Examination of the electropolishing behaviour of 73 brass in a 70% H ₃PO₄ solution using a rotating disc electrode

The electropolishing behaviour of 73 brass in a 70% H₃PO ₄ solution was studied using a rotating disc electrode (RDE). Based on the results of an anodic polarisation test, a transition peak from kinetic-controlled to diffusion-controlled dissolution was detected when a blue Cu²⁺-rich layer was developed on the RDE. The blue Cu ²⁺-rich layer impedes the anodic dissolution rate of the brass-RDE and is essential for levelling during electropolishing. Potentiostatic polishing at different locations on the limiting-current plateau was studied. By polishing at the transition-peak potential, grain boundaries were preferentially etched. In contrast, a well-polished surface with a nanosized surface roughness (R_a) was achieved after electropolishing in the middle of the limiting-current plateau. By polishing at the end of the limiting-current plateau at a relatively low rotational speed, nipple-like and flow-streak features were observed. The formation of these features is attributed to the evolution of oxygen bubbles on the RDE surface. Mechanisms for the formation of the above-mentioned features were proposed.