In situ 2D visualization of hydrogen entry into Zn-coated steels in NaCl solutions: Roles of Zn dissolution and potential distribution

The hydrogen entry behavior of a partially Zn-coated steel sheet in NaCl solutions was investigated employing a polyaniline-based hydrogenochromic sensor, electrochemical hydrogen permeation tests, and potential measurements using a scanning Kelvin probe. While the Zn coating mitigated corrosion of the steel substrate, it simultaneously accelerated the hydrogen entry. The hydrogen entry occurred at the bare steel surface regions exposed to the NaCl solution, with the hydrogen flux exhibiting non-uniform distribution: higher near the dissolving Zn coating. While no significant differences in Zn dissolution behavior or galvanic current were observed between 0.1 and 0.01 M NaCl solutions, the total hydrogen flux decreased with decreasing Cl − concentration. This reduction was attributed to a potential gradient induced by differences in electrolyte conductivity. The results demonstrate that potential distribution, rather than galvanic current, is a dominant factor influencing hydrogen entry under the investigated conditions. Graphical abstract • Hydrogen entry site was identified as the steel substrate exposed to NaCl solutions. • Non-uniform distribution of hydrogen flux existed across the steel substrate. • Hydrogen flux was considerably higher near the dissolving Zn coating. • Cl − concentration influenced the potential distribution but had minimal effect on galvanic current. • Hydrogen flux distribution was affected by potential distribution on the steel substrate.