Tetrathiotungstate and tetrathiomolybdate compounds as corrosion inhibitors for protection of Al-Cu alloy

Abstract

The inhibition of corrosion on an Al-Cu alloy (2014-T6) in sodium chloride solution by compounds containing either tetrathiomolybdate or tetrathiotungstate anions has been investigated. Total immersion tests conducted in quiescent 1, 10, 100, and 600 mmol L⁻¹ sodium chloride solutions have shown that, at a 2 mmol L⁻¹ concentration, the soluble ammonium and tetramethylammonium tetrathiotungstate compounds prevented the corrosion of the Al-Cu alloy in each of the chloride solutions, giving inhibitor efficiencies of 99% and above, comparable with levels obtained for chromates. The sparingly soluble compound methyltriphenylphosphonium tetrathiotungstate also inhibited corrosion in each of the chloride solutions when present at its maximum concentration of 0·62 mmol L⁻¹. In contrast, the ammonium tetrathiomolybdate compound at a concentration of 2 mmol L⁻¹ was found only to provide inhibitor efficiencies exceeding 90% in the dilute chloride solutions and appeared to have little effect in the 100 and 600 mmol L⁻¹ sodium chloride solutions. The minimum concentrations of the tetrathiotungstate inhibitors required to prevent corrosion of the Al-Cu alloy in 600 mmol L⁻¹ sodium chloride solution were found to be 0·6 mmol L⁻¹ for the methyltriphenylphosphonium salt and 0·20 mmol L⁻¹ for both the ammonium and tetramethylammonium salts. The results of electrochemical measurements support a mechanism by which the tetrathiotungstate species inhibits the cathodic corrosion reduction reactions, possibly by the formation of an absorbed or precipitated layer at the cathodic sites on the alloy surface.