Corrosion Kinetics and Morphology in AA7075/SS316 Galvanic Couples under Environmental and Mechanical Loading.

RIP2025-00132: Aluminum alloys are commonly used in aircraft applications due to their high strength-to-weight ratio.  Due to the susceptibility of high-strength aluminum alloys to hot cracking after welding, structural components are joined using fasteners. Often the fasteners are of a dissimilar metal that, along with coating damage from operation, introduces the risk of galvanic corrosion between the aluminum alloy and the fastener.  Corrosion and cracking in areas of known stress concentrators such as fasteners seen in naval aircraft teardowns suggests the need to explore the effect of stress on galvanic corrosion kinetics. Therefore, the goal of this work is to quantify the effect that static stress has on corrosion reactions in environments relevant to atmospheric corrosion of airframe materials.

AA7075 panels with a countersunk SS316 fastener were exposed to static mechanical loads and environmental conditions of relevance to naval applications. Panels were treated with a non-chrome conversion coating (MIL-DTL-81706 Type 2), primer (MIL-PRF-23377 Class N Type I), and topcoat (MIL-PRF-85285 Class H Type IV) and tested at 3 static load conditions (0σy, 0.4σy, 0.9σy) and 3 RH values (60 %, 75%, 95%). Panels were held at test conditions for 2 weeks with an initial application of a 300 µL droplet of ASTM D1141 simulated seawater solution on the fastener head. Metallographic imaging of the through-hole cross-section and analysis with an in-house Python code allowed characterization of the corrosion morphology in the fastener through-hole. The corrosion morphology was characterized through optical and SEM imaging to investigate the effects of environmental and mechanical loading on features susceptible to cracking. Increased corrosion pitting events and max pit height were seen in stressed conditions.

This work will provide insight into the effect of static stresses on galvanic corrosion kinetics and morphology, which will assist with the design of future aircraft structures to mitigate galvanic corrosion risks as well as improve maintenance predictions.