Titanium anodized, corrosion rate

Furthermore, the restrictions on operating voltage that apply to titanium in a marine enviroment are not always relevant to titanium in soils free of chloride contamination. Coke breeze is, however, an integral part of the groundbed construction and ensures a lower platinum consumption rate. However, for some borehole groundbeds, platinised niobium is preferred, particularly in the absence of carbonaceous backfill or in situations where the water chemistry within a borehole can be complex and may, in certain circumstances, contain contaminants which favour breakdown of the anodic Ti02 film on titanium. In particular, the pH of a chloride solution in a confined space will tend to decrease owing to the formation of HOCl and HCl, and this will result in an increase in the corrosion rate of the platinum. 

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...

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... 

Ebonex is a conducting ceramic material mainly composed of the Magneli-phase titanium oxides, Ti407 and TisOg, resistant to anodic corrosion in a wide range of media. Electrodeposition of a metal changes the low electron transfer rates of the ceramic material to that of the bulk metal [169]. 

The corrosion rates of the titanium casing under cathodical conditions were smaller than that of stainless steel in nitric acid ( 0,05 mm Ti/y). This value as well as the corrosion rate of the platinized tantalum anode is within the expected order of magnitude (< 0,005 mm/y). 

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. 

Valve Metals Titanium is extensively used in the chemical process industry owing to its excellent corrosion resistance. However, anodic protection of titanium is required for certain environments. For example, anodic protection has been recommended widely for application of titanium in sulfuric acid applications. Experiments indicated that anodic protection considerably increased the corrosion resistance of titanium. In sulfuric acid up to 65% concentration at 65 C, the corrosion rate of anodically protected titanium was found to be 0.025 mmyr . Even at a higher temperature of 90 °C, for sulfuric acid concentration of 57%, the corrosion rate under anodic protection was found to be only 0.13 mm yr [18,19]. A... 

The corrosion rate and corrosion potential are estimated using electrochemical kinetic parameters such as exchange current density for hydrogen evolution reaction on titanium and platinum, reversible potentials, and cathodic and anodic slopes. 

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. 

The corrosion rate of anodized titanium (solution 60 ml ethanol, 35 ml water, 10 ml lactic acid, 5 ml phosphoric acid, 5 mg citric acid and 5 mg oxalic acid 45V, 45s) is much lower than that of passivated titanium (40%... 

In the absence of suspended particles, the corrosion rate of passive metals such as stainless steel or titanium in neutral media is not markedly affected by hydrodynamic conditions (Table 10.26). However, when exposed to slurries, these metals are subject to erosion corrosion because the suspended particles that impinge on the surface damage the passive film. As a consequence an anodic partial current flows which serves for film repair and repassivation of damaged areas. In the presence of aggressive anions such as chloride, passive film damage can lead to metal dissolution by pitting [23]. 

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 ... 

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). 

The metallization for some integrated circuits with submicron features is made up of the Ti/TiN/Al/TiN stack. Lateral undercutting of up to 20 pm of aluminum from the middle of the stack has been observed (Fig. 9-16). Residual chloride from the reactive-ion etch initiates the aluminum corrosion process that is stimulated galvanically by titanium nitride (Jones, 1992). Titanium nitride, being more noble, serves as the cathode in this electrochemically driven process. The corrosion rate of the more active metal increases as the cathode-area/anode-area ratio increases. At the time of the rinse... 

Anodic protection possesses unique features. For example, the applied current is usually equal to the corrosion rate of the protected system. Thus, anodic protection not only protects but also offers a direct means for monitoring the corrosion rate of a system. The main advantages of anodic protection are (1) low current requirements (2) large reductions in corrosion rate (typically 10,000-fold or more) and (3) applicability to certain strong, hot acids and other highly corrosive media. It is important to emphasize that anodic protection can only be applied to metals and alloys possessing active-passive characteristics such as titanium, stainless steels, steel, and nickel-base alloys. 

Figure 9.19 Corrosion rate vs anode potential for titanium in 40°C (wt) sulfuric acid at 60°C. (From I.M.I. (Titanium Information Bulletin), Witton Birmingham, UK)...

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