Third generation Aluminum-lithium (Al-Li) alloys such as AA2060 are attractive for aerospace applications due to their high strength-to-weight ratios and their corrosion resistance during seacoast exposures.¹ However characterizing these alloys in laboratory corrosion tests can be difficult as the results from conventional accelerated tests do not always agree with seacoast results. For example, ASTM G34 is a standard test for exfoliation corrosion, which is a form of localized attack that produces blisters when corrosion products due to intergranular attack produce a wedging pressure and lift un-attacked grains upward. This test was developed for alloys 7075 and 7178,² but is considered applicable to 2xxx and 7xxx series aluminum alloys.³ However when Al-Li alloys 2099 and 2060 are tested in ASTM G34, the trend in exfoliation susceptibility over different aging conditions is opposite to the trend observed after seacoast exposures.⁴ This kind of disagreement between laboratory testing and seacoast exposure slows down the process of optimizing aging practices for new products.

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 (E_corr) and polarization resistance (R_p) 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 R_p as well as extent of attack. When the test was run with a lower bound of 60% RH, R_p 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, R_p 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%.

Acknowledgements

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.

References

1. R.J. Rioja, J. Liu, Metal. Mater. Trans. A 43 (2012): p. 3325-3337
2. S. Lee, B.W. Lifka, “Modification of the EXCO Test Method for Exfoliation Corrosion Susceptibility in 7XXX, 2XXX, and Aluminum-Lithium Alloys,” in New Methods for Corrosion Testing of Aluminum Alloys, eds. V.S. Agarwala, G.M. Ugiansky, STP 1134 (West Conshohocken, PA: ASTM International, 1992), p. 1-19.
3. ASTM G34-1, “Standard Test Method for Exfoliation Corrosion Susceptibility in 2XXX and 7XXX Series Aluminum Alloys” (West Conshohocken, PA: ASTM International, 2001), p. 1-7.
4. J.P. Moran, F.S. Bovard, J.D. Chrzan, P. Vandenburgh, “Corrosion Performance of New Generation Aluminum-Lithium Alloys for Aerospace Applications,” 13th Int. Conf. on Aluminum Alloys, held June 3-7, 2012 (Hoboken, NJ: John Wiley & Sons, Inc., 2012).
5. ASTM G85-A2, “Standard Practice for Modified Salt Spray (Fog) Testing” (West Conshohocken, PA: ASTM International, 2002), p. 1-14.