Evaluation of red mud as surface treatment for carbon steel prior painting

Red mud (RM) is the waste product of the Bayer process for obtaining alumina from bauxite. The alkaline nature of RM suspensions together with the presence of Fe3+ species point towards the possibility of using RM as a good corrosion inhibitor for carbon steel. Based on this idea, the possible use of RM suspensions as pre-treatment for carbon steel was studied by recording the electrode potential and the electrochemical impedance spectroscopy (EIS) evolution with immersion time. Different parameters regarding the steel surface finishing (grinding, pickling or degreasing) and RM suspension condition (stirred or steady, decanted or filtered) have been considered in the study; the passivation was obtained when ground samples were immersed in decanted RM suspensions and subjected to continuous stirring. The influence of chlorides and pH were analysed by potentiometric titration. The obtained results indicate that treated samples depassivate at lower Cl−/OH− ratio than untreated ones. Regarding the pH parameter, treated samples remain passive at lower pH values than the untreated ones. Finally, some treated and untreated samples were painted and subjected to cathodic polarisation experiments, including an artificial defect in the coating. The comparative study was done using EIS technique; the impedance diagrams would indicate an effective passivation of the steel surface for treated samples.