Environmentally Assisted Cracking: Theory and Practice : EAC of Aluminum Alloys
Sponsored by: TMS Corrosion and Environmental Effects Committee
Program Organizers: Jenifer Locke, Ohio State University; Wenjun Cai, Virginia Polytechnic Institute and State University; Bai Cui, University of Nebraska Lincoln; Srujan Rokkam, Advanced Cooling Technologies, Inc.; Kaila Bertsch, University of Wisconsin-Madison

Tuesday 2:00 PM
November 3, 2020
Room: Virtual Meeting Room 30
Location: MS&T Virtual

Session Chair: Zachary Harris, University of Virginia; Wenjun Cai, Virginia Polytechnic Institute and State University


2:00 PM  Invited
The Role of SCC in Corrosion Fatigue Kinetics of AA5456-H116: David Schrock1; Jenifer (Warner) Locke1; 1The Ohio State University
    The role stress corrosion cracking (SCC) plays in accelerating corrosion fatigue (CF) kinetics (da/dN) of 5xxx series aluminum is investigated. 5xxx series alloys with as-fabricated microstructures are corrosion resistant. However, at temperatures 40 °C and higher, 5xxx series alloys containing greater than ~3 wt.% Mg precipitate anodic β-phase on α-aluminum grain boundaries. This phenomenon, called sensitization, increases environment assisted cracking susceptibility. Experiments on AA5456-H116 were conducted in 3.5 wt.% NaCl at different maximum stress intensities (Kmax), but same stress intensity range, to probe the effect of SCC on da/dN. Results establish that heavily sensitized microstructures exhibit da/dN with inverse f-dependence. The severity of this inverse f-dependence increases when Kmax values are within stage II for SCC. Conversely, increasing Kmax for lightly sensitized microstructures, which are SCC resistant, does not cause an inverse f-dependence. Findings support a hypothesis that strong inverse f dependent da/dN is driven by superposition of SCC.

2:40 PM  
Microstructure and Beta Phase Distribution Effects on Environmental Fracture Susceptibility in Al-Mg Alloys: Matthew McMahon1; William Golumbfskie1; 1Naval Surface Warfare Center, Carderock Division
     Al-Mg alloys are popular structural materials in marine applications due to their high strength-to-weight ratio and general corrosion resistance. However, extended time in service has demonstrated that these alloys may become sensitized through precipitation of β phase on the grain boundaries, which enables intergranular corrosion and/or stress corrosion cracking (IG-SCC). New alloy tempers such as – H128 are promising for slowing this intergranular precipitation, but questions remain concerning the IG-SCC susceptibility of this heat treatment once a significant sensitization level has been reached (Nitric Acid Mass Loss Test (NAMLT) value greater than 15 mg/cm2). The present research compares the IG-SCC susceptibility of AA5083-H128 to other common AA5xxx-series alloys at constant sensitization level. Slight differences in IG-SCC susceptibility are evident due to microstructural differences, such as percent recrystallization, as well as differences in yield strength. These differences are resolved based on understanding of hydrogen concentration in the fracture process zone.

3:00 PM  
Directional Sensitization Responses in 5XXX Series Aluminum Alloy Microstructures: Likun Sun1; Matthew Steiner1; 1University of Cincinnati
    The extent of intergranular corrosion in sensitized 5XXX series aluminum alloy plates is highly dependent on the exact geometric configuration of networked high-angle grain boundaries available for propagation normal to the exposed surface. Utilizing adaptations of the Nitric Acid Mass Loss Test (NAMLT) standard commonly used to assess bulk sensitization in these alloys, we will present the isolated directional intergranular corrosion responses from different rolled plate microstructures as a function of their time-temperature history. We will show how both the magnitude and kinetics of the directional sensitization responses exhibited by these microstructures contributes to variations in the bulk measurements of nominally identical 5XXX series aluminum alloy plates, despite their shared alloy-temper designation. Lastly, we will highlight how the directional sensitization responses correlate with features of the grain boundary configurations in ways that render the microstructural data predictive of the intergranular corrosion behavior, with direct implications to stress corrosion cracking.

3:20 PM  
Corrosion Fatigue Testing of AA7085-T7451 in Complex Atmospheric Environments of Varied Humidity with Surface Salt Loading: Brandon Free1; Austin Burns2; Jason Niebuhr2; Sarah Galyon Dorman2; Jenifer Locke1; 1The Ohio State Uniersity; 2SAFE Inc.
    AA7085-T7451 is used in aerospace environments with fluctuating relative humidity (RH), variable temperatures (T), ozone, UV light, and surface salt. Traditional corrosion fatigue (CF) experiments utilizing lab air, water vapor, or full immersion may not adequately model in-service environments. The aim of this study is to measure CF crack growth rates (da/dN) in environments of varied RH and T where surface salts are present. Testing is completed by monitoring crack length during intervals of fixed loading and environmental parameters. Experiments that changed loading frequency for each interval show samples loaded with 300 μg/cm2 of NaCl and exposed to 80% RH exhibit similar da/dN to samples immersed in 0.06 M NaCl. Experiments that varied RH show da/dN does not immediately rise with increasing RH or fall when RH is decreased below the efflorescent point. Effects of wet/dry cycling and da/dN changes due to changing crack tip electrolyte content are under investigation.

3:40 PM  
Understanding Pitting Corrosion in a High-performance Aluminum Alloy by 4D X-ray Microtomography: Daniel Sinclair1; Sridhar Niverty1; Nikhilesh Chawla1; 1Arizona State University
    Aluminum alloys are commonly used for engineering applications due to their high strength to weight ratio and low cost. Pitting corrosion, accelerated by saltwater environments, can lead to fatigue cracks and stress corrosion cracking during service. Two-dimensional (2D) characterization methods are typically used to identify and characterize corrosion. These methods, however, are destructive and do not enable an efficient means of quantifying mechanisms of pit initiation and growth. In this study, we have used lab-scale x-ray microtomography to non-destructively observe, quantify, and understand pit growth in three dimensions over a 20-day corrosion period in an AA7075-T6 alloy. Pit depths were quantified over time using renderings of sample volumes, leading to an understanding of how inclusion particles, oxide formation and breakdown, and other corrosion mechanisms impact the growth and morphology of pits.