Anodic oxides breakdown potential

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

By virtue of the high breakdown potential of the oxide film (approximately 155 V in sea water and 280 V in low conductivity water of pH = 7) tantalum has found use as a substrate for platinum in impressed-current cathodic-protection anodes, which can be used at high impressed voltages (50 V) and high current densities. However, because of its lower cost, niobium is preferred for systems that have to operate at high voltages... 

The formation of deposits on platinised anodes can cause anode degradationThus dissolved impurities present in water which are liable to oxidation to insoluble oxides, namely Mn, Fe, Pb and Sn, can have a detrimental effect on anode life. In the case of MnOj films it has been stated that MnOj may alter the relative proportions of Cl, and O, produced and thus increase the Pt dissolution rate Fe salts may be incorporated into the TiO, oxide film and decrease the breakdown potential or form thick sludgy deposits. The latter may limit electrolyte access and iead to the development of localised acidity, at concentrations sufficient to attack the underlying substrate . 

There have been instances reported in the literature where the breakdown potential for Nb and Ta in seawater has been found to be lower than the generally accepted value of 120 V, with reported values in extreme instances as low as 20- V . This has been attributed to contamination of the niobium surface from machining operations, grit blasting or traces of copper lubricant used in anode manufacture. These traces of impurities, by becoming incorporated in the oxide film, decrease its dielectric properties and thus account for the lower breakdown voltage. Careful control of surface contamination in the manufacture of platinised niobium is therefore essential to minimise the lowering of the breakdown potential of niobium. 

Fig. 7.14 Effect of chloride-ion concentration on the anodic polarization of type 304 stainless steel. Dashed lines indicate breakdown potentials, Eb pit. Curves A and B are schematic representations of polarization of cathodic reactions of relatively (A) high and (B) lower oxidizing strength. Based on Ref 27...

Fig. 27 Breakdown potential Ubd at T = 298 K for the anodic polarization of anodic metal-insulator-nnetal (MIM) systems (AI/AIOx/Ag) as a function of the film thickness d. The formation potential for anodically formed oxides is given in the upper abscissa for comparison [21]. 

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. 

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