Carburizing Treatment And Anti-corrosion Property Of Tial Alloys

Abstract:Porous Ti-46.5A1(at.%) alloy in recent years got very good application for solid-liquid separation in metallurgical chloride (e.g. titanium sponge), hydrometallurgy (such as metal cobalt, zinc, manganese), and the recovery of by-products in the production process (such as furnace gas recycling for the manufacture of sulfuric acid, molybdenum and other precious metals in ammonium molybdate effluents). Its application has brought considerable economic and social benefits. As the same time, it was found in practical application that the corrosion resistance of the porous TiAl alloy in certain harsh environment must be improved, which limits its wider scope of its application.Chemical surface treatment is an effective method to improve the corrosion resistance of metal materials. However, the complex pore structure of the porous TiAl alloy enhances significantly the difficulty in dealing directly with it. What’s more, at present there is very little literature about surface treatments of the porous metal materials. Therefore, solid carburizing of the compact TiAl alloys were firstly performed in this paper, the carburizing mechanism of the TiAl alloys were analyzed. The influence mechanism of carburizing process parameters, microstructure and Al content on the structure of the carburized TiAl alloys and carburizing mechanism were explored. On the basis, the appropriate process parameters were selected for the solid-gas carburizing of the porous TiAl alloy, carburizing behavior of the porous TiAl alloy was revealed. The influence of carburizing process parameters and pore size of porous TiAl alloy on the carburized microstructure were also researched. Finally, the corrosion resistances of the dense and porous TiAl alloy were researched in details in the corresponding corrosive medium. The main findings are as follows:(1) The carburizing mechanism of TiAl alloys was clarified, in which the precedential placeholder theory of active C atoms in the crystal of TiAl alloy, its diffusion pathways in TiAl alloy and the phase transformation of TiAl alloys in the carburizing process were involved. The influence law of Al content on the microstructure of the carburized TiAl alloy is that the lower Al content lead to create the thicker binary carbide layer and the thinner ternary carbide layer. The single-phase ternary carbides Ti2AlC can generate on the surface of the TiAl alloy with certain Al content. The results of the carburized TiAl alloys lay a foundation for the selection of the preferred process parameters for porous TiAl alloy carburizing, and then the carburizing behavior of porous TiAl alloy and influencing factors were analyzed. These results help to control the conditions for generating the continuous and uniform carburized layer on the surface and around the pore wall of porous TiAl alloy.(2) The corrosion behavior of the untreated and the carburized TiAl alloy were analyzed from the difference of the microstructure characteristics between the untreated and carburized TiAl alloy in HCl solution. It was found that the crystal defects of TiAl alloy and the specific adsorption of Cl ion causes the pitting and crevice corrosion. The carburized layer of the carburized dense TiAl alloy at the temperature (≤930℃) is compact structure. The main phase Ti2AlC of the carburized TiAl alloy has the unique crystal structure, which leads the formation of passive film showing excellent corrosion resistance.(3) The passivation characteristics of the dense TiAl alloy in sulfuric acid were explored. The compact TiAl alloys have different corrosion behavior in sulfuric acid comparing in HCl acid and is easy to be passive. The results show that the passive films of the untreated and carburized dense TiAl alloys in sulfuric acid are n-type semiconductor, the difference is that the passivation film of the carburized dense TiAl alloy has a low carrier density in the depletion layer resulting in a low corrosion rate and the stability for a long time.(4) The corrosion behavior of porous TiAl alloys was described in sulfuric acid. It is difficult for the untreated porous TiAl alloy to generate passive film at the corrosion potential, which leads its corrosion resistance is poor. The passive film can generate on the surface of the untreated TiAl alloy under the applied voltage by intermediates adsorption and cover the active sites improving corrosion resistance, but the passive film is unstable and has poor corrosion resistance. The carburized porous TiAl alloy at the corrosion potential and the applied potential can form a stable passive film with an excellent corrosion resistance.87figures,4tables and187references.