Corrosion Resistance of Structural Materials in Tungstate Solutions
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CORROSION RESISTANCE OF STRUCTURAL MATERIALS IN TUNGSTATE SOLUTIONS O. O. Smyrnov,1 T. E. Shepil,1 V. Yu. Kozin,1 A. O. Bezhenko,1 K. S. Rutkovska,2 and O. I. Pylypenko2,3 It is shown that the corrosion activity of tungstate solutions is caused by the presence of fluoride ions and by the total alkalinity. We determine the optimal mode of welding of carbon, chromium, and nickel-chromium steels. In the original tungstate solution, the maximum corrosion rate is recorded for specimens of VSt3sp carbon steel with welded joints. The electrochemical studies show that, in the original solution at room temperature, this type of steel corrodes in the passive state. As temperature or the concentrations of components of the solution increase to the values corresponding to the evaporated solution, the analyzed steel begins to corrode in the active state. Chromium and nickel-chromium steels (08Kh13, 08Kh22N6T, and 12Kh18N10T) preserve their passive state both in the original solution and in the evaporated solutions. The PON-A, PON-B, and PON-V paronites and TMKShch rubber are chemically stable and, therefore, suitable for the production of gaskets intended for operation in the original Na 2 WO 4 solutions. Keywords: tungsten concentrate, evaporation installation, chromium and nickel-chromium steels, corrosion resistance, polarization dependence.
Introduction In the process of high-temperature autoclave-soda processing of tungsten concentrates, it is possible to extract at least 10% of tungsten, as compared with the technology of sintering of scheelite with sodium carbonate [1–3]. In this case, the input raw material is a water suspension of crude tungsten concentrate obtained from the concentrating division of the concern and dry sodium carbonate. The main chemical reactions running in the course of processing of the concentrate are connected with the interaction of scheelite and molybdenite, which leads to the formation of sodium tungstate and sodium molybdate. After decomposition, the solution is cleared from admixtures and evaporated in order to get a commercial solution of sodium tungstate. Its corrosion activity is determined by the presence of fluorine ions and total alkalinity. It is known that fluorine ions, like other halide ions [4, 5] are strong activators leading to the local corrosion fracture of metals susceptible to passivation [6–12]. The alkalinity of solutions is also a dangerous factor because it causes the corrosion fracture of carbon, low-alloy, and austenitic stainless steels, as well as of high-strength martensitic and ferritic structures [13–19]. The corrosion cracking is most often observed in welded joints and deformed elements. It is 1
“UKRNAFTOKHIMPROEKT, Ukrainian Institute for Design of Petroleum-Refining and Petrochemical Plants,” JSC, Kharkiv, Ukraine. 2 “Kharkiv Polytechnic Institute” National Technical University, Kharkiv, Ukraine.. 3
Corresponding author; e-mail: [email protected]
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 55, No. 5, pp. 49–55, September–October, 2019.
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