12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

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 overlay coating can act as a hydrogen barrier, and also the Fe-Zn intermetallic layer inhibits hydrogen diffusion out of the metal. As a result, an excess of diffusible hydrogen remains in the steel substrate and can subsequently leads to a possible embrittlement.

In this work, the effects of the coating nature on the hydrogen diffusion and embrittlement of a 980MPa 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. Indeed, if the behaviour of classical galvanized zinc coating with respect to hydrogen is well known, it is however not the case for its alternatives.

The hydrogen permeability of the coatings is first assessed through degassing experiments at room temperature on samples prepared with a hot dip galvanizing process simulator. The influence of cracks developed in the coating of the galvannealed samples is investigated, as these are not present in GI and Zn1.2Al1.2Mg coatings. Secondly, hydrogen uptake of these coated samples due to their corrosion in cyclic SAE J2334 corrosion testing is analysed. Finally, mechanical performances of corroded samples are evaluated. In all cases, the diffusive hydrogen content is measured through a thermal desorption analysis, with a critical assessment of the methodology.