Study on the microstructure, mechanical properties, tarnish and corrosion resistance of sterling silver alloyed with manganese

The objective of this research was to investigate the effect of manganese in as-cast sterling silver alloys used in jewellery industry on microstructure, mechanical properties, and tarnish and corrosion resistance. In order to determine the suitable manganese content, manganese in the specimens varied from 0, 0.38, 0.76, 1.3, 1.7, 2.1, 2.6 and 3.0 wt%. The microstructure consisted of both silver-rich solid solution, as a matrix, and eutectic structure. The amount of manganese examined in the eutectic region was higher than those in the matrix. By increasing manganese content, the amount of eutectic structure decreased, and therefore, tensile and yield strengths and hardness of the alloys also decreased. It was obvious that the hardness of 3.0% Mn alloys (52.1 HV) was lower than that of manganese-free alloys (66.8 HV). The tarnish resistance improved when the amount of manganese increased. The tarnish films were studied quantitatively using a spectrophotometer. For 3-h tarnishing test time, the colour difference, DE*, of the manganese-free alloy was 24.98 and higher than that of the 3.0% Mn alloy, which was 5.19. The anodic polarization curves, from which the corrosion potentials (Ecorr), the primary passive potentials (Epp), the pitting potentials (Ep) and passive current density (Ip) were evaluated, of the test alloys were measured in 1% NaCl and H2S-saturated 1% NaCl solutions. In 1% NaCl solution, increasing of manganese content not only promoted the noble shift in corrosion potentials but also reduced the average passive current density. In H2S-saturated 1% NaCl solution, the passive region was not observed and the effect of manganese in sterling silver on corrosion potential was not clear, but the corrosion current decreased with the increase of the manganese content. The tarnish and corrosion resistance of sterling silver were improved with manganese. This may be due to the decrease of eutectic structure, of which the presence of copper content was higher than that of matrix.