| In recent years, the study of surface preparation has attracted considerable attention due to its potentially important effect on properties of TiNi alloy. Electropolishing has been used in TiNi alloy in several fields for its special characteristics, but its essential details and electroplishing mechanism have not been reported yet as a demand from business competition, which, to a great degree, restricts the application and extension of the electropolishing technology. In this paper, the electropolishing processes of TiNi alloy is explored, the electropolishing mechanism of the TiNi alloy in organic solutions is studied and the effect of surface preparation on the properties is analyzed. The judgement of the results of electropolishing lies in the changes of the surface qualities, mainly weighted through two quantitative parameters: surface roughness and lustrousness, which are inconsistent in many cases and thus makes it hard to judge which one has a better surface. As a uniform, comparatively comprehensive judgement standard, a new parameter (Rlr , the relative surface luster and roughness) is constructed in a way. Where a and b (a+b=1)are weight number whose value is determined by the degree of the study emphasizing particularly the roughness and luster. R a0: the original surface roughness; Ra: the final surface roughness 0R e: the original surface luster; Re: the final surface luster Rlr has a physical meaning on two layers as of the charge in material's surface state. If Rlr is positive, the surface is considered a better one and the bigger Rlr is, the better the surface is; if Rlr is negative, the surface is consitered a worse one, and the smaller Rlr is, the worse the surface quality is. The judgement of the results of electropolishing lies in the value of Rlr, combining the value of surface roughness and luster. Studies on eight different electrolytes show that the methanol-perchloric acid system, amidosulphonic acid-formamide system and methanol-sulfuric acid system have a comparatively better effect on the electropolished surface. Then, with the orthogonal experiment method, experiments depending on L9(34) orthogonal table are carried out, whose results show that it will be led to a better electropolishing results using the methanol-perchloric acid system. And the four factors in turn according to importance are: solution composition, temperature, current density and time. The best combination is A3B1C2D2, that is, the methanol-perchloric acid system, 10 oC, 45s and 0.7A/cm2, which leads to a better electropolished surface with Ra 117nm and reflectivity 53.17%。The methanol-perchloric acid system with certain additives (patent pending) is studied for a better electropolished surface. The four factors in turn according to importance are: additions, current density, temperature, spacing between anode and cathode. The best combination is A3B1C2D2, that is, certain additives 90ml/l, 10 oC, 45s and 1.8cm, which leads to a much more better electropolished surface with Ra 82nm and reflectivity 54.1%。Besides the electrolyte, other factors that influence the electropolishing are temperature, current density, spacing between anode and cathode, electrolyte stirring, etc. Studies on the effect of the temperature on the electropolishing process show that the higher the temperature is, the bigger the electropolishing rate is, following a near Gauss law. The relationship between the temperature and surface roughness follows near parabolic law, and the relationship between the temperature and surface reflectivity follows near sigmoidal law . The relationship between the electropolishing voltage and current density follows a near parabolic law, while that between the electropolishing rate and current density a near linear law. The relationship between the electropolishing rate and time follows near sigmoidal law. Cyclic voltammetry and polarization curve are used for the first time for study of the electrochemical mechanism of the TiNi alloy in organic systems. The anodic polarization curves and the Tafel curves are obtained. The dynamical parameters of the NiTi alloy in the amidosulphonic acid-formamide system are: a:5.6399,b:0.7229,i0:1.5784×10-8 A/cm2 , nβ: 0.08 ; while the dynamical parameters of the NiTi alloy in the methanol-perchloric acid system are: a:5.0805,b:1.1958,i0 :5.6416×10-5A/cm2,nβ:0.048. The results of cyclic voltammetry show that the anodic reactions of the NiTi alloy in both systems belong to an inreversible process, an oxidation process with 7 electrons gained at one step, that is: TiNi -7e →Ti4++Ni3+. For the Ni3+ is unstable in the acid solution, so another reaction occurs: Ni3+ + e →Ni2+. Electropolishing mechanism of NiTi alloy has been studied, and it is found that different from the typical one, the electropolishing characteristic curve of NiTi alloy would not show electric current plateau. When the specimen is electropolished at low voltage, the...
|
PDF Full Text Request