Analysis of Intergranular Stress Corrosion Cracking in Al-Mg Alloys under Atmospheric Exposure Conditions

Steiner, Patrick, Materials Science - School of Engineering and Applied Science, University of Virginia
Burns, James, EN-Mat Sci/Engr Dept, University of Virginia

Intergranular stress corrosion cracking (IG-SCC) is a continuous problem for many alloy systems, especially for high strength Al-Mg alloys in aggressive saline environments. It is believed that a combined process starting with anodic dissolution of the intergranular β phase (Al3Mg2) coupled with hydrogen enhanced decohesion ahead of the crack tip is the underlying mechanism, with crack growth rates being shown to be highly dependent on both crack tip electrochemical potential and solution chemistry. The majority of these studies have been performed in electrochemically controlled full immersion environment where the bulk solution has both a large volume, fixed solution concentration, and the potential is fixed via a potentiostat. This differs from atmospheric environments, which involve lower solution geometries, higher ion concentrations, and no governing potentiostat. These environments are more typical of real-life service conditions and may exhibit different underlying IG-SCC mechanisms and crack growth rates compared to analogous immersion studies. However, atmospheric environments complicate testing techniques (e.g. linear elastic fracture mechanics (LEFM) and direct current potential drop (dcPD)) used to characterize IG-SCC behavior. As such the objective of this dissertation is to systematically isolate and understand the various distinct factors that are unique to atmospheric environments to better inform alloy development, prognosis modelling, and potential IG-SCC mitigation strategies.

The objective of this dissertation is to address these knowledge gaps by focusing on the following research questions:
1. What are the potential limitations and modifications imposed upon the IG-SCC mechanism by transfer of the cathode from an infinite supply to solely the bulk surface of the sample surface?
2. How do various solution geometries of reduced volume and solution layer thickness effect the IG-SCC mechanism development and/or propagation in Al-Mg alloys in saline environments, and what is the governing factor(s) at play?
3. How do various solution chloride concentrations effect the IG-SCC mechanism development and/or propagation in Al-Mg alloys when exposed to varying atmospheric saline environments?
4. Are metal-rich primers a viable IG-SCC mitigation strategy in atmospheric environments, and if so, what is their relative potency as compared to the existing full immersion results?
5. In IG-SCC testing protocols that require exposed dcPD current wires, what are the potential effects caused by application of the dcPD current on the measured or applied electrochemical potential?

Considering the practical impact of the current dissertation, results indicate that across all atmospheric environments the use of IG-SCC kinetics for full immersion testing at an appropriate set potential and the highest anticipated chloride concentration remain conservative across all conditions for Al-Mg alloys. Critically, these results do not account for the transient effects of wetting and drying, external contaminants, or other environmental effects such as UV radiation. Furthermore, the use of metal-rich primers to mitigate IG-SCC growth in atmospheric environments was found to result in a reduction of the IG-SCC susceptibility similar to that seen in full-immersion conditions, albeit at reduced effectiveness. Lastly, the method of dcPD current wire attachment was found to be critically important for environmental setups that require exposed dcPD current wires to solution. Considering scientific impacts, the development and benchmarking of multiple archetypal environments compatible with LEFM-based testing and electrochemical monitoring were designed and verified for the IG-SCC testing of samples in atmospheric environments. These protocols are easily transferable to other material systems/geometries and establish a rigorous method to qualitatively assess atmospheric environmental effects. Finally, key insights into the proposed IG-SCC mechanism for Al-Mg alloys were determined such as the governing effects of the cathodic current and close relation between the bulk surface potential and the anticipated anodic demand.

PHD (Doctor of Philosophy)
Al-Mg alloy, stress corrosion cracking, atmospheric environments, metal-rich primer
Sponsoring Agency:
U.S. Office of Naval Research
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