An RDE Approach to Investigate the Influence of Chromate on the Cathodic Kinetics on 7XXX Series Al Alloys under Simulated Thin Film Electrolytes

Abstract

The effect of sodium chromate on the cathodic current availability pertinent to the micro-galvanic-induced corrosion of high-strength AA7XXX alloys under simulated thin electrolyte films representative of atmospheric conditions was investigated utilizing a combination of electrochemical and surface characterization techniques. The rotating disk electrode technique provided a means to simulate the effects of water layer thickness to differentiate thin film conditions from full immersion conditions, and enabled the study of the mass-transport-limited oxygen reduction reaction (ORR) on AA7XXX alloys as a function of chromate concentration. The ORR current density decreased by up to two orders of magnitude upon addition of 10 mM chromate, however, the degree of inhibition was observed to depend on the Cu content of the alloy. Chromate was reduced irreversibly to form a Cr3+-rich film on the alloy surface that blocked cathodic sites and hindered ORR. This film was confirmed by X-ray photoelectron spectroscopic characterization of the chemistry and thickness of the chromate-induced layer formed on the specimens after exposure to chromate. The layer was approximately 13 nm in thickness and consisted of mixed Cr3+/Cr6+ oxides with some metallic Cr. Studies on a Pt electrode demonstrated the intrinsic ability of chromate as an effective inhibitor for ORR.

Effect of pH and Al Cations on Chromate Inhibition of Galvanic-Induced Corrosion of AA7050-T7451 Macro-Coupled to 316SS

“…This observation is consistent with our previous work on AA7050 in 0.6 M NaCl at natural pH. 19 Under pH 3 conditions, Cr(III) hydroxide was also found to be the dominant Cr species in the surface film, making up 65% of the total peak area. Cr(III) oxide constituted 19%, and adsorbed and unreduced Cr(VI) constituted a considerable 16%.…”

“…Although chromates are known to be excellent inhibitors for Albased alloys, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] a number of investigations have demonstrated that macro-galvanic coupling can defeat chromate inhibition of these alloys. 2,17,[28][29][30] It is noted that many of these studies were carried out on chromate-coated, scribed Al-based panels containing bare SS fasteners, and a measured concentration of chromate that might have leached out into the scribes was not reported.…”

Accelerating FEM-Based Corrosion Predictions Using Machine Learning

“…[6][7][8][9] These modeling frameworks have enabled analysis of the rate and extent of corrosion given a combination of environmental, physicochemical, and geometric variables such as the RH, the temperature, the thickness of the electrolyte layer, as well as the electrochemical kinetics on the alloy surface. 8,[10][11][12][13] As such it can be noticed that the corrosion of metal alloys is dependent on a complex combination of many different alloy parameters and exposure conditions. Unfortunately, existing computational frameworks are ill-equipped to rapidly assess the multi-dimensional space to identify areas/zones in the input space where corrosion is more (or less) severe, despite their demonstrated accuracy given that FEM models rely on discretization of the domain into small elements on which complex, non-linear electrochemical and kinetic Eqs.…”

“…Quiescent polarization scans were performed with the working electrode perpendicular to gravity which permits the assumption of an equivalent diffusional boundary layer thickness for natural convection. 12 Cathodic polarization measurements were performed on SS304L in 1, 3, and 5.3 M (saturated) NaCl at scan rates of 0.167 mV s −1 respectively after 1 h of open circuit potential (OCP) equilibration and stabilization. The scans were performed from OCP to −1.2 VSCE.…”

Application of Finite Element Modeling to Macro-Galvanic Coupling of AA7050 and SS316: Validation Using the Scanning Vibrating Electrode Technique

“…2 As an alternative, high-strength, noble fasteners such as stainless steels (SS) are often used in joining the Al alloy (AA) parts in aerospace structures. In natural corrosive environments, macro-galvanic interactions between the more noble fastener and the AA, in addition to the micro-galvanic coupling within the base AA microstructure, 3 can exacerbate localized corrosion of the Al-based structures. [4][5][6][7][8][9][10][11] Many studies of galvanic corrosion processes on Al alloys have utilized a number of experimental techniques, including the zero resistance ammeter (ZRA) technique, the coupled microelectrode array (CMEA), and the scanning vibrating electrode technique (SVET).…”