Effect Of Hydrogen On The Passive Film Of Steel

A typical Devanathan-Stachurski device was used in this paper to investigate the effectof hydrogen on the passive film of X80pipeline steel and Alloy600. The working surfaceof the test sample in the oxidation cell was in a stable passive state while the other side ofthe test sample in the charging cell was charged continuously with a current density of1mA/cm2. With or without continuous hydrogen charging, corrosion potential (Ecorr)monitoring, potential dynamic scanning, electrochemical impedance spectra (EIS) andMott-Schottky (MS) were employed to study the changes in electrochemical behavior ofthe test samples. Moreover, scanning electron microscopy (SEM) and X-ray photoelectronspectroscopy (XPS) surface analysis method were used to study the composition of thepassive film and the surface morphology of the test sample. The mechanism involved wasalso discussed.The results show that hydrogen has a similar effect on both X80pipeline steel andAlloy600. As the hydrogen charging process proceeds, the corrosion potential graduallydecreased until stable, and the impedance of the system decreased significantly. Afterhydrogen charging, anodic polarization curves indicate that the X80electrode is in anactive state, while the Alloy600electrode is still in the passive region, but its passivationperformance has been weakened obviously. Mott-Schottky curves show that thesemiconductor type of the passivation film on the X80is not changed by charged hydrogen,but it increases the donor density, shiftes the flat band positively, and decreases thethickness of the space-charge layer of the passive film. Compared with the unchargedcoupons, the surface of the X80electrode was covered by a porous corrosion product filmafter hydrogen-charging, as was indicated in the SEM images. XPS results also show thatthe hydrogen atoms diffuse into the interior of the metal, reactes with the components of theoxide film resulting in the changes in its composition. Finally, a free radical mechanism isproposed: under the corrosion potential, the hydrogen diffuses into the passive film in theform of atoms, and is combined with the adsorbed oxygen in the film to a highly oxidizingradical, which react with components of the oxide film resulting in the changes in thecomposition and structure of the passive film.