Laboratory and Computational Study on Galvanic and Local Corrosion of Aluminum Alloy 6061-T6 Coupled to Non-Passivating and Passivating Alloys

Wohner, Natalie Yvonne Danielle

Laboratory and Computational Study on Galvanic and Local Corrosion of Aluminum Alloy 6061-T6 Coupled to Non-Passivating and Passivating Alloys

Files

Wohner_hawii_0085A_11587.pdf (94.58 MB)

Date

2022

Authors

Wohner, Natalie Yvonne Danielle

Advisor

Hihara, Lloyd H.

Department

Mechanical Engineering

Abstract

Corrosion of Aluminum Alloy (AA) 6061-T6 coupled to non-passivating and passivating alloys was studied concerning galvanic effects on local corrosion. In this work, local and galvanic corrosion was quantified, the effects of cathode material on local electrolyte pH were explored, and a relationship between pH and self-corrosion of AA6061-T6 was established. In addition, a finite element thin film model for simulating the galvanic corrosion of Aluminum Alloys based on pH-dependent corrosion kinetics was developed to show trends in corrosion rates for acidic electrolytes. Marine and aerospace structures often combine lightweight aluminum alloys with dissimilar metals to optimize mechanical performance and reduce costs. Unfortunately, the exposure of such dissimilar couples in a harsh environment can cause severe corrosion damage to the aluminum structure due to its position in the galvanic series. Atmospheric field tests are typically performed to estimate the performance of galvanic couples in natural environments; however, these tests are time-consuming and costly. In addition, field tests only provide limited insights into the accelerated, localized corrosion damage to aluminum alloys when coupled to dissimilar metals. Computational modeling offers a complementary approach to studying the corrosion behavior of galvanic couples at a lower cost and an enhanced understanding of localized corrosion. Experimental and numerical studies quantified galvanic corrosion of AA6061-T6 coupled to 316 stainless steel, copper, titanium alloy Ti6Al4V, and 316 stainless steel coated with titanium nitride, chromium nitride, and a sol-gel nano-coating. To validate the thin film model, numerical results were compared with laboratory tests of galvanic couples exposed for 21 days in a controlled environment at 90% relative humidity and 30◦C. Galvanic currents were measured during the exposure time, and the aluminum alloy’s total mass loss was determined to quantify the corrosion damage. Exposure tests showed that galvanic corrosion accounts for less than 15% of the total corrosion of AA6061-T6 and that most aluminum corrosion damage was caused by local corrosion. In addition, immersion experiments of AA6061-T6 galvaniccouples in gelled 3.15 wt.% NaCl solutions showed that galvanic coupling influences the evolution of electrolyte pH leading to severe acidification at the aluminum anode surface and alkalization around the cathode. The solution pH at the aluminum surface was decreased by galvanic action and depended on galvanic couple materials and design. To quantify the effect of acidity on self-corrosion of AA6061-T6, potentiodynamic polarization tests were performed in aerated and deaerated 3.15 wt.% NaCl solution adjusted to different pH. Anodic dissolution reactions and cathodic oxygen reduction reactions show significantly higher anodic and cathodic currents for more acidic solutions due to the instability of the passive oxide film of aluminum. This film is stable in near-neutral solutions and unstable in highly acidic solutions. As a result, the breakdown of the passive film increases the effective area contributing to anodic or cathodic currents. Observations from experimental work were implemented into the finite element thin film model using COMSOL Multiphysics to predict the self-corrosion and galvanic interaction of aluminum alloy AA6061-T6 coupled to noble metals in severe marine environments. The model results show galvanic interaction accounts for most corrosion in neutral pH. However, in acidic solutions, the corrosion of aluminum is mainly caused by local corrosion, which results in accelerated corrosion rates. The numerical result agrees with our experimental findings and underlines the importance of accounting for local corrosion when predicting the galvanic compatibility of aluminum alloys.

Keywords

Materials Science, AA6061, Aluminum Alloy, corrosion, finite element methods, galvanic corrosion, potentiodynamic polarization

URI

https://hdl.handle.net/10125/104644

Extent

150 pages

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All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.

Collections

Ph.D. - Mechanical Engineering

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