Influence of the acidity of the medium and the activity of chloride ions on kinetics of partial electrode reactions on steel with a superhydrophobic surface in chloride media

• E.Yu. Shel¹, V.I. Vigdorovich^1,2, L.E. Tsygankova³ and M.V. Vigdorowitsch⁴

¹ Tambov State Technical University, ul. Sovetskaya, 106, Tambov, 392000, Russian Federation
² All-Russian Scientific Research Institute of Use of Machinery and Oil Products, Novo-Rubezhnyi per., 28, Tambov, 392022, 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: The electrochemical behavior of carbon steel with a superhydrophobic surface in weakly acid solutions with a constant ionic strength equal to 1 and a complex composition of the electrolyte x M HCl + (1–x) M NaCl with x = 0.0005–0.01 mol/L has been studied. The effect of the concentration (activity) of Cl^– anions was evaluated in NaCl solutions with variable concentrations in the range 0.0005–0.01 mol/L of the salt. To obtain a superhydrophobic coating, the metal surface was textured by IR laser radiation of nanosecond duration followed by chemisorption of MAF (methoxy-{3- [2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy]propyl}silane from a solution of n-decane. The thickness of the superhydrophobic coating, including the textured metal layer and the adsorbed nanosize layer of the superhydrophobization agent (2–3 nm), was of the order of 100 μm. The wetting angle is 168.8±1.5°. The kinetic parameters of the hydrogen evolution reaction, accompanied by cathodic reduction of dissolved oxygen, and anodic ionization of the metal as a function of the acidity of the medium were obtained. With increasing duration of exposure, corrosion potential (E_cor) shifts to the region of more negative potentials and the most negative E_cor is observed at minimal HCl concentration. The steady state is achieved only after 120 hours and the corrosion potentials of the steel that corrodes in the active state assumes very negative values (–0.550 to –0.580 V) due to inhibition of the cathodic reaction. In media containing 0.0005–0.01 mol/L sodium chloride, the relationship between the kinetic parameters of the same electrode reactions and the activity of chloride ions (a_Cl-) has been studied. The anodic reaction is accelerated and the corrosion rate of the steel increases with an increase in the activity of chloride ions (1.0≤d lg i_a/d lg a_Cl-≤2, 1.0≤d lg i_cor/d lg a_Cl-≤1.3). This may be a consequence of the participation of halogen anions in the anodic process.

Keywords: superhydrophobic coating, carbon steel, kinetic parameters, hydrogen evolution, anodic ionization, corrosion rate, ionic strength, activity

Int. J. Corros. Scale Inhib., 2018, 7, no. 4, 528-541 PDF (829 K)
doi: 10.17675/2305-6894-2018-7-4-4

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