Influence of heat treatment on microstructure and passivity of Cu–30Zn–1Sn alloy in buffer solution containing chloride ions

Tin as an alloying element is of great interest in brasses for dezincification impediment. In this paper, Cu–30Zn–1Sn alloy was submitted to three different heat treatments, viz. A (heating up to 800 °C for 20 h, held at 200 °C for 20 h in salt bath and air cooled), B (heating up to 800 °C for 20 h and water quenched) and C (heating up to 600 °C for 20 h and water quenched). The influence of heat treatment on microstructure was evaluated by OM and SEM–EDS analysis. The corrosion resistance in buffer solution (pH 9), H3BO3/Na2B4O7 ·10H2O, with various concentrations of chloride ions was evaluated by potentiodynamic polarization curves and compared with multi-component Pourbaix diagrams. A correlation between the heat treatment, microstructure and passivity of the heat treated samples was observed. The results indicated that all heat treatment procedures led to formation of α, and γ-Sn-rich phases as microstructure constituents with a small fraction of β′ phase in A. Sn-rich phase appears in grain boundaries and its morphology was slightly changed due to heat treatment. Beneficial influence of low concentration chloride ions on passivity was associated with the formation of copper oxides/hydroxide and chloride complexes. Deterioration was observed at concentrations higher than 0·05 M NaCl due to accelerated dissolution of copper by formation of CuCl\(₂^-\). As a result of dezincification process, preferential corrosion attack and copper redeposition on α phase (matrix) were observed. However, Sn-rich (γ1) phase in grain boundaries was not attacked due to SnO2 formation. In buffer solution, the higher passivity current density in A was related to the presence of small amount of β′ phase. On the other hand, in 1 M NaCl, lower critical current density for passivation in B and A (about two times lower than C) was attributed to the grain size effect.