Some aspects of kinetics and mechanism of corrosion and passivation of metals in aqueous media

This thesis deals with the kinetics and mechanism of corrosion of zinc in concentrated sodium hydroxide solution and iron in phosphoric acid; the kinetics and mechanism of passivation of zinc in dilute NaoH solution and steel in sodium phosphate solutions. Corrosion and passivation of zinc: Kinetics and mechanism of corrosion of zinc in 1 N to 10 N Naoh solutions were studied by steady state and transient measurements. Both current step and potential step methods were used for transient measurements. The individual electrode reactions giving rise to the observed corrosion were studied in detail for transfer coefficients, reaction orders with respect to Na+ ion OH- ion, zinc(OH)4-2 ions and the rate determining steps identified. Using these equations relating icorr and Ecorr to solution parameters were derived and compared with the experimental dependence. The rate determining step for the anodic reaction obtained from the steady state measurements and this is explained in terms of mechanism of the anodic reaction. The kinetics and mechanism of passivation of zinc in dilute NaoH solutions was studied. The potential step method has been used to detect the number of minelayers formed on solid zinc before passivation. The analysis of the first peak shows two dimensional formation of passivation film with instantaneous nucleation. Dependence of the rate constant for the film growth on OH- ion concentration and potential shows that the rds is the discharge of OH- ions. Corrosion and passivation of iron and steel: Studies on the anodic and cathodic reactions involved in the corrosion of iron in H3PO4 which shows that the rate determining steps for the two reactions are similar to those reported for corrosion of iron in H2SO4. However the dependence of icorr and Ecorr on time and solution parameter shows that an additional factor in the corrosion of iron in H3PO4 Is the slow coverage of the surface with phosphate resulting in the decrease of corrosion rate with time. During the passivation of steel in NaH2PO4¬ solutions in the pH range of 3 to 5, three current peaks were observed. These are identified as peaks due to growth of phosphate film on the iron surface resulting in passivation of the metal. It is shown that in this case also film growth takes place in two dimensions with instantaneous nucleation. The dependence of the rate constant for film growth at pH, phosphate ion and ferrous ion concentration shows that the rds is Fe2+ surface Fe2+ lattice