Anodisation of copper in thiourea-containing acid solution

Abstract

The formation of anodic films during the anodisation of copper, at different applied potentials E, in aqueous 0.5 M sulphuric acid containing different amounts of dissolved thiourea was investigated following the corroding electrode profile by on line in situ imaging. For E<0.07 V (vs. SCE) the electro-oxidation of thiourea to formamidine disulphide and the electrodissolution of copper to Cu(I)–thiourea complexes, including the formation of a polymer-like Cu(I)–thiourea complex (film I), take place. For E>0.07 V, the main reactions are the electro-decomposition of formamidine disulphide and Cu(I)–thiourea complexes yielding a copper sulphide-containing film (film II) and the electrodissolution of copper as aqueous Cu(II) ions through film II. The relative contribution of these processes depends on thiourea concentration in the solution, the applied electric potential and anodisation time. The growth kinetics of films I and II were determined from the evolution of the average film height〈h〉obtained from in situ imaging. The kinetics of film I fit a parabolic rate law, whereas those of film II approach a linear〈h〉versus anodisation time relationship. The rupture of film II assists the localised corrosion of copper. Likely physical mechanisms for the formation of these anodic films are discussed.