Titanium Substrates

The coating process, outlined in Figure 6, starts either with the specification of appropriate titanium substrate material for new anodes, or with... 

Anode Applications. Graphite has been used as the primary material for electrolysis of brine (aqueous) and fused-salt electrolytes, both as anode and cathode. Technological advances, however, have resulted in a dimensionally stable anode (DSA) consisting of precious metal oxides deposited on a titanium substrate that has replaced graphite as the primary anode (38—41) (see Alkali and chlorine products). 

Pt electrodeposits may also be produced from molten salt electrolytes. Such a high-temperature process has the advantage that the deposits are diffusion bonded to the titanium substrate and thus have good adhesion, and, if necessary, thick deposits can be produced. However, they have the disadvantage that because of the complexity of the process there is a limitation on the size and shape of the object to be plated, and the resultant deposits are softer and less wear resistant than those from aqueous solutions... 

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. 

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 ... 

It has now gained acceptance as an impressed current anode for cathodic protection and has been in use for this purpose since 1971. The anode consists of a thin film of valve and precious metal oxides baked onto a titanium substrate and when first developed was given the proprietary name dimensionally stable anode , sometimes shortened to DSA. Developments on the composition of the oxide film have taken place since Beer s patent, and this type of anode is now marketed under a number of different trade names. 

At present only titanium substrates are coated in this way because at the temperatures encountered in the anode manufacturing process, niobium would oxidise. Tantalum can be coated with a mixed oxide but this is a relatively expensive process. 

Magnetite may also be used in combination with lead or electrodeposited onto a titanium substrate". The latter anode system has been shown to exhibit good operating characteristics in seawater but at present it is only of academic interest. 

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. 

Bonding is used extensively on steel. The reaction occurs with a high hydrogen dilution of the BCI3 to prevent substrate attack. An iron boride is formed. 1 1 Not all metals, however, are suitable to bonding. For instance, the bonding of titanium by CVD in a chloride-based system is more difficult since the titanium substrate is highly susceptible to HCl attack and the rate of diffusion is low. 

The layer of titanium and ruthenium oxides usually is applied to a titanium substrate pyrolytically, by thermal decomposition (at a temperature of about 450°C) of an aqueous or alcoholic solution of the chlorides or of complex compounds of titanium and rathenium. The optimum layer composition corresponds to 25 to 30 atom % of ruthenium. The layer contains some quantity of chlorine its composition can be written as Ruq 2sTio 750(2- c)Cl r At this deposition temperature and Ru-Ti ratio, the layer is a poorly ordered solid solution of the dioxides of ruthenium and titanium. Chlorine is completely eliminated from the layer when this is formed at higher temperatures (up to 800°C), and the solid solution decomposes into two independent phases of titanium dioxide and ruthenium dioxide no longer exhibiting the unique catalytic properties. 

The passive layer is subsequently formed from Ti02+ and was described as Ti02 x H20. During corrosion CO or C02 is formed from the carbon of the carbide. The electrochemical behaviour of TiC in acid electrolytes was reinvestigated with respect to the depassivation of titanium substrates for anodic PbOzdeposition by M. Cappa-donia et al. [122] using XPS. 

S.A. Marzouk, Improved electrodeposited iridium oxide pH sensor fabricated on etched titanium substrates. Anal. Chem. 75, 1258—1266 (2003). 

Typically, the electrodes are of lead dioxide on a titanium substrate in the form of horizontal perforated plates, usually 5-40 mm apart, depending on the conductivity of the liquid. A potential difference of 5-10 V may be applied to give current densities of the order of 100 A/m2. Frequently, the conductivity of the suspension itself is adequate, though it may be necessary to add ionic materials, such as sodium chloride or sulphuric acid. Electrode fouling can usually be prevented by periodically reversing the polarity of the electrodes. Occasionally, consumable iron or aluminium anodes may be used because the ions released into the suspension may then assist flocculation of the suspended solids. 

N. Wan, C. Wang, and Z. Mao. Titanium substrate based micro-PEMFC operating under ambient conditions. Electrochemistry Communications 9 (2007) 511-516. 

H.W. Kim, H.E. Kim, J.C. Knowles, Fluor-hydroxyapatite sol-gel coating on titanium substrate for hard tissue implants, Biomaterials 25 (2004) 3351-3358. 

Under potentiostatic conditions, CdSe Aims were deposited on titanium substrate from a bath containing sodium seleno-sulflte, cadmium sulfate, and sodium citrate [190]. 

On many metal substrates, CO dissociates into individual atoms that in some cases still produce an ordered monolayer. The resulting geometry has been investigated for a titanium substrate and for an iron substrate. 

Ir Ir02 electrodes (commercially available from Cypress Systems, Lawrence, KS) can measure pH in harsh environments or microscopic spaces [S. A. M. Marzouk, Improved Electrodeposited Iridium Oxide pH Sensor Fabricated on Etched Titanium Substrates, Anal. Chem. 2003, 75, 1258 A. N. Bezbaruah and T. C. Zhang, Fabrication of Anodically Electrodeposited Iridium Oxide Film pH Microelectrodes for Microenvironmental Studies, Anal. Chem. 2002, 74. 5726 D. O. Wipf. F. Ge, T. W. Spaine, and J. E. Baur, Microscopic Measurement of pH with Ir02 Microelectrodes, Anal. Chem. 2000, 72, 4921]. For pH measurement in nanoscopic spaces, see X. Zhang,... 

Development of chlorine electrode materials has benefited from the experience of chlor-alkali electrolysis cell technology. The main problem is to find the best compromise between cycle life and cost. Porous graphite, subjected to certain proprietary treatments, has been considered a preferable alternative to ruthenium-treated titanium substrates. The graphite electrode may undergo slow oxidative degradation, but this does not seem to be a significant process. 

For use over a wide temperature range, it is necessary to match the thermal expansion coefficients of electrode and insulation sheath. RRDEs of glassy carbon embedded in borosilicate glass for use up to 450° C [123] and gold sputtered on to a chromium or titanium substrate on a Macor ceramic cylinder for use up to at least 125°C [124] are examples. 

Fig. 3.7 Typical micro-Raman (a), XRD (b) and SEM (c) characterization results of diamond film deposited on titanium substrate...

Gerger, I., Haubner, R., Kronberger, H. and Fafilek, G. (2004), Investigation of diamond coatings on titanium substrates for electrochemical applications. Diam. Relat. Mater., 13(4-8) 1062-1069. 

Guo, L. and Chen, G. (2007a), High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method. Diam. Relat. Mater., 16(8) 10. 

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