corrosion and microstructure evaluation of nickel-aluminum bronze alloys with varying compositions and cooling media for marine propellers

Nickel-aluminum bronze (nab) alloys are one of the primary alloys used to manufacture ship propellers due to their high strength, fracture toughness, and excellent corrosion resistance in a seawater environment. among the practical methods to enhance the surface properties of this alloy, the friction stir processing (fsp) is an optimal process. in this research, the effect of adding alloying elements and cooling medium during friction stirring processing on electrochemical properties and immersion corrosion tests was investigated. for this purpose, three alloys cu-10al, cu-10al-5ni and cu-10al-5ni-5fe were cast and friction stirring processing was carried out on them in both air and underwater conditions, and also the corrosion properties of all samples in 3.5% nacl solution discussed. by adding the nickel to cu-10al alloy, the detrimental phase to the corrosion properties in the microstructure is reduced compared to cu-10al alloy and the intermetallic compound nial was also observed in the structure. by adding the iron element to cu-10al-5ni alloy, the detrimental phase was completely eliminated from the microstructure, resulting in the formation of complex intermetallic compounds. due to dynamic recrystallization phenomena occurring in all three alloys during the friction stir process (fsp), the coarse and inhomogeneous microstructure of the cast samples was transformed into a very fine-grained, homogeneous, and equiaxed structure. furthermore, precipitates that were locally dispersed in the cast microstructure became homogeneously and randomly distributed in the stir zone (sz) after the fsp. the results of electron backscatter diffraction (ebsd) indicated the influence of the process and alloy composition on the microstructure and grain boundary density. the sz of the cu-10al-5ni-5fe alloy, which underwent submerged-fsp, exhibited a higher grain boundary density compared to the other samples, and its orientation displayed no visible texture. due to the similarity in the potentiodynamic polarization (pdp) curves, it was found that the polarization behavior of all the samples is almost the same and they have the same anodic and cathodic branches. in the cu-10al and cu-10al-5ni alloys, fsp resulted in a decrease in corrosion current density of 0.002 and 0.0003, respectively. on the other hand, in the cu-10al-5ni-5fe alloy, fsp led to an increase in corrosion current density of 0.005. fittings of the electrochemical impedance test data and nyquist curves revealed that the cu-10al-5ni-5fe cast alloy sample, with an rp value of 5121 ohms per square centimeter, exhibited the best electrochemical corrosion properties, while the cu-10al alloy sample fsped at ambient temperature, with an rp value of 846 ohms per square centimeter, demonstrated the weakest properties compared to the other samples. after conducting immersion tests, it was evident that the presence of homogeneous and fine grains significantly influenced the long-term corrosion properties of the samples. all the cast samples experienced the highest weight loss, while the samples that underwent submerged-fsp exhibited the lowest weight loss and corrosion rates. after 30 days, the submerged-fsped cu-10al-5ni-5fe sample, with a corrosion rate of 0.01 mm/year, displayed the best weight loss properties, while the as-cast cu-10al sample, with a corrosion rate of 0.06 mm/year, exhibited the weakest weight loss properties. from the investigations conducted on the corrosion product films formed on the surface all the samples, it was found that the chemical composition of these films is similar for all samples and the product films on the samples which were fsped are more effective.