Corrosion of Iron-Chromium-Nickel Alloys in Hot Concentrated Caustic Soda Solution

Abstract

The corrosion behaviour of iron-chromium-nickel alloys was studied by measurements of potentiodynamic polarization curves, the change in natural electrode potential with time, and average penetration. The corrodent used was 48%NaOH solution at temperatures between 140 and 200°C, and the following results were obtained. Anodic dissolution of iron-chromium-nickel alloys in the active state decreased with increasing nickel and chromium content. However, as the potential was raised, nickel in the alloys easily dissolved as the Ni²⁺ ion at a low potential of Eh=−0.65 V. The passivation current density did not change with the nickel content in the alloys. Addition of nickel to the alloys gave excellent inhibition effects on the dissolution of FeO₄²⁻ and CrO₄²⁻ ions at potentials more than +0.6 V. Then, the natural electrode potential of the alloys showed a noble potential with increase in nickel content, but that of the alloys containing 12% nickel remained at low corrosion potentials in the active region. Immersion tests showed that the corrosion resistance of iron-chromium alloys containing 25% chromium or less increased by the addition of nickel, but that the corrosion resistance of the alloy containing about 30% chromium decreased when nickel was added to the alloy.