Passivation of Carbon Steel in a High Temperature Concentrated LiBr Solution Containing Hydroxide and Molybdate Ions

Abstract

Carbon steel has been utilized well as a structural material in a commercial adsorption refrigerating system with help of corrosion inhibitors : most typically, hydroxide and molybdate ions. In this study, X-ray photoeletron spectroscopy (XPS) analysis has been carried out on the films formed on carbon steel exposed in the 17.3 mol kg^-1 LiBr+0.1 mol kg^-1 LiOH+0.01 mol kg^-1 Na₂MoO₄ solution at 393 and 433K. Carbon steel passivates well in the solution at 433K, forming a doubly layered film. The inner layer is mainly composed of Fe₃O₄ and the outer one of MoO_x, where the value of x decreases from 3 to 2 with immersion time. The film thickness increases in proportion to the cubic root of immersion time suggesting that movement of ions through inner oxide layer is governing the film formation process. Carbon steel exposed in the solution at 393K suffers from pitting occasionally. The film formed on a specimen without pits shows incorporation of Mo species within it whereas that with pits shows no incorporation. Molybdate ion would inhibit corrosion through 1) competitive adsorption with Br^-, 2) acceleration in formation of the protective inner layer and 3) restriction for anions to permeate trough the outer layer. It can be concluded that higher temperature is advantageous for molybdate ion to work better because the chemical process for 2) proceeds more efficiently.