Halfcell Cathodic Protection Galvanic anode design to AS 2832.3, supply, dive installation and half-cell potential surveys for marine and buried steel — Fremantle to Kwinana.

Steel in seawater corrodes because parts of it act as anodes and give up metal. Bolt on a lump of a less noble metal — aluminium, zinc, magnesium — and the lump becomes the anode instead. It gives up its metal, the steel doesn’t, and you replace the lump every so often. That’s galvanic, or sacrificial, cathodic protection. The whole engineering question is how many kilograms of lump, and that’s what the calculator above works out.

Because the anodes only have to supply current to the steel the water can actually reach. A coating in good order might expose 5 % of the area; an aged one, 30 %; bare or heavily corroded steel, all of it. AS/NZS 2312 sets breakdown allowances for exactly this. The current demand scales straight off that percentage, and the anode mass scales off the current — so on 180 m² over fifteen years in open seawater the difference between a new coating and an aged one is 51 kg against 301 kg of aluminium. Coating first is almost always the cheaper job.

It’s the protection criterion in AS 2832.3: steel is considered protected when its potential is at or more negative than −800 millivolts measured against a silver / silver-chloride (seawater) reference half-cell. For buried steel AS 2832.1 states the same potential against a copper / copper-sulfate cell, which reads −850 mV — same steel, different reference electrode. We measure it with a half-cell on a cable, traversing the structure and logging depth and chainage. A design predicts a potential. Only the survey measures one.

Because they won’t work and we’d be selling you metal. Fresh water sits around 8,000 Ω·cm — hundreds of times more resistive than seawater. Aluminium passivates in it and stops delivering current at all; zinc simply hasn’t the driving voltage to push useful current through that resistance. The anodes would look like protection on the drawings and measure like nothing on the half-cell. What that tank wants is an impressed-current system with a transformer-rectifier, which we don’t design. Select “fresh water” above and the calculator refuses instead of pricing.

By survey, not by calendar. A design life is an estimate made from an assumed mean current density — the real consumption depends on what the coating actually did, how much stray current is around, and whether someone bolted a bronze fitting to your steel. We survey potentials, we inspect the anodes for remaining mass, and we tell you how long you’ve got. Typically an aluminium system designed for fifteen years gets its first real check at year five and a decision at year ten.

We don’t design impressed-current systems — different discipline, and there are two firms in Perth who do it properly; ask and we’ll give you their numbers. We’re not coating applicators, so when the honest answer is “coat it before you anode it”, we say so and hand the job to someone else. And we don’t do ultrasonic thickness surveys: what your steel has left is a corrosion engineer’s judgement off wall-thickness readings, and it is not something we or any calculator can tell you.