Effect of retrogression re-aging treatment on corrosion behavior of 7055 Al-Zn-Mg alloy

Abstract

The effect of retrogression re-aging treatment (RRA) on the corrosion behavior of 7055 Al-Zn-Mg alloy was studied in this paper. Results provided that corrosion resistance could be greatly improved by RRA. After the RRA treatment, the isolated precipitates occurred on grain boundaries (GBs) and the low angle grain boundaries (LAGBs) presented a larger fraction compared to that of single-stage peaking aging treatment. The samples after RRA treatment also show better corrosion resistance than that of single-stage peak aging treatment. In addition, of the electrochemical impedance spectroscopy (EIS) analysis showed that the impedance spectrum was consisted of semi-infinite layer diffusion impedance and stagnant Weber impedance. The semi-infinite layer diffusion impedance corresponded to a limited retention layer on the electrode surface while the Weber impedance represented a stagnant layer of corrosion products generated by the anode branches. The aluminum alloy after RRA treatment achieved higher Rf and low Cf, Cp values and its corrosion current density was ten times smaller compared to the single-stage peak aging samples with the 10% hardness losing.

Evaluation of the Effect of Dual-Stage Aging and RRA on the Hardening and Corrosion Resistance of AW7075 Alloy

“…High-strength 7000 series aluminum alloys have been widely used in the aerospace industry due to their unique properties [1][2][3][4][5][6][7]. Their high strength properties are obtained through heat treatment, during which conventional methods of precipitation hardening based on supersaturation and subsequent aging are usually used.…”

“…Single-stage aging treatment to the T6 state provides high strength but is associated with high susceptibility to stress corrosion cracking (SCC) [2,6,7,11,12]. The SCC mechanisms in high-strength 7000 series alloys have been associated with hydrogen-induced cracking, anodic dissolution-assisted cracking, and passive layer cracking [5,13,14].…”

“…The higher corrosion susceptibility of the T6 alloy is attributed to the continuous distribution of GBPs providing corrosion channels for anodic dissolution. These continuous chain-like precipitates are distributed along grain boundaries and make the alloy susceptible to stress corrosion cracking [2,7].…”

“…The third Retrogression and Reaging (RRA) heat treatment involving multi-stage aging treatments provides better control over the resulting precipitates responsible for strengthening, and thus increases the chance of obtaining a microstructure that will provide a favorable combination of mechanical strength and corrosion susceptibility [3,4,7,19,20,24]. Retrogression involves heating of the previously precipitation-strengthened alloy at temperatures in the range of 200-260 • C for a short time (120s), followed by Reaging typical of T6 peak aging [1,2,6].…”

“…Reaging aims to restore the alloy strengthening, which decreases during the retrogression treatment as a consequence of the increase in precipitations [1]. The application of RRA treatment makes it possible to obtain a microstructure characterized by the presence of finely dispersed MgZn 2 matrix precipitates (MPs) in the grains that are characteristic for the T6 state (and by fragmented, discontinuous precipitates occurring along grain boundaries (GBPs) typical of T7X overaging [2,6,7,13,20,25]). Thus, the grain boundary line previously blocked by continuous GBPs precipitates is transformed into a state where η phase precipitates are coarse and discontinuous [4,6,24,26].…”