Effect of thiosulfate on the passivation of zinc-alloyed anodes, (Z32120 and Z13000) at 80°C after 288h immersion in 3.5% NaCl solution

Abstract

In this research, the inhibition of passivation on Zn alloys (Z32120 with Al) and Z13000 without Al) in artificial seawater (3.5% NaCl) mixing with different thiosulfate concentrations (100ppm, 150ppm, 200ppm) were investigated electrochemically after 288h immersion at 80°C and compared the results with that of testing without thiosulfate in the environment. According to the Potentiodynamic polarization test results, it is found that the presence of thiosulfate (S2O32-) in the solution hinders the passivation that was formed during the immersion of Zn alloys between 120h and 288h at 80°C in the absence of thiosulfate. Thiosulfate effect promotes Cl- penetration for not only hinderance of the passivation but also generating the depassivation of passive films resulting in retarding repassivation. It is confirmed by the comparison between polarization test results of testing without thiosulfate and with thiosulfate in the solution. However, although all thiosulfate concentrations used in the study impedes the passivation, the different thiosulfate concentration gives the different effectiveness depending on the immersion time and temperature. Also, Al presence in the Zn alloys supports the different effectiveness. The lower thiosulfate concentration (100ppm) exhibits the decrease of current density at longer immersion for Z32120 whilst Z13000 exhibits the decrease of the current density at higher thiosulfate concentration (200ppm). Regarding the results investigated in this study, Z32120 is effective with 150ppm and 200ppm thiosulfate meanwhile Z13000 is better result with 100ppm and 150ppm. Therefore, 150ppm is considered to be the proper and average value for both Zn alloys. The effect of thiosulfate on passivation of both Zn alloys was discussed based on immersion time, temperature, pH, and the ratio of Cl- and S2O32-.