Corrosion and Protection

The corrosion cell

Metal corrodes because it returns to the oxide or other compound it came from. Four things are needed for a corrosion cell: an anode (where the metal dissolves, Fe → Fe²⁺ + 2e⁻), a cathode (where the electrons are used, usually O₂ + 2H₂O + 4e⁻ → 4OH⁻ in neutral water), a conducting path between them in the metal and an electrolyte (water with dissolved salts) to carry the ions. Take away any one of the four and the corrosion stops: coat the metal, keep it dry, break the electrical path or remove the oxygen.

Forms of corrosion

Form	What happens	Where it appears	Cure
Uniform	even loss over the surface	steel in air or water	allowance, coating, galvanizing
Galvanic	the less noble metal of a couple is attacked	dissimilar metals joined in a wet place	isolate, same metal, sacrificial anode
Pitting	small deep holes	stainless steel in chlorides, aluminum	better alloy (higher PREN), avoid stagnant chloride
Crevice	attack under gaskets, washers and deposits	flanges, lap joints, bolts	seal or design out crevices, flush
Intergranular	attack along grain boundaries	sensitized stainless (weld decay), some aluminum	low-carbon (316L) or stabilized grades, solution anneal
Stress corrosion cracking	cracks from tension plus a specific environment	stainless in hot chlorides, brass in ammonia, 7075 aluminum	reduce stress, change material, shot peen
Erosion-corrosion	flow removes the protective film	elbows, pumps, copper pipe at high velocity	lower velocity, harder alloy
Hydrogen embrittlement	absorbed hydrogen makes high-strength steel brittle	plated high-strength bolts (above about 1 000 MPa)	bake after plating, avoid acid pickling, use coatings that do not generate hydrogen

How fast: Faraday and the rate formula

corrosion rate (mm/year) = 3.27 × 10⁻³ × i_corr (µA/cm²) × EW ÷ ρ (g/cm³)

At the same current density of 10 µA/cm²: steel (EW 27.9, ρ 7.87) corrodes at 0.116 mm/year; zinc (EW 32.7, ρ 7.13) at 0.150 mm/year; aluminum (EW 9.0, ρ 2.70) at 0.109 mm/year. A tolerance of 0.1 mm/year is usually reckoned as the limit of a useful life for steel in service; 0.1 mm/year on a 6 mm plate leaves 3 mm after 30 years from both faces.

Galvanic corrosion

When two different metals touch in an electrolyte, the one that is lower in the galvanic series (more negative potential) becomes the anode and corrodes faster, and the more noble one is protected. The corrosion current also depends on the area ratio: a small anode next to a large cathode has all the current concentrated on it, so it is eaten away quickly; a large anode with a small cathode corrodes slowly.

Worked example: bolts and plates

Stainless steel and pitting

Stainless steels owe their resistance to a thin chromium oxide film (at least 10.5 percent chromium). The film heals in oxygen but is attacked by chlorides (salt, bleach, seawater), which cause pits and crevices. The pitting resistance equivalent number PREN = %Cr + 3.3 × %Mo + 16 × %N ranks alloys: 304 about 19, 316 about 25, 2205 duplex about 35, 2507 super duplex about 43. Roughly, 304 is for fresh water and food, 316 for coastal and mild chemicals, a PREN above 32 to 35 for seawater service (with a temperature limit of about 25 °C for 316 in seawater against 40 °C or more for the duplex grades). Keep the surface clean (pickle and passivate after welding, remove iron contamination from tools), avoid crevices, and use low-carbon grades (316L) near welds.

Coatings, galvanizing and paint

• Hot-dip galvanizing (EN ISO 1461): a zinc coating of 45 to 85 µm or more on steel, protecting it by a barrier and by sacrificial action at scratches. The zinc loss rate depends on the corrosivity (ISO 9223): for an 85 µm coating:

Environment	Zinc loss rate	Life of an 85 µm coating
C2 (rural, dry)	0.4 µm/yr	about 213 years to first rust
C3 (urban, light industry)	1.4 µm/yr	about 61 years to first rust
C4 (industrial, coastal)	3.0 µm/yr	about 28 years to first rust
C5 (heavy industrial, marine)	6.0 µm/yr	about 14 years to first rust

• Paint systems: a typical C4 system (ISO 12944) is zinc-rich primer, epoxy build coat and polyurethane topcoat: 60 + 120 + 60 = 240 µm dry film thickness on blast-cleaned steel (Sa 2½). Surface preparation is 80 percent of the result.
• Plating and conversion coatings: zinc, nickel, chrome, anodizing (aluminum: a controlled oxide, 10 to 25 µm), phosphating, black oxide; most thin platings only delay rust.
• Duplex systems (galvanizing plus paint) last 1.5 to 2.3 times the sum of the lives of each.

Uncoated structural steel in a C3 environment loses about 25 µm a year per face, so a 6 mm plate would lose half its thickness in about 60 years from both sides: protection is needed for any long life or thin section.

Cathodic protection

Cathodic protection makes the whole structure the cathode of a cell, either with sacrificial anodes (zinc, aluminum, magnesium bolted or welded on) or an impressed current from a power supply. It protects buried pipes, tanks, ship hulls and offshore structures. The anode mass follows from the current needed and Faraday’s law: m = I × t ÷ (capacity × utilization).

Designing against corrosion

• Keep water out and let it drain: avoid pockets, ledges and horizontal faces that hold water; seal or weld closed any crevice; ventilate hollow sections.
• Choose the material for the environment (not the cheapest in the dry shop): stainless, aluminum or coated steel; check the galvanic compatibility of every pair (Aluminum, Copper, Titanium and Other Alloys).
• Allow for loss where coatings are impractical: a corrosion allowance of 1 to 3 mm on pressure equipment, or the inspection schedule.
• Weld and coat together: grind welds smooth, round the edges (paint thins at sharp edges), coat before assembly where fasteners sit.
• Inspect and maintain: keep drains open and the paint intact; repair damage at once; record the thickness of critical items by ultrasonic gauge.

Related

• Previous: Welding Processes and Weld Design. Next: Casting, Forging and Powder Metallurgy.
• Temperature, Coatings and Fit Problems for coating thickness on fits.
• Materials and Processes: the whole guide.

FAQ

Does stainless steel never rust?

It is stain-less, not stain-proof: chlorides, iron contamination and crevices can start rust and pitting. Choose the grade for the environment.

Why does aluminum corrode when it touches steel?

Aluminum is anodic to steel and most other metals in a wet joint, so it corrodes if the contact is bridged by water. Isolate, seal and coat.

What does a salt spray test tell me?

A comparative ranking of coatings under an accelerated, harsh test (ASTM B117, ISO 9227): hours do not convert directly to years outdoors.

How do I stop a high-strength bolt snapping after plating?

Hydrogen embrittlement: bake it at 190 to 220 °C within hours of plating, or use mechanically applied zinc or a coating that does not use acid or cathodic hydrogen.

References

• Fontana, M. G., Corrosion Engineering, McGraw-Hill; Jones, D. A., Principles and Prevention of Corrosion, Pearson; ASM Handbook, Volume 13: Corrosion.
• ISO 9223 (corrosivity of atmospheres), ISO 12944 (protective paint systems), ISO 1461 (galvanizing), NACE and AMPP standards for cathodic protection.
• Davis, J. R. (ed.), Stainless Steels, ASM Specialty Handbook; Nickel Institute design guides.

Last reviewed: October 2026. Corrosion rates and potentials are typical; real rates vary widely with the environment.