Penetration Rate from Corrosion Current Density

Corrosion is an electrochemical reaction, and electrochemical reactions are counted in electrons. That single fact is what lets a corrosion rate be measured electrically, and it is Faraday's law of electrolysis — published in 1834, three decades before anyone understood why it should be true — that supplies the exchange rate between coulombs and grams.

The bridge is the equivalent weight: the molar mass divided by the number of electrons each atom gives up. It is the mass of metal that one mole of electrons removes. Iron dissolving to Fe²⁺ has an equivalent weight of 55.845/2 = 27.92 g per mole of electrons; the same iron going to Fe³⁺ has 55.845/3 = 18.62, and the metal lost per coulomb falls by a third. Divide the mass by the density and you have a volume; divide by the area and you have a thickness. That chain — current to charge, charge to moles of electrons, moles to mass, mass to thickness — is the whole derivation, and no step of it is empirical.

The number worth memorising is that 1 A/m² on ordinary steel is about 1.16 mm/yr, or roughly 46 mils per year. Everything else scales linearly from it: 10 mA/m², a very low corrosion current, is 0.012 mm/yr and would take a century to consume a pipe wall; 10 A/m² would eat the same wall in a year. Carrying that one figure lets you sanity-check any electrochemical corrosion measurement in your head, and it is the reason experienced people can tell instantly that a quoted result is off by a factor of a thousand.

Two places where the arithmetic is exactly right and the answer is still wrong. The first is the assumed reaction: this equation cannot tell you whether the iron is going to Fe²⁺ or Fe³⁺, whether an aluminium alloy is dissolving as Al³⁺ throughout, or whether a stainless steel is losing chromium, nickel and iron in the proportions of the bulk alloy. Those are chemistry questions and they have to be answered outside the calculation. The second is uniformity. Like every rate expressed as a penetration, this one spreads the metal loss evenly across the measured area. If the current is actually concentrated in a few pits — and localised attack is exactly the case where an electrochemical measurement is most tempting, because it is so hard to see — then the real penetration at the pit is the calculated rate multiplied by the ratio of total area to pitted area, which can be a factor of hundreds.

The practical value of the equation is that it runs in both directions. Given a current, it gives a rate you can compare against a coupon. Given a coupon result, it gives the current that must have been flowing, which is a check on an instrument. When the two disagree by more than a factor of two, one of them is measuring something other than what you think.