Dimensionally Stable Anodes (DSAs), also known as mixed-metal-oxide anodes, are essential to chlor-alkali production, metal electrowinning, electroplating, wastewater treatment, cathodic protection, and proton exchange membrane water electrolysis. They replaced consumable graphite electrodes, which eroded rapidly, causing unstable current distribution, frequent shutdowns, and increased energy consumption.
A typical DSA consists of a commercially pure titanium substrate covered with a microscopic catalytic coating. Titanium provides mechanical strength, electrical conductivity, corrosion resistance, and dimensional stability. Expanded titanium mesh is particularly common because its open geometry promotes electrolyte circulation, distributes current effectively, and allows chlorine or oxygen bubbles to disengage from the electrode surface.
The active coating contains combinations of iridium oxide, ruthenium oxide, titanium oxide, and tantalum oxide. Ruthenium oxide is highly active for chlorine evolution, while iridium oxide performs better in strongly acidic, oxygen-evolving environments. In an iridium oxide–tantalum oxide coating, iridium provides the catalytic sites, while tantalum improves structural stability, limits iridium dissolution, and reinforces the oxide matrix. Depending on the application, iridium loadings may range from approximately 2.5 to more than 10 grams of elemental metal per square meter of geometric surface area.
MMO coatings are generally manufactured through repeated thermal-decomposition cycles. Metal-salt precursor solutions are applied to cleaned and etched titanium, dried, and calcined, often at temperatures between 400°C and 600°C. This procedure may be repeated 15 to 20 times to create a final coating only about 6 to 15 micrometers thick. Thermal processing produces the porous “mud-crack” surface associated with many MMO coatings. Controlled microcracking increases the electrochemically active surface area, although excessive cracking can expose the titanium and accelerate deterioration.
An MMO anode is normally removed from service when operating voltage rises sharply, and the electrode can no longer maintain the required current economically. This condition is often interpreted as proof that the precious-metal coating has been completely consumed. In reality, deactivation results from several interacting mechanisms and may occur while significant quantities of iridium or ruthenium remain on the titanium.
Some catalytic material is gradually lost through anodic dissolution. High potentials can convert iridium and ruthenium oxides into soluble species that enter the electrolyte. Excessive current density, low or fluctuating pH, current reversals, thermal cycling, and contaminants such as fluorides or bromides can accelerate this process. Vigorous gas evolution also creates hydrodynamic stress that may cause microscopic coating particles to crack or flake from the surface.
The most important failure mechanism, however, is often titanium passivation. Electrolyte penetrates the pores and fissures in the MMO coating, reaching the underlying titanium. Under anodic polarization, the titanium forms a dense titanium dioxide layer. Although this passive film protects the substrate from corrosion, it also has high electrical resistance. As the titanium dioxide interlayer thickens, the voltage required to maintain current production increases. The resulting heat and electrochemical stress can promote further passivation and coating degradation.
The anode eventually becomes electrically or economically unusable when interfacial resistance exceeds the capacity of the process or power supply. This can happen long before the catalytic layer has disappeared. Consequently, there is no universal residual-iridium concentration at which every anode fails. Anodes containing similar amounts of precious metal may perform differently because of variations in passivation, coating distribution, contamination, current flow, and operating history.
A deactivated MMO anode rarely appears valuable. It may look dull, dark, scaled, discolored, or pitted, while its remaining precious-metal coating is too thin to resemble bulk iridium or ruthenium. Because nearly all of the anode’s physical weight is titanium, plant personnel and general scrap dealers frequently classify the entire component as ordinary titanium scrap. Generic scrap yards usually calculate payment based on gross weight and base-metal grade.
They may lack the analytical equipment needed to identify a thin MMO coating and the metallurgical infrastructure required to recover refractory platinum-group metals. A facility accepting a titanium-based offer may therefore surrender valuable iridium and ruthenium without receiving compensation for them. The same risk applies to specialized rhenium-bearing components, whose residual surface value can substantially exceed the value of the titanium substrate.
This loss has strategic as well as financial consequences. Iridium and ruthenium are exceptionally rare and are recovered primarily as by-products of platinum and palladium mining. Most primary production is concentrated in a small number of geographic regions, leaving supply vulnerable to energy shortages, labor disruptions, infrastructure constraints, trade restrictions, and changing base-metal economics. Because these metals are by-products, even a sharp increase in iridium prices does not automatically result in increased production.
Demand is also growing. Iridium oxide is one of the few mature catalysts capable of sustaining oxygen evolution at the acidic anode of a PEM water electrolyzer, making it critical to the development of green hydrogen. Ruthenium remains important in chlorine-evolving anodes, chemical catalysts, electronics, and data-storage technologies. Recovering these metals from spent industrial equipment reduces dependence on constrained primary sources and strengthens the circular supply chain.
Visual inspection cannot determine the value of a spent MMO anode. Coating wear is uneven because current distribution, electrolyte flow, gas bubbles, connector placement, edge effects, and contamination vary across the electrode. Accurate valuation must therefore combine material identification, representative sampling, and validated laboratory analysis. X-ray fluorescence spectroscopy is useful for preliminary screening. XRF can quickly identify iridium, ruthenium, tantalum, titanium, platinum, rhodium, or rhenium without destroying the material. Multiple readings across the mesh can reveal differences between heavily worn and less exposed areas. A single reading, however, is not sufficient for final financial settlement. XRF examines only a shallow surface region, and titanium oxide, electrolyte scale, organic deposits, and uneven coating thickness can obscure the true precious-metal concentration.
Final valuation generally requires representative destructive sampling. Sections should be systematically collected from multiple areas of the anode or lot and then cut, shredded, milled, or chemically stripped to produce a sufficiently homogeneous sample. Sampling is often the greatest source of uncertainty because even the most precise laboratory instrument cannot correct an assay based on an unrepresentative section of mesh.
Iridium and ruthenium also resist ordinary dissolution methods. Complete decomposition may require alkaline peroxide fusion, controlled chlorination, or alloying with a collector metal. The resulting solution can then be measured using inductively coupled plasma mass spectrometry or optical emission spectrometry. Traditional fire assay can support bulk analysis, but procedures designed mainly for gold and silver may understate iridium or ruthenium because these metals can remain in the slag or form volatile oxides. A qualified laboratory should therefore use PGM-specific methods supported by recovery checks, reference materials, and appropriate quality controls.
Assay reports must also clearly distinguish between oxide content and elemental metal content. Iridium dioxide is approximately 85.7 percent iridium by mass, while ruthenium dioxide is approximately 76 percent ruthenium. Confusing oxide loading with elemental-metal loading can produce a substantial valuation error. Final settlement should be based on the elemental metal content, agreed-upon payable percentages, treatment charges, and a clearly defined market price reference.
Spent anodes may retain strong acids, heavy metals, plating residues, organic contaminants, or other hazardous materials from their operating environment. Their storage, transportation, and processing may therefore be governed by hazardous waste laws and international rules governing transboundary waste movements. Sending contaminated anodes to an unlicensed or unqualified scrap dealer can expose the original owner to environmental, financial, and reputational liability.
Specialized precious-metal refiners can provide both compliant handling and accurate valuation. Phoenix Refining, for example, offers testing for qualifying iridium-, ruthenium-, and other PGM-bearing industrial samples. Its stated program requires prior approval, accurate documentation, secure packaging, and adherence to sample-weight limits.
Submitted samples are not returned and are treated as material offered for sale or recycling. If recoverable precious metals are identified, the company may provide an offer for the remaining inventory. Current shipping requirements, assay methods, treatment charges, permits, payment terms, and settlement procedures should always be confirmed directly before sending material.
The end of an MMO anode’s operating life is not necessarily the end of its economic value. Because interfacial passivation can cause failure before the catalyst is exhausted, deactivated titanium mesh may retain significant quantities of iridium, ruthenium, or other strategic metals.
Facilities should not sell these anodes solely by titanium weight without first obtaining representative, PGM-specific analysis. Proper testing and specialized refining can convert an apparent waste liability into revenue while supporting regulatory compliance and preserving some of the world’s scarcest industrial resources.
