorr) and polarization resistance (Rp) during standard and modified versions of each test in-situ. After each exposure, samples were also cross-sectioned and examined with optical microscopy. Results indicate that for immersion tests (ASTM G34 and ANCIT), testing temperature has a significant impact on anodic kinetics, while solution pH affects cathodic kinetics. The higher testing temperature of ANCIT led to faster anodic kinetics and faster formation of exfoliation in the T36 temper compared to ASTM G34. For salt spray testing (ASTM G85-A2), it was found that the lower bound of relative humidity (RH) during RH cycling had an impact on Rp as well as extent of attack. When the test was run with a lower bound of 60% RH, Rp remained relatively constant throughout the salt spray, dry air purge, and dwell portions of the cycle. However when the test was run with a lower bound of 20% RH, Rp increased (corresponds to a decrease in corrosion rate) during the dry air purge and dwell periods. This time at very low RH resulted in much slower development of exfoliation on the T36 temper compared to the test with a lower RH bound of 60%.
This work is supported by the Office of the Undersecretary of Defense Corrosion University Pilot Program under the direction of Dr. D. Dunmire, Rolls-Royce, and the Virginia Space Grant Consortium.
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