What Coatings are Used for MMO Anode in Cathodic Protection?

What Coatings are Used for MMO Anode in Cathodic Protection?

MMO anodes are charactered by low consumption rate, long service life (comparing with the sacrificial anode), dimensional stability almost during the whole operating life, and lightweight. The uses of mixed metal oxide (MMO) catalytic coated titanium anodes have been growing over decades. MMO anodes can work in the soil, carbonaceous backfill, freshwater, brackish water, seawater, and other working environments. It can be shaped in ribbon, tubular, rod, wire, etc.

The coating for MMO anode in cathodic protection is mainly divided into two types, iridium oxide (IrO2-x) and ruthenium oxide (RuO2-x). In some cases, a mixture of these is used. In addition, titanium oxide (TiO2-x) and tantalum oxide (Ta2O5) are also used as bulk materials.  Iridium bases MMO coating is more used in electrolyzes to evolve oxygen. MMO coating is applied on titanium substrate as a liquid form of metals salt and then is thermally decomposed to form an adherent layer of mixed oxides at temperatures typically ranging 400 to 600°C. The MMO coatings are applied in many layers and heat treated after each coat.

Ruthenium based MMO coating was initially developed to produce hypochlorite for the chemical industry. Iridium based MMO coating is generally used in highly acidic environments, such as electro galvanizing, anodizing of aluminum, recovery of metals, electrosynthesis, and membrane processes, precious metal plating, etc. which rapidly fails due to the instability of RuO2.

One of the purposes of MMO coating on titanium substrate is to reduce the anodic potential of the titanium so that the MMO coated titanium anode can be operated at higher voltages without break down.