Application of pure oxygen for the passivation of mild steel in high temperature and pressure Bayer liquor under conditions of disturbed flow

The passivation characteristics of sulfuric acid cleaned mild steel in spent Bayer liquor (pH 14.4 and 160 °C) are examined using a high pressure nickel flow loop. An entrained atmosphere of 99.90% v/v oxygen gas is incorporated as an experimental variable along with Reynolds number (141,700 and 50,950) and intensity of fluid flow disturbance. State of passivation has been defined using criteria derived from transient polarisation resistance measurements and large-scale polarisation, linear sweep voltammetry. In the majority of cases, oxygenation introduces instantaneous passivation of the mild steel on contact with the Bayer liquor. In comparison to de-oxygenated and aerated electrolytes, this rapid rate of passivation can lead to up to an order of magnitude reduction in the quantity of charge associated with metal dissolution over 20 h. Although relative rates of corrosion when passive are low and largely independent of the level of flow disturbance, dissolution rates when passive are somewhat larger at the higher Reynolds number.