Role of Chromate in the Cathodic Current Availability in an Al Alloy-316 Stainless Steel Galvanic Couple Under Simulated Atmospheric Conditions

摘要

Numerous studies have demonstrated that the Cu-rich intermetallic particles (IMPs) are the major facilitators of localized corrosion of high strength Al alloys in chloride-containing environments. In the context of cathodic activity, these Cu-rich IMPs catalyze the fast oxygen reduction reaction (ORR) rates required to sustain anodic dissolution of the peripheral Al matrix and/or preferential dealloying of active elemental constituent(s) of the Cu-rich IMPs. The latter process results in Cu replating on the surface which increases the cathodic surface area. The situation is exacerbated when these Al alloys are coupled with more noble fasteners such as stainless steel (SS) as found in aircraft structures; macro-galvanic interactions between the Al plates and SS fasteners lead to substantial damage, including within the fastener holes. The SS fastener is speculated to increase the driving force for Cu replating, and the replated Cu in turn sustains fast cathodic reaction kinetics. Traditionally, soluble chromates have been used in conversion coatings and as pigments in protective coatings to mitigate localized corrosion of Al alloys. Chromate cathodically reduces to form an insoluble Cr~(3+)-rich film on the alloy surface that blocks cathodic sites and hinders the ORR as well as the dealloying of the S-phase (Al_2CuMg). While chromate is well known to be carcinogenic, and efforts are underway to replace it with an environmentally-friendly inhibitor, efforts to date on potential alternatives have proven generally unsuccessful. Consequently, chromate remains the choice inhibitor for Al alloy applications particularly in the aerospace industry. As such, it remains of utmost academic and industrial interest to continue to gain more in-depth understanding of chromate inhibition mechanism(s), especially as it pertains to the corrosion behavior of complex structures with micro- and macro-galvanic couples with the intent to enhance the knowledge required to facilitate the establishment of non-toxic chromate replacement systems.