Effects of inorganic sulphide and ammonia on microbial corrosion behaviour of 70Cu–30Ni alloy in sea water

Abstract

A laboratory electrochemical investigation was carried out on the effects of inorganic sulphide (1 ppm) and ammonia (10 ppm), as sea water pollutants, on the corrosion of 70Cu–30Ni alloy owing to sulphate reducing bacteria (SRB). Potential–time and linear polarisation measurements revealed that the presence of SRB makes the corrosion potential more active by ∼250 mV and increases the corrosion rate by a factor of ∼6·5. The addition of inorganic sulphide to sea water inoculated with SRB caused a significant ennoblement of the corrosion potential which was associated with a decrease in the corrosion rate, while the addition of ammonia did not affect the corrosion potential although it caused a significant increase in the corrosion rate. Sulphate reducing bacteria had a pronounced effect on the potentiodynamic polarisation through shifting the corrosion potential to a more active value and eliminating the active–passive transition. However, the active–passive transition peak was retained when sulphide was added to sea water inoculated with the bacteria. In the presence of ammonia, the hysteresis loop was retained in the absence of a well defined active–passive transition. With increase in the exposure time to sea water inoculated with SRB, the impedance and phase angle peak decreased. In the presence of sulphide, the impedance increased in the high frequency region and the frequency dependence of the phase angle showed two time constants. In the presence of ammonia, the impedance behaviour experienced significant changes in terms of a decrease in impedance and a shift in the phase angle peak towards lower frequencies. Scanning electron microscopy (SEM) examinations revealed the formation of a patchy layer of bacterial biofilm and corrosion products during exposure to SRB containing sea water. The formation of this layer was associated with depletion of nickel from the alloy. In the presence of inorganic sulphide, micropits and intergranular attack were seen within crevices in an adherent corrosion product layer. In the presence of ammonia, the alloy initially suffered from shallow localised attack which gradually spread to cover the whole metal surface, revealing grain boundaries, twin boundaries, and slip steps.