Corrosion Simulation
Theory: current distribution decides corrosion
Overview
We used stainless bolts on a ship's steel part, and the steel corroded badly just around the bolts. Why?
That's galvanic corrosion. When metals with different potentials are connected in seawater, the less noble one (steel) becomes the anode and dissolves, while oxygen is reduced on the more noble one (stainless). The steel must supply by dissolution not only the current for its own surface but also for the stainless surface. So the larger the noble metal's area, the faster the steel corrodes. Corrosion simulation solves the potential distribution in the seawater to find where current gathers and how much.
Governing equations
Charge is conserved in the electrolyte, so potential $\phi$ obeys Laplace's equation ($\kappa$ is conductivity). On metal surfaces, current flows according to the difference between metal potential $E$ and solution potential. That relation is the polarisation curve, embodying corrosion tendency and the rate of oxygen supply.
From current to corrosion rate
Faraday's law converts current density into thinning rate. When the cathodic reaction is limited by oxygen diffusion (limiting current $i_L$), anodic current density is set by the cathode-to-anode area ratio.
Copper-sheathed ships and iron nails
In the 18th century the Royal Navy began sheathing wooden warship hulls in copper to protect against shipworm and barnacles — only to see iron nails holding the sheets and iron fittings on the hull corrode rapidly. In 1824 the chemist Davy showed that attaching pieces of zinc or iron to copper protected it, often cited as the beginning of cathodic protection. Both the danger of dissimilar metals and the protection that uses the same principle in reverse have been explained by the same electrochemistry for two centuries.
Worked example: area ratio and corrosion rate
In seawater, the cathodic reaction (oxygen reduction) is taken as diffusion-limited at 0.1 A/m². Steel thins 1.16 mm a year per 1 A/m². Years to lose 3 mm are also shown:
| Noble / steel area ratio | Steel corrosion rate | Versus alone | Years to lose 3 mm |
|---|---|---|---|
| Steel alone | 0.12 mm/y | ×1 | 25.8 y |
| 1 | 0.23 mm/y | ×2 | 12.9 y |
| 10 | 1.28 mm/y | ×11 | 2.3 y |
| 100 | 11.7 mm/y | ×101 | 0.3 y |
I thought small stainless bolts were fine — but it's large stainless against small steel that's dangerous.
Yes — a small anode with a large cathode is worst. Steel bolts in a stainless plate would dissolve quickly. With stainless bolts in a steel plate, as here, the area ratio should be small — but on a painted steel plate, small coating defects expose tiny bits of steel that become small anodes and dissolve intensely. The table values are idealised upper bounds; in reality seawater's resistance concentrates current near the contact, so you need the distribution, not an area average. That's why the potential field is solved with finite elements or similar.
Example 2: sacrificial anode mass
Protect 10 m² of bare steel at 0.1 A/m² for ten years: current 1.0 A, charge 87,660 Ah. With zinc at 780 Ah/kg and utilisation 0.85, about 132 kg of zinc is needed. If coating reduces exposed area to a tenth, about 13 kg suffices.
Analysis workflow
- Set electrolyte conductivity, temperature and flow (oxygen supply).
- Prepare polarisation curves for each metal from tests or literature.
- Solve the electrolyte potential with the geometry to get surface current distribution.
- Convert current to corrosion rate and compare life and protection options (insulation, coating, anodes).
- Consider coating defects and surface changes over time (films, deposits).
“Painted, yet it holed through”
A steel tank connected to copper-alloy piping had only its steel side carefully painted for protection, yet within a few years small holes appeared in the steel. Steel exposed at tiny coating defects became small anodes against the large unpainted copper-alloy surface, and current concentrated there. Solving the potential field showed current density at the defects tens of times that of an unpainted tank. When combining dissimilar metals, the rule is to coat the cathode side (noble metal) too, or insulate electrically.
Common mistakes
Mistakes and fixes
| Mistake | Effect | Fix |
|---|---|---|
| Coating only the anode | Current concentrates at defects | Coat the cathode or insulate |
| Area-averaged assessment | Local attack near contact missed | Solve the potential field |
| Fixed polarisation curves | Temperature and flow effects wrong | Prepare per condition |
| Ignoring conductivity | Current spread misjudged | Include solution resistance |
| Ignoring time evolution | Film and deposit effects missed | Account for changes over time |
I'd like to try related calculations.
Try the corrosion rate calculator, galvanic series tool, cathodic protection calculator and Nernst equation calculator. Related electrochemical analyses include corrosion analysis basics, electroplating and electrochemical machining.