The electrochemical corrosion and passivation behaviour of Monel (400) in concentrated acids and their mixtures

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The electrochemical polarisation behviour of Monel (400) in different compositions of binary and ternary solution mixtures of concentrated phosphoric, sulphuric, formic and acetic acids has been studied by potentiostatic polarisation technique at 25°C. The cathodic polarisation curves are almost linear and similar in all the binary and ternary solution mixtures and cathodic reaction is hydrogen evolution. The anodic polarisation curves reveal active, passive and transpassive behaviour in all the solution mixtures. The magnitude of the critical current density (ic) and passivity current density (ip) is higher in binary solution mixtures of phosphoric-formic acid than binary solution mixtures of either phosphoric-acetic acid or phosphoric-sulphuric acid. In the ternary solution mixtures of phosphoric-sulphuric-formic acids and phosphoric-sulphuric-acetic acids, the passivity is improved in comparison to their binary solution mixtures. Addition of sulphuric acid is found to improve the passivity in case of ternary solution mixtures. The effect of temperature on polarisation behvaiour of Monel (400) in a selected composition of solution mixture (60% H3PO4 + 4% H2SO4 + 36% HCOOH) has been studied at 25, 37, 45 and 56°C. Higher temperature increased the corrosion and enhanced the surface reflection. The stability of the passive film formed on the surface of the alloy has been examined by current decay method. Examination of the surface of working electrode, after polarisation studies was done using scanning electron microscopy (SEM) and atomic force microscopy (AFM) in contact mode.

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