Types of Corrosion and How to Prevent Them

Corrosion costs industrialised economies an estimated 3 to 4% of GDP each year, through replacement, maintenance, lost production and failures. It weakens bridges, pipelines, tanks, ships and plant equipment, and can cause leaks, fires and collapses. Recognising each type of corrosion is the first step to preventing it.

The electrochemistry of corrosion

Aqueous corrosion is an electrochemical cell with four parts:

• Anode: metal oxidises and dissolves, for iron Fe → Fe²⁺ + 2e⁻.
• Cathode: electrons are consumed, typically by oxygen reduction O₂ + 2H₂O + 4e⁻ → 4OH⁻ in neutral water, or hydrogen evolution in acids.
• Electrolyte: water or moisture that carries ions.
• Metallic path: the metal itself carries electrons from anode to cathode.

Removing any one of these stops corrosion, which is the logic behind all prevention methods.

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The main types of corrosion

1. Uniform (general) corrosion

Metal is lost fairly evenly over the exposed surface, as in rusting steel in the atmosphere or acid attack. It is predictable, measured as a corrosion rate in mm per year, and usually managed by a corrosion allowance, coatings or material choice.

2. Galvanic corrosion

When two dissimilar metals are electrically connected in an electrolyte, the more active (less noble) metal in the galvanic series corrodes faster, while the nobler one is protected. Examples: steel bolts in a copper alloy plate, aluminium in contact with stainless steel in seawater. Risk increases with a large cathode to small anode area ratio. Prevention: choose compatible metals, insulate them, coat the cathode, or make the anode large.

3. Pitting corrosion

Highly localised attack forming small, deep pits, typical of passive metals such as stainless steel and aluminium in chloride environments. Pits can perforate a wall while the surface looks almost untouched. Resistance of stainless steels is ranked by the pitting resistance equivalent number, PREN = %Cr + 3.3 × %Mo + 16 × %N.

4. Crevice corrosion

Localised attack inside narrow gaps, such as under gaskets, washers, deposits and lap joints, where stagnant solution becomes depleted in oxygen and enriched in chlorides and acidity. Prevention: avoid crevices in design, use welded rather than bolted joints, seal gaps, keep surfaces clean.

5. Intergranular corrosion

Attack along grain boundaries, for example in austenitic stainless steels sensitised by heating in the range of about 450 to 850 °C during welding, which forms chromium carbides and depletes chromium near the boundaries. Prevention: low carbon grades (304L, 316L), stabilised grades (321, 347) or solution annealing.

6. Stress corrosion cracking (SCC)

Cracking caused by the combined action of tensile stress (applied or residual) and a specific corrosive environment, such as chlorides with austenitic stainless steel, caustic with carbon steel, or ammonia with brass. Cracks can grow with very little metal loss and cause sudden failure. Prevention: resistant materials (duplex steels), stress relief, reducing temperature or chlorides, and compressive surface stresses by shot peening. Closely related is hydrogen embrittlement, especially in high strength steels and sour (H₂S) service.

7. Erosion corrosion and cavitation

Fast or turbulent flow removes protective films and accelerates attack, typically at pipe bends, pump impellers and valve trims. Cavitation damage from collapsing vapour bubbles in centrifugal pumps is a related mechanical and corrosive attack. Prevention: lower velocities, larger radius bends, harder or more resistant alloys.

8. Selective leaching (dealloying)

One element is removed from an alloy, such as zinc from brass (dezincification) or iron from grey cast iron (graphitic corrosion), leaving a weak, porous structure.

9. Corrosion fatigue

Cyclic stress in a corrosive environment shortens fatigue life and removes the fatigue limit of steels. See fatigue failure.

10. Microbiologically influenced corrosion (MIC)

Bacteria such as sulphate reducing bacteria create local corrosive conditions under biofilms in water systems, tanks and pipelines. Prevention: biocides, flow and cleaning, pigging, and material selection.

Preventing the types of corrosion

• Material selection: stainless steels, duplex steels, nickel alloys, titanium, copper alloys or polymers and composites for aggressive environments.
• Design: avoid crevices and water traps, allow drainage and ventilation, avoid galvanic couples, provide access for inspection and maintenance.
• Protective coatings: paints and coating systems, hot dip galvanising, metallic and ceramic coatings, linings. Surface preparation is the most important factor in coating life.
• Cathodic protection: sacrificial anodes or impressed current systems for buried pipelines, ship hulls, offshore structures and tank bottoms.
• Inhibitors: chemicals added to cooling water, oil and gas streams or closed systems that slow corrosion reactions.
• Environmental control: dehumidification, deaeration of boiler water, pH control, removing chlorides.
• Inspection and monitoring: thickness measurement with ultrasonic NDT methods, corrosion coupons and probes, and intelligent pipeline pigging.

Estimating remaining life

For uniform corrosion, remaining life is:

$$t_\text{remaining} = \frac{t_\text{actual} - t_\text{minimum}}{\text{corrosion rate}}$$

Corrosion in engineering practice

Corrosion engineering combines chemistry, metallurgy and design. Most failures are not caused by unknown mechanisms but by known types of corrosion that were not considered: an unexpected galvanic couple, a crevice under a gasket, a sensitised weld or chlorides on hot stainless steel. Checking each type of corrosion during design and selecting engineering materials suited to the environment prevents most problems.