Influence of Aeration on the Passivation Characteristics of AS/NZS 3679.1-300 Mild Steel in High-Temperature Bayer Liquor Under Conditions of Disturbed Flow

The flow-enhanced corrosion of mild steel has been examined in high-temperature spent Bayer liquor as a function of Reynolds number, fluid flow disturbance, and electrolyte aeration. The electrochemical characteristics of the steel in this electrolyte was derived using a high-pressure nickel flow loop at 160°C (total gauge pressure = 0.7 MPa to 0.9 MPa). The influence of both a sudden pipe expansion and a pipe contraction on both corrosion rate and polarization behavior was quantified. Considering the mechanism of the corrosion reaction, electrolyte mass transfer did not directly control the electrochemical rates of either the cathodic or anodic currents at the corrosion potential. It is concluded that chemical modification and/or dissolution/erosion of passivating layers may have a strong influence over the metal loss of the steel observed in some alumina plants. Aeration of the liquor was found to be a significant factor regarding passivation rates and the corrosion characteristics of acid-treated steel, and it caused an overall increase in the rate of active and passive steel dissolution relative to the deoxygenated state for all but one electrode. However, the time to passivation was relatively low at high fluid velocities (Re ≅ 141,500), which led to a 27% to 50% reduction in metal loss over a caustic exposure time of 19 h. At low velocities (Re ≅ 50,900), time to passivation in the aerated liquor was also lowered, but not to the same order as that observed in the high-velocity area. Therefore, as a result of elevated reaction rates in the presence of oxygen, total corrosive losses over this short operating period were up to 37%, higher than that of the deoxygenated electrolyte. The results were essentially independent of the level of flow disturbance.