| The effects of the major solution compositions, temperature, surfactants, and agitation rate on the dissolution behavior of titanium alloy in corrosion processing were studied by E-t, polarization curve and EIS. Scanning electron microscopy (SEM) and atomic force microscope (AFM) were used to discuss the relationship between the morphology and corrosion dissolution. Corrosion dynamic law in corrosion processing solution was investigated by testing anodic and cathodic reaction rate. Finally fatigue properties after corrosion and mechanical processing were compared.Corrosion potential of titanium alloy moves rapidly to the positive direction, and then becomes negative slightly in solution with low concentration hydrofluoric acid at the initial period. However, for high concentration hydrofluoric acid, corrosion potential becomes negative quickly, and then shifts positive slowly. These phenomena are caused by the easier damage of oxide film formed in the air on the surface of titanium alloy in high concentration hydrofluoric acid. In the corrosion processing, the passivation and dissolution exist simultaneously. Corrosion potential remains stable in solution when the passivation and dissolution rate are the same. An active-passive area can be observed apparently in polarization curve. As the acid concentration increases, corrosion potential becomes negative, corrosion current density and passivation current density increase continuously. A capacitive loop appears in high-frequency region of electrochemical impedance spectroscopy, the real part becomes narrow as acid concentration increases. There is an inductive loop in mid-low frequency region, which presents the decreasing trend with the increase in acid concentration as the protective effect of passive film becomes weak. Through AFM and SEM observation, the recessed area becomes shallow while protruding area is smoother as nitric acid concentration increases. Titanium alloy has relatively low roughness value when the volume ratio of hydrofluoric acid to nitric acid is1:3.Adsorption isotherm of surfactants SN, SR, SF and mixed-type surfactants SN and SR follows to Langmuir adsorption isotherm curve. In solution with surfactants SR2mL/L and SN0.16g/L, the polarization curve results showed that corrosion potential of TC1and TC4titanium alloy has the most negative values, corrosion current density is the maximum, surface tension has a minimum value28.23×10-4N·m-1while the maximum adsorption quantity is15.10×10-6mol·m-2. A good synergistic effect of mixed-type surfactants SR and SN can be seen.Titanium alloy corrosion tendency is promoted with increasing temperature, passivation current density presents a linear increase law. The activation energy of TCI and TC4titanium alloy is2.526,1.748,0.232KJ/mol and1.368、1.111、0.205KJ/mol respectively when hydrofluoric acid concentration is20mL/L、40mL/L、80mL/L. Arrhenius curves indicate that TC4titanium alloy has a stronger corrosion tendency compared to TC1in the same condition. Limit diffusion current density rises with the increasing rotation rate, titanium alloy is dissolved seriously, the passive state is more difficult to occur with high-speed rotation rate.In the corrosion processing, cathodic and anodic reaction dynamic tests showed that the gas producing rate of cathodic reaction is basically identical with the anodic dissolution rate. At the initial period, the gas producing rate rises relatively slow, then declines slightly, finally the curve tends to be stable gradually. Anodic corrosion dissolution rate increases promptly in the first seconds, then drops slightly and remains stable. With the extension of time, anodic dissolution rate shows a decrease tendency. Surfactants CE, SN and SR can accelerate the anodic dissolution rate. CO2, N2and H2are the eventual gases of titanium alloy in corrosion processing. In the case of temperature30℃, the efficiency of acid frog suppression of TC1and TC4titanium alloy is73.1%-76.1%in solution with hydrofluoric acid100mL/L, the volume ratio of hydrofluoric acid to nitric acid is1:2. Surfactants SN and SR has a good ability of acid fog suppression.Titanium alloy after corrosion processing performs a better fatigue property than that after mechanical processing. The fatigue cycles of corrosion processing samples are more than that of mechanical processing in the same loading stress level. TCI fatigue limit is300MPa,350MPa while TC4were450Mpa,400Mpa after corrosion processing and mechanical processing. Fatigue cracks initially appear in R round-angle near the root area of titanium alloy surface. |
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