Influence of Zn-based Coating Alloys on Hydrogen Diffusion and Corrosion Resistance in a DP Steel

Abstract

In advanced high strength automotive steels, small amounts of diffusible hydrogen can lead to a deterioration of mechanical performances, especially a loss of ductility, in the simultaneous presence of internal stresses and of a sensitive microstructure. In the hot-dip galvanizing process, hydrogen is mainly absorbed during high temperature operations in hydrogen-containing atmospheres before hot-dipping when the solubility of hydrogen in steel is the highest. After hot-dipping, the metallic zinc-based coating can impede hydrogen diffusion out of the metal. As a result, an excess of diffusible hydrogen remains in the steel substrate and can subsequently lead to a possible embrittlement. In this contribution, the effects of the coating nature on the hydrogen diffusion of a 980\u202fMPa dual phase steel (DP980) are investigated. The attention is focused on three Zn-based coating alloys: galvanized (Zn‑0.23%Al), galvannealed (Fe-Zn based on Zn‑0.12%Al) and Zn1.2Al1.2Mg. The hydrogen permeability of the coatings is first assessed through degassing experiments at room temperature and highlights the tightness of all coatings at room temperature. Secondly, cyclic SAE J2334 corrosion testing is performed and enables to highlight a stronger corrosion of GI and GA samples with respect to Zn1.2Al1.2Mg-coated samples, which is however the sole coating that seems to promote a hydrogen uptake during corrosion experiment.