Copper protection with sodium salts of lower dicarboxylic acids in neutral aqueous solution

The adsorption of sodium salts of malonic, ethylmalonic and succinic acids on copper in neutral buffer solutions with pH 7.40 and the inhibition of copper corrosion in an aqueous solution containing 0.01 M NaCl by these compounds were studied. It was shown that the highest free adsorption energy (Delta G(a, max)(0)) on pre-oxidized copper (E=0.0 V) in borate buffer is shown by succinate anions, while that on oxide-free surface (E=-0.60 V) is shown by ethylmalonate anions. The Delta G(a, max)(0) values on oxidized copper for sodium malonate, ethylmalonate and succinate are 47.7; 69.4 and 77.4 kJ/mol, respectively. Based on these values, one can assume that they are chemisorbed. Since a plateau is established on all isotherms at certain anion concentrations, it was regarded as a monolayer adsorbate coating, and the thickness of the respective monolayer was calculated from these data. Comparing this thickness with the size of the inhibitor molecules, we concluded that the anions have a planar orientation on the oxidized copper surface. On reduced copper surface, the Delta G(a, max)(0) values are noticeably smaller i.e., 38.3; 64.2 and 53.1 kJ/mol for malonate, ethylmalonate and succinate, respectively. This indicates that at least succinate and ethylmalonate anions are chemisorbed on reduced copper surface. Copper corrosion tests in water containing 10 mmol/L NaCl also showed the advantage of sodium succinate and ethylmalonate over sodium malonate. The highest degree of copper corrosion protection is provided by 3 mmol/L sodium ethylmalonate: Z = 91%, while for sodium succinate and malonate it is Z = 83 and 77%, respectively.