Development of the Noble Metal/Oxide Coated Titanium Electrode PART 111: COATED TITANIUM ANODES in WIDELY RANGING OXYGEN EVOLVING SITUATIONS by €? C
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Development of the Noble Metal/Oxide Coated Titanium Electrode PART 111: COATED TITANIUM ANODES IN WIDELY RANGING OXYGEN EVOLVING SITUATIONS By €? C. S. Hayfield Bickenhill, West Midlands, England The first two parts of this paper on titanium-based electrodes concentrated on the development of coatings comprising noble metals and their oxides suitable for use in chloride-bearing electrolytes. This concluding part deals with simi- lar types of electrodes which act as counter electrodes in commercial processes where the product is made on the cathode. Many electrolytes in this category involve sulfuric acid, which means that oqgen evolves at the anodes. Applications for which these electrodes are used range from those operating at very high current density, such as electrogalvanising, to others at much lower current densitr, such as impressed current cathodic protection of rebar in concrete. When platinised titanium-type electrodes were type chlorine cell technology. first envisaged for commercial use, manufac- The electrolyte is usually strong sulfuric acid turers fixed their sights almost exclusively on (- 150 g 1.') at a temperature above ambient, using them as a possible replacement for graphite usually - 40°C, but at a significantly lower tem- in high current density chlor-alkali processes, perature than the electrolyte in chlor-alkali cells. in order to effect a lowering of the cell voltage The electrolyte is more likely to have an and therefore produce cost savings (26).In the organic content. author's experience, other applications, for There may be a high manganese level (10 instance replacing lead-based anodes in elec- to 15 g 1.'). trowinning, were not initially considered. Such Some chloride and fluoride may be present. applications, mostly involving sulfuric acid-based As the cathodes are periodically removed electrolytes, call for durability under oxygen to recover the metal, there are more disconti- evolving electrochemical conditions. Present nuities in operation than in chlor-alkali pro- day, large-scale applications for titanium-based duction. electrodes, including electrogalvanising and There may also be anodekathode shorting. impressed current cathodic protection of rebar All in all, this is rather a formidable environ- in concrete, did not then exist. ment in which to introduce viable titanium- based electrodes, and the incentives for change Electrowinning are therefore not straightforward. However, it In electrowinning, lead-based electrodes are can be advantageous to produce a cathodic prod- used to extract metal from a solution of its salts uct containing only low levels of lead, and while by electrolysis, with the electropositive metal there may be some savings in electrical cost, being deposited on the cathode. In this indus- high levels of manganese in solution usually try conditions differ from the chlor-alkali indus- make this difficult because of manganese diox- try in numerous ways: ide deposition on the surface of the anode. Current densities, in general, are about an The 'Bunker Hunt' affair in 1980, when the order of magnitude lower than in diaphragm- price of silver escalated, meant that some users Platinum Metals Rev., 1998, 42, (3), 116-122 116 Fig. 12 A 20-cell plating section from a Gravitel electrogalvanisingplant in Usiminas, Brazil. The capacity is 360,000 tonneslannum. A 20 cell unit will contain around 400 electrodes, each of size - 0.5 ni x 1 m Reproduced by courtesy of Audritz AG Ruthner Surface Technologies switched to titanium-based electrodes to commercial importance, the characteristics of capitalise on the silver inventory in their lead- the electrocatalysts, iridium oxide and iridium silver anodes. In fact, the requirements of each oxidehantalum oxide mixtures, were studied in plant needed to be considered individually; but more detail. Usually, the favoured means of apart from a few plants which operated with depositing pure iridium oxide is by the paint/ chloride-based electrolytes and then switched thermal decomposition route using chlor-iridic to titanium-based electrodes, the majority of acid dissolved in an alcohol. If the decomposi- the world metal winning industry still uses lead- tion temperature is too low then water-soluble based electrodes. iridium chloride remains, but if the decompo- sition temperature is too high the iridium oxide Durable Electrocatalysts for Oxygen is converted to less catalytically-durableiridium Evolving Anodes metal. The window for the correct decompo- A good place to start looking at present-day sition temperature is small, only about 20°C. In oxygen electrode technology is the early de Nora the preferred iridium oxide form, the electro- patent assigned to Bianchi and colleagues, where catalyst has a tendency to change &om an attrac- the preferred electrocatalyst was IrO,/Ta,O, tively-low to an undesirably-high oxygen over- codeposit (27). It is said that electrodes with potential. Factors that cause passivation include: this coating were incorporated into a Sulfomat length of time under anodic polarisation cell installed in Japan by the U.S. company Ionics increasing current density and Inc., of Watertown, Massachusetts, but the lives decreasing acidity. of these electrodes were reputedly much shorter The inventors of the de Nora patent seem, like than those of anodes in chlor-alkali cells. Beer with his Ru02/Ti0, patents, to have found As oxygen electrodes became of greater the key parameters back in 1975. Subsequently, Platinum Metals Rev., 1998, 42, (3) 117 Comninellis and coworkers have clarified much precipitation, which caused such thick film of the detail (28,29). Perhaps the most impor- formation. At more elevated temperature, 70°C tant finding is that mixing tantalum oxide with and above, structural corrosion may still occur the iridium oxide acts, in some way, to slow but the continuing localised titanium dissolu- down the reduction of the iridium oxide to irid- tion no longer precipitates to form film. ium metal. Thus, while iridium oxide by itself An early warning of dire events that could hap- requires a decomposition temperature of pen to titanium-based electrodes was implicit 380 to 4OO0C, a codeposit of Ir02/Ta205can be in a paper by Antler and Butler who reported safely put into a furnace at 450 or even 5OO0C, degradation mechanisms in platinum- and and this also results in the expulsion of more rhodium-plated titanium electrodes at accel- residual chloride ion. erated high current densities during high tem- In retrospect, the platinum in a 70/30 Pt/Ir perature testing in simulated chlorate liquor coating appears to slow down the reduction of (31). Thick titanium oxide was found to have iridium oxide. Thus, with paints comprising formed underneath the noble metal coating. chlor-platinic and chlor-iridic acids in alcohol, How could this occur if the local anodising decomposition temperatures of 500°C can safely potential on such specimens was never more be used, producing coatings comprising fine than a few volts? As no similar electrode failures scale mixtures of Pt + IrO,. were found in sodium chlorate plants, the pos- Some evidence for the less powerful nature of sibility of thick film formation under noble iridium oxide as an oxygen electrocatalyst, com- metal/oxide coatings was put to one side for pared with ruthenium oxide as a chlorine elec- many years. trocatalyst, can be gauged from the finding that optimised IrO,/Ta,O, codeposits for oxygen evo- Electrogalvanising: The Andritz lution are always high in iridium oxide. A paint Ruthner Experience with a high iridium oxide content, such as 95/5 The next episode in this story came when the or 90/ 10, tends to favour the suppression of chlo- Austrian company, Andritz Ruthner of Vienna, rine evolution in electrolytes containing trace came up with a novel method of coating zinc chloride, while an optimised 'work horse' com- onto one side of continuous coils of steel strip, position is nearer 50/50 dwtper cent. principally for the automobile industry, but also With hindsight, an improved understanding for domestic appliances, see Figures 12 and 13. of the titanium substrate was needed to explain This Gravitel process also finds application for accumulating experiences with titanium-based coating both sides of steel strip to different thick- oxygen electrodes. A prerequisite for any com- nesses, see Figure 14. mercial coated titanium electrode must be an The Gravitel process involved zinc deposi- adherent and durable coating. In most chloride- tion at high current density (equivalent to mer- bearing electrolytes, even when quite acidic, the cury-type chlorine cells at 10 to 20 kA m-') from titanium is adequately durable to general cor- made-up electrolyte compositions. Initial eval- rosion, provided the design gives freedom from uation seemed to show that a pure iridium oxide anodic breakdown corrosion. Highly concen- electrocatalyst on a titanium substrate with an trated sulfuric acid is generally more aggressive overlay of tantalum oxide would sufEce to uprate towards titanium. In engineering terms anodic corrosion resistance in hot sulfuric acid. polarisation (protection) allows practical usage Accordingly, a massive quantity of electrodes for many years. In one reported situation (30), was made for - almost simultaneous - use in however, titanium, polarised at 12 volts in 10 three plants. However, their operation showed per cent sulfuric acid at 35"C, was found to be that the nominally identical anodes behaved quite covered with film of thickness 30 to 40 p.Later