Nanocomposites and nanomaterials

Increasing the titanium corrosion resistance by tungsten coating deposition
Malyshev V.V., Gab A.I., Lukashenko T.F., Rogovyi V.V.
University ”Ukraine”, 23 Lvivska St., Kyiv-03115, Ukraine. 
E-mail: viktor.malyshev.igic@gmail.com

This work was aimed on study of the electrochemical and corrosion behavior of titanium with tungsten nanocrystalline coatings deposited by high-temperature electrochemical synthesis (HTES) in sulfuric acid solutions. Tungsten was electrodeposited onto titanium surface by HTES from Na2WO4 – 5 mol.% WO3 melt. Corrosion studies were carried out in sulfuric acid (9.5 ppm) solution at 343353 K through recording of time dependence of potential and corrosion rate variation. The corrosion rate was determined by photocolorimetry. The corrosion potential of tungsten- coated titanium increases greatly, from 0.590.61 up to 0.260.31 V, passing to the titanium passive-active region and only by 0.020.04 V not reaching the tungsten corrosion potential. The solution photocolorimetry was performed after 5, 10, 15, and 30 h of experiment. The average titanium dissolution rate obtained for these time intervals is constant for the coated specimen and is 0.005 0.01 g/(m2h) in contrast to the titanium dissolution rate at the free-corrosion potential equal to 19.1 19.3 g/(m2h). As the result of the coating deposition, the titanium dissolution rate decreases by 20004000 times. The uncoated titanium dissolution rate at the steady-state potential of the tungsten-coated specimens (300÷ 310 mV) was 0.41÷0.50 g/(m2h).
The average tungsten coating corrosion rate decreased from 0.030.04 g/(m2h) during the first 5 h of experiment down to 0.0070.011 g/(m2h) after 30 h of exposition with simultaneous corrosion degree increase. A similar average corrosion rate variation with time was observed studying the tungsten corrosion and electrochemical properties in sulfuric and hydrochloric acids solutions of different concentration. According to anodic polarization curves, the titanium free-corrosion potential under experimental conditions is -(0.590.61) V. It is characterized by an active- dissolution region with relatively low critical passivation potential -(0.27  0.29) V and high critical current. For tungsten specimen, there is no active-dissolution region, and tungsten is passive up to the transpassivation potential. The anodic polarization curve for tungsten-coated titanium is similar to those for tungsten specimen and almost coincides with it at a polarization of over 0.1 V. The coated specimen potential is close to the tungsten transpassivation potential. The anode current in the passive and transpassive regions is caused mainly by the oxidation of tungsten.