Influence of Sulfide Dispersed in Lead-Free Bronze Casting on Electroplating

Abstract

Recently, a newly developed lead-free bronze casting alloy was designated as CAC411 under Japanese Industrial Standards. CAC411 has a unique metallographic feature: tiny sulfide（Cu₂S and ZnS）particles are dispersed in the bronze matrix, unlike conventional copper alloys. Earlier research results demonstrated that the sulfides have different electrochemical properties from those of the bronze matrix. At or under −1.20 V vs. Ag/AgCl, Cu₂S suffered reduction and dissolution in neutral NaCl aqueous solution under cathodic polarization. The Cu₂S dissolution reaction is regarded as affecting the alloy surface morphology in cathodic treatments such as electroplating. This study was conducted to elucidate effects of the sulfide on Cu-, Ni-, and Cr-electroplatings on CAC411. Results show that Cu-plating and Ni-plating affected the sulfide dissolution only slightly. Their plating surfaces were smooth and bright. However, the Cr-plated alloy surface was fogged. The fogged surface resulted from Cr cluster deposition on the sulfides. Presumably, the sulfide dissolved under the cathodic conditions was oxidized to HSO₄^- by Cr⁶⁺. The HSO₄^- behaved as catalyst for Cr deposition. Cu-plating or Ni-plating before the Cr-plating was found to be effective for obtaining a smooth and bright Cr-plating surface on CAC411.