MMO Coating Explained: Why Ru-Ir, Ir-Ta and Platinum on Titanium Are Not Interchangeable

Introduction: the coating does the work, titanium just holds it there

A titanium anode is often sold as if the metal were the product. It is not. Titanium's job is to stay dimensionally stable and form a passive oxide film. That same film is why bare titanium fails fast as an anode: it blocks current. The part that actually drives the reaction sits on the surface - a mixed metal oxide, or MMO, coating applied as a formulated precursor and baked on through repeated coats and thermal decomposition.

This distinction matters because it explains the single most common sourcing mistake: assuming that two anodes described as "titanium with ruthenium" will behave the same. They will not. Coating composition, loading, number of coats, firing temperature and substrate pretreatment all change the result. Two parts can look identical and still sit on opposite sides of the economic choice.

 

 

What an MMO coating actually is

Think of the coating as a thin, crystalline, electrically conductive ceramic. It must do three things at once: pass current, catalyse the intended reaction, and survive thousands of hours in an aggressive environment.

Under direct current, chloride ions are oxidised at the anode surface while water is reduced at the cathode. The useful product is hypochlorous acid and hypochlorite; the unavoidable companions are oxygen evolution, heat, and a gradual change in coating structure. A coating's value is therefore not its ability to start a reaction. It is its ability to keep running at a stable voltage while side reactions stay under control.

"Stable" is the operative word. As a coating wears, cell voltage rises. The rectifier compensates with more current. Heating and oxygen evolution increase. What began as a slow drift can end as a runaway. A buyer who only compares the price per piece has missed the metric that pays for the anode: cost per kilogram of product delivered over the anode's working life.

Ru-Ir: the chlorine-evolution workhorse

Ruthenium dioxide is a strong chlorine-evolution catalyst. Add iridium and titanium oxides in the right proportion and you get a coating widely used where chlorine evolution dominates: brine electrolysis, on-site hypochlorite generation, chlor-alkali, and many electroplating and anodising duties.

Ru-Ir coatings are valued for high current efficiency towards chlorine and relatively low operating voltage. They are also the coating most often mis-specified, because "Ru-Ir" covers a wide range of formulations. A coating optimised for warm, dilute brine is not the same proposition as one run in concentrated chloride at high current density.

Use Ru-Ir when the anodic reaction is predominantly chlorine evolution and the electrolyte is chloride-bearing. Do not assume it is right for a duty that is mostly oxygen evolution, or for an application where platinum's surface behaviour is required. And do not accept a Ru percentage as the whole story: a 30% Ru formulation from one shop is not comparable to 30% from another.

Ir-Ta: built for oxygen-evolution and harder conditions

Iridium and tantalum oxides shift the coating's character towards oxygen evolution and improve resistance to aggressive oxidation. Ir-Ta systems are commonly associated with acidic media, high oxygen-evolution fractions, and duties where the coating must tolerate polarisation or periodic reverse current.

Tantalum contributes corrosion resistance and structural stability; iridium contributes electrocatalytic activity and durability. The trade is that an Ir-Ta coating may operate at a higher voltage than a Ru-rich one in a chlorine-evolution cell. Paying for Ir-Ta in a pure chlorine-evolution duty can be paying for capacity you do not use.

A useful rule of thumb: if the dominant anode gas is oxygen, ask for an oxygen-evolution-oriented formulation. If it is chlorine, ask for a chlorine-evolution-oriented one. If the duty swings between the two - as many industrial cells do - the answer is a compromise formulation and a tested duty cycle, not a catalogue part number.

Platinum on titanium: a different material logic

Platinized titanium anodes use a thin platinum surface layer, typically applied by electroplating or other deposition, sometimes over an intermediate layer. Platinum offers excellent conductivity, a stable electrochemical surface and strong resistance to many aggressive media. It is a rational choice where its surface behaviour justifies the material cost.

It is not a universal upgrade. Platinum is expensive, and a thin platinum layer can be vulnerable to local damage, especially where current density is uneven or where particles scratch the surface. It is also not automatically the right answer for every chloride or every high-chlorine-duty. Some of the most expensive premature failures we see are platinum anodes specified by a buyer who equated "more expensive raw material" with "longer life."

Platinum on titanium deserves consideration for applications that value a stable, low-noise electrochemical surface and where the operating envelope fits. It should be selected from a duty specification, not from a price ladder.

How to read a coating specification without getting lost

A credible coating conversation should cover six questions. None of them is optional if you want a result you can budget.

Which reaction dominates - chlorine evolution, oxygen evolution, or a mix?

What is the electrolyte: composition, pH, temperature, and how dirty is it?

What current density and run/stop duty is expected, including start-up and polarity events?

What active area and geometry are required, and how even is current distribution?

What failure criterion counts: voltage rise, coating loss, product purity, or visual condition?

What does the warranty actually cover, and on what basis is life measured?

Notice what is missing: "make it last five years." A coating life expressed only in years is not a specification. It is a hope. Life depends on salt, water, temperature, current density, stop/start cycles, cleaning practice, and what the buyer considers "failed."

The questions buyers should ask instead of asking for a discount

What is the coating family, and why was it chosen for this duty? A supplier who cannot name the intended reaction should not be sizing your cell.

What is the coating loading, in grams per square metre of active area? Loading is not everything, but a refusal to disclose it is a signal.

What is the guaranteed active area? Total titanium area and active coated area are different numbers. The second one is the one that does the work.

What is the failure criterion? A 10% voltage rise, a visible bare spot, or loss of product concentration are three different definitions of "worn out." They produce three different replacement cycles.

What does recoating cost, and what is reusable? An anode is not always scrap when the coating fails. The substrate may be recoatable. The answer depends on whether the titanium was damaged, warped, or contaminated during service.

What is the cost per unit of output? Divide the installed anode cost, including recoating and downtime, by the kilograms of product delivered over the warranted life. That number is comparable across suppliers. The unit price is not.

Application map: where each coating tends to land

Electroplating and metal finishing often involve chloride or mixed electrolytes, complex geometry, and uneven current distribution. Ru-Ir is common, but the geometry and rack or basket design can matter more than the coating chemistry.

PCB plating is unusually sensitive to current distribution and contamination. A small variation in deposit uniformity can become a reject rate. Here the conversation must include anode shape, masking, edge effects and bath purity, not coating alone.

Water treatment and on-site hypochlorite generation are chloride-rich and chlorine-evolution duties. Ru-Ir is the natural starting point, provided salinity, temperature and cell design are controlled. Hard water is an anode killer in this sector: scale raises voltage and creates hot spots.

Cathodic protection in seawater uses MMO anodes because they are dimensionally stable and do not sludge. The coating must tolerate long service, variable current and marine growth. This is not a plating-anode duty in a different shape.

Electrolysis, chlor-alkali and specialised electrochemical processes cover a wider range. Coating selection follows the anode gas and the impurity tolerance of the process. This is where a generic "titanium electrode" claim has the least value.

What the anode should not be asked to do

An anode cannot compensate for bad water. Hardness, iron, manganese and silica deposit on surfaces and drive voltage up. If the softener is not sized or maintained, the best coating in the world is being asked to survive a condition the specification never included.

It cannot compensate for the wrong polarity. Some duties reverse polarity; some do not. A coating designed for one direction can fail abruptly when used for the other. State the duty cycle plainly.

It cannot compensate for a cell that runs dry, a pump that stalls, or a rectifier with excessive ripple. These are electrical and mechanical failures dressed up as coating failures. They should be investigated as such.

And it cannot be judged by colour. Dark, light, speckled, smooth, rough - none of these observations establishes coating quality. The only evidence is performance under the actual duty, measured against an agreed criterion.

 

 

Conclusion: buy the reaction, not the ingredient list

Ru-Ir, Ir-Ta and platinum on titanium solve different problems. Ru-Ir is a strong chlorine-evolution choice. Ir-Ta is oriented towards oxygen-evolution and demanding oxidation conditions. Platinum on titanium offers a stable, high-value surface where the duty and budget justify it. None is "best." Each is a response to a duty specification.

The right question is not "which coating is strongest?" It is "which coating, at what loading, on what active area, under my electrolyte, at my current density, with my stop/start cycle, fails in a way I can measure and afford?"

For a site-specific coating recommendation, send Dinoer your electrolyte data, target output, geometry, current density and failure criterion. We will return a specification and a project schedule - not a generic part number. Please contact us.