Study Of Boronized Layer On Titanium Surface By Molten Salt Pulse Electrolysis

Boronizing layer was prepared on pure titanium by molten salt pulse electrolysis.The influence rules of electrolyte temperature, electrolyte time, current density and duty cycle on the boronizing layers’ composition, microstructure, phase were studied by the single factor way. The effect of boronizing on hardness and corrosion resistance of the samples were researched. The diffusion behavior of boron in pure titanium was also investigated in the aspect of dynamics, and the diffusion coefficient accompanied by diffusion activity energy of boron in titanium during boronizing was caculated.Meanwhile, the possible reactions and reaction products were discussed by means of the thermodynamic calculation. The results are showed as follows:1) The higher electrolyzing temperature and the longer electrolyzing time, the faster deposition rate and the thicker boronized layer as well as the bigger grain size. Overdeposition time may lead to the appearance of cracks on the surface of samples; Under the low temperature(800℃and 840℃) and short time(60 min) leads a better boronizing result: grain size is small, dense and smooth. The smaller the duty cycle is, the greater the deposition rate will be, whereas the duty ratio plays no role in the thickness of boride zone. When the current density is 60 m A/cm2, the boronizing layer grains are fine with no crack. Boronizing layers obtained under various process parameters consisted of Ti B and Ti B2, in which Ti B grows with preferred orientation of(111).2) Boronization renders an enhancement of the microhardness and corrosion resistance for the titanium matrix: there exists 6 times improvement in microhardness and it is likely to reach a maximum value of 2259.3HV0.05 compared to the substrate. All data derived from electrochemical methods, including open circuit potential(OCP),potentiodynamic polarization curve and electrochemical impedance spectroscopy(EIS),indicating that the corrosion resistance of test specimen with boronizing treatment has been remarkablely improved.3) The Gibbs free energy(g) calculated through thermodynamic analysis for three reactions comporizing Ti+B=Ti B, Ti+2B=Ti B2 and Ti B+B=Ti B2 were all negative, and clearly illustrating that these three reactions could probably occur and Ti B and Ti B2 phase both exist. Verifing the main phases of the layer on titanium surface are Ti B and Ti B2.The diffusion coefficient of boron in the titanium matrix under different temperatures obtained by dynamic calculation are K800=2.63×10-15m2/s, K840=4.45×10-15m2/s, and K880=6.26×10-15m2/s. The fitting diffusion activation energy Q on the basis of Arrhenius equation for the boron in titanium is about 111KJ/mol.