Abstract
A series of anodic polarization experiments with mild steel in 300 C (572 F) LiOH over the concentration range 0.0016N to 5N have been conducted using a tubing capsule electrochemical cell so designed that the cell wall itself serves as the working electrode. Spontaneous passivation, as marked by an abrupt rise in corrosion potential, was observed for all solutions during the 30 minute prepolarization temperature equilibrating period, and active-passive transitions were not found when the specimens were subsequently polarized, the current-potential curves indicating a passive corrosion mode even for the 5N solutions. Protective spinel oxide films, possibly containing LiFe5O8, formed on the tubing interior with 1N and lower LiOH. A predominantly LiFeO2 film was produced with 5N LiOH which, although initially protective, gradually lost protectiveness after an induction period of about 1.5 hours. This loss of protectivity occurred irrespective of applied potential and evidently results from a chemical reaction between the oxide film and 5N LiOH which leads to spalling. Anodic polarization did not yield α-Fe2O3 with any of the LiOH solutions, contrary to earlier experiments in 300 C NaOH where α-Fe2O3 was produced in the transpassive regions. The α-Fe2O3 transformation is apparently displaced (or obscured) by reactions giving the more stable lithium containing oxides. Precipitous current rises were observed during anodic polarization in 0.008N and 0.0016N LiOH which appeared to be associated with the occurrence of localized disruptions in the surface oxide films. These disruptions were not associated, however, possibly because of the presence of LiOH, the crack like pits produced with current breakaway in dilute NaOH.
