Effects of Sulfate and Hydrogen Carbonate Ions on Pitting Corrosion of Copper in the Presence of Scale Dispersants and Corrosion Inhibitors

We focused on phosphonic acid (HEDP) as a scale dispersant and benzotriazole (BTA) as a copper corrosion inhibitor to prevent carbon film-dependent pitting corrosion of copper pipes used in air conditioning systems. In the presence of HEDP and BTA, we also found that silicate SiO44−, calcium Ca2+, and bicarbonate HCO3− ions were effective in inhibiting copper corrosion, while chloride Cl− and sulfate SO42− ions have been reported to be involved in the promotion of copper corrosion as corrosive anions. Here, we investigated the effects of the corrosive anions SO42− and HCO3−, which are effective as corrosion inhibitors, in the presence of HEDP, BTA, SiO44−, Ca2+, and Cl− on copper pitting corrosion. Natural electrode potential measurements demonstrated decreases in the potential at stability and the amount of pitting corrosion with increasing SO42− concentration. When HCO3− was added to the system, the potential at stability became lower than when only the SO42− concentration was varied, and the amount of pitting corrosion decreased. In anodic polarization curve measurements, the potential range during a gradual increase in current density was wider when SO42− and HCO3− concentrations were varied than when only the SO42− concentration was varied. In addition, the range of potentials at the time of a gradual increase in the current density was wider, and there was a trend toward a more stable increase in current density value. These results suggested the involvement of SO42− in the progression of corrosion of copper in the presence of HEDP and BTA, while HCO3− had an inhibitory effect.