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

• N.P. Andreeva, Yu.I. Kuznetsov, M.O. Agafonkina and I.A. Kuznetsov

Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31-4, Leninsky prospect, 119071 Moscow, Russia

Abstract: The adsorption of sodium malonate and its alkyl-substituted derivatives on zinc from a borate buffer solution with pH 7.4 (BBS) and their protective effect on the oxidized surface in an aqueous solution of sodium chloride with its concentration С_NaCl=0.01 mol/L was studied. It was shown that the adsorption of these compounds on zinc at a potential of E=0.2 V is described by the full Temkin isotherm. The highest standard free energy of adsorption
(-ΔG⁰_a,max)=76.7 kJ/mol is possessed by sodium nonylmalonate anions, which is almost 2 times higher than for the anions of malonate itself (38.7 kJ/mol). Such high values of (-ΔG⁰_a) indicate the chemisorption interaction of these organic anions with the oxidized zinc surface. Corrosion tests of zinc plates in a chloride solution with the addition of a three-component mixture of sodium malonate + ethylmalonate with 2-mercaptobenzimidazole (2-MBT) yielded excellent results: at Cin=0.004 mol/L, complete protection of zinc was achieved under conditions of natural aeration of 0.01 mol/L NaCl aqueous solution for 7 days. The thickness of films, formed from inhibited solutions, was calculated using the ellipsometry. The thickness of the adsorption films of the inhibiting mixture {[malonate + ethylmalonate (4:1)] + 2-MBT} (9:1) formed on the zinc surface over 30 min in 0.05 mmol/L chloride-containing inhibited BBS was 90 Å, and after 20 hours already 500 Å.

Keywords: corrosion inhibitors, neutral borate buffer, sodium salts of alkylmalonic acids, adsorption, zinc, passivity, local depassivation potential, degree of protection, Temkin isotherm, ellipsometry

Int. J. Corros. Scale Inhib., 2025, 14, no. 4, 2083-2098
doi: 10.17675/2305-6894-2025-14-4-18

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