MMO Anode Coating & Recoating
The titanium substrate is the easy part. What determines whether an anode lasts three years or fifteen is the coating — its composition, its loading, and whether it was specified for the duty it actually runs in.
There is no universal coating specification. Anyone who quotes you a loading in g/m² before asking what current density you run at is selling from stock rather than specifying for your cell.
This page explains what actually drives the specification, so you can brief a supplier properly — including us.
The Four Variables
Coating specification is determined by these, in this order:
1. Duty — chlorine evolution or oxygen evolution
This decides the coating system, and the two are not interchangeable.
| Duty | Coating system | Typical applications |
|---|---|---|
| Chlorine evolution | RuO₂-IrO₂ (ruthenium-iridium), sometimes with tin | Sodium hypochlorite generation, seawater chlorination, chlor-alkali, ballast water treatment |
| Oxygen evolution | IrO₂-Ta₂O₅ (iridium-tantalum) | Electroplating, electrowinning, cathodic protection, water electrolysis, wastewater treatment |
Running a ruthenium-iridium coating in an oxygen-evolving cell will fail early. It is one of the most common specification errors in this product category.
2. Current density
The single biggest driver of loading and life. A cathodic protection anode at low current density and an electrowinning anode at high current density need different coatings even when both are oxygen-evolving.
3. Design life
Loading and life move together. Doubling required life is not a small increment in precious metal content — it is the main cost variable in the anode.
4. Electrolyte chemistry and temperature
Composition, concentration, pH, temperature and the presence of fluoride or organics all affect coating selection and expected life.
Industry Reference Ranges
These are publicly published industry figures, given so you can sanity-check any quotation you receive. They are not a specification for your cell, and they are not a claim about a particular production line.
| Parameter | Publicly cited range |
|---|---|
| Ru-Ir coating (chlorine evolution) | Coating thickness around 10–15 μm, precious metal loading in the region of 15–25 g/m² |
| Ir-Ta coating (oxygen evolution) | Coating thickness around 8–12 μm, loading in the region of 20–35 g/m² |
| Low-duty chlorine evolution electrodes | Total noble metal loading as low as 2–8 g/m² |
| IrO₂ content in RuO₂-based coatings | Roughly 10–30%, higher proportions improving resistance to oxygen corrosion |
| Ru-Ir maximum current density | Up to around 3000 A/m² |
| Chlor-alkali service life | Commonly quoted above 8 years |
How to use this table: if a quotation gives you a low-duty loading figure for a high-current-density oxygen-evolving application, or quotes a long life without a loading that supports it, the numbers do not hang together. Ask how the life figure was established — accelerated life testing conditions, or field reference.
Recoating Spent Anodes
When the coating is consumed, the titanium substrate is usually still sound. Stripping and recoating typically costs materially less than a new anode, which makes it a recurring requirement for electrolysis, plating and cathodic protection operators rather than a one-off.
What determines whether your anodes are recoatable:
- Remaining substrate thickness — the substrate loses material at each strip
- Pitting depth — deep pits will not recoat to full performance
- Conductor bar and weld condition — a sound coating on a failed connection is useless
- Deformation — mesh and plate that have distorted in service may not fixture correctly
Send photographs and the service history of a spent anode and it can be assessed before you ship anything.
What We Need to Quote
These are the same questions any competent coating shop will ask you. Having them ready shortens the process considerably:
- Application and duty — what is the cell doing, and is it evolving chlorine or oxygen?
- Working current and current density (A/m²)
- Required or expected service life
- Electrolyte — composition, concentration, temperature, pH
- Anode form and dimensions — mesh, plate, rod, tubular; drawing if available
- Conductor arrangement — connection type, cable or bar specification
- New anodes or recoating of existing units
If you do not have all seven, send what you have. Items 1 and 2 are the minimum needed for a meaningful discussion; the rest can be established together.
Related
Anodes are supplied as finished units or as coated components for your own assembly. See titanium anodes and electrodes and titanium anode baskets for the product forms, and titanium mesh for substrate material.