Concentrated Chloride Effects on Chromate Inhibition of Al Alloy-316 Stainless Steel Galvanic Couple-Induced Corrosion

Abstract

Dissimilar metal assemblies are frequently encountered in complex structures, such as in aerospace applications, and pose a major challenge in terms of galvanic-induced localized corrosion. One example of such an assembly is that of high-strength aluminum alloy (AA) components joined with stainless steel (SS) fasteners. Defects in the corrosion protection coating systems at these joint locations facilitate the wicking of aggressive electrolyte species into the confined fastener crevices, creating a hidden macro-galvanic corrosion cell which can aggravate attack and degrade the fatigue life of the localized corrosion-prone base AA component.[1-2] In this situation, inhibitor release from the coating system, and transport into the fastener crevice, would be crucial in mitigating the localized attack. Previous studies on inhibitor activity on AA-SS galvanic couples have mainly focused on the boldly exposed surface conditions and associated chemistries due to the difficulties encountered in reconstructing and/or simulating complex fastener geometries. As a result, there is limited understanding of the physical, electrochemical, metallurgical, and geometric factors that govern both the location and mode of corrosion damage in a fastener crevice, let alone the rationalization of how an inhibitor, such as chromate, can influence these factors. Recent downhole analyses of an AA7050-316SS fastener crevice utilizing X-ray tomography and coupled multi-electrode arrays (CMEAs) showed that although there was a significant decrease in fissure density with no (near) surface fissures on addition of chromate, galvanic attack on AA7050 was still occurring at the bottom of the fastener hole.[3] This result raised questions surrounding the possible effects of crevice depth on solution chemistry in the fastener crevice which may have impacted the speciation and/or activity of chromate in solution.The present work will build on the work of Rafla and Scully[3] to advance current knowledge of the role of chromate on the interaction of the various factors that govern damage in AA plate-SS fastener systems. This work will utilize a combination of electrochemical techniques – rotating disk electrode (RDE), zero resistance ammetery (ZRA), potentiodynamic polarization experiments – and surface characterization techniques - optical microscopy and scanning electron microscopy (SEM) - to assess the performance of chromate on AA7050-316SS galvanic couples in elevated chloride environments with the addition of Al3+ simulating deep fastener crevice environments. The results will reveal whether or not chromate suppresses HER kinetics on 316SS that may be occurring deep inside a fastener crevice and/or if there is a critical [Cl-]/[CrO4 2-] above which chromate does not stifle damage propagation on AA coupled to SS in aggressive chloride environments. References V. N. Rafla et al., Corr. 74, 2018, p. 5-23R. S. Marshall et al., Corr. 75, 2019, p. 1461-1473V. N. Rafla and J. R. Scully, Corr. 75, 2019, p. 587-603 Acknowledgement This work is support...