How is corrosion rate obtained from current?

Corrosion is metal giving up electrons to become ions, so current maps directly to dissolved metal (Faraday's law). For steel (iron dissolving as divalent ions), 1 µA/cm² equals 11.6 µm of thickness loss per year. In seawater with oxygen supply limited to 15 µA/cm², that gives 0.174 mm/yr.

Why does contact between dissimilar metals speed corrosion?

The lower-potential metal (steel) becomes the anode and the higher one (copper) the cathode, and the anode must supply all electrons for oxygen reduction on the cathode surface. When oxygen supply controls the reaction, anode corrosion current is proportional to the combined area of cathode and anode. With copper area 100 times the steel's, steel dissolves 101 times faster than alone, at 17.6 mm/yr.

How much zinc does cathodic protection need?

The protected area sets the current. Protecting 100 m² of bare steel at 15 µA/cm² needs 15 A, or 131,400 Ah per year; zinc (780 Ah/kg in practice, 0.85 utilisation) comes to about 198 kg per year. Coating that leaves 2% bare cuts this to about 4 kg, so coatings and cathodic protection are combined in practice.

Corrosion Simulation

Category: Coupled Analysis – Electrochemistry | Revised 2026-10-01

Theory: corrosion is an electrochemical cell

Overview

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What does a corrosion simulation actually calculate?

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It calculates current flowing over metal surfaces. When iron rusts, at some sites iron gives up electrons and becomes ions (anodic reaction), and elsewhere those electrons reduce dissolved oxygen (cathodic reaction). Corrosion is thus a small battery, and the current equals the dissolved metal. In salty water such as seawater, the cathodic reaction is often capped by how fast oxygen reaches the surface. When two metals touch, the lower-potential one becomes the anode, taking on the oxygen reduction on its partner as well, and dissolves quickly. Simulation solves the potential field in the electrolyte together with surface reaction rates to find where and how much metal dissolves.

Current to thickness loss

$$ \frac{dx}{dt} = \frac{i\,M}{nF\rho}\qquad (\text{steel: } 1\ \mu\mathrm{A/cm^2} \approx 11.6\ \mu\mathrm{m/yr}) $$

M is molar mass, n the number of electrons and ρ density; for steel M = 55.85 g/mol, n = 2, ρ = 7.87 g/cm³.

Galvanic coupling under oxygen-supply control

$$ i_a A_a = i_L\,(A_a + A_c)\ \Rightarrow\ i_a = i_L\left(1 + \frac{A_c}{A_a}\right) $$

i_L is the oxygen-limited current density (15 µA/cm² here); A_a and A_c are anode and cathode areas. The larger the cathode, the more current the anode must dissolve.

Coffee Break Trivia

Copper hulls and zinc

In 1824 Davy proposed attaching blocks of iron or zinc to the copper sheathing on Royal Navy hulls to stop its corrosion — sacrificing a lower-potential metal to protect copper, the start of modern cathodic protection. Corrosion stopped, but an unexpected problem appeared: with copper no longer dissolving, the barnacles and weed it had kept away covered the hulls. It is remembered as a case where stopping corrosion also stopped another useful effect.

Worked examples

Example 1: steel–copper contact and area ratio (seawater)

Copper/steel area	Steel corrosion current	Steel corrosion rate	Potential rise
Steel alone	15 μA/cm²	0.174 mm/yr	—
0.1	16.5 μA/cm²	0.19 mm/yr	+0.002 V
1	30 μA/cm²	0.35 mm/yr	+0.018 V
10	165 μA/cm²	1.91 mm/yr	+0.062 V
100	1,515 μA/cm²	17.6 mm/yr	+0.120 V

Potential rise follows the Tafel slope of steel dissolution (60 mV per decade). A steel plate with copper rivets (ratio 0.1) is barely affected, but steel rivets in a copper plate (ratio 100) dissolve at 17.6 mm/yr.

Example 2: cathodic protection with zinc

Condition	Protection current	Charge per year	Zinc consumed
Bare steel 100 m²	15 A	131,400 Ah	about 198 kg/yr
Coated (2% bare)	0.30 A	2,628 Ah	about 4.0 kg/yr

Zinc's theoretical capacity is 820 Ah/kg; 780 Ah/kg in practice with 0.85 utilisation was used.

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If a ratio of 100 gives 100 times the corrosion, does copper far from the joint count too?

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Good catch. Example 1 is an upper bound assuming every part of the cathode receives electrons equally. In reality current must travel through the electrolyte to reach distant cathode, and its resistance drops potential. In well-conducting seawater, cathode far away still counts, so the area effect shows almost fully. In thin films of rain or condensation, conductivity is poor and only cathode right next to the joint works — but then corrosion concentrates right beside the joint as a deep groove. How much cathode is effective depends on electrolyte conductivity and water thickness. Corrosion simulation therefore solves the Laplace equation for potential in the electrolyte with surface reaction rates as boundary conditions to obtain the current distribution.

Modelling workflow

Measure polarization curves (potential vs current) of each metal in the service water.
Set electrolyte conductivity and water-film thickness.
Solve electrolyte potential with surface polarization curves as boundary conditions.
Convert computed current to thickness loss with Faraday's law.
If needed, update geometry and re-solve over time.

Coffee Break Trivia

“A steel fitting joined to stainless piping”

In a plant's cooling-water piping, only a short carbon-steel fitting inserted in stainless pipe perforated within a few years. Its inner surface was tiny compared with the whole line, and the long stainless surfaces around it acted as cathode. Potential-distribution analysis showed corrosion concentrated at both ends of the fitting at the stainless boundary, matching the actual holes. The fitting was replaced with stainless, and insulating flanges were installed to separate dissimilar metals electrically where unavoidable.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Judging by potential difference only	Area effect missed	Evaluate area and current
Treating all cathode as effective	Distribution wrong in thin films	Solve electrolyte potential
Average loss only	Local perforation missed	Resolve current distribution
Polarization curves from another water	Rate badly wrong	Measure in service water
Sizing protection on bare area when coated	Zinc oversized	Include coating breakdown

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