Titanium standard electrode potential

In general, the higher the oxidation potential the lesser the tendency to corrode. However, some metals corrode less than other metal with higher redox potential. For example, chromium (—0,74 V), zinc (—0,76 V), titanium (—0,89 V), aluminum (—1,71 V) etc. withstand corrosion much better than iron (—0,42 V). This is due to the fact that the surface of these metals coats with an insoluble very thin layer, just a veil, of hard-bitten oxide not reactive at all that, at variance with rust, passivizes the surface blocking the prosecution of corrosion. Table 13.2 provides a synoptic picture of the standard potentials, the so called electrode potential, relative to oxidation reactions of various metals. The standard electrode potential, abbreviated as , is given in volts and is the measure of the potential of any individual metal electrode which is with solute at an elfective concentration of 1 mol/dm at 1 atm of pressure. These potentials are referred to a hydrogen electrode whose reference potential is assumed equal to zero. This is because it is not possible to measure experimentally the value of the dilference of potential Ay between an electrode and its solution as, for example, in the case of zinc reaction (13.16), because any device used for making the measurement must be inserted in the circuit with two electrodes of which one is put in contact with the metal electrode of interest and the other with the solution. Now, this second electrode creates necessarily another interface metal-solution and the potential difference provided by the system is that between the two metals, without any possibility to infer the absolute value of each of them. This is why it is necessary to introduce a reference electrode, which any other potential can be referred to. To... 

Platinum—The high standard reduction potential for platinum makes it an ideal anode material (although Pt corrosion does occur during some oxidations reactions, such as the Kolbe oxidative coupling reaction [29, 58, 59]). For anode potentials greater than about 0.50 V (on the hydrogen scale), an oxide film covers the Pt surface, so the electrode material is often platinum oxide. Large anodes are often titanium coated with platinum in order to reduce costs. 

Fig. 12. Rotating (45 Hz) ruthenium dioxide/titanium dioxide electrode (35%w/w ruthenium dioxide) in 1.0 M NaCl solution, (a) Standard rate constant-potential curve assuming a constant Tafel slope of 70mV. Dcl = 5 x 10 6cm s-1, Dc = 7 x 10 6cm s-1, E° = 1050mV SCE, and R = 0.8 ohm cm2. (b) Standard rate constant-potential curve assuming a constant Tafel slope of 40mV. DC1 = 5 x 10 8cm s 1, Z)ct2 = 7 x 10 8cm s 1, E° = 1050mV SCE, and R = 0.8ohm cm2. (c) Common experimental and calculated current-potential curve using the parameters of Fig. 12(b). (d) Double layer capacity curve.

However, the active dissolution of titanium depends markedly on temperature in acid solution. At lower temperatures, the picture is not so clear. It is necessary to have a quantitative measure of the rate of the hydrogen reaction and the titanium dissolution reaction. The complete set of current-potential and impedance-potential data has been tested against the theory given above. The best strategy seems to be to fit to a single electrode reaction and then to look for deviations from the expected behaviour for a perfect redox reaction. A convenient way of doing this is to represent the electrochemical data as a standard rate constant-potential curve in conjunction with a double layer capacity-potential curve [21]. 

Fig. 7. Analysis of the experimental steady-state current-potential and impedance-potential data from E = - 1300 mV to E = — 600 mV for a titanium rotating-disc electrode (45 Hz) in a solution of 2 M hydrochloric acid, (a) Standard rate constant-potential curve calculated for the hydrogen evolution reaction on titanium assuming that DA = 7.5 x 10 5cm"1s 1 and E° = - 246 mV. The Tafel slope 6C = 211 mV and the measured ohmic resistance was 0.4 ohm cm2. The potentials are the "true potentials, (b) High-frequency double layer capacity-potential curve. The potentials are the measured potentials.

It has been demonstrated that the half-wave potential for the reduction of Ti(IV) to Ti(III) is -0.81 V (against the standard calomel electrode) in 0.1 M HCl [27]. The further reduction of Ti(III) to Ti(ll) can be observed in alkaline media, but this reaction has no useful analytical significance. In these methods, oxalate, tartrate, or citrate buffer systems are used as supporting electrolytes to prevent the hydrolytic precipitation of hydrated titanium oxides. In the presence of tartrate buffer, well defined waves are obtained only at pH values less than 2, or between 6 and 7. The Ti(lV)-Ti(III) couple is reversible only in tartrate buffer at pH values less than 1. 

In non-carbonated concrete without chlorides, steel is passive and a typical anodic polarization curve is shown in Figure 7.3. The potential is measured versus the saturated calomel reference electrode (SCE), whose potential is +244 mV versus the standard hydrogen electrode (SHE). Other reference electrodes used to measure the potential of steel in concrete are Ag/AgCl, CU/CUSO4, Mn02, and activated titanium types. From this point on in the text, unless explicitly stated otherwise, potentials are given versus the SCE electrode. 

Figure 17.9 Average value in time and standard deviation of potential of activated titanium electrodes embedded in alkaline and carbonated concrete (Portland cement, iv/c 0.65, outside exposure, unsheltered) [23]...

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