Effects of Chromate and Molybdate Ions on Scratch Repassivation Behavior of Precipitation Hardened Aluminum Alloys

Abstract

Scratch depassivation of AA2024-T351, AA7075-T6, and 99.999% aluminum in molybdate (MoO4 2-) and chromate (CrO4 2-) containing NaCl solutions was used to examine electrochemical repassivation kinetics over a range of potentials and inhibitor concentrations. Single frequency impedance measurements were taken in the capacitive region of frequency response. Capacitance measurements enabled the observation of the effects of inhibitor additions on oxide growth as protective oxide films formed on the scratched electrode surface. Chromate was found to suppress scratched electrode current transients at high potentials on both AA2024-T351 and AA7075-T6. Capacitance measurements revealed the rapid growth of a thick, protective oxide when concentrations as low 5 mM CrO4 2-E were in solution. Molybdate did not suppress transient current density in AA2024-T351 or AA7075-T6. In some cases the addition of molybdate increased the total charge passed after the scratch. However, capacitance measurements in the presence of molybdate indicate the rapid growth of a passive film.

Effect of chloride concentration on the corrosion–fatigue crack behavior of an age-hardenable martensitic stainless steel

“…This enhanced chloride concentration exacerbates the H-embrittlement process by destabilizing the crack tip passive film [118] and enhancing the H-activity [149]. The film stability is relevant to the cracking process where active crack tip strain can rupture the protective passive film [150,151]. The kinetics and stability of the film regrowth [95,152] at the crack tip will influence dissolution (thus metal ions available for hydrolysis) and can act as a hydrogen permeation barrier to limit ingress [153,154].…”

The interaction of corrosion fatigue and stress-corrosion cracking in a precipitation-hardened martensitic stainless steel

“…61 For non-cyclic loading, many researchers have noted that the crack-tip strain rate associated with active increase in loading, creep, or crack advance will augment the pure electrochemical effects on the film stability. 24,50,51,59,[61][62][63][64] The concept of cyclic loading-induced rupture of a crack-tip protective film that enables HE (or other environmentally induced processes) has been postulated to explain how loading frequency and corrosion inhibitors influence various alloy-environment systems, which contain crack-tip passive films. [51][52][53][65][66][67][68][69][70] Low-amplitude cyclic loading at stress ranges insufficient for cyclic-damage accumulation to dominate (e.g., below the ΔK TH ) have been referred to as ripple loads.…”