Stability of oxide films formed on mild steel in turbulent flow conditionsof 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 d etermine the rates of erosion-corrosion of the equipment. In the present st udy 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 be en investigated by employing electrochemical techniques of cyclic voltammet ry 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 t hat 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 appea rs the films are compact and more stable, and therefore provide good protec tion. At higher potentials (> -0.4 V-SHE) in the passive region, the result s suggest that film formation and dissolution occur simultaneously and the increase in temperature and turbulence causes a breakdown of the passive fi lm resulting in a situation similar to nonprotective magnetite growth. (C) 2001 Elsevier Science Ltd. All rights reserved.