Stability of oxide films formed on mild steel in turbulent flow conditions of alkaline solutions at elevated temperatures

In the industries involving alkaline solutions in different process streams, the nature and stability of oxide films formed on the metallic surfaces determine the rates of erosion-corrosion of the equipment. In the present study the characteristics of the oxide films formed on AISI 1020 steel in a 2.75 M sodium hydroxide solution at temperatures up to 175 degreesC, have been investigated by employing electrochemical techniques of cyclic voltammetry and chronoamperometry. The experiments were carried out in an autoclave system based upon a 'rotating cylinder electrode' geometry to determine the effects of turbulence on the stability of the films. The results suggest that little protection is afforded in the active region (at about -0.8 V-SHE). In the passive region at low potentials (-0.6 V to -0.4 V-SHE), it appears the films are compact and more stable, and therefore provide good protection. At higher potentials (> -0.4 V-SHE) in the passive region, the results suggest that film formation and dissolution occur simultaneously and the increase in temperature and turbulence causes a breakdown of the passive film resulting in a situation similar to nonprotective magnetite growth. (C) 2001 Elsevier Science Ltd. All rights reserved.