Early stages of copper protection in chloride solution containing ferrous sulphate

The films formed on Cu surfaces in 4% NaCl with 10 ppm FeSO4 solution for short immersion times of up to 4 h were analysed using X-ray photoelectron spectroscopy and Auger electron spectroscopy. Iron was detected on the copper surface after immersion for 30 min. The presence of Fe(OH)2 and α-FeOOH species on the surface supports the proposal that Fe2+ ions are transported to the cathodic sites to form Fe(OH)2 which is subsequently oxidised to α-FeOOH (lepidocrocite) by dissolved oxygen in solution. The presence of a peak corresponding to the approximate empirical formula HCuFe2O10 suggests that the iron is incorporated into Cu2O which is formed at the moment of immersion. Both the Fe/Cu ratio on the surface and the film thickness increased with increasing time of immersion up to 2 h and then decreased again because it depends on the concentration of dissolved oxygen in solution and the residual amount of Fe2+ in solution.