Titanium anodic polarization

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...

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... 

Ti, Ta, Zr, and Nb are preferred, because of their ability of forming stable compact oxides film during anodic polarization. Compared with other metals, titanium has the lowest density, easy machinability, high anticorrosion and quick repassivation electrochemical performance, and relatively low cost (Leyens and Peters 2003). It becomes a preferred choice as the substrate material of diamond film electrode (Drory and Hutchinson 1994 Chen andLin 1995 Chen et al. 2003 Hian et al. 2003 Gerger et al. 2004 Chen et al. 2005 Guo and Chen 2007a). 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

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. 

C. Hall Jr., N. Hackerman, Charging process on anodic polarization of titanium, J. Phys. Chem. 77 (1953) 262-268. 

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 )...

While the previous examples were limited in the anodic polarization potential either by transpassive dissolution or by oxygen evolution valve metals can be polarized to potentials of up to 100 V and above. Examples are aluminum, titanium, tantalum, hafnium, and zirconium. Formation characterization and properties of these oxides were treated in Chapter 9. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

Y. Okazaki, A. Ito, T. Tateishi, and Y. Ito, Effect of alloying elements on anodic polarization properties of titanium alloys in acid solution, Materials Transactions, JIM, 35, 58-66 (1994). 

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. 

The Ti02 film, being an n-type semiconductor, is electronically conductive. As a cathode, titanium permits electrochemical reduction of ions in an aqueous electrolyte. On the other hand, very high resistance to anodic current flow through the passive oxide film (i.e., significant anodic polarization) can be expected in most aqueous solutions. Elevated anodic pitting (breakdown and repassivation) potentials can also be expected with many titanium alloys. 

Immersion testing wiU generate weight loss data, or corrosion current measurements can be obtained from stan-deird electrochemical polarization tests (see ASTM G 5, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements see also Ref 27). Corrosion rates in millimeters per year (mpy) for titanium alloys can be calculated from sample weight loss data as follows ... 

Electrochemical oxidation of organic conpounds requires an anode that is stable under anodic polarization, and that hs a low catalytic efficiency for oxygen evolution. Noble metals, such as platinum, are resistant to oxidation, but they have hi catalytic efficiencies for oxygen evolution and are prone to fouling (d, 7). Dimensionally stable anodes, such as titanium coated widi active or inactive catalysts, are less prone to oxygen evolution and fouling, but ftiey do suffer from leaching of the catalyst from e electrode surface. 

Titanium in sulfuric and hydrochloric acids easily undergoes corrosion, but easily passivates during anodic polarization. For example, anodic protection in 40% H2SO4 at 60 °C decreases the corrosion rate of titanium by 1100 times. Also, anodic protection of this metal is applied in solutions containing chloride ions, especially in hydrochloric acid. The corrosion rate of titanium in 30% HCl at 80 °C after the application of protection decreases by approximately 800 times. More information can be found in the works of Locke (1987) and Kuzub and Novitski) (1984). 

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. 

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