Study of corrosion behaviour of ductile iron by electrochemical polarisation techniques

The use of electrochemical methods to characterize materials for their corrosion behaviour is well known. Both linear polarisation and Tafel extrapolation techniques are made use of to study the corrosion behaviour of ductile iron (DI) in natural sea water, 5 v/o H2SO4 and 5 w/o NaOH. Samples were tested in the as cast, annealed and cold worked conditions at 300 K, 325 K and 340 K. It was found that annealed material had the least corrosion rate. This is explained on the basis of the proportion of phases in the microstructure, the Df has on annealing. On annealing as cast DI, certain amount of cementite in the pearlite is converted into ferrite. Since: a single phase material is always more corrosion resistant than a multiphase one, ferrite phase increases corrosion resistance. High degree of cold working is detrimental with respect to corrosion resistance as it increases the stored energy and dislocation density in the material thereby enhancing the kinetics of corrosion reactions. The activation energy for DI was found to be lowest in sulphuric acid medium and highest in sodium hydroxide medium. The corrosion rates for DI in H2SO4 medium were the highest while in NaOH they were the lowest irrespective of material condition and technique of rate measurement.