Physico-Chemical nanomaterials science

IV-VIB groups metals borides and carbides nanopowders corrosion resistance in nickeling electrolytes
Goretskiy O.O.1, Shakhnin D.B.1, Malyshev V.V.1
1 Institute for Engineering & Technology, University “Ukraine”. Lvivska St. 23, Kyiv-03115, Ukraine. 
E-mail: goretskiy@email.ua

As study objects, zirconium, titanium, vanadium, chromium, molybdenum, and tungsten borides and carbides nanopowders were used, as well as silicon carbide, obtained by plasmochemical and high-temperature electrochemical synthesis. The electrolytes acidity was controlled by the concentrated sulfuric acid addition. Concentration of carbides and borides powders was 10 kg/m3 in all experiments. The nanopowders corrosion resistance was evaluated depending on the electrolyte acidity, the temperature, and the exposition time.
In both groups of compounds, the corrosion resistance of the materials is comparable and is primarily due to the acidity of the electrolyte. In acidic electrolytes (pH = 2.0 ÷ 3.0), nanopowders dissolve rapidly. So, after 3 h at T = 323K, the borides dissolution degree was 15.6 ... 9.5%, after 24 h - 38.2 ... 31.0%, and after 240 h - 89.9 ... 75.1%. Nanopowders of metal-like carbides have a higher corrosion resistance: the dissolution degrees similar to borides are achieved in 24, 120, and 360 h, respectively. For all materials, there was corrosion resistance decrease with increasing temperature and specific surface growth during dissolution, which is 2000 ... 10000 m2/kg with the particles shape preservation. This indicates the layered nature of the process. The only exception is the silicon carbide nanopowder, which dissolution degree throughout the studied range of pH and temperature values did not exceed 7 ... 10%.
Calculated on the results obtained, in electrolytes with pH = 2.5, the time for which half of the initial disperse material dissolves was 32 ... 49 h for borides and 68 ... 88 h for carbides; in electrolytes with pH = 3.0, this time was 92 ... 112 h and 138 ... 167 hs, respectively, and in electrolytes with pH = 5.0, it was practically unlimited. The comparison of the obtained kinetic characteristics with known values for coarse-grained powders shows that the nanopowders dissolution rate is 3 ... 5 times higher. 
Thus, the corrosion resistance of zirconium, titanium, vanadium, chromium, molybdenum, and tungsten borides and carbides in electrolytes solutions is similar within each compounds group and is determined mainly by the medium acidity.