About corrosion siliconized titanium alloys in sulfuric acid

Corrosion behavior of titanium alloys VT1-0, OT4, and VT14 with diffusion silicide coatings in 80 % sulfuric acid have been investigated. The diffusion coatings obtained by saturation of the samples in the powders of silicon with halide activation. The silicide coatings obtained in the powder of silicon (Si- coatings), in the powders of silicon and copper (Si-Cu- coatings), in the powders of silicon, copper and zinc (Si-Cu-Zn- coatings) and in the powders of silicon, copper and ligature containing rare earth elements (Si-Cu-REE- coating). Corrosion testing was carried out at room temperature for 300 hours. It has been established that the corrosion process occurs in two stages. In the first stage the corrosion kinetics obeyed an exponential law. In the second stage weight loss of the samples increased linearly. The lowest corrosion rate of the alloy with Si coatings was observed on $\alpha$- alloys VT1-0 and OT4. ($\alpha + \beta$)- alloy VT14 was korrodiruet with more rate, but the corrosion rate after 300-hour testing was in 625 times lower than that of the alloy without coating. The corrosion rate of the unprotected alloy VT14 was equal to 5.5 g/m$^2$h. After 300-hour testing, the lowest mass loss of samples with Si-Cu-coatings was observed in (a+b)-alloy VT14, and the greatest in commercial titanium VT1-0 The higher resistance of this alloy is associated with greater thickness as of the coating, and so of the silicide layer TiSi$_2$ in the coating. The corrosion rate of siliconized alloy VT1-0 at the end of the testing amounted to 0.003 g/m$^2$h, alloy OT4 - 0.0002 g/m$^2$h, alloy VT14 - 0.00005 g/m$^2$h. At the initial stage of the testing alloys with a Si-Cu-Zn-coating highest corrosion rate was observed on the alloys VT1-0 and OT4, with increasing duration of the testing the corrosion rate rapidly decreased and after a 65-hour exposure in the acid corrosion of alloys VT1-0 and OT4 goes with almost constant speed. The corrosion rate of alloy VT14 at the initial time was almost an order of magnitude lower than that of $\alpha$-alloys, but it decreased more smoothly. The subsequent decrease of corrosion rate of alloy VT14 was very slowly until the end of the testing. The corrosion rate of siliconized alloy VT1-0 at the end of the testing was 0.005 g/m$^2$h, alloy OT4 - 0.007 g/m$^2$h, alloy VT14 - 0.009 g/m$^2$h. The data for corrosion of alloys with Si-Cu-REE-coatings quantitatively coincide with those obtained for Si-coating.