Effect of cooling rate and grain structure on the exfoliation corrosion susceptibility of AA 7136 alloy

Abstract

The effect of cooling rate and grain structure on exfoliation corrosion susceptibility of AA 7136 alloy was investigated by open-circuit potential (OCP) measurement, immersion test, optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), scanning transmission electron microscopy (STEM), and scanning Kelvin probe force microscopy (SKPFM).The results showed that quench-induced η phase formed with a decrease in the cooling rate from 244 °C/s to 1.3 °C/s after solution heat treatment, thereby increasing exfoliation corrosion susceptibility. The quench-induced impact is the lowest, intermediate, and highest in the specimen with large elongated recrystallized grains (large-REX), fine recrystallized grains (fine-REX) and elongated unrecrystallized grains (elongated-UNREX), respectively. The Volta potential difference between the matrix and grain boundary precipitates was the lowest (68.3 ± 6.6 mV) in large-REX specimens, intermediate (88.6 ± 11.3 mV) in fine-REX specimens, and highest (137.6 ± 27.3 mV) in elongated-UNREX specimens. The reason has been discussed in detail based on the features of grains, and quench-induced change in the microstructural and microchemical features at grain boundaries in different specimens.