Evaluation of Long Term Seawater Exposure Corrosion Specimens and Electrochemical Studies of Nickel Aluminum Bronze Alloys

Cast nickel aluminum bronzes (NAB) are widely used in submarine seawater handling systems. However, they are susceptible to corrosion, especially selective phase corrosion (SPC), at welds and under marine growth. This leads to high repair or replacement costs. A first step in reducing these costs is to understand the effect of local environmental factors. In order to assess local environmental factors affecting the corrosion performance of NAB alloys, a long-term seawater exposure experiment was conducted, in which four sets of NAB castings, including welded specimens, were immersed in natural seawater at Bedford Basin, Nova Scotia for 7, 13 and 39 months. Corrosion performance was evaluated through visual inspection, weight loss and cross-section metallographic examination. In addition, electrochemical techniques were used to study the effect of decaying biological matter on the corrosion behavior of NAB castings.This paper presents the analysis results of the long-term exposure experiments including general corrosion and SPC in the weld heat-affected zone and under crevices formed by nylon nuts and bolts, along with local environmental conditions (dissolved oxygen, temperature, and salinity). The electrochemical study on the effect of decaying biological matter on the corrosion behavior of NAB alloys is also presented and discussed.