Mixed metal oxide coating

Precious metals and oxides platinised titanium, platinised niobium, platinised tantalum, platinised silver, solid platinum metals, mixed metal oxide-coated titanium, titanium oxide-based ceramics. 

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. 

The composition of the mixed metal oxide may well vary over wide limits depending on the environment in which the anode will operate, with the precious metal composition of the mixed metal oxide coating adjusted to favour either oxygen or chlorine evolution by varying the relative proportions of iridium and ruthenium. For chlorine production RuOj-rich coatings are preferred, whilst for oxygen evolution IrOj-rich coatings are utilised. ... 

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. 

The active coating consists of a thermally deposited mixed metal oxide coating, the composition of which is considered proprietary information, although it is known that certain filler materials, e.g. Ta, may be added to the mixed metal oxide to reduce the precious metal content of the coating, and hence the cost of the anode. 

Mixed Metal Oxide Coated Titanium As an alternative to platinised titanium, these materials are finding increasing use in seawater and soil based deep well groundbed applications. 

The DSA-type anodes are inert , coated anodes made of a valve metal (titanium, niobium, or tantalum) base coated with an electrochemically active coating. The active coating is made either of noble metals or of mixed metal oxides. Noble metals in active coatings are usually platinum or platinum alloys. Mixed metal-oxide coatings contain active oxides and inert oxides the active components are usually ruthenium dioxide (R.UO2) and iridium dioxide (IrC>2) and the inert components are mostly titanium dioxide (TiC>2) and other oxides such as tantalum... 

One of the most successful commercial anodes is the expanded titanium mesh with an activated precious or mixed metal oxide coating. This also comes in the form of an expanded titanium mesh, strips and other configurations. It is fixed onto the surface, usually with plastic fixings and a cementitious overlay applied. 

This anode consists of an expanded titanium mesh with a catalytic mixed metal oxide coating. The anode is fixed to the concrete surface and then overlaid with concrete. This is usually dry spray shotcrete on vertical and soffit surfaces or cast on top surfaces and decks. It is also available in a number of mesh sizes and as a ribbon (described separately). 

Figure 7. II (a) A mixed metal oxide coated titanium mesh anode being overlaid on a bridge in Baltimore MD. Courtesy Jack Bennett, (b) Extreme example of an overlay failure on a titanium mesh anode. Oregon Inlet N. Carolina USA.The whole bridge was swept away in a storm a few years after the author took this photograph. 

As stated in the anode descriptions earlier, there are also two NACE test methods for cathodic protection anodes. These are TM 0294 on embeddable anodes (mixed metal oxide coated titanium, mesh, ribbon, tnbes, rods and conductive ceramic tubes) and TMO1105-2005 on organic-based condnc-tive coating anodes. In addition there is a specification for applying thermal sprayed zinc anodes to concrete American Welding Society (2002). 

For the electro winning of nickel, a tubular anode constructed from titanium with a mixed metal oxide coating is employed. In a standard design EMEW cell, the anode is approximately 50mm in diameter. The cathode is a stainless steel tube approximately 151 mm in diameter. Both the anode and cathode are fixed in the cell. A thin stainless steel starter sheet is rolled and inserted in the cell to deposit nickel. The deposited nickel cathode is removed from the cell as a tube. The stainless steel starter sheet then springs off the deposit to release the nickel cathode. 

Mixed metal oxide coated anodes, also called dimensionally stable anodes (DSA), are based on electrode technology developed in the early 1960s for the production of chlorine and caustic soda. The mixed metal oxide films are thermally applied to a noble metal such as titanium, niobium, and tantalum as substrate materials and are available in a variety of sizes and shapes. These oxide coatings have excellent conductivity, are resistant to acidic environments, are chemically stable, and have relatively low consumption rates. Groundbed installation in soils usually specifies that the anode be prepackaged in a canister with carbonaceous backfill material. 

---