Recent efforts have highlighted a potential issue in the currently accepted ASTM G-1 mass loss standard as an effective means for evaluating corrosion damage on aluminum alloy 7075-T6. It was found that the use of the current standard can result in users reporting mass loss values that are inconsistent with the visual corrosion assessment, including mass gain instead of loss on obviously corroded samples. It is hypothesized that the current method of repeated immersion in acidic solution followed by mechanical cleaning is not effective at dislodging corrosion product from the pits formed on a corroded aluminum surface. To address this issue, studies have been completed to evaluate a two-step method for corrosion analysis. First, ultrasonic cleaning of coupons in nitric acid more effectively removes corrosion from the corroded surface. Second, follow-on analysis of the cleaned coupons with both mass loss and optical profilometry demonstrated that there is significant amount of corrosion information which analysis via mass loss alone will not reveal. The present work focuses on the refinement of a new cleaning protocol for aluminum and the use of profilometry to assess coupons corroded in outdoor exposure and accelerated corrosion tests.
Measuring the severity of corrosion on a specific alloy is often accomplished via mass loss using ASTM G-1.1 These processes work well and provide high fidelity data for many materials, especially steels. However, recent internal findings and disclosures from other research groups have highlighted a potential issue with using mass loss techniques to measure the damage on some aluminum alloy surfaces.2 Mass loss analysis of AA 7075-T6 coupons demonstrated extensive visual indicators of corrosion but provided minimal mass loss measurements or even mass gain after applying the techniques outlined in ASTM G-1. Follow-on discussions with the University of Dayton Research Institute identified the following issues with the procedures for aluminum mass loss in ASTM G-1. They are, in no specific order:
1) The use of harsh chemicals, especially boiling phosphoric acid mixed with chromic acid, results in the aggressive removal of non-corroded material.3
2) The mass loss techniques do not specify how many cleaning cycles should be employed. This allows a user to continue cleaning a coupon until mass loss is observed, even if the material loss is caused by the removal of pristine material.
3) The use of mechanical scrubbing or shot-peening does not effectively remove corrosion product from subsurface corrosion commonly seen in aluminum alloys
