J.P. Frayret

Behaviour of titanium in concentrated hydrochloric acid: dissolution-passivation mechanism

Abstract Titanium dissolved in concentrated hydrochloric acid under valences III and IV. The stationary current-voltage curve is consistent, in a wide potential range, with a reaction model similar to that proposed for the fluorinated sulphuric media. Nevertheless there exist discrepancies between the behaviours in the two types of media. In particular an oxide formation and a particular variation with the potential of the product R t × I (transfer resistance × stationary current) are specific to the hydrochloric media.

Anodic behaviour of titanium in acidic chloride containing media (HCl NaCl). Influence of the constituents of the medium—III. Analysis of the electrochemical impedance. General dissolution — passivation mechanism

Analysing the electrochemical impedance diagrams plotted in a large frequency range (10−3 Hz104 Hz) enables to complete and support the dissolution—passivation scheme of titanium elaborated from steady state measurements. The analysis of the Rt. I product (charge transfer resistance by stationary current) reveals the presence of two active dissolution paths in tervalen titanium, one of which depends on the presence of the hydride (TiH2). Moreover it confirms the participation of the tetravalent oxide TiO2 in the establishment of the stable passivity. A complete reaction model specifying the chemical nature of all the intermediates and final products is proposed.

Anodic behaviour of titanium in acidic chloride containing media (HClNaCl). Influence of the constituents of the medium—I. Study of the stationary current. Calculation of the overall reaction orders

Abstract In concentrated hydrochloric acid solutions, the overall dissolution rate depends on the acid concentration. Using mixed electrolytes (HClNaCl) we have established the influence on the overall anodic stationary current, of each of the H+ and Cl− ions. Considering the logarithmic derivative of the overall current with respect to the activities in the various constituents of the medium reveals the complexity of the reaction mechanism especially in the activity and passivation ranges.

On the anodic dissolution of titanium between 15°C and 100°C in deaerated 2 m hydrochloric acid

Abstract We have demonstrated in previous investigations, under usual conditions of temperatures and pressures, that titanium dissolution, in media including chloride acid, is consistent with a reaction model having a “bifurcation”, and involves six or nine kinetic parameters. This result was obtained mainly via the utilization of polarization curves and an optimization calculation. In this paper, we suggest extending the study in order to include temperatures up to 100°C and pressures up to 70 atm. A specific apparatus was developed to ensure a single phase medium. Calculation of activation energies will be used to investigate the influence of temperature on the kinetics of dissolution-passivation of titanium.

Anodic behaviour of titanium in acidic chloride containing media (HClNaCl). Influence of the constituents of the medium—II. Calculation of the partial reaction orders

Abstract Analyzing the stationary current—potential curves with an original simplified reaction scheme of the dissolution—passivation of titanium, we have determined the partial reaction orders of the elementary rate determining steps of the mechanism. The discussion of a negative order with respect to H + ions observed for the passivation electrochemical reaction leads to propose a modified reaction scheme including two passivation paths, one of the two implying the tetravalent oxide TiO 2 , responsible of the stable passivity.

On the corrosion resistance of the submerged arc welded titanium in concentrated hydrochloric acid 8 N

Abstract The corrosion behaviour of the submerged arc welded T-35 in concentrated 8 N HCl has been investigated. The weld metal is characterized by a martensite type structure, a hardness superior by 60 HV and an oxygen content about two and a half times higher than the base metal. In order to underline the part played by oxygen and structural transformations intervening during the welding operation, an analysis of the anodic current-potential curves by a reaction model involving four determining steps has been carried out. It has been shown that the two reaction rates which determine the passivation and the passivity of the metal depend on the oxygen content as well as the metallurgical structure. In the active dissolution range, whereas one of the two determining rates depends on the oxygen content, the other one is controlled by the metallurgical structure of the metal.

Influence of cathodic reduction on the photoelectrochemical behaviour of titanium dioxide under UV radiation measurement of the photocurrent in chloride medium

Abstract It is established that a cathodic pretreatment of thermic titanium dioxide increases considerably the photoresponse to TiO 2 under uv radiation in chloride solution. The effects of the duration of the pretreatment and of the nature of the acidic media used to perform the cathodic pretreatment are investigated. The variation law of the increase of the photocurrent with the pretreatment duration, which is the sum of two exponential laws at least, reveals the presence of various phenomena in the improvement of the energetic efficiency. The various mechanism involved there are discussed according to different hypotheses.