Galvanic Currents and Hydrogen Permeation Currents Caused by Galvanic Coupling

Galvanic coupling and hydrogen permeation experiments were performed in a Devanathan-Stachurski cell to measure simultaneously the galvanic currents and the hydrogen permeation currents in an attempt to determine the extent of hydrogen charging into noble metals as a result of galvanic coupling. Large amounts of hydrogen permeated through the more noble element of the couple. The magnitude of the hydrogen permeation flux varied depending on the galvanic coupling conditions. No simple direct relationship between galvanic currents and hydrogen permeation currents was observed, although large hydrogen permeation currents were usually developed whenever large galvanic currents were established. The noble metals Hastelloy⁽¹⁾ C-276 (UNS⁽²⁾ N10276) and Monet⁽³⁾ 400 (UNS N04400) coupled to less noble metals gave hydrogen permeation fluxes ranging from 6.24 × 10¹¹to 1.25 × 70¹⁵ atoms·cm⁻²·s⁻¹, depending on the type of electrolyte, the kind of less noble metal, and the temperature. Even galvanic coupling in tap water caused significant hydrogen permeation fluxes through the more noble electrode. NaCl added to the electrolytes during galvanic coupling caused oscillations in the galvanic currents and always an increase in the hydrogen permeation currents. On the other hand, additions of Na₂S and As_sO₃ increased the permeation currents under certain conditions only. Hydrogen charging at constant applied cathodic current densities on palladized alloy 400 and on alloy C-276 was also done. The resultant hydrogen permeation curves and the effective hydrogen diffusivities calculated from them were generally different than those obtained by galvanic coupling.