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

Abstract

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.[1-4] 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.[5-8] The SS fastener is speculated to increase the driving force for Cu replating, and the replated Cu in turn sustains fast cathodic reaction kinetics.[4] Traditionally, soluble chromates have been used in conversion coatings and as pigments in protective coatings to mitigate localized corrosion of Al alloys.[9-10] Chromate cathodically reduces to form an insoluble Cr3+-rich film on the alloy surface that blocks cathodic sites and hinders the ORR as well as the dealloying of the S-phase (Al2CuMg).[10-13] 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.[14] 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.In the present work, the rotating disk electrode (RDE) technique is used for a comparative study of the inhibitive effect of sodium chromate on the ORR on polished and pre-corroded AA7050 electrodes. The results will serve as boundary conditions for the computational modeling of galvanic corrosion inhibition of AA7050 coupled to 316SS in a plate-fastener arrangement. The RDE is used to simulate thin electrolyte films with varying diffusion boundary layer thicknesses as rotation rate is increased. In the inhibitor-free chloride solution, a Pt working electrode is used to determine boundary layer thickness as a function of rotation rate, eliminating the effects of a surface oxide film. Initial results on polished electrodes show that at optimal chromate concentrations where the cathodic current density is significantly reduced on AA7050 and 316SS, the ORR kinetics on pure Cu is still substantial. This implies that higher inhibitor concentrations may be required to suppress the ORR on corroded AA7050 replated with Cu. AA7050 pre-corrosion is simulated by employing two approaches: 1) free corrosion at the open circuit potential in inhibitor-free solution and 2) electroplating Cu onto AA7050. The results are compared to the ORR kinetics on pure Cu. Furthermore, the scanning vibrating electrode technique (SVET) will be carried out on a typical AA7050/316SS galvanic couple to map local current density distributions as a function of chromate concentration. Surface characterization will be performed using optical microscopy and scanning electron microscopy (SEM) and surface film analyses will be conducted utilizing x-ray photoelectron spectroscopy (XPS).