Platinum Metals Rev., 1960, 4, (1), 15
Tantalum-Platinum and Titanium-Platinum Bi-Electrodes
Anodic Behaviour In Electrolytes Of Low Conductivity
Previous studies of the anodic polarisation of titanium in chloride solutions have demonstrated that, when the potential across the oxide exceeds a critical value (12 to 14 volts), film growth ceases owing to breakdown of the oxide film and intense corrosion or pitting occurs at localised areas. Pitting of the titanium can, however, be prevented by having platinum in contact with the metal surface. In these circumstances the entire current will be conducted through the platinum and pitting of the titanium will occur only if the breakdown potential is exceeded. A continuous coating of platinum is not essential, and very high current densities can be applied to a titanium-platinum bi-electrode in sea-water, or other highly conducting electrolytes containing chlorides, without exceeding the breakdown potential (1, 2). The use of platinised titanium as an anode for cathodic protection of structures in sea-water is now firmly established.
The situation is, however, quite different if the electrolyte has a very high electrical resistance as, in these conditions, it is possible to exceed the critical potential with consequent film breakdown and pitting of the exposed titanium. A rod of titanium, coated with platinum over its lower half only, that has been anodically polarised in fresh water (specific conductivity 3 × 103 μ mhos/cm–1), using a cathode of equal length placed one inch from the anode, at 25 amp/ft2 shows appreciable pitting. Owing to the geometry of the system and the very high IR drop through the electrolyte the potential of the titanium will increase with increasing distance from the platinum and, at a certain point, the breakdown potential will be exceeded. It has been observed, however, that the titanium adjacent to the platinum shows no evidence of pitting. If the experiment had been carried out in sea-water the titanium would have remained unattacked.
Tantalum, unlike titanium, can be anodically polarised in sea-water without the occurrence of pitting, and film growth, as shown by the interference colours, proceeds until the potential is ~ 160 volts, when the film breaks down.
The thick oxide produced in these circumstances is not, however, firmly adherent to the metal and can be readily detached with Sellotape.
The potential of tantalum during anodic polarisation can be demonstrated visually by the brilliant interference colours. If a tantalum-platinum bi-electrode (1 × 1 inch tantalum with platinum, 0.1 × 0.1 inch, welded to the centre) is anodically polarised in sea-water only a first-order blue film will be formed uniformly on the tantalum surface and current will then proceed through the platinum (Fig. 2a). In a low-conductivity water, however, the potential distribution is not uniform and the thickness of the oxide will increase with increasing distance from the platinum microelectrode. This results in a wedge-shaped oxide film (Fig. 2b) which gives rise to five orders of interference as a series of concentric rings around the platinum, as shown in Fig. 1. Since the thickness of the oxide is a function of the potential (14.5 Å/V for tantalum) it is evident that the potential increases with increasing distance from the platinum.
A similar experiment with a corresponding titanium-platinum bi-elcctrode resulted in only 1 to 2 orders of interference, as breakdown of the film, at points in the surface where the potential exceeded ~12 volts, precluded further film thickening (Fig. 2c). Again, however, it is necessary to observe that in no circumstances could pitting be produced in the immediate vicinity of the platinum. Although the potential at which pitting occurs usually corresponds to 12 to 14 volts, this value can be exceeded by careful preparation of the metal surface. Other conditions being equal, a roughly machined surface pits at a lower potential than a surface that has been carefully prepared by polishing or by pre-anodising in a non-corrosive electrolyte.
The bi-electrode previously described represents an extreme case of exposure of titanium to the electrolyte and is unlikely to be encountered in practice. Short-term laboratory experiments have demonstrated that appreciable areas of platinum (0.5 inch diameter) can be removed from platinum-coated titanium without consequent pitting when the metal is used as an anode in a natural fresh water. It would appear from laboratory tests that although platinised tantalum would be preferred, platinised titanium could be used with some confidence as an anode for cathodic protection of systems containing natural fresh waters. It is essential, however, that, if high voltages are to be used, the titanium should be completely coated with platinum.
In conclusion it should be observed that these experiments have been conducted using “commercial purity” titanium. It has recently been suggested (3) that, as a result of research into the breakdown voltage of this purity titanium, anodes may shortly be available which will permit the use of higher voltages than 12 to 14 volts.
Acknowledgments are made to Mr M. Giles of Metal & Pipeline Endurance Limited for carrying out the experimental work.
- 1J. B. Cotton Chem. & Ind., 1958, p. 492 – 493
- 2J. B. Cotton Platinum Metals Rev., 1958, 2, 45 – 47
- 3J. B. Cotton Private communication