Corrosion characteristics of beta prime in nickel aluminum bronze

Nickel aluminum bronze (NAB) is a copper-based alloy used in marine turbines and propellers. Understanding corrosion behavior is important in prevention and maintenance. The alloy contains the primary alpha phase, four kinds of kappa precipitates, and the retained beta phase (beta '). The latter exists due to incomplete transformation from high temperatures. These phases have different chemical compositions resulting in various rates of corrosion attack. In marine or NaCl solutions, it has been observed that corrosion occurs in alpha adjacent to kappa phases. However, this study presents strong evidence that the corrosion of NAB in a 3.5 wt% NaCl solution is initiated in the beta ' Al-rich phase, which reveals numerous nanometer-size particles distributed throughout the beta ' area. The crystallography of the beta ' region was characterized using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and scanning transmission electron microscope (STEM). Energy dispersive x-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) was used to determine the compositions of the nanoparticles, which were found to be based on (Fe,Ni)Al. It is suggested that the preferential corrosion of the beta ' phase could be accelerated by micro-galvanic cells among these nanoparticles and the beta ' matrix. Therefore, corrosion could be alleviated by eliminating the beta ' phase from the alloy. In terms of processing conditions, it is recommended to choose a heat treatment at a high enough temperature to suppress the formation of the beta ' phase.