Operando Assessment of Galvanic Corrosion Between Al-Zn-Mg-Cu Alloy and a Stainless Steel Fastener Using X-ray Tomography

Abstract

Operando x-ray tomography was used to study the galvanic corrosion between aluminum alloy 7050-T7451 and Type 304 stainless steel using a simulated fastener covered with droplets of either 4 M NaCl or 2 M MgCl2. The simulated rivet was an x-ray transparent cylindrical 1 mm pin with a 250 μm diameter rivet hole in the center. The rivet hole was embedded with a 500 μm, in length, stainless steel wire. The rivet hole formed an alkaline crevice between the stainless steel and AA7050-T7451 owing to the proximity of anodic and cathodic sites. The corrosion fissures followed an intragranular path and did not follow bands of intermetallic particles. The x-ray tomography indicated that multiple corrosion fissures propagated over the galvanic couple potential range at all depths examined suggesting the presence of multiple strong proximate cathodes. The volume loss was converted to anodic charge using Faraday’s Law. Cathodic reaction rate studies were conducted on Type 316 stainless steel, copper replated on AA7050, pure Cu, and other matrix secondary phases in simulated crevice environments to assess the ability of each to support fissure corrosion. The substrates for fast cathodic reactions capable of supporting the growth of these fissures were found to be the stainless steel fastener, copper replating on the AA7050-T7451 surface, and dealloyed S-phase. Moreover, removal of the stainless steel fastener stifled fissure growth. The damage morphology assessed with x-ray tomography was compared to laboratory-produced accelerated exposures, as well as field data, and were found to have similar morphologies.