Cast nickel-aluminium bronze (UNS C95800), CuAl9Fe5Ni4Mn, is extensively used in seawater environments because of its castability, high toughness and erosion-corrosion resistance. This investigation has for the first time studied the evolution of nickel-aluminium bronze pitting corrosion during prolonged exposure to natural (estuarial) seawater. The corrosion performance has been linked to the complex nickel-aluminium bronze microstructure, principally the influence of the a+Kiii eutectoid on the initiation of selective phase corrosion. Initially, a protective oxide film forms as the copper-rich a-phase corrosion penetrates further into the microstructure leaving the unattacked k-phases to create an adherent skeletal lattice. At localised sites selective phase corrosion can also occur and due to the continuous nature of the Kiii-phase develops into a 'pitting' type phenomenon after prolonged exposures. The pitting mechanism is characterised by very wide although relatively shallow corrosion features. This mechanistic understanding provides further insights into the development of pitting corrosion experienced by NAB after long-term exposures to natural seawater for up to 3 years.
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