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Understanding the Discrepancies in Wet Bottom and Dry Bottom MASTMAASIS Testing Results of AA2060

Picture for Understanding the Discrepancies in Wet Bottom and Dry Bottom MASTMAASIS Testing Results of AA2060
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In the current study, in-situ measurements were used to deconstruct the testing environment of dry-bottom (DB) ASTM G85-A2 to provide an understanding of what makes this test successful when others are not.

Product Number: 51317--9171-SG
ISBN: 9171 2017 CP
Author: Mary Parker
Publication Date: 2017
Industry: Science of Corrosion
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Aluminum-lithium (Al-Li) alloys are attractive for aerospace applications because of their improved strength-to-weight and stiffness-to-weight ratios fracture toughness and corrosion resistance compared to legacy alloys. These alloys can suffer from various forms of localized corrosion when exposed to corrosive atmospheric conditions during service. Localized corrosion susceptibility and attack rates are difficult to predict so accelerated laboratory testing is used for lot acceptance new alloy and temper development and material lifetime predictions. Many standardized accelerated tests are currently used for high-strength aluminum alloys (ASTM standards G34 G85 G110 and B117) but these tests can produce drastically different results for the same alloy. This disagreement among tests which are all aimed at assessing localized corrosion resistance limits their utility and hampers alloy and temper development. Understanding the testing variables that control localized corrosion development is a goal of this work.One test of interest is ASTM G85 Annex 2 also known as MASTMAASIS. This cyclic acidified salt spray test has two variations: dry bottom (DB) and wet bottom (WB). The single difference between the variations is that during WB testing several inches of water are maintained on the bottom of the salt spray chamber but during DB testing the chamber bottom is dry. Although this difference seems trivial it has a drastic impact on testing results. Testing of Al-Li alloys has shown that DB MASTMAASIS correctly distinguishes between exfoliation susceptible and resistant tempers but WB MASTMAASIS does not. In the current work potential relative humidity and time of wetness measurements were made during both variations of MASTMAASIS with the goal of explaining the discrepancies in testing results.

Key words: ASTM G85-A2, Aluminum Lithium Alloy 2060, Exfoliation, Salt Spray Testing

Aluminum-lithium (Al-Li) alloys are attractive for aerospace applications because of their improved strength-to-weight and stiffness-to-weight ratios fracture toughness and corrosion resistance compared to legacy alloys. These alloys can suffer from various forms of localized corrosion when exposed to corrosive atmospheric conditions during service. Localized corrosion susceptibility and attack rates are difficult to predict so accelerated laboratory testing is used for lot acceptance new alloy and temper development and material lifetime predictions. Many standardized accelerated tests are currently used for high-strength aluminum alloys (ASTM standards G34 G85 G110 and B117) but these tests can produce drastically different results for the same alloy. This disagreement among tests which are all aimed at assessing localized corrosion resistance limits their utility and hampers alloy and temper development. Understanding the testing variables that control localized corrosion development is a goal of this work.One test of interest is ASTM G85 Annex 2 also known as MASTMAASIS. This cyclic acidified salt spray test has two variations: dry bottom (DB) and wet bottom (WB). The single difference between the variations is that during WB testing several inches of water are maintained on the bottom of the salt spray chamber but during DB testing the chamber bottom is dry. Although this difference seems trivial it has a drastic impact on testing results. Testing of Al-Li alloys has shown that DB MASTMAASIS correctly distinguishes between exfoliation susceptible and resistant tempers but WB MASTMAASIS does not. In the current work potential relative humidity and time of wetness measurements were made during both variations of MASTMAASIS with the goal of explaining the discrepancies in testing results.

Key words: ASTM G85-A2, Aluminum Lithium Alloy 2060, Exfoliation, Salt Spray Testing

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