Titanium Coating on Carbon Steel: Direct-Current and Impulsive Electrodeposition. Physicomechanical and Chemical Propert
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TITANIUM COATING ON CARBON STEEL: DIRECT-CURRENT AND IMPULSIVE ELECTRODEPOSITION. PHYSICOMECHANICAL AND CHEMICAL PROPERTIES V. V. Malyshev1,2 and D. B. Shakhnin1
UDC 541.135:621.357.1:546.271
We study the direct-current and unipolar impulsive electrochemical deposition of titanium from halide melts on steel materials. The corrosion resistance of titanium deposits in solutions of sodium chloride and nitric acid at room temperature is evaluated. Keywords: titanium, coatings, electrodeposition, carbon steels, physicomechanical and chemical properties.
The electrodeposition of titanium is an important method of corrosion protection in seawater and chemical media. Numerous attempts were made to obtain electrodeposits of titanium from organic [1, 2], aqueous [3, 4], and molten salt media [5–18]. However, pure titanium was obtained only from molten salt media; in particular, from chloride [5–8], fluoride [9–14], and chloride–fluoride melts [15–18]. In chloride melts, predominantly powder [6] or dendritic [7, 8] deposits are formed due to the existence of three low-stability oxidized states and disproportionation reactions. In fluoride melts, only two oxidized states of titanium exist, and the high stability of titanium–fluoride complexes leads to the reduction of titanium in two stages [13, 14]: TiF62− + e − → TiF63− ;
TiF63− + 3e − → Ti + 6F − . Dense homogeneous deposits of pure titanium are obtained by the direct-current electrolysis of NaF–K2TiF6 melts within the range 1223–1273°K [9, 10] and LiF–NaF–KF–K 2 TiF6 melts at 973°K [11, 13, 14] and 1073°K [12]. However, these coatings become rough and sometimes dendritic if the cathodic current density and/or time of deposition increase. In chloride–fluoride melts, dense titanium coatings [15, 17] can be obtained in the same way as powder and dendritic deposits [18] by varying the ratio [Fe − ]/[Cl − ] . Depending on the metallic substrate and temperature of electrolysis, intermetallic compounds
may form in molten salts under the external titanium layer. The same phenomena were observed on copper [13], nickel [14], and iron [19, 20]. In the technological aspect, the corrosive action of fluoride and chloride–fluoride melts (with substantial amounts of fluorides) upon the structural materials of the electrolyzer should be taken into account. The impulsive coatings in aqueous [21] and molten [22–24] media improve the properties of deposits. Thus, the dense nondendritic coatings can be obtained because, for the impulsive current, the concentration polarization is minimal. In what follows, we present results of getting impulsive-current electrodeposited titanium coatings on carbon steel in a KCl–NaCl–10 wt.% KF–TiCl 3 melt and compare the efficiencies of cathodic currents, morphol-
ogy, and microhardness of the deposits obtained by applying direct and impulsive currents. The electrochemical behavior of electrodeposits was evaluated in NaCl and HNO 3 solutions at room temperature. 1 2
Vernads’kyi Institute of General and Inorganic Chemistry, Ukrainian National Academy of Sc
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