ampere per square meter

Current density is the honest form of an electrochemical current. Faraday's laws tell you how much metal a given charge dissolves, but whether that matters depends entirely on how thinly the charge is spread. One amp delivered to a square metre of pipeline is a mild protective current; the same amp concentrated at a coating holiday the size of a coin is a hole.

The conversions worth carrying: $1\ \mathrm{A/m^2} = 1000\ \mathrm{mA/m^2} = 100\ \mu\mathrm{A/cm^2}$, and $1\ \mathrm{A/cm^2} = 10{,}000\ \mathrm{A/m^2}$. That last one catches people constantly, because a fuel cell running at a perfectly ordinary 1 A/cm² is at a current density ten thousand times the number a corrosion engineer would call high.

In cathodic protection the design current density is what sets everything downstream — the anode mass, the anode count, and the life of the system. Bare steel in seawater typically wants 100 mA/m² or so and well-coated steel a fraction of that, which is the whole economic argument for coating a structure before protecting it.

The same quantity drives the corrosion rate itself. One amp per square metre on iron dissolves metal at about 1.16 mm per year, a figure worth remembering because it turns an electrochemical measurement directly into a wall-thickness prediction.