Corrosion Observatory / specimen 04-A
Oxide
Iron surrendering to air. A single mild-steel coupon, watched as a reaction-diffusion rust front nucleates at its scratches and consumes bright metal over compressed decades.
Live corrosion render unavailable — the plate rusts in software.
The reaction
Bare iron carries a thin, passive film. Break it — a scratch, a dropped tool, a grain boundary at the edge — and water and oxygen reach raw metal. There, iron gives up electrons. It does not return them.
What follows spreads. Each corroding site feeds its neighbours: dissolved iron and hydroxide diffuse outward, seeding the next reaction ring. That is why rust does not appear as a stain but as a front — a bloom with a bright, active edge and a dead, flaking interior.
The plate above runs that chemistry as a bistable reaction–diffusion field — an Allen–Cahn front. Every point is either metal or rust; diffusion drags the boundary between them into fresh steel, so corrosion advances as a filled, blooming front. Where the metal is tougher the front slows, stranding bright islands until they too are taken.
bright metal → hydrated iron oxide · rust
- ANucleationA defect breaks the passive film. Oxidation begins as a point — a pit no wider than a scratch.
- BPropagationReagents diffuse to fresh metal. The corroded zone widens as a glowing front, one ring feeding the next.
- CFlakingBehind the front, oxide bulks, cracks and lifts, exposing metal beneath. The interior darkens to spent rust.
- DConsumptionFronts merge. The last bright islands close over. The coupon is returned, in full, to the air.
Reading the plate
Three states, one field. Colour is not decoration here — it is the value of the reaction at each cell.
Specimen record
Coupon 04-A
Filed under atmospheric corrosion · accelerated timescale
ASTM A36
× 3 mm plate
ISO 9223 · C5
per year
× 5 defects
bistable front
~84 % · reseeds
— surface