Long-Term Ennoblement Studies on Ni-Cr-Mo Alloys

Abstract The present work demonstrates that seawater corrosion potential ennoblement can persist during long-term immersion exposures for several corrosion resistant Ni-Cr-Mo alloys. The present study considers the implications of long-term and short-term seawater ennoblement on Alloy 625 (UNS N06625), Alloy 276 (UNS N10276), Alloy 59 (UNS N06059), Alloy 2000 (UNS N06200), and Alloy 686 (UNS N06686). When compared to freshly exposed specimens not influenced by ennoblement biofilms, seawater ennoblement of Ni-Cr-Mo alloys are shown to increase the seawater cathodic current capacity of such alloys at potentials more positive than –400mV, but show a diminshed oxygen reduction capacity at potentials below this threshold. The work also explores the ability of seawater ennoblement biofilms to respond to changing cathodic demand. The degree of cathodic current adaptation is lower for mature ennoblement biofilms than for ennoblement biofilms formed on freshly exposed specimens, although the increase in cathodic current capacity – the cathodic current density available to drive galvanic corrosion – is not insignificant for long-term specimens.