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. 

A capacitor, previously called a condenser, stores electrical energy based on the relationship between voltage (V) and stored charge (Q) in coulombs as shown in the equation C = QU. One farad of capacitance is a coulomb per volt of stored charge. The voltage limit of a capacitor is determined by the breakdown potential of the dielectric material. 

Fig. 1.55 Breakdown potentials of Fe-18Cr-8Ni stainless steel in 0-1 M NaCl plus various concentrations of Soj ions during potentiostatic anodic polarisation (after Leckie and...

Pourbaix, on the basis of the breakdown potential Ey, and the protection potential E, distinguishes between the following states of a metal surface, which have been incorporated in the potential-pH diagram shown in Fig. 1.566 ... 

Table 2.24 Breakdown potentials (mV) for 316 stainless steel, titanium and cobalt-chromium-molybdenum alloy in oxygen-free 0.17 m NaCl solution at 37°C using a silver/ silver chloride reference electrode.

Table 2.25 Breakdown potentials for 316S12 stainless steel (cold worked), high nitrogen stainless steel (cold worked), titanium-6Al-4V and cast-cobalt-chromium-molybdenum alloy in continuously aerated aqueous acidified chloride solution 0.23 m [C1 ] pH 1.5 at 25°C. ...

Material British standard Condition Breakdown potential mV(s.c.e.) Rest potential mV(s.c.e.)... 

Increasing concentrations of bicarbonate tended to raise the breakdown potentials but also increased the corrosion potentials. This, in combination with a high chloride concentration, high bicarbonate concentrations may raise the corrosion potentials such that they border on passivation breakdown. The increase in hysteresis loop size on potentiodynamic cycles with increasing bicarbonate concentration shows a lowered resistance to pitting attack and crevice corrosion. 

Variations in pH promoted increases in corrosion potentials from acid pH levels to neutral pH thereafter, however, corrosion potentials were lowered in alkaline solutions to more active values. Decreasing pH caused a lowering of breakdown potentials in the presence of Cl and an increase in the current densities for passivation. 

In 3% sodium chloride solution at 60°C the austenitic irons again show superior characteristics to the ferritic. The breakdown potentials determined in this environment, which provide a relative measure of the resistance to attack in neutral chloride solutions, are generally more noble for the austenitic irons than for the ferritic (Table 3.47). This indicates that the austenitic irons should show better corrosion resistance in such environments. 

Table 3.47 Breakdown potentials evaluated from E-i curves in 3% NaCl at 60°C...

Figure 4.36 shows the influence of pH on the breakdown potential of nickel in alkaline solutions containing Cl ions, and it is apparent that the breakdown potential becomes more positive as the pH increases, i.e. breakdown is unlikely unless the solution has a very high redox potential. 

Fig. 4.36 Influence of pH and Cl ion on (he breakdown potential of commercial nickel in alkaline solutions (0-001-5 m NaOH) de-aerated with N2 (after Postlethwaite )...

Fig. 4.37 Influence of the chromium content of Ni-Cr alloys on the breakdown potential in 0-1 M NaCI at 25°C de-aerated with N2 (after Horvath and Uhlig )...

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... 

Table 10.16 Breakdown potentials of cominercially pure titanium in various environments...

Reference Electrolyte and conditions Breakdown potential of commercially pure titanium (V)... 

Platinised titanium anodes may be operated at current densities as high as 5 400 Am however at these current densities there is the possibility that the breakdown potential of titanium may be exceeded. The normal operating current density range in seawater is 250-750 Amwhilst that in brackish waters is given as 100-300 Am with values within the range... 

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 . 

These anodes are considerably more expensive than platinised titanium, especially when expressed in terms of price per unit volumeIndeed, since niobium is cheaper than tantalum the use of the latter has become rare. The extra cost of Nb anodes may be offset in certain application by their superior electrical conductivity and higher breakdown voltages. Table 10.17 gives the comparitive breakdown potentials of Ti, Nb and Ta in various solutions under laboratory conditions. 

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. 

The critical breakdown potential, which is the positive potential limit of stability of the oxide film. At this potential and more positive potentials, the oxide film is unstable with respect to the action of anions, especially halide ions, in causing localised rupture and initiating pitting corrosion. 

All of these three properties of the oxide films on metals are influenced by the anion composition and pH of the solution. In addition the potential of the metal will depend on the presence of oxidising agents in the solution. Inhibition of corrosion by anions thus requires an appropriate combination of anions, pH and oxidising agent in the solution so that the oxide film on the metal is stable (the potential then lying between the Flade potential and the breakdown potential), and protective (the corrosion current through the oxide being low). 

Potentiostatic tests " have been used and Wilde and Williams in potentiokinetic studies of the critical breakdown potential of stainless steels (Types 430 and 304) in 1 -0 mol dm" NaCl, showed that the nature of the gas used to purge the solution has a pronounced effect on the value of... 

Fig. 19.41 Dependence of breakdown potential of Fe-Cr alloys (containing 13% Cr, in 0-1 mol dm HBr + Kj SO4 solution) on the ratio of sulphate and bromide concentration in...

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