Anodic behaviour of carbon steel in concentrated NaOH solutions

Abstract

A study has been made of the anodic behaviour of carbon steel in deaerated 1–19 N NaOH solutions at 25–80°C over a range of potentials from active to transpassive regimes. A rotating steel disk electrode was used to obtain the anodic polarization curves at a scan rate of 1 mV s −1. The soluble dissolution products during the potential scan were analyzed with a rotating ring-hemisphere electrode. The results indicate that in the active dissolution potential regime, steel dissolved primarily via a two-electron transfer reaction to HFeO − 2 ion. As the potential approached the active—passive transition potential, a parallel reaction occurred, which produced a soluble tri-valent iron species of FeO − 2 ion. The formation of FeO − 2 ion was caused by the oxidation of an intermediate Fe(OH) 2 surface species in the active dissolution reaction. At the active—passive transition potential, the Fe(OH) 2 intermediate was further oxidized to a passive Fe 3O 4 film, causing the current to drop with further increase in potential. In the course of current drop from the active to passive state, the Fe 3O 4 film was oxidized to a Fe 2O 3 film, resulting in a small second current peak on the anodic polarization curves. The passivity of steel was unstable in NaOH solutions at the concentration greater than 4 N and temperature higher than 40°C. The Fe 2O 3 film reacted with hydroxyl ion and dissolved in concentrated NaOH solutions at high temperatures. The exposed steel then anodically dissolved into the electrolyte to form FeO − 2 ions at the passive potentials, and the current—potential curve exhibited a humped shape. The steel dissolution reaction in the hump potential regime was a first order reaction with respect to the activity of hydroxyl ions. The rate of the dissolution reaction in the hump potential regime was limited by the ohmic resistance of an intermediate Fe(OH) 3 surface layer at low overpotentials. At high overpotentials, the reaction became limited by mass transfer of the dissolution product, FeO − 2, to the bulk electrolyte.