Electrochemical regularities of the corrosion-mechanical fracture of titanium alloys
DOI:
https://doi.org/10.31734/agroengineering2021.25.113Keywords:
titanium alloys, corrosion process, electrode potential, polarization, repassivation, deformation, mechanism, electrochеmical kineticsAbstract
The mechanism and kinetics of corrosion-fatigue fracture of titanium alloys of different phase composition (a, pseudo-a, a+b) are considered using the electrochemical investigations, namely the change of electrode potential E during specimens deformation (dependences E–lgN), external polarization. The electrochemical processes on the fresh surface (FS) for time t = 5×10-3 s of their interaction with sodium chloride solutions of different concentration (0.1; 0.5; 1.0 and 1.5N solution) are studied. The FS potential of titanium alloys for the time of formation t @5×10-3s shifts into cathodic region of values and approaches to the reverse potential of anodic reaction Ti+H2O=TiO++2H++ , j°a=-1.31V ambiguously depends on Cl--ions concentration in the range from 0,1 to 1,5N NaCl solutions. In the first seconds, repassivation of the surface occurs by the linear law, later - by the exponential dependence, and finishes completely to 1 hour. The main stages of corrosion fatigue of titanium alloys of different structural and phase state have been indentified, on the basis of the correlation changes of the surface and the electrode potential. It has been shown that FS of the alloys demonstrates all the properties of the material that can pass into the passive state. Activation of the alloy surface is accompanied by significant discharge of the electrode potential (–1,2 V) and a sharp increase of current (159 А/m2). The repassivation is characterized by a three-stage change of both the potential and the current: existence of the period after surface activation, when the potential changes slightly and lasts from 0.01 to 0.02s; decrease by the linear law in the first seconds; retardation and stabilization. The first period is most clearly observed on the curve “time vs passivation current”. Time of almost constant current (up to 0.01 to 0.02s) does not change under cyclic deformation effect. The test results, presented in semi logarithmic coordinates (dependences E-t and i-t) show that the current changes more significantly with time than the potential as well as differentiation of curves under deformation effect is more clear in the investigated environment. The character of i-t curves changes in deformation and without deformation is similar. However, under the effect of the last one, a more rapid decrease of current and its stabilization take place. The deformed alloy passes into a passive state more quickly than the undeformed one. The variation of the curve “current–time” with deformation and without deformation is the same, whereas, under the deformations influence the current decreases more rapidly. The kinetics and the mechanisms of corrosion-fatigue fracture of titanium alloys are analyzed.
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