Titanium cathodic protection

T. Nijland, L. Bertolini, Acid formation in the anode/concrete interface of activated titanium cathodic protection systems for reinforced concrete and the implications for service life , ICC 15 Int. Corrosion Congress, Granada, 22—27 September 2002 (CD-ROM). 

Titanium has potential use in desalination plants for converting sea water into fresh water. The metal has excellent resistance to sea water and is used for propeller shafts, rigging, and other parts of ships exposed to salt water. A titanium anode coated with platinum has been used to provide cathodic protection from corrosion by salt water. 

Cathodic Protection Systems. Metal anodes using either platinum [7440-06 ] metal or precious metal oxide coatings on titanium, niobium [7440-03-17, or tantalum [7440-25-7] substrates are extensively used for impressed current cathodic protection systems. A prime appHcation is the use of platinum-coated titanium anodes for protection of the hulls of marine vessels. The controUed feature of these systems has created an attractive alternative... 

The internal cathodic protection of pipes is only economic for pipes with a nominal width greater than DIN 400 due to the limit on range. Internal protection can be achieved in individual cases by inserting local platinized titanium wire anodes (see Section 7.2.2). 

In common with many of the alloy-environment systems described so far, if the alloy is not susceptible to stress-corrosion cracking under constant stress or stress intensity, then little or no effect of environment on fatigue crack growth is observed. In these cases, frequency, R ratio and potential within the passive or cathodically protected ranges for titanium have no effect on growth rates. 

Copper-base alloys will corrode in aerated conditions. It is, therefore, sometimes appropriate to consider cathodic protection. It becomes particularly relevant when the flow rates are high or when the design of an item causes the copper to be an anode in a galvanic cell (e.g. a copper alloy tube plate in a titanium-tubed heat exchanger). Corrosion can be controlled by polarisation to approximately — 0-6V (vs. CU/CUSO4) and may be achieved using soft iron sacrificial anodes. 

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. 

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. ... 

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. 

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. 

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. 

The most recently developed anode for the cathodic protection of steel in concrete is mixed metal oxide coated titanium mesh The anode mesh is made from commercially pure titanium sheet approximately 0-5-2mm thick depending upon the manufacturer, expanded to provide a diamond shaped mesh in the range of 35 x 75 to 100 x 200 mm. The mesh size selected is dictated by the required cathode current density and the mesh manufacturer. The anode mesh is supplied in strips which may be joined on site using spot welded connections to a titanium strip or niobium crimps, whilst electrical connections to the d.c. power source are made at selected locations in a suitably encapsulated or crimped connection. The mesh is then fitted to the concrete using non-metallic fixings. 

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. 

Hayfield, P. C. S. and Warne, M. A., Titanium Based Mesh Anodes in the Cathodic Protection of Concrete Reinforcing Bars , presented at UK Corrosion, Brighton (1988)... 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

Use Aluminum alloys for structural parts, die-cast auto parts, missiles, space vehicles powder for pyrotechnics and flash photography, production of iron, nickel, zinc, titanium, zirconium antiknock gasoline additives magnesium compounds and Gri-gnard syntheses cathodic protection reducing agent desulfurizing iron in steel manufacture precision instruments optical mirrors dry and wet batteries. 

Other macrocell effects. A special case of macrocell effects has been observed on structures contaminated by chlorides where an activated titanium mesh anode was installed in order to apply cathodic protection when the cathodic protection system is installed but is not in operation, locahzed corrosion on steel can be slightly enhanced by the presence of the distributed anode [4]. 

Anodic acidification. At the anode surface, the anodic process of oxygen evolution takes place 2H2O —>62 + 4H + 4e . In the presence of chlorides, even chlorine develops 2Cr —> CI2 + 2e . Such processes may directly or indirectly produce acidity and may thus lead to destruction of the cement paste in contact with the anode [34]. Experience shows that such deterioration is negligible for activated titanium mesh anodes if the anodic current density does not exceed 100 rtiA/m (or values 3-4 times greater for brief periods). Design of the anodes for cathodic prevention and cathodic protection must respect these hmits. 

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