Microstructural and corrosion studies of 9Cr-1Mo steel in acidic methanol solutions

The corrosion behaviour of 9Cr-1Mo ferritic steel has been studied in methanol containing different concentrations of sulphuric, hydrochloric acids and their mixtures using potentiostatic and potentiodynamic techniques at 30 degrees C. The influence of various heat treatments: furnace cooled (FC), air cooled (AC) and water quenched (WQ) on the electrochemical polarization behaviour has also been studied for a few selected compositions of these acids and their mixtures in methanolic solutions. The cathodic reaction is observed to be hydrogen evolution. The alloy showed active, passive and transpassive behaviour in different concentrations of sulphuric acid in methanol. The anodic polarization curves of the alloy revealed a characteristic 'cathodic loop' in methanol at higher concentrations of sulphuric acid (0.1 and 1.0 M). Stable passivity was also observed which became ustable at higher potential due to pitting. Well defined anodic Tafel region followed by a limiting current density region, in different concentrations of methanolic solution of hydrochloric acid, was observed. However, in the mixture of different concentrations of sulphuric acid and hydrochloric acids in methanol and at higher concentration of sulphuric acid (1.0 M) the alloy showed active, passive and transpassive behaviour. Such behaviour was not observed below 1.0 M concentration of either acid in methanol, rather a limiting nature was observed. It seems that in the acidic mixture, the SO42- ions act as inhibitor and decreases the adsorption of Cl- ions and at sufficiently high concentration it effectively hinders the adsorption of the Cl- ions on the surface. The anodic polarization curves for FC specimen shifted towards higher current density region and the curves for AC and WQ specimens shifted towards lower current density region in comparison to as-received one in methanolic solution of sulphuric acid. However, the anodic polarization curves for FC, AC and WQ specimen shifted towards higher. current density region as compared to as-received specimen in methanolic solution of hydrochloric acid and in the mixture of hydrochloric and sulphuric acids in methanol. Examination of the sample surface was done using scanning electron microscopy (SEM).