Same Anode, Different Life: What Operating Duty Explains That the Datasheet Does Not

A buyer calls with a familiar complaint: the same anode, same coating, same supplier, yet one cell runs for years and the other fails inside a season. The datasheet cannot answer this, because the datasheet describes the part. It does not describe Monday morning.

The part is only half the story. The other half is duty - what actually happens between the terminals once the cell is running, and nobody is looking.

Current density is where the gap opens first.

Two cells can both be "Ru-Ir coated Grade 1 titanium, 100 mm × 500 mm." In one, current density sits near the lower end of the design range and the coating wears slowly, predictably, along the flats. In the other, a poor rack layout, a smaller active area than the drawing claims, or a rectifier set to hold a fixed voltage through a thickening deposit, pushes the real current density well above what the coating was tested for. The coating does not fail because it is badly made. It fails because nobody asked what the current density would be on the day the part shipped.

If the number on the datasheet and the number on site disagree, the datasheet is not the one controlling the result. Measure it. A clamp meter and a stopwatch cost less than a replacement campaign.

Temperature is the accelerator nobody budgets for.

Coating loss rates are usually quoted at a reference temperature, often ambient or a modest operating range. Real cells run hotter. Resistive heating, exothermic reactions, a cooling loop that was never commissioned, a summer in a shed with no extraction - any of these moves the actual duty outside the tested envelope.

The relationship is not linear. A coating that is comfortable at 35 °C can be in a very different place at 55 °C, because side reactions, dissolution and thermal stress all move in the same direction. The failure shows up as a voltage drift, then a local hot spot, then a bare patch. By the time it is visible, the decision was made months ago at the commissioning bench.

State the operating temperature as a range, not a number. And state whether that range includes startup, which is usually the worst minute of the cell's life.

Start-stop is a different duty from continuous run.

A rectifier that runs flat out for eight thousand hours is one kind of test. A cell that starts and stops forty times a week is another. Thermal expansion and contraction work the coating at the edges. Polarity events, even brief ones, can reverse the local chemistry. A deposit that formed during a long idle changes the first current pulse of the morning.

Coatings rated on continuous-run data are being asked a different question when they land in an intermittent process. If the application is batch, tell the manufacturer how many cycles, how long the stops, and whether the cell drains or stays wet. "Continuous duty" on the order form is not a description of the process. It is a hope.

Electrolyte purity is a coating variable, not a housekeeping issue.

The same chemistry can behave differently depending on what else is in the tank. Iron, manganese, silica, organics, suspended solids, stray chloride from a neighbouring process - these change what forms on the surface and what the coating has to survive. A part tested in clean synthetic brine is being judged on a different examination from the one it faces in a working plating line.

This is why a sample that passes in the lab and fails on site is not necessarily a coating failure. It may be that the sample was tested against the wrong water. Send the real electrolyte, or a representative sample of it, with the contamination levels you actually measure. A clean test is a clean answer to a question nobody is asking.

Geometry decides where the current goes, and the coating only follows.

A coating is applied to a surface. It does not redesign the surface. Edges, corners, narrow gaps, the last few centimetres of a long rod, the inside of a bend - current crowds in these places regardless of what the coating specification says.

So a perfectly formulated Ru-Ir coating on a part with poor current distribution fails at the edges while the flat faces are almost unused. The coating did its job. The geometry did not. Before ordering, ask where the current actually goes, not where the drawing says it should. A rough current-density map is worth more than another thickness certificate.

What the spec should say instead.

Skip the adjectives. Write the operating envelope: electrolyte composition, pH range, temperature range including startup, current density, duty cycle with starts and stops, polarity events, target geometry and active area, titanium grade, coating system, coating loading or performance target, connection method, inspection scope, quantity and destination.

Then add a failure criterion. A ten per cent voltage rise, a visible bare spot, loss of product concentration, and an agreed consumption rate are four different definitions of worn out. They produce four different replacement cycles and four different prices.

Finally, ask the question that separates suppliers from parts vendors: what is the cost per unit of output over the warranted life? Take the installed cost, including recoating and the downtime, and divide it by the kilograms, ampere-years or protected square metres delivered. That number compares across shops. The unit price does not.

A failed anode on the bench is evidence, not a complaint. Send it back with the operating log, the water analysis, the rectifier settings and the date the coating was approved, and the answer will be an engineering one. Send it back with only the words "coating bad," and you will get a replacement part - and a few months later, the same conversation.

For a site-specific coating recommendation, share the duty rather than the shape.