The invention of the dimensionally stable anode (DSA) transformed both chlor-alkali production and electrochlorination. Instead of consumable graphite electrodes, these systems use catalytic mixed metal oxide (MMO) coatings on titanium substrates, delivering greater efficiency, less maintenance, and longer operating life. In different applications, the MMO coatings contain ruthenium dioxide (RuO₂), iridium dioxide (IrO₂), and stabilizing oxides such as titanium dioxide (TiO₂) or tantalum pentoxide (Ta₂O₅).
That doesn't mean materials said to be dimensionally stable are completely resistant to degradation. Under an apparently undamaged surface, mechanical stress, chemical dissolution, and changes at the coating-substrate interface gradually impair performance. Rather than wearing off uniformly, MMO coatings typically deteriorate in patterns that depend on current distribution, electrolyte chemistry, and fluid flow. Understanding how these mechanisms interact is essential for extending anode life and identifying the remaining valuable precious metal in an electrode that can no longer function efficiently.
Most commercial MMO coatings are made by applying a precursor solution multiple times and then thermally decomposing it. As the solvents evaporate and the precursor salts turn into oxides, the coating shrinks. Combined with the difference in thermal expansion between the coating and the titanium substrate, this process creates extra stress during heating and cooling. The resulting surface resembles dried clay, with interconnected microcracks that divide the catalytic material into small islands. This 'mud-crack' structure is not always a manufacturing defect, however. In fact, its textured, porous nature can increase the electrochemically accessible surface area, enabling the electrolyte to reach catalytic sites inside the coating.
However, this same structure can also lead to deterioration, since cracks tend to concentrate mechanical stress and allow the electrolyte to reach the titanium interface. Their impact varies with depth, connectivity, and the surrounding coating's condition. It is therefore critical to carry out proper substrate preparation. Carefully controlled blasting, cleaning, and etching help create a surface that supports coating adhesion. If preparation is inadequate, contamination is present, or processing conditions are inappropriate, weak interfaces may remain and lead to premature failure under operating stresses.
During electrolysis, chlorine or oxygen form bubbles that constantly nucleate, grow, and detach from the anode. This disturbs the surrounding electrolyte and exerts varying forces on the coating surface. Additional shear is introduced by bulk flow, especially in the vicinity of edges, restrictions, and other geometric discontinuities. For a sound coating, normal gas evolution is typical under service conditions. However, if a coating has been weakened by dissolution or has poor adhesion, repeated bubble activity and the fluid forces involved can spread cracks and dislodge particles. This type of physical coating loss is called spallation. Confined gas growth can also apply stress to accessible pores and fissures. Nevertheless, ordinary bubble detachment during electrolysis should not automatically be interpreted as violent cavitation collapse or micro-jetting; those mechanisms depend on specific conditions and require supporting evidence.
As a result, mechanical wear is uneven: areas with higher local current density tend to release more gas, and flow patterns affect both bubble removal and surface loading. When a small portion of the catalytic material is lost, the current shifts to adjacent areas, increasing their workload and promoting additional damage.
Mechanical loss occurs alongside electrochemical dissolution; the stability of oxides containing ruthenium and iridium depends on electrode potential, local pH, electrolyte composition, temperature, and coating structure. In chloride-containing systems, chlorine evolution competes with oxygen evolution. If chloride supply to the surface is insufficient because of low concentration, high current demand, or limited transport, the anode potential may increase, increasing oxygen evolution. This change can be especially harmful to coatings that are rich in ruthenium. Under sufficiently oxidizing conditions, ruthenium can form soluble or volatile high-valence species, removing the active metal from the coating. Although iridium oxides generally prove to be more durable in many oxygen-evolving environments, they are not entirely immune to dissolution.
Studies also link lattice-oxygen involvement to instability in certain oxide catalysts. While this does not explain all commercial MMO failures, it shows an important principle: catalytic reactions can cause structural changes that undermine the material's performance. When the active metal is lost, a residue rich in less conductive oxide may remain. The coating may still appear to be there, but its capacity to conduct current and catalyze the desired reaction will have decreased. The remaining active areas then have to carry more current, reinforcing localized degradation.
Titanium's corrosion resistance comes from a protective oxide film. However, under an MMO coating, excessive growth of this film can increase the electrical resistance between the substrate and the catalyst. This voltage increase commonly causes anode deactivation, even when some precious metal remains measurable. Polarity reversal also presents another possible source of stress. Although some electrochlorination systems use reversal to control scale formation, anodes must be designed to perform this function. Cathodic polarization may change the catalytic oxides and, in adverse conditions, can cause hydrogen to be taken up by the titanium. Hydride formation and embrittlement may weaken the bond between the substrate and the coating. Reversal schedules should therefore be based on the equipment and coating specifications rather than on a general cleaning rule.
Electrolyte impurities can also accelerate degradation. Fluoride is especially worrying because, in acidic conditions, it can break down titanium's protective oxide layer by forming a soluble titanium–fluoride compound. Acceptable levels vary with operating conditions, and no single concentration ensures safety in all cases. Deposits linked to manganese, lead, or other contaminants may likewise block the catalytic sites and alter the local current distribution. Taken together, these factors explain why you must assess water chemistry and operating conditions alongside the coating itself.
You cannot safely judge an anode's condition by appearance alone, since scale can hide a sound coating. At the same time, a dark remaining layer may contain so little active material that it cannot function effectively. Calibrated X-ray fluorescence (XRF) can map residual ruthenium and iridium levels. Take measurements at representative sites, such as the edges, high-current areas, and any visibly damaged areas. When interpreting the results, account for the coating's composition, the substrate's effect, the geometry, and instrument calibration. For commercial recovery, representative sampling followed by confirmatory laboratory analysis may also be required. Decisions about recoating should be based on a separate evaluation of substrate thickness, pitting, distortion, and cracking. Although accelerated life testing can help compare replacement coatings, laboratory results do not automatically indicate field life without validation for the specific application.
Finally, MMO degradation is a combined process: microcracks allow access, dissolution reduces the catalytic structure's strength, and mechanical forces remove the damaged material. If current demand matches electrolyte transport, operators can control contaminants, monitor voltage, and track coating condition to break the cycle driving this degradation before the titanium base material becomes unusable. Even when anodes reach the end of their useful life, operators can still recover the precious metals they contain. Phoenix Refining purchases spent MMO-coated materials and provides free testing, giving operators a realistic basis to assess material that would otherwise be regarded as ordinary scrap.
