Corrosion (Rust)

How rust really happens

The textbook reaction "iron + oxygen = rust" hides the actual mechanism. Rust is not a direct combination. It's a galvanic cell that lives on every wet iron surface. The cell has four parts running at the same time:

Anode (small patch):     Fe → Fe²⁺ + 2 e⁻              E° = −0.44 V
Cathode (nearby patch):  ½ O₂ + H₂O + 2 e⁻ → 2 OH⁻    E° = +0.40 V (pH 7)
Electrons through:       the bulk metal itself
Ions through:            the water film on top
Net cell EMF:            ~+0.84 V — strongly spontaneous

Once Fe²⁺ ions diffuse outward and meet dissolved O₂, they oxidize again to Fe³⁺ and precipitate as a hydrated oxide:

2 Fe²⁺ + ½ O₂ + 2 H₂O  →  Fe₂O₃·H₂O + 4 H⁺
                          ─────────────
                         the orange flake

The whole thing is just electrochemistry running in parallel with no external wire. Anodes and cathodes can be millimetres apart on the same nail, separated by a microscopic geometry difference (a scratch, a stress concentration, or a drop of water sitting on a paint flaw).

Three things rust needs

Strip any one and corrosion stops:

1. Iron (or another oxidizable metal). Pure metal in its zero oxidation state is the fuel.
2. Oxygen. The cathodic reaction needs an oxidant. Buried steel below the water table corrodes much slower because O₂ runs out.
3. Water. Acts as the electrolyte. Bone-dry iron in a desert vault basically doesn't rust — see ancient artifacts at Luxor.

Add a fourth optional accelerant — chloride ions — and rates jump tenfold. Sea spray, road salt, and coastal humidity all push the curve up. The Pourbaix diagram (potential vs pH) tells you the same story: iron is thermodynamically immune in alkaline anoxic water and aggressively unstable in chloride-rich neutral water.

Forms of corrosion compared

	Uniform	Pitting	Galvanic	Crevice	Stress-corrosion cracking	Erosion
Geometry	Whole surface, even rate	Tiny deep holes	At the metal-metal junction	Inside narrow gaps and seals	Crack propagation under tension	Where flow erodes passive film
Trigger	General exposure	Cl⁻ breaks passive film locally	Two metals in contact + electrolyte	O₂ depletion in stagnant pocket	Tensile stress + corrosive species	Turbulent fluid scrubs oxide
Detectability	Very visible	Easy to miss until perforation	Visible at junction	Hidden under bolts and gaskets	Often catastrophic with no warning	Visible scoured tracks
Rate	Predictable, ~mm/year	µm/day local — pit grows in days	Faster than either metal alone	Slow but persistent	Crack growth ~mm/hr near failure	Material-loss like sandblasting
Famous case	Steel ship hull	Stainless in seawater chloride	Statue of Liberty Cu/Fe armature	Aircraft lap joint, Aloha 243	Boiler-tube SCC, brass "season cracking"	Pump impellers, condenser tubes
Counter-measure	Coating, sacrificial anode	Mo-alloyed stainless, lower Cl⁻	Insulation, similar metals	Seal design, drain paths	Stress relief, change alloy	Lower velocity, harder facing

In practice, real failures combine modes. The Aloha 737 fuselage rip-out in 1988 was crevice corrosion that primed a fatigue crack — pure SCC in a salt-air environment that normal NDT had missed.

Worked example: marine vs urban exposure

ISO 9223 classifies atmospheres by how fast they eat carbon steel:

• C1 (controlled indoor): < 1.3 µm/year. A bank vault.
• C2 (rural arid): 1.3–25 µm/year. Sahara, high desert.
• C3 (urban, low-humidity): 25–50 µm/year. Madrid, Denver.
• C4 (industrial or coastal): 50–80 µm/year. Pittsburgh in 1960; Lisbon today.
• C5 (severe coastal/industrial): 80–200 µm/year. Aberdeen, Mumbai monsoon shore.
• CX (offshore / splash zone): 200–700 µm/year. North Sea oil-platform splash zones.

So a 6 mm steel plate left unpainted at the splash zone of an offshore rig perforates in roughly 6000 / 500 = 12 years. The same plate inside a heated office is still useful in 4000 years — barring scratches.

Prevention strategies

• Barrier coatings. Paint, epoxy, polymer wrap. The cheapest line of defense, but breaks down at edges and pinholes — every car body has them.
• Galvanizing. Hot-dip the steel in molten zinc. Zinc is anodic to iron (E° = −0.76 V), so even at a scratch the zinc oxidizes preferentially and protects the bare iron underneath. Standard galvanized steel: 50–80 µm Zn, ~50-year life in C3.
• Sacrificial anodes. Bolt slabs of zinc, magnesium, or aluminum to the protected structure. Ship hulls carry 50–100 kg of zinc per metre of waterline. Replaced every 1–3 years.
• Impressed-current cathodic protection (ICCP). A DC power supply pushes the structure to a more negative potential than its corrosion potential. Used on long pipelines, harbour piers, and even reinforced-concrete bridge decks.
• Alloying. Stainless steel (≥10.5% Cr) forms a self-healing Cr₂O₃ passive layer 1–2 nm thick. 316L adds 2–3% Mo to resist chloride pitting.
• Inhibitors. Add a chemical (chromate, phosphate, organic film-formers) that adsorbs onto the surface and slows the anodic or cathodic step. Engine coolants are inhibitor cocktails.
• Environmental control. Lower humidity, deoxygenate (boilers run at <5 ppb O₂ via hydrazine), shift pH alkaline.

Real-world case studies

• Golden Gate Bridge. Salt fog plus 100% humidity ate the original lead-and-linseed paint in 30 years. Since 1965, a permanent crew of ~30 painters has been continuously stripping and repainting with epoxy zinc primer + intermediate + topcoat ("International Orange"). They never finish — when they reach one end they start over at the other.
• Statue of Liberty (1980s restoration). Galvanic corrosion between the copper skin and the iron armature behind it — the asbestos shellac insulation had failed. The iron was replaced with stainless 316L, with PTFE spacers between metals.
• Trans-Alaska Pipeline. 800 miles of buried steel, protected by ~10 000 sacrificial Mg anodes plus impressed-current rectifier stations every 30 km. Wall loss after 45 years: under a millimetre on average.
• Boston "Big Dig" rebar. Chloride ingress from de-icing salt drove pitting in unprotected reinforcing bar; epoxy-coated rebar and ICCP retrofits cost the project several hundred million extra.
• Silver Bridge collapse, 1967. Stress-corrosion cracking in a single eyebar pin link. 46 dead. The disaster created the modern US bridge inspection program.
• Sunken WWII battleships. Hulls below ~30 m and below the seabed lose only ~25 µm/year because dissolved O₂ runs out. This is why "low-background steel" — pre-1945 wreck plate — is salvaged for radiation-sensitive instruments.

Variants and edge cases

• Microbiologically influenced corrosion (MIC). Sulfate-reducing bacteria in anaerobic biofilms generate H₂S that pits steel from underneath. Notorious in oilfield piping and ship ballast tanks.
• Hydrogen embrittlement. Atomic H produced at the cathode diffuses into high-strength steel, accumulates at grain boundaries, and embrittles it. Bolts that crack months after installation are the classic case.
• Filiform corrosion. Hairline rust threads creeping under paint, driven by humidity gradients. Cosmetic on cars, structurally serious on aluminum aircraft skins.
• Cold-worked iron resists. Cold-rolling raises dislocation density and surface stress, but oddly it can shift corrosion potential to be slightly more noble in some media. Engineers don't trust this; it's a curiosity.
• "Self-healing" rust on weathering steel. COR-TEN forms a tight, adherent rust patina (rich in Cr, Cu, P) that slows further attack. Used for sculpture and exposed-frame buildings; depends on alternating wet-dry to seal.

Common pitfalls and misconceptions

• "Stainless can't rust." 304 stainless pits readily in seawater. The chromium oxide film breaks down where Cl⁻ ions concentrate; specify 316L or duplex steel for marine use.
• "More paint is better." Thick, brittle coats crack first; thin elastic ones flex with the substrate. Modern coating systems specify dry-film thickness in tens of microns per layer, not millimetres.
• "Galvanic series tells me everything." Position in the seawater galvanic series sets which metal becomes anodic, but rate depends on area ratio. A small steel rivet in a large copper plate corrodes catastrophically; reverse the ratio and the steel barely moves.
• "Rust is purely chemical." It's electrochemical. Without electron and ion paths, the reaction stops. That's why a shielded patch of iron sealed under perfect lacquer in a vacuum doesn't rust.
• "Sealing the surface is enough." If water gets in through any crack, the anode is now trapped under the coating with no oxygen — perfect crevice geometry. Coatings must drain, or flooding will accelerate, not stop, attack.