A corroding metal is a short-circuited battery. Iron dissolves at the anodic sites, giving up electrons; oxygen takes them at the cathodic sites a few micrometres away. The electrons that flow between them are a current, and a current can be measured in minutes instead of the ninety days a coupon takes.

$P = \dfrac{i\,M}{n\,F\,\rho}$ — P equals i M, over n F rho. $P$ is the penetration rate in mm/yr. $i$ is the corrosion current density in amps per square metre — the current per unit of corroding surface. $M$ is the metal's molar mass in g/mol. $n$ is the valence, a bare count of the electrons one atom gives up as it dissolves. $F$ is the Faraday constant, 96 485 coulombs per mole of electrons — the price of one mole of charge, and the only number here you never enter. $\rho$ — rho — is the metal's density in kg/m³. Solve forward for the rate, or backwards for the current an acceptance limit allows.

Read the shape and it is a chain of conversions. Charge buys moles of electrons; $M/n$ turns those into a mass of metal; the density turns the mass into a volume; the volume over one square metre is a thickness. The checkpoint every corrosion engineer carries is worth memorising: 1 A/m² on ordinary steel is about 1.16 mm/yr, or roughly 46 mils per year. If your answer is not near that ratio on a steel, the equivalent weight or the density is wrong.

$n$ is the one input that has to be KNOWN rather than looked up. Iron going to Fe²⁺ and iron going to Fe³⁺ are the same metal at the same current, and the penetration rates differ by a third. For an alloy the honest figure is the weighted equivalent weight of the elements that actually dissolve, not the base metal's.

Then the push. $\Delta E = E_c - E_a$ — delta-E equals E-sub-c minus E-sub-a. $\Delta E$ is the galvanic driving voltage in volts. $E_c$ is the potential of the cathode, the more noble metal, and $E_a$ that of the anode, the more active one — both measured against the SAME reference electrode, or the subtraction means nothing. Against a copper/copper-sulphate reference, ordinary field values are steel about −0.65 V, zinc −1.10, magnesium −1.55, copper −0.20.

Two things this voltage does not tell you, and both matter more than it does. It is not a constant: a galvanic series is measured in one electrolyte, and steel and zinc famously REVERSE polarity in hot domestic water, so the galvanising on a tank becomes the cathode and the steel beneath it corrodes. And it is not a rate. Rate is set by the circuit resistance and, above all, by the area ratio: a large cathode wired to a small anode concentrates the whole cell's current onto very little metal, which is why a steel bolt in a copper plate fails quickly and a copper bolt in a steel plate is almost harmless.