Chapter 5 - Corrosion Behavior Investigations of Traditional Structural Metallic Materials in Electrochemical Plant Media, Taking Into Account Attack by Leakage Currents

Abstract

In the early stages of the development of electrochemical plants, carbon steel equipment was used in the zones of action of leakage currents coming from the electrolyzers. In these cases, the external anodic current, which attacked the actively corroding metal, was spent almost completely on its dissolution, in accordance with Faraday's laws. Therefore, using carbon steel in neutral solutions in electrochemical plants under conditions of attack by leakage currents is inadmissible without complete isolation of the metallic structures from contact with an aggressive environment. Under attack by external anodic currents, the potential of carbon steel shifts to the area of oxygen evolution, where the corrosion rate of carbon steel in alkaline solution becomes significant. In chloride-alkali solution the corrosion rate of carbon steel further increases. Corrosion tests have shown that the rate of corrosion of carbon steel St 3 in this solution at a temperature of 90ºC, in the absence of an external current, was less than 0.1 g/m2 h. The open circuit potential of carbon steel under these conditions was 0.65 to 0.75 V. This value is much more negative than the potential value in 1N NaOH solution and carbon steel had a passive state at this potential, which explains its corrosion stability. The corrosion stability of stainless steel in concentrated alkali solutions is quite low, even in the absence of attack by external anodic currents. The stability decreases still further under the action of both low and high external anodic currents at which steel undergoes corrosion at the potentials of either transpassivation or of oxygen evolution. At the same time, nickel maintains its corrosion stability in a large range of potentials and current densities, owing to its stable passive state.