Environmental factors affecting the marine corrosion performance of nickel aluminium bronze
Nickel-aluminium bronze (NAB) alloys are extensively used in seawater environments because they have good castability, toughness and erosion-corrosion performance. However, NAB alloys may encounter corrosion related problems under in-service conditions, i.e. different local environments, leading to variability in corrosion performance worldwide. Importantly, NAB alloys have complex microstructures consisting of up to 6 different phases and as a consequence they are susceptible to selective phase corrosion. The current study has primarily concentrated on the corrosion performance of cast NAB, both uncoupled and galvanically coupled, in a marine environment, namely the natural seawater at the National Oceanography Centre Southampton. With particular emphasis given to the galvanic compatibility of NAB (naval specification NES 747 Part 2), the seasonality of environmental factors such as biofouling, plus an in-depth laboratory investigation was made into the erosion-corrosion performance of cast NAB using a test slurry containing 3.5% NaCl solution and sand. A study of the corrosion performance of uncoupled NAB revealed that initially corrosion was confined mainly to the eutectoid regions with slight attack of the copper-rich -phase within the +III eutectoid. However, for prolonged exposures an adherent corrosion film formed as corrosion of the copper-rich -phase penetrated further into the NAB microstructure leaving the unattacked -phases to create an adherent skeletal lattice primarily due to the continuous nature of the III-phase. In addition, prolonged exposures resulted in selective phase corrosion which was predominately associated with an attack of the III–phase, which may also develop into pit initiation involving the formation of solid CuCl in an otherwise protective oxide film. The galvanically coupled NAB, with either itself, a wrought high strength copper-nickel alloy or a commercially pure titanium (Grade 2) showed accelerated galvanic corrosion within the first year of exposure to natural seawater which diminished in the following years. This behaviour coincided with the seasonal biofilm activity and increased likelihood of biofilm formation and kinetics. The role of bacterial metabolites/enzymes in the biofilm has been associated with an initial modification of the cathodic oxygen reduction kinetics, thus increasing the corrosion activity during the first biofilm season after exposure. Mass transfer kinetics for the anodic and cathodic reactions, as a function of flow rate, were studied in a wall-jet electrode for freshly polished pure copper and NAB. The diffusion coefficient of oxygen during cathodic reduction increases with increasing concentration oxygen. The cathodic reaction is the rate controlling step in the corrosion process. The high mass transfer coefficient for the oxygen has been attributed to the higher turbulence intensity within the wall-jet cell compared with either rotating disc or cylinder electrodes. Under flow corrosion, erosion and erosion-corrosion conditions (jet impingement) the cast NAB demonstrated superior performance than HVOF NAB coatings. The flow corrosion rates for cast NAB and HVOF NAB coating was found to be 0.5-0.8 and 0.8-1.5 mm y-1, respectively. Compared with the cast NAB, the porous nature of the HVOF coating and the presence of coating impurities undermined the corrosion performance. Likewise, the erosion behaviour of HVOF NAB coating was due to the high flaw density and splat boundaries, which act as crack initiators. The high kinetic energy exponent in HVOF NAB coating suggested a combination of both ductile and brittle erosion mechanisms possibly due to the porosity. In contrast, the cast NAB resulted in an energy exponent close to unity consistent with ductile materials. A SEM study revealed the cast NAB had undergone plastic deformation at the maximum erosion depth while microcutting was seen outside the centre of erosion scar. The synergistic effect based on the gravimetric, electrochemical and surface hardness measurements are also presented. The negative synergy in general infer to the good properties of the material.