Titanium anodic polarization curve

Figures 16.8 and 16.9 show only the anodic polarization curves for corrosion cells. The important question is, where do these curves intersect with the polarization curves for likely cathodic reactions, such as hydrogen evolution or oxygen absorption The intersection point defines the corrosion current density icorr and hence the corrosion rate per unit surface area. As an example, let us consider the corrosion of titanium (which passivates at negative Eh) by aqueous acid. In Fig. 16.10, the polarization curves for H2 evolution on Ti and for the Ti/Ti3+ couple intersect in the active region of the Ti anode. To make the intersection occur in the passive region (as in Fig. 16.11), we must either move the H+/H2 polarization curve bodily...

Figure 6. Anodic polarization curve for titanium with superimposed schematic...

For metals such as titanium and chromium, the active peak in the anodic polarization curve may occur below the half-cell potential for the... 

The approximate anodic polarization curves for iron, nickel, chromium, and titanium in 1 N H2SO4 are shown in Fig. 5.42. The cathodic reactions are for the environments shown and are representative of curves obtained on platinum. Since they may be displaced significantly when the reactions occur on the other metal surfaces, particularly the shift of the oxygen curves to lower potentials and current densities, the following discussion is qualitative. The conclusions drawn, however, are consistent with observations on the actual metal/environment systems. 

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. 

The corrosion resistance of TiN can be presented by anodic polarization curves. The polarization result of TiNo.gr sintered body determined at room temperature is shown in Figure 11.3.1 [11]. Since its electrode potential directly after immersion in dilute sulfuric acid is positive (4-0.016 V), it is not soluble for the dilute sulfuric acid. The current density increases rapidly with increasing the voltage, and then decreases reversibly from 0.1 to 0.5 V, showing passivation of TiN. This behavior resembles to anodic polarization curve of titanium. This would be due to substoichiometric composition TiN with excess amount of titanium. Dissolution of TiN is possible in hot fluoric nitric acid solution alone. 

Besides alloy composition (Table 2-19), the corrosion behavior of titanium in reducing acids is very dependent on acid concentration, temperature, and impurities in the acid (Schutz and Thomas, 1987). Anodic polarization curves of titanium in sulfuric acid solutions showed that the critical current for passivation at fixed temperature increased with the acid concentration (Levy, 1967 Peters and Myers, 1967) and with temperature at a given acid concentration (Levy, 1967). Fig. 2-29 shows the rates of corrosion of Ti Gr 2 and other alloys as a function of the concentration of pure hydrochloric acid at the boiling temperatures. As the acid concentration increases the rate of corrosion of Ti Gr 2 increases rapidly. 

Figure 8-7 shows the anodic and cathodic polarization curves observed for a redox couple of hydrated titanium ions Ti /Ti on an electrode of mercury in a sulfuric add solution the Tafel relationship is evident in both anodic and cathodic reactions. FYom the slope of the Tafel plot, we obtain the symmetry factor P nearly equal to 0.5 (p 0.5). 

Fig. 10-16. Polarization curves for anodic oxygen and cathodic hydrogen redox reactions on an n-type semiconductor electrode of titanium oxide in the dark and in a photoex-cited state i = anodic current in the dark (zero) = anodic current...

Figure 10-16 shows polarization curves observed for the anodic ox en reaction (anodic hole transfer) and for the cathodic hydrogen reaction (cathodic electron transfer) on an n-type semiconductor electrode of titanium oxide. The data in Fig. 10-16 show that the anodic current due to the transfer of holes (minority... 

The direct electrochemical measurement of such low corrosion rates is difficult and limited in accuracy. However, electrochemical techniques can be used to establish a database against which to validate rates determined by more conventional methods (such as weight change measurements) applied after long exposure times. Blackwood et al. (29) used a combination of anodic polarization scans and open circuit potential measurements to determine the dissolution rates of passive films on titanium in acidic and alkaline solutions. An oxide film was first grown by applying an anodic potential scan to a preset anodic limit (generally 3.0 V), Fig. 24, curve 1. Subsequently, the electrode was switched to open-circuit and a portion of the oxide allowed to chemically dissolve. Then a second anodic... 

In deaerated 1 N H2SO4 (pH = 0.56), hydrogen-ion reduction is the cathodic reaction with the cathodic polarization curve intersecting the iron, nickel, and chromium curves in the active potential region. Hence, active corrosion occurs with hydrogen evolution, and the corrosion rates would be estimated by the intersections of the curves. The curves predict that the titanium will be passivated. However, the position ofthe cathodic hydrogen curve relative to the anodic curves for titanium and chromium indicates that if the exchange current density for the hydro-... 

The sequence of reactions involved in the overall reduction of nitric acid is complex, but direct measurements confirm that the acid has a high oxidation/reduction potential, -940 mV (SHE), a high exchange current density, and a high limiting diffusion current density (Ref 38). The cathodic polarization curves for dilute and concentrated nitric acid in Fig. 5.42 show these thermodynamic and kinetic properties. Their position relative to the anodic curves indicate that all four metals should be passivated by concentrated nitric acid, and this is observed. In fact, iron appears almost inert in concentrated nitric acid with a corrosion rate of about 25 pm/year (1 mpy) (Ref 8). Slight dilution causes a violent iron reaction with corrosion rates >25 x 1()6 pm/year (106 mpy). Nickel also corrodes rapidly in the dilute acid. In contrast, both chromium and titanium are easily passivated in dilute nitric acid and corrode with low corrosion rates. 

Figure 7.14 Polarization curves for the CFRP-metal cells in simulated sea water. 1 = Composite cathodic curve at 25°C, 2 = composite cathodic curve at 50°C, 3 = aluminium anodic curve at 25 °C, 4 = brass anodic curve at 25°C, 5 = stainless steel anodic curve at 50°C, 6 = stainless steel anodic curve at 25°C, 7 = titanium anodic curve at 25°C. (Reproduced from [114] by kind permission of TV Chukalovskaya and lAPC Nauka )...

As shown in Fig. 3-6, both anodic and cathodic polarization curves of aluminum have simple shapes. There is no active-passive range as there is for stainless steel and titanium, for which the polarization behavior is characterized by an approximately linear relationship between electrode potential and current density. With acceptable error Eqs (3-10) and (3-11) can be simplified to ... 

Polarization curves are shown for four titanium materials in boiling 1 Msodium chioride with 1 iWhydrochioric add. The corrosion potentials of both 71-0.15 Pd and PdO/nOj-Ti, where no anodic peaks occur, are more noble than those of CP titanium and Ti-0.3Mo-0.8Ni. Source B. Satoh eted., The Crevice Corrosion Resistance of Some Titanium Materials, Plat. Met. Rev., Vol 31,1987, p 115-121... 

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