Sacrificial Anode Mass for a Required Life

A sacrificial anode is a battery that discharges into your structure, and this equation asks the same question you would ask of any battery: how much charge does the duty need, and how much metal does it take to store it? The charge is the current times the life. The metal follows from the alloy's current capacity, which is how many ampere-hours a kilogram is worth.

Faraday's law sets the ceiling on that capacity, and it is a ceiling nobody reaches. Zinc's theoretical capacity is 820 A·h/kg, aluminium's 2980, magnesium's 2200. The practical figures quoted for commercial alloys — around 780 for zinc, 2500 for activated aluminium, 1100 for magnesium — are lower because part of the anode's dissolution does not deliver current to the structure at all. It self-corrodes, producing hydrogen and heat instead of protection. That ratio is the current efficiency, and it is roughly 95% for zinc, 85–90% for the aluminium–zinc–indium alloys and only about 50% for magnesium. It is also not fixed: an aluminium anode that passivates because the water is too fresh, or a magnesium anode in high-resistivity soil, can deliver far less than its catalogue number.

The utilisation factor is a separate deduction and a more physical one. An anode stops working long before it is gone. As it wastes, the metal around the steel core insert loses its mechanical connection and falls off, taking still-usable alloy with it, and the remaining stub can no longer pass current. Slender stand-off anodes with the core running their full length manage about 0.90; bracelet anodes and flush-mounted shapes are conventionally taken at 0.80, and shapes that leave an unsupported skirt do worse. A design that quietly assumes 1.0 is short by a fifth on day one.

The current in the numerator has to be the mean over the design life, and this is where anode calculations most often go astray. A coated structure demands very little current when new and progressively more as the coating degrades, so the initial, mean and final demands are three different numbers that can differ by an order of magnitude. Anode mass is sized on the mean, because mass is consumed over the whole life. Anode count and geometry are sized on the final demand, because that is when the system has to be able to deliver the most current through the highest resistance. Sizing both on the same number gets one of them wrong.

Two failure modes close the loop. An anode can have ample metal and still fail to protect, because the circuit resistance is too high for it to push its current — that is a separate calculation and both have to pass. And an anode can be consumed far faster than designed because something is stealing its current: a shorted casing, an unintended metallic contact with a neighbouring structure, or a bond that should not exist. An anode that wastes at twice its predicted rate is usually not a bad anode. It is a symptom.