銅陽極の不働態化面での硫酸銅塩析出機構

Abstract

A proposed mechanism leading to precipitation of a CuSO4salt on the surface of a passivating copper anode was discussed.A special technique has been developed to facilitate extracting an electrolyte sample from inside the anode diffusion layer at any given time of anodization, and it has been shown that an electrolyte extracted near the onset of passivity typically contains (g/l) 96 Cu, 18 Ni and 113 H2SO4. The concentration of H2SO4assayed here is represented, in the greater part, by HSO4-.In such a highly concentrated electrolyte in terms of Cu2+and HSO4-ions, the stability of hydrated Cu (H2O) 42+ions near the surface of a passivating anode may gradually decrease, and in its place the following sequential substitution reactions become more predominant;Cu (H2O) 42++HSO4-→Cu (H2O) 3 (HSO4-) ++H2O (1)Cu (H2O) 3 (HSO4-) ++HSO4-→Cu (H2O) 2 (HSO4-) 2°+H2O (2)and so on.The productant in equation (2) is presumed to be a weakly associated ionic pair, which autodecomposes when its concentration exceeds a certain critical point, likeCu (H20) 22+…(HSO4) 22+→CuSO4+H2SO4+2H2O (3)to produce CuSO4 (s), H2SO4and some water. Thewater molecules discharged through this process seem to play a key role in redissolving the precipitated CuSO4salt while Cu2+and HSO4-ions are newly supplied to this site of reaction to be ready for the subsequent process (2) to reoccur. Thus a certain cyclic relation is predicted to build up between the processes (2) and (3) at a near-saturation state of the electrolyte near the passivating anode.Electrolyte properties at a saturated state were also thoroughly reviewed to support the possibility of this proposed model.