Protection of copper against corrosion in neutral solutions by salts of 2-alkylmalonic acids

The electrochemical and corrosion behavior of copper in aqueous solutions of sodium salts of alkylmalonic acids with alkyl lengths of n=0, 2, 4, 7 and 9 was studied by ellipsometry, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and corrosion tests. Addition of alkylmalonic acid salts at a concentration of C-inh= 0.002 mol/L to borate buffer solution (pH 7.4) containing 0.01 mol/L NaCl slows down the anodic dissolution of copper, increases its local depassivation potential and inhibits the cathodic oxygen reduction. The greater the alkyl length of the inhibitor, the more expressed these effects are. It has been shown that the adsorption strength of alkylmalonate increases with increasing alkyl length and is adequately described by the full Temkin isotherm equation. The standard free energy of adsorption delta(0)(Gads) of these anions on the oxidized copper surface at E = 0.0 V is 47.7 kJ/mol for malonic acid and 83.9 kJ/mol for nonylmalonic acid, which suggests a chemical nature of adsorption. EIS measurements of copper in 0.01 mol/L NaCl solution without and with addition of 0.002 mol/L sodium nonylmalonate showed that after 20 h of tests the protection degree (Z, %) calculated from the values of polarization resistances reaches 99%. Seven-day corrosion tests of copper in 0.01 mol/L NaCl solution performed in the presence of alkylmalonic acid salts with n= 0, 2, 4, 7 and 9 have shown that the protective effect increases both with increasing C-inh and with increasing alkyl length. In the C-inh range of 0.5-3 mmol/L the degree of copper protection by malonic acid anion increases from 26 to 76% and by nonylmalonic acid from 66 to 95% which confirms the highest efficiency of sodium nonylmalonate among the studied dicarboxylates at copper corrosion inhibition.