Corrosion of Ni-Resist cast irons in sea water

Abstract

The corrosion behaviour of Ni-Resist cast irons in sea water has been investigated at various temperatures and dissolved oxygen concentrations. The investigations involved a case study of the failure of a Ni-Resist cast discharge column of a brine recycling pump from a sea water desalination plant and laboratory tests on two Ni-Resist cast irons. Weight loss measurements, electrochemical polarisation techniques, and metallographic methods of analysis were used to determine the corrosion rates and to investigate the nature of corrosion in Ni-Resist irons. It has been established that the discharge column of the brine recycling pump failed by stress corrosion cracking (SCC). The cracks initiated at pores and the cracking pattern was characterised by the emanation of fine cracks from large cracks. The region near the weld/base metal interface wasfound to be a preferential site for the initiation and propagation of SCC owing to the presence of higher concentrations of chromium carbide. The corrosion rates of Ni-Resist type 2 (T2) iron, which has a graphite flake structure, are higher than the corresponding D2 alloy which has a graphite nodule structure. The corrosion rates of Ni-Resist irons are strongly dependent on the dissolved. oxygen content of the aqueous environment. The corrosion rates are least in deoxygenated conditions although no passive region is observed in the potentiodynamic polarisation and open circuit corrosion potential–time plots. Under fully oxygenated conditions, only a region of limited passivity is invariably observed because of the poor adherence of the oxide scales.