Platinised anodes

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. 

Electroplating. Platinised titanium-on-niobium anodes are preferred for use ia electroplating precious metals. These anodes find wide apphcation ia the electronics iadustry and ia the creation of fine jewelry. 

Pla.tinum, Platinum plating has found appHcation in the production of platinised titanium, niobium, or tantalum anodes which are used as insoluble anodes in many other plating solutions (see Metalanodes). Plating solutions were often based on platinum "P" salt, which is diamminedinitroplatiniim (IT). A dinitroplatinite sulfate—sulfuric acid bath has been used to plate direcdy onto titanium (129). This bath contains 5 g/L of the platinum salt, pH adjusted to 2.0 with sulfuric acid. The bath is operated at 40°C at 10—100 A/m. Other baths based on chloroplatinic acid have been used in both acid and alkaline formulations the acid bath uses 20 g/L of the platinum salt and 300 g/L hydrochloric acid at 65° C and 10—200 A/m. The alkaline bath uses 10 g/L of the platinum salt, 60 g/L of ammonium phosphate and ammonium hydroxide to give a pH of 2.5—9.0. The alkaline bath can be plated directly onto nickel-base alloys acid baths require a gold strike on most metals. 

Cathodic protection applications in fresh water include use of ferrite-coated niobium , and the more usual platinum-coated niobium . Platinised niobium anodes have been used in seawater, underground and in deep wells " and niobium connectors have been used for joining current leads Excellent service has been reported in open-seawater, where anodic potentials of up to 120V are not deleterious, but crevice corrosion can occur at 20 to 40V due to local surface damage, impurities such as copper and iron, and under deposits or in mud ... 

In aqueous chloride where it is necessary to use platinised titanium anodes coated over only part of their surface, e.g. titanium rod tipped with a thin platinum film, it may be necessary to limit the applied voltage to 12 V. 

Certain organic compounds form complexes with platinum, and this accounts for the fact that the thin coating of platinum on titanium is rapidly corroded when platinised titanium is used as an anode in plating baths containing organic addition agents. 

The anodic behaviour of platinum and certain of its alloys is of considerable technical importance, since they can be employed under a wide range of conditions without appreciable corrosion, and often in circumstances where no other metal can be used. Their use industrially has recently been extended by applying them as thin coatings to a substrate of a passive metal such as tantalum or, more commonly nowadays, titanium, to reduce the cost. Platinised titanium anodes are discussed in detail in Section 11.3. 

A number of bi-electrodes have been studied for application as insoluble anodes in electroplating platinised titanium, Ti-Pt, Ti-Cu and Ti-Ag. Anodic polarisation measurements in various copper, nickel, chromium and tin plating solutions together with passivation current densities are used to discuss performance and suitability. 

Anodes made from platinised titanium or niobium fall in this category. 

Canister anodes consist of a spirally wound galvanised steel outer casing containing a carbonaceous based extender which surrounds the primary anode element which may be graphite, silicon iron, magnetite, platinised titanium, mixed metal oxide-coated titanium or platinised niobium, etc. 

Groundbeds consist of a carbonaceous extender generally coke breeze and graphite, silicon-iron scrap steel, platinised titanium or niobium anodes. 

Co-axial anodes These are copper-cored anodes of lead silver, platinised titanium and platinised niobium. 

It is a valve metal and when made anodic in a chloride-containing solution it forms an anodic oxide film of TiOj (rutile form), that thickens with an increase in voltage up to 8-12 V, when localised film breakdown occurs with subsequent pitting. The TiOj film has a high electrical resistivity, and this coupled with the fact that breakdown can occur at the e.m.f. s produced by the transformer rectifiers used in cathodic protection makes it unsuitable for use as an anode material. Nevertheless, it forms a most valuable substrate for platinum, which may be applied to titanium in the form of a thin coating. The composite anode is characterised by the fact that the titanium exposed at discontinuities is protected by the anodically formed dielectric Ti02 film. Platinised titanium therefore provides an economical method of utilising the inertness and electronic conductivity of platinum on a relatively inexpensive, yet inert substrate. 

Cotton was the first to publish results on platinised titanium as an anode material, and the first commercial installation utilising platinised titanium anodes was completed in 1958 at Thameshaven for the protection of a Thames-side jetty. 

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 effect of temperature on the consumption rate of platinised titanium anodes has not been found to be significant over the ranges normally encountered in cathodic protection installations, although at elevated temperatures of 90-95°C, consumption rates of 570mg A y in 0-02<7o Na2S04 and 12<7o NaCl solutions have been reported. ... 

Early failures of platinised titanium anodes have been found to occur for reasons other than increased consumption of platinum or attack on the titanium substrate caused by voltages incompatible with a particular electrolyte. The following are examples ... 

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 . 

The superimposition of a.c. ripple on the d.c. output from a transformer rectifier can under certain circumstances lead to increased platinum consumption rates and has been the subject of considerable researchIndeed, when platinised titanium anodes were first used it was recommended that the a.c. component was limited to 5% of the d.c. voltage . 

Attention must be paid to field end effects, particularly on cantilever anodes, e.g. on long anodes that extend away from the cathode surface. Under these circumstances the anode surface close to the cathode may be operating at a considerably higher current density than the mean value, with the exact values dependent upon the system geometry. The life of the platinising in this region would then be reduced in inverse proportion to the current density. 

Platinised-titanium installations have now been in use for 30 years for jetties, ships and submarines and for internal protection, particularly of cooling-water systems . For the protection of heat exchangers an extruded anode of approximately 6 mm in diameter (copper-cored titanium-platinum) has shown a reduction in current requirement (together with improved longitudinal current spread) over cantilever anodes of some 30% . This continuous or coaxial anode is usually fitted around the water box periphery a few centimetres away from the tubeplate. 

Platinised-titanium anodes may also be used in soils when surrounded by a carbonaceous backfill. Warne and Berkeley " have investigated the performance of platinised-titanium anodes in carbonaceous backfills and conclude that the anodes may be successfully operated in this environment at a current density of up to 200 AmThis also supplements the findings of Lewis, who states that platinised-titanium anodes may be used in carbonaceous backfill without breakdown of the titanium oxide film. Success with platinised-titanium anodes has been reported with anodes operating at a few tens of Am and failures of anodes have often been attributed to operation at high current densities . 

Furthermore, the restrictions on operating voltage that apply to titanium in a marine enviroment are not always relevant to titanium in soils free of chloride contamination. Coke breeze is, however, an integral part of the groundbed construction and ensures a lower platinum consumption rate. However, for some borehole groundbeds, platinised niobium is preferred, particularly in the absence of carbonaceous backfill or in situations where the water chemistry within a borehole can be complex and may, in certain circumstances, contain contaminants which favour breakdown of the anodic Ti02 film on titanium. In particular, the pH of a chloride solution in a confined space will tend to decrease owing to the formation of HOCl and HCl, and this will result in an increase in the corrosion rate of the platinum. 

The high cost of platinised materials for use in borehole groundbeds as opposed to conventional silicon-iron anodes may also be offset by the reduction in required borehole diameter, hence lower installation cost, with the relative economics between the different systems dependent upon a combination of both material and installation costs. 

The principle of these anodes is similar to that of platinised titanium since they are all valve metals that form an insulating dielectric film under anodic polarisation. 

Platinum electrodeposition on to tantalum had been carried out as early as 1913 and the use of platinised tantalum as an anode suggested in 1922 , whilst platinum electrodeposition on to niobium was first successfully carried out in 1950 . 

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 relative merits of platinised titanium and niobium in a deep-well environment, in comparison with those of other anode materials, have been given by Stephens . 

With the advent of hul mounted anodes this material has been replaced by the superior platinised titanium and niobium anodes and is now seldom used. 

The oxide coatings are porous and therefore the limitations on operating voltage for platinised titanium anodes apply as well to the oxide-coated titanium electrodes. It has been reported that breakdown of mixed metal oxide anodes may occur at 50-60 V in low-chloride concentration water but at only 10 V in chloride-rich environments . 

These anodes, like platinised Ti may be supplied in different forms e.g. rod, tube, mesh, wire, etc. They may be used for the cathodic protection of offshore structures, heat exchangers, or even pipelines as they can be installed in the soil surrounded by carbonaceous backfill, and are comparable in cost to platinised titanium. ... 

Lead dioxide on graphite or titanium substrates has been utilised as an anode in the production of chlorate and hypochlorites and on nickel as an anode in lead-acid primary batteries Lead dioxide on a titanium substrate has also been tested for use in the cathodic protection of heat exchangers and in seawater may be operated at current densities up to lOOOAm" . However, this anode has not gained general acceptance as a cathodic protection anode for seawater applications, since platinised Ti anodes are generally preferred. 

These consist of a number of parallel slots cut into the concrete surface. Each slot is then filled with a secondary anode of carbon/graphite fibres embedded in a conductive polymer grout. The current to each of these secondary anode systems is provided by a primary anode of platinised niobium wire placed in slots filled with conductive polymer which acts as the primary anode, these slots intersecting each slot of graphite fibre/conductive polymer at right angles. 

Conductive paints (resins) have recently been used for the cathodic protection of steel reinforcing bars in concrete, but they are always used in conjunction with a primary anode material, e.g. platinised-niobium or platinised-titanium wire or a conductive polymer rod. 

In recent years, there has been interest in using zinc as a power-impressed anode for the cathodic protection of steel in concrete. The zinc is flame sprayed onto a grit blasted concrete surface to a final film thickness of approximately 250 m. A primary anode is necessary. Early systems used brass plates as the primary anode, but more recent systems used platinised titanium or niobium wire anodes as the primary current conductor. 

Jacob, W, R., paper 5, Substrate Materials for Platinised Anodes , Proc. Symposium on Cathodic Protection, London, May (1975)... 

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