Corrosion and its prevention

Why it matters

Corrosion costs industrial economies several percent of GDP every year through replaced parts, leaks, contamination and failures. Production engineers meet it when choosing materials and fasteners, specifying coatings and plating, designing joints between dissimilar metals, welding stainless steel and setting corrosion allowances, so knowing how corrosion works and how to stop it is part of everyday design.

Key ideas

Electrochemical nature. In water or moist air, metallic corrosion is an electrochemical cell with four elements: an anode where metal oxidises (Fe → Fe²⁺ + 2e⁻), a cathode where a reduction consumes the electrons (O₂ + 2H₂O + 4e⁻ → 4OH⁻ in neutral water; 2H⁺ + 2e⁻ → H₂ in acid), an electrolyte that carries ions, and an electrical path for electrons through the metal. Remove any one and corrosion stops — the basis of every prevention method. Rust forms as Fe²⁺ meets OH⁻ and oxygen. Dry high-temperature oxidation (scaling) is a separate, chemical process.

Electrode potentials and the galvanic series. The standard EMF series ranks pure metals in 1 M solutions of their own ions (Au most noble, Zn and Mg very active: Zn −0.76 V, Fe −0.44 V, Cu +0.34 V against hydrogen). For engineering use, the galvanic series in seawater ranks real alloys and includes passive states (e.g. passive stainless steel sits near the noble end). When two metals are coupled, the one more active in the series becomes the anode and corrodes faster.

Forms of corrosion.

• Uniform (general): even thinning; predictable, handled by a corrosion allowance.
• Galvanic: dissimilar metals in electrical contact in an electrolyte. The area effect matters: a small anode coupled to a large cathode (steel rivets in copper sheet) corrodes very fast; the reverse is much milder.
• Crevice: inside gaps under gaskets, washers or deposits, where oxygen is depleted and the stagnant solution becomes acidic and chloride-rich.
• Pitting: deep, narrow local attack, often in chloride on passive metals (stainless steel, aluminium); dangerous because little metal loss can perforate a wall. Mo in 316 stainless improves resistance.
• Intergranular: attack along grain boundaries. In austenitic stainless steel heated in about 500–800 °C (e.g. the heat-affected zone of a weld), chromium carbides precipitate at grain boundaries and deplete the nearby metal of Cr (sensitisation, weld decay). Prevent with low-carbon grades (304L, 316L), stabilised grades (321 with Ti, 347 with Nb) or solution annealing.
• Selective leaching: one element removed from an alloy, e.g. dezincification of brass, graphitisation of grey cast iron.
• Erosion–corrosion and cavitation: flowing or turbulent fluid strips protective films (pump impellers, pipe bends).
• Stress-corrosion cracking (SCC): cracking under a sustained tensile stress (applied or residual) in a specific environment: chlorides with austenitic stainless steel, ammonia with brass (season cracking), caustic with steel. Stress relief, compressive surface stresses and material change help.
• Corrosion fatigue: cyclic stress plus a corrosive environment; there is no endurance limit.
• Hydrogen embrittlement: atomic hydrogen from pickling, plating, welding or cathodic reactions enters high-strength steel and causes brittle delayed cracking; bake parts after electroplating.

Passivity. Cr, Ni, Ti, Al and stainless steels form a thin, adherent, self-healing oxide film that makes them behave as noble metals. Chlorides and lack of oxygen can break passivity locally (pitting, crevice).

Prevention.

1. Material selection: stainless steels, Cu–Ni, titanium, plastics, low-carbon stainless for welding.
2. Design: avoid crevices and water traps, allow drainage, avoid dissimilar-metal contact or make the anode large, insulate joints (dielectric unions, nylon washers), avoid sharp bends and turbulence.
3. Coatings: paints and epoxy; metallic coatings — anodic (zinc galvanising protects steel even when scratched, sacrificially) or cathodic (tin, nickel, chromium protect only while intact; a scratch creates a small anode on the steel); conversion coatings (phosphating, chromating, anodising of aluminium).
4. Cathodic protection: make the structure the cathode. Sacrificial anodes of Zn, Mg or Al are bolted on and corrode instead (ship hulls, water heaters); impressed-current systems use a DC source and inert anodes (buried pipelines, jetties).
5. Anodic protection: hold passivating metals in the passive potential range (acid tanks).
6. Environment control: inhibitors, deaeration, pH control, dehumidification.

Formulas

m = I·t·M / (n·F) Faraday's law: m = mass of metal dissolved (g); I = corrosion current (A); t = time (s); M = molar mass (g/mol); n = electrons per atom; F = 96 485 C/mol.

CPR = K·W / (ρ·A·t) Corrosion penetration rate: W = mass loss; ρ = density; A = exposed area; t = time. With W in mg, ρ in g/cm³, A in cm² and t in h, K = 87.6 gives CPR in mm/year.

i = I / A Corrosion current density (A/m²); for the same current, a small anode area means a high dissolution rate.

E°cell = E°cathode − E°anode Standard cell potential (V) from standard electrode potentials (data).

Worked examples

Example 1 (standard): penetration rate and allowance. Given: a steel coupon, A = 100 cm², loses W = 2.5 g in one year (8760 h); ρ = 7.86 g/cm³.

1. CPR = 87.6 × 2500 / (7.86 × 100 × 8760).
2. = 219 000 / 6 885 360 = 0.032 mm/year.
3. A 20-year design life needs a corrosion allowance of about 0.032 × 20 = 0.64 mm (plus a margin).

Example 2 (GATE level): Faraday's law and sacrificial anodes. (a) A corrosion current of 2 A flows from a buried steel structure for one year. Mass of iron lost (M = 55.85 g/mol, n = 2):

1. t = 365 × 24 × 3600 = 3.154 × 10⁷ s.
2. m = 2 × 3.154 × 10⁷ × 55.85 / (2 × 96 485) = 18.3 kg of iron. (b) A 10 kg zinc anode (M = 65.38 g/mol, n = 2) must supply a protection current of 0.5 A. Its life, assuming 100 % efficiency:
3. t = m·n·F / (I·M) = 10 000 × 2 × 96 485 / (0.5 × 65.38) = 5.90 × 10⁷ s.
4. Life = 5.90 × 10⁷ / 3.154 × 10⁷ = 1.87 years (real anodes are less efficient, so the replacement interval is shorter). (c) Zn–Fe couple: E°cell = −0.44 − (−0.76) = 0.32 V; zinc is the anode.

Common mistakes

• Thinking the more noble metal corrodes in a galvanic couple; the more active one does.
• Ignoring the area ratio: small anode, large cathode is the dangerous combination.
• Treating a scratched tin or nickel coating like galvanising; cathodic coatings accelerate attack at the defect.
• Using the EMF series for alloys in service conditions instead of the galvanic series.
• Welding ordinary 304 stainless steel for corrosive service without considering sensitisation.
• Forgetting the time conversion to seconds in Faraday's law.

For GATE PI

Expect one-mark questions identifying the form of corrosion from a description (weld decay, dezincification, season cracking, pitting), choosing a prevention method (sacrificial anode, galvanising vs tinning, 304L), and the electrochemical cell elements. Numericals use Faraday's law, penetration rate from weight loss and simple cell potentials.

Quick check

1. In a steel–copper couple in seawater, which metal corrodes?
2. Why does galvanised steel resist rust at a scratch?
3. What causes sensitisation in austenitic stainless steel?
4. Name two metals used as sacrificial anodes.
5. A metal loses 0.1 mm per year. How long until a 3 mm allowance is used up?

Answers: 1. Steel; 2. Zinc is anodic to steel and corrodes sacrificially; 3. Chromium-carbide precipitation at grain boundaries at about 500–800 °C; 4. Zinc and magnesium (also aluminium alloys); 5. 30 years.