Nickel-aluminium bronze pitting corrosion in seawater

1. I  IV II III 20 mm (a) (c) (b) Research impact The results and conclusions derived from the collaboration has been passed to many MOD Integrated Project Teams (IPTs) where NAB issues constitute a major, and costly, area of concern. Knowledge gained from biofouling interactions are also currently being promulgated and exploited through existing corrosion work groups. Environment and Mitigation Nickel-aluminium bronze pitting corrosion in seawater R.C. Barik1,3, J.A. Wharton1, R.J.K. Wood1, K.R. Stokes1,2 1 national Centre for Advanced Tribology at Southampton (nCATS), School of Engineering Sciences, University of Southampton, Highfield, SO17 1BJ, UK. 2 Physical Sciences Department (Dstl), Dstl Porton Down, Salisbury, Wiltshire, SP4 0JQ, UK. 3 Current address: School of Engineering and Electronics, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JL, UK. Introduction and background Nickel-aluminium bronze (NAB) is widely used in marine applications because of its high toughness and erosion-corrosion resistance • NAB is used for high performance propellers and seawater handling systems – seawater valves and heat exchangers. • Pitting and crevice corrosion for copper-based alloys is often attributed to a metal-ion concentration cell. • Areas exposed to high copper-ion concentrations are considered to act as cathodic sites (with copper deposition sometimes observed). NAB may encounter corrosion related problems under in-service conditions: • NAB can encounter variability in corrosion performance worldwide, i.e. different local environments. • NAB components in naval vessels can be affected by their operational cycles (open / dock type seawater) – long periods in the dock compared with commercial vessels. • Corrosion problems can lead to expensive repairs to NAB propellers and seawater intakes. • Propeller replacement costs can be £0.25 million (€0.31 million) and dry dock costs are £20k / day (€25k / day). • NAB is known to be susceptible to localised corrosion, e.g.: • Crevice corrosion rates as high as 0.7 to 1.0 mm.y–1 have been reported - compared with 0.25 mm.y–1 for type 304stainless steel. • NAB is prone to selective phase corrosion (SPC). • SPC may initiate pitting corrosion. Test material Casting NAB (CuAl9Ni5Fe4Mn – NES 747 Part 2 spec.) used for naval applications is typically given a heat treatment at 675 C for 2 to 6 hours. NAB is susceptible to selective phase corrosion due to its complex microstructure: Results and discussion The SEM study in Fig. 2 shows the NAB surface corrosion was initially confined to the eutectoid regions with slight attack of the copper rich -phase within the +III eutectoid. While the eutectoid -phase was preferentially attacked the -grains show very little attack, this is a form of selective phase corrosion. The accumulation of Cu2O deposits at these locations will limit the diffusion (mass transport of species including: copper-ions, chloride and dissolved oxygen) towards and away from the NAB surface, thus there is the potential for a microenvironment to develop beneath the deposit. If the pH of this microenvironment becomes acidic, e.g. below pH 4.0, the III-phase becomes anodic to the -phase and corrodes preferentially. • Copper-rich a-phase. • Plus, b-phase (retained martensite) and a series of k-phases based on Fe3Al and NiAl (with one exception - I may be based on Fe3Al or FeAl). SPC sites Pit site Fig. 1: (Left) NAB phase micro- structure and (Above) after 15-months seawater immersion. Fig. 2: Backscattered SEM micrographs of the NAB exposed to estuarial seawater: (a) initial attack at the -phase within the +III eutectoid, (b) development of protective oxide film (uniform corrosion) and (c) selective phase corrosion and the accumulation of corrosion products. • Conclusions • On exposure the copper-rich a-phase was initially (during the first 6 months) susceptible to corrosion leaving the unattacked k-phases to create an adherent skeletal lattice. • At localised sites, SPC occurred and due to the continuous nature of the III-phase this resulted in the accumulation of corrosion products / deposits at these locations. • The formation of a micro-environment beneath the deposit developed into a ‘pitting’ type phenomenon after prolonged exposures (within 15 months). • Overall, the pitting mechanism is characterised by very wide although relatively shallow corrosion features. Fig. 3 illustrates the accumulation of initial corrosion products from SPC acting as a site for ‘pitting’ type attack. Cu2O is porous and electrically conducting layer with the upper surface acting as a cathode and the lower surface as an anode. In contrast to stainless steels, there is no preferred cathodic reactions on the surface surrounding the corrosion activity, thus, resulting in the observed rapid peripheral growth of corrosion around the pit. Fig. 3: Schematic of NAB pitting corrosion showing the arrangement of corrosion products and the reactions involved. Worked sponsored by