Unraveling the passive film characteristic and pitting mechanism of the 7xxx high-strength Al alloy with different microstructures: Experimental and FEM simulation

Abstract

The pitting mechanism of 7 series high-strength aluminum (Al) alloys (7xxx) is investigated experimentally and numerically (finite element method, FEM) by comparing the characteristics of passive films and pitting kinetics of two types of 7xxx Al alloys. The point defect model (PDM) is used to evaluate the passive film properties in the two varieties of 7xxx Al alloy, and the FEM is employed to calculate the point defect diffusion process in detail. The results reveal that the two 7xxx Al alloys exhibit various microstructures, particularly the 7xxx-1, characterized by a robust (111) fiber texture and a lower proportion of low-angle grain boundaries than the 7xxx-2. The diffusivity D of point defects in 7xxx-1 (3.85 × 10−18–52.74 × 10−18 cm2 s−1) is lower than that in 7xxx-2 (5.47 × 10−18–66.61 × 10−18 cm2 s−1) and the diffusion duration of 7xxx-1 is longer than 7xxx-2, which is closely associated with the rupture of passive film. Additionally, FEM proved that the initial shapes of pits dictate the progression of pitting, manifesting that the shallow dish shape grows laterally while the deep hole shape grows longitudinally.