Anti‐tarnish treatment of brass for coinage

Abstract The anti‐tarnish film was formed on brass for coinage by 1‐phenyl‐5‐mercaptotetrazole (PMTA), which was studied by accelerated corrosion test (metallic coatings‐thioacetamide corrosion test), polarization curves and electrochemical impedance spectroscopy (EIS) in synthetic sweat solution. The accelerated corrosion test showed that PMTA exhibited better anti‐tarnish performance on brass than that of traditional anti‐tarnish agent benzotriazole (BTA). The polarization measurements showed that PMTA could be classified as a mixed inhibitor for it could restrain both anodic and cathodic reactions. EIS measurements indicated that the inhibition efficiency of PMTA on brass was over 98.4% in synthetic sweat solution. All these results showed that PMTA was an excellent anti‐tarnish agent of the brass for coinage, the optimum treating conditions of which were 0.5 gL −1 , pH = 3, 55°C–85°C, 7 min.