Titanium breakdown potentials

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. ...

The Operational Characterisics of Platinised-Titanium Anodes Platinised-titanium anodes have the disadvantage that the protective passive him formed when titanium is made anodic in certain solutions can breakdown. This could result in rapid pitting of the titanium substrate, leading ultimately to anode failure. The potential at which breakdown of titanium occurs is dependent upon the solution composition, as is evident from Table 10.16. 

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... 

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. 

Table 9.21 shows that the corrosion resistance of stainless steel increases upon coating with hydroxyapatite. The presence of calcium phosphate in solution, due to dissolution of hydroxyapatite, seems to be the cause for these changes. The same table indicates that calcium phosphate is detrimental to the corrosion resistance of titanium, both in terms of film breakdown potential and corrosion rate under passive conditions. 

Platinized titanium is often used in marine environments. To avoid the dissolution of titanium at unplatinized locations on the surface, the operating voltage of the anode is limited by the anodic breakdown potential of titanium which is in the range of 9 to 9.5 V in the presence of chlorides. Hence the maximum recommended operating voltage of platinized titanium anodes is 8 V. The corresponding maximum current... 

Some metals and alloys have low rates of film dissolution (low /p) even in solutions of very low pH, e.g. chromium and its alloys, and titanium. In these cases the value of /p is quite low, and although it increases as the temperature increases, a maximum is reached when the solution boils. The maximum current is below and breakdown does not occur. However, in certain alloys, e.g. Cr-Fe alloys, the protective film may change in composition on increasing the anode potential to give oxides that are more soluble at low pH and are therefore more susceptible to temperature increases. This occurs in the presence of cathode reactants such as chromic acid which allow polarisation of the anode. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Thus titanium by itself cannot function as an efficient anode for the passage of positive direct current into an electrolyte. The surface film of oxide formed upon the titanium has, however, a most useful property while it will not pass positive direct current into an electrolyte (more correctly, while it will not accept electrons from negatively charged ions in solution), it will accept electrons from, or pass positive current to, another metal pressed on to it. Hence a piece of titanium which has pressed on to its surface a small piece of platinum will pass positive direct current into brine and into many electrolytes, at a high current density, via the platinum, without undue potential rise, and without breakdown of the supporting titanium . ... 

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