Assessment of Chromate Effects on the Galvanic-Coupling-Induced Localized Corrosion of AA7050-T7451
Dr. Petra Reinke, MSE, Chair
Dr. Jim Fitz-Gerald, MSE
Dr. Bi-Cheng Zhou, MSE
Dr. Andres Clarens, ESE
Dr. John Scully, MSE, Advisor
Dr. Robert Kelly, MSE, Advisor
Dissimilar metal assemblies are frequently encountered in complex structures such as in aerospace applications and pose a major challenge involving galvanic-induced corrosion. One example of such assemblies is that of precipitation-strengthened aluminum alloy (AA) components and stainless steel (SS) fasteners. These precipitation-strengthened AA components are inherently susceptible to localized corrosion due to micro-galvanic interactions that develop between the Al matrix and the constituent (or intermetallic) particles that form as a result of the strengthening process. In addition, the strengthening process renders these AAs unsuitable for traditional welding, and as an alternative, they are commonly joined with 316SS fasteners. Although the structures are coated to mitigate galvanic-induced localized corrosion of the AA component, defects in these coating systems are inevitable during the operational and maintenance life cycles of the AA-based structures. In natural corrosive environments, including the thin electrolyte films present under atmospheric conditions, this situation would often create a macro-galvanic cell in the vicinity of the fastener joint, including within the confined fastener crevice, between the localized corrosion-susceptible base AA component – the anode in the galvanic cell – and the 316SS fastener – the cathode in the galvanic cell – in which the AA component may be polarized above critical potential thresholds (e.g., pitting and repassivation potentials), resulting in greater localized corrosion than if there was no external 316SS cathode.
These aggravated attack sites, particularly the hidden deep fissures within the fastener crevice, can serve as locations for crack initiation with the potential for accelerated crack propagation rates, which would be detrimental to the service and fatigue life of the AA-based structure. This situation has warranted an increasing body of research with regards to the macro-galvanic-induced corrosion of these AAs with the intent to develop an understanding of the physical, electrochemical, and metallurgical factors that govern both the location and mode of corrosion damage in a fastener geometry, including the rationalization of how an inhibitor, such as chromate, can influence these factors. Although extensive work has been carried out on chromate effects on the micro-galvanic corrosion behavior of base precipitation-strengthened AAs, information is generally lacking on the impact of chromate on the damage distribution and morphology pertinent to complex geometries such as plate-fastener configuration and how the unique fastener crevice environments affect the activity of chromate compared to the boldly exposed surface conditions under atmospheric conditions. Although the dangers of chromate have been widely recognized, with a mandate to phase out chromate-based inhibitor systems, a comprehensive understanding of its role in the galvanic-induced corrosion phenomena is crucial for the optimization of testing frameworks and design of environmentally-friendly inhibitor technologies needed to replace toxic chromate-based corrosion mitigation systems.
This work aims to improve the mechanistic understanding of the galvanic-induced localized corrosion of AA7050-T7451 coupled to 316SS in simulated atmospheric environments from the perspectives of damage distribution dependence on cathodic activity and inhibitor action on the main cathodic contributors. This understanding is accomplished by means of a combination of experimental and modeling techniques through three main areas: 1) characterization of the baseline cathodic behavior of AA7XXX in an attempt to establish the factors that affect the degree of suppression of cathodic reactions on this class of AAs, 2) assessment of the current capacities of the individual cathodic contributors to drive anodic dissolution of AA7050 in simulated atmospheric environments, 3) correlation of the findings from areas 1 and 2 to those attained on actual AA7050-316SS galvanic couples under various environmental conditions pertinent to atmospheric exposure.
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