Effects of Applied Cathodic Current on Hydrogen Infusion, Embrittlement, and Corrosion-Induced Hydrogen Embrittlement Behaviors of Ultra-High Strength Steel for Automotive Applications

The effects of the electrogalvanizing conditions (a combination of plating current and time) on hydrogen infusion, embrittlement, and corrosion-induced hydrogen embrittlement (HE) behaviors of ultra-high strength steel were examined. A range of experimental and analytical methods, including electrochemical impedance spectroscopy, hydrogen permeation, polarization, and slow strain rate test, were employed. The results showed that the applied cathodic current density during electrogalvanizing had an inverse relationship with the Zn crystalline size. A smaller cathodic current density and longer plating time led to a larger crystalline size, resulting in a higher infusion rate of hydrogen atoms, and HE-sensitivity (i.e., mechanical degradation with larger density of secondary crack in fracture surface). On the other hand, a larger cathodic current density and shorter plating time caused a higher anodic dissolution rate and smaller polarization resistance of the coating when exposed to neutral aqueous environments. Hence, a higher rate of galvanic corrosion between the coating and exposed steel substrate (e.g., locally damaged areas around coating layer) resulted in a higher infusion rate of hydrogen atoms and HE-sensitivity. This study provides insight into the desirable plating conditions for electro-Zn plating on ultra-high strength steels with enhanced resistance to hydrogen infusion and embrittlement, induced by aqueous corrosion.