Corrosion inhibition of Mg90 alloy by mixtures based on sodium oleate. Influence of oxide film thickness

• V.A. Luchkina, Yu.I. Kuznetsov, D.B. Vershok, A.A. Chirkunov, N.A. Salavatov and O.V. Dement’eva

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation

Abstract: The effect of the thickness of the oxide layer formed on commercial magnesium Mg90 on the efficiency of its anticorrosive protection with sodium oleate (SOl) and formulations based on it was studied. It was shown that in the absence of treatment with the corrosion inhibitor (CI), an increase in the thickness of the oxide-hydroxide film increases the corrosion resistance of Mg90. The longest protection is provided by a film formed in 10 minutes of degreasing in an alkaline phosphate solution, which is associated with incorporation of the inorganic inhibitor into the coating. Treatment of Mg90 samples coated with different oxide-hydroxide films in aqueous solutions of the CI improves the corrosion resistance. The protection efficiency was estimated in three ways: based on anodic polarization curves recorded in a borate buffer (pH 9.2) containing 1.0 mM NaCl; from EIS results obtained in the same solution but at the free corrosion potential, and by accelerated corrosion tests at 100% relative air humidity with forced daily moisture condensation on the samples. According to the EIS data, the formulation of 12 mM SOl + 4 mM vinyltrimethoxysilane (VS) was the best mixed CI, in whose solution a one-hour exposure of Mg90 samples chemically preoxidized in 5 М NaOH provided a high degree of protection, Z=99.89%. The mixture of 12 mM IFKhAN 25F+4 mM VS was slightly less efficient (Z=99.83), but it proved to be the best СI in accelerated corrosion tests. FTIR analysis of the Mg90 surface confirmed the presence of CI components on it, and in the case of 12 mM IFKhAN 25F+4 mM VS, a band corresponding to the stretching vibrations of the Si–O–Si bonds was found in the range of 1000–1100 cm–1, indicating the formation of a network of siloxane bonds. This fact, together with the existence a band with a maximum at 3280 cm–1 due to the stretching vibrations of the silanol groups, supports our earlier assumption that an increase in the passivation efficiency upon incorporation of VS into the mixture is due to the ability of such a network to prevent the desorption of the main CI.

Keywords: magnesium, mixed corrosion inhibitors, sodium oleate, vinyltrimethoxysilane, electrochemical impedance spectroscopy, FTIR spectroscopy

Int. J. Corros. Scale Inhib., 2020, 9, no. 4, 1607-1629 PDF (1 001 K)
doi: 10.17675/2305-6894-2020-9-4-26

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