Influence of tannin extracts on the corrosion resistance of 20 steel in tap water and in 5% HCl
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INFLUENCE OF TANNIN EXTRACTS ON THE CORROSION RESISTANCE OF 20 STEEL IN TAP WATER AND IN 5% HCl Z. V. Slobodyan,1 R. B. Kupovych,1 and L. A. Mahlatyuk 1, 2
UDC 620.197.3
We study the influence of tannin extracts obtained from natural raw materials on the corrosion resistance of 20 steel. Keywords: tannins, corrosion rate, depth corrosion index, protective action.
Tannins are esters or compounds of nonetheric type containing phenol residues known as passivating inhibitors of iron and its alloys [1]. A group formed by two or more molecules of gallic acid is typical of esteric tannins. For nonetheric tannins, the same role is played by phenol residues connected by – C – C – or – C – O – C – bonds, i.e., by the so-called catechines [2]. The ability of tannins to form stable tannate complexes with corrosion products on the surfaces of ferrous metals is used as basic for the preliminary treatment of metallic surfaces prior to the application of lacquer coatings [2]. We can also mention the ability of tannin solutions to partially dissolve the deposits of hardness salts. Thus, it is of significant interest to analyze the possibility of application of tannins in removing the deposits of hardness salts and corrosion products from the heat-exchange surfaces of boilers. However, the influence of the nature and concentration of tannins on the rate of corrosion of mediumcarbon steels is investigated quite poorly. The aim of the present work is to obtain natural tannins from the tea leaves and oak bark and study the influence of these extracts on the corrosion resistance of 20 steel in tap water and in 5% HCl. Materials and Procedure A mixture of tannins was extracted from tea leaves Tn. (1) and oak bark Tn. (2) at 100°C by continuous stirring for 4 h. Then the mixture was settled, squeezed, and filtered [3]. The pH values were measured with an I-160 ion meter. The corrosion rate was found by the gravimetric method [4] on spherical specimens of 20 steel with a diameter of 2 cm both in concentrated and diluted (1 : 1, 1 : 2, and 1 : 3) solutions of tannins. In 5% HCl, we used tannins diluted 1 : 2. The electrode potentials were measured by using an IPC-Pro potentiostat with silver–silver-chloride reference electrode. Results and Discussion The Tn. (1) and Tn. (2) aqueous tannin extracts are subacid. The pH values are equal to 4.917 for Tn. (1) and 4.825 for Tn. (2) as compared with 7.370 for tap water. The stationary potential of steel in tannin extracts shifts in the negative direction by – 647 and – 652 mV for concentrated Tn. (1) and Tn. (2) and by – 659 and – 650 mV for the diluted Tn. (1) and Tn. (2) solutions. For the tap water, this shift is equal to – 563 mV. 1 Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. 2 Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 45, No. 4, pp. 120–121, July–August, 2009. Original article submitted August 9, 2008. 1068–820X/09/4504–0609
© 2009
Springer Science+Business Media, Inc.
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