Effect of Humidity of Air on Hydrogen Absorption into Fe with Rust Layer Containing MgCl₂ during Atmospheric Corrosion

Abstract

An Fe plate, whose one side was electro-polished and the other was covered with the rust layer containing 25.7 g·m⁻² MgCl₂, was used as the specimen to investigate the effect of humidity on the hydrogen absorption of the plate during atmospheric corrosion. The specimen was subjected to an electrochemical hydrogen-absorption test during which the rusted surface was exposed to the air with controlled relative humidity (RH) and atmospheric corrosion occurred on it. When the rusted surface was subjected to dry (RH 0%)–wet (RH 27%) cycle tests for 10.8 ks each, the anodic current density corresponding to the hydrogen-absorption rate was measured on the hydrogen detection surface. The maximum current density was almost independent of the cycle during the first 10 cycles, after which it decreased with an increase in the cycle, reaching almost a steady value after about 40 cycles. After 55 cycles of the dry–wet cycle test, the specimen was subjected to an electrochemical hydrogen-absorption test to obtain the relationship between the steady-state hydrogen-absorption rate and RH. Hydrogen absorption was observed at RH over about 15%, and the absorption rate increased rapidly with an increase in RH, reached a maximum at RH of about 30%, and then decreased rapidly. When RH increased beyond 40%, the absorption rate increased again, reached a maximum value at RH of 80%, and then decreased gradually. The specimen with the rust layer containing 39.8 g·m⁻² MgCl₂ also showed two peaks in the hydrogen-absorption rate versus RH plot.