Using data of weight tests and impedance spectroscopy to evaluate the protective effectiveness of zinc-rich oil coatings in corrosion of carbon steel

• V.I. Vigdorovich^1,2, L.E. Tsygankova³, N.V. Shel², A.O. Golovchenko¹, V.V. Ostrikov¹ and M.V. Vigdorowitsch⁴

¹ All-Russian Scientific Research Institute of Use of Machinery and Oil Products, Novo-Rubezhnyi per., 28, Tambov, 392022, Russian Federation
² Tambov State Technical University, ul. Sovetskaya, 106, Tambov, 392000, Russian Federation
³ Derzhavin State University, ul. Internatsyonalnaya, 33, Tambov, 392000, Russian Federation
⁴ Angara GmbH, In der Steele 2, D-40599, Düsseldorf, Germany

Abstract: Studies have been carried out in a 3% aqueous solution of NaCl, acidified in some cases to pH 3 or 4. The protective efficacy of zinc-rich coatings based on waste motor oil has also been studied in a thermal moisture chamber and natural conditions. Partial rates of oxidation of zinc from the coating and iron are estimated on the base of chemical analysis of saline solution for Zn²⁺ and Fe²⁺ + Fe³⁺ ions. The protective effect of zinc as a function of its concentration in the coating, which varies with time as a result of its anodic dissolution, is estimated. The effect of oil-zinc-containing (40, 50 and 60 wt.% Zn) coatings on the charge transfer resistance in partial electrode reactions occurring during steel corrosion has been investigated by impedance spectroscopy. It is shown that the protective effect (Z, %) of the oil compositions reaches 88, 86 and 90% for corrosion tests in the chloride salt solution, thermal moisture chamber and under natural conditions, respectively. However, in 3% NaCl solution it sharply decreases after a zinc concentration decrease in the oil film to 16–20%. The charge transfer resistance in the cathodic reaction (R_c) of unprotected steel is 45 Оhm·cm² immediately after immersion in a solution with pH 6.5 and decreases to 33 Оhm·cm² after 24 h exposure. When the pH of the corrosive medium drops to 3.0, the picture is analogous. At the initial stage, R_c equals 40 Ohm·cm2 and decreases to 24 Ohm·cm2 after 24 hours (in situ). The magnitude of the charge transfer resistance in the anodic reaction is 20–470 times greater than R_c under the indicated conditions. The presence of 60% zinc in the surface oil coating has a relatively weak effect on the R_c value and increases R_a 45–50 fold under comparable conditions (pH = 3.0, 24 h exposure). Corrosion of steel begins on the tenth day after immersion in a solution. At that the initial concentration of Zn (C_initial = 60 wt.%) decreases twofold and steel corrosion rate increases with a further decrease in the zinc concentration in the oil coating.

Keywords: steel, corrosion, saline solution, thermal moisture chamber, natural conditions, charge transfer resistance, anode process, cathodic reaction, protective effect

Int. J. Corros. Scale Inhib., 2019, 8, no. 2, 212-224 PDF (864 K)
doi: 10.17675/2305-6894-2019-8-2-4

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