How Activated Titanium Anodes Are Used in Concrete Structures for Cathodic Protection?

How Activated Titanium Anodes Are Used in Concrete Structures for Cathodic Protection?

Activated titanium anodes, coated with mixed metal oxides, for use embedded into new concrete structures or applied to existing structures cathodic protection. Applications include anodes within overlays, cast into slots or drilled holes or fixed to the surface under GRP casings. In the appropriate form, these anodes can be embedded in new concrete that is then buried or immersed, as well as the more common applications to atmospherically exposed concrete. The life of the coating is determined by its composition and the amount of coating deposited on the substrate, the “coating loading”. Coatings were proprietary and patent protected, but there are now many producers of these “MMO/Ti” anodes.

Test procedures are available for this anode type which enable accelerated testing to demonstrate that the anode life is equivalent to the design life at the maximum design anode current density plus any safety margin determined necessary by the design in respect of possible non-uniform distribution of current.

For all forms of anode of this type, a minimum anode/reinforcement or other steel distance of 15 mm should be maintained.

The titanium substrate may be in the form of an expanded mesh, a strip (solid or expanded), wire, tube, etc., as required by the design and the magnitude and distribution of current. The coatings are proprietary, wholly or in part protected by patents and frequently their composition is not disclosed.

Activated titanium anodes may be operated at current densities of up to 400 mA/m2 of anode surface area for short periods; the limiting factor is acid attack of the surrounding concrete. Generally, the anode current density is limited by design to a long-term maximum of 110 mA/m2 but current densities of up to 220 mA/m2 may be permitted. Dependent upon electrocatalytic coating loading and composition, the calculated life of the anode is in the range 25 years to 100 years. Anode lifetime should be determined considering a non-uniform current distribution.

Surface-mounted activated titanium mesh and grid anodes require cementitious overlays and the surface preparation, pretreatment, design and application of the cementitious overlay is critical to the total performance of the system (see Clauses 6 and 7). Some failures have occurred due to disbondment of the overlay; these failures are generally accepted to be attributable to deficiencies in substrate surface preparation, pretreatment and/or application procedures not related to cathodic protection.

Activated titanium (MMO/Ti) tubular discrete anodes have been used relatively extensively to protect steel in atmospherically exposed concrete, directly cast into mortar in drilled holes. These applications have been designed for operation at anode/mortar current densities of up to 800 mA/m2, but are unlikely to operate continuously at that level.

MMO/Ti and previously platinized titanium (Pt/Ti) have also been used in discrete anodes as the primary anode/conductor to a graphite paste backfill around the primary anode, filling the holes drilled into the concrete for the anode installation. These applications have operated at carbon paste/concrete current densities of up to 800 mA/m2.

Uncoated titanium conductor strips used to connect activated titanium anodes and also various discrete anodes may be at risk of corroding by pitting in chloride environments if operated at in excess of approximately 8 V metal/electrolyte potential.