Effect of wire drawing and heat treatment on the exfoliation corrosion mechanism of Al-Zn-Mg-Zr-V wires

Abstract

To ensure the use of 7000 series for safety-relevant applications in automobiles, such as screws, a comprehensive understanding of their exfoliation corrosion (EFC) behavior is requested. The EFC behavior of Cu-free Al-Zn-Mg-Zr-V wires in solution annealed (T4), peak-aged (T6) and overaged (T7) state is investigated by EXCO immersion tests combined with (quasi) in situ measurements of the dissolved species using inductively coupled plasma mass spectrometry (ICP-MS) and electrochemical impedance spectroscopy (EIS). Supplemented ex-situ investigations of the EFC progress are performed using X-ray computed tomography. The EFC behavior is linked to microstructural features that are investigated using i. a. scanning (transmission) electron microscopy (SEM/STEM) coupled with energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). Wire drawing and heat treatment have a significant effect on the microstructure, the EFC susceptibility and the corresponding EFC mechanism. EFC susceptibility decreases from T4 to T7 state and is less pronounced in the wire core compared to the rim zone, while in T7 it is the opposite. Dependent on heat treatment and related microstructure, the corrosion mechanism changes from intergranular corrosion (T4) to pitting (T7). Quasi in situ measurements of the electrolyte species are linked to the corrosion mechanisms.