Roles of Chloride Ions in the Formation of Corrosion Protective Films on Copper

Abstract

Unambiguous evidence is presented that the chloride ions play a dual role in the formation of a micrometre thick film of polymerized [Cu-Cl-MBI] n . This occurs when the copper is exposed to 3 wt.% NaCl solution containing 1 mM of mixture of inhibitors 2-mercaptobenzimidazole, MBI, and octylphosphonic acid, OPA, in the molar ratio MBI:OPA of 9:1. The chloride ions act simultaneously as a promoter of polymerized [Cu–MBI] n /[Cu–Cl–MBI] n film formation and a reactant that is incorporated in the film, as confirmed by time-of-flight secondary ion mass spectrometry. Also, formation of a Cu2O film under the Cu-inhibitor film was proven by focused ion beam microscopy, with chemical analysis being employed at the cross-section of the thick polymerized film. The Cu(I) oxide underlayer, together with the porous straw-like morphology of the [Cu–Cl–MBI] n overlayer, is believed to be responsible for the excellent corrosion protection of copper, even in a chloride environment without the reservoir of MBI+OPA. We also report a new insight into the mechanism of degradation of the Cu–MBI/Cu–Cl–MBI film that results in the formation of (MBI)2 dimers. The inhibitor layer, formed in NaCl solution and containing the synergistic combination of MBI and OPA, showed outstanding resistance to degradation.