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 ion 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.