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

One of the most important aspects for a material used in automotive industry is the reliability in service regarding corrosion resistance, safety and functionality. However, hydrogen embrittlement can cause a dramatic deterioration of mechanical properties, especially in case of advanced high strength steels (AHSS). Thus, it is essential to clarify the mechanisms of hydrogen insertion into the material as well as the effect of hydrogen on the mechanical behavior. Corrosion is one source for hydrogen in steel. Therefore, the present work is based on a comprehensive approach using electrochemical and thermal desorption techniques to study the hydrogen insertion into hot-dip galvanized dual-phase steel during corrosion. The effect of hydrogen from corrosion at defects as well as at cut edges is investigated and discussed. Results from hydrogen determination experiments are compared with those from mechanical tests to reveal the impact on the mechanical properties.

One focus of the present work is the influence of the corrosion conditions: pH sensitive hydrogels are used to visualize pH changes at these special areas where galvanic corrosion between steel and coating contributes to the overall corrosion process. A second focus is put on the influence of thermal and mechanical sample pre-treatment on the hydrogen entry into the material. Therefore, coated samples were pre-strained to uniform elongation and/or annealed, to simulate the heating cycle in the cathodic dip painting process, before immersion into sodium chloride solution to induce corrosion. Surface analysis of the differently pre-treated samples before and after corrosion was performed via scanning electron microscopy (SEM). Via thermal desorption mass spectrometry (TDMS) the amount of diffusible hydrogen after immersion of coated samples in aqueous sodium chloride solution was determined. Constant load test (CLT) and step load test (SLT) revealed the effect of corrosion and inserted hydrogen on the mechanical properties of the steel. Scanning Kelvin probe (SKP) and scanning Kelvin probe force microscopy (SKPFM) enabled in situ detection of inserted hydrogen during corrosion. In this manner, the role of defects in the coating as well as diffusion pathways of hydrogen within the steel microstructure was studied.