Corrosion, Degradation & Protection

Why metals return to their ores — and how to stop them. Learn electrochemical corrosion (anodic and cathodic reactions), electrode potentials and the Nernst equation, the galvanic series and galvanic corrosion, Pourbaix diagrams, corrosion rate from current density (Faraday), forms of corrosion (uniform, pitting, crevice, intergranular, stress-corrosion cracking, erosion, microbial), passivity and stainless steels, high-temperature oxidation, polymer and ceramic degradation, and protection (materials selection, coatings, galvanising, cathodic protection, inhibitors, design) — with Python for Nernst potentials, corrosion rate and anode life.

Corrosion costs an estimated 3–4% of GDP in many countries, including India — through replaced pipelines, bridges, vehicles and plant downtime, and sometimes catastrophic failures.

Most metal corrosion is electrochemical: metal atoms lose electrons (anode) while oxygen or hydrogen ions gain them (cathode). Understanding this cell lets engineers predict and prevent corrosion.

An analogy

A corroding pipe is a tiny battery that eats itself: one area becomes the anode and dissolves, sending electrons to a cathode area where oxygen is consumed. Cathodic protection is like connecting a 'sacrificial' battery that dies instead of the pipe.

The corrosion cell

Four elements are needed: an anode (metal oxidises), a cathode (reduction of O₂ or H⁺), an electrolyte (water, soil, moisture film) and an electrical connection. Remove any one and corrosion stops. Rust (hydrated iron oxides) forms where Fe²⁺ meets OH⁻ and oxygen.

Forms of corrosion

Recognise the type to fix it:

Passivity and stainless steels

Some metals form thin, protective oxide films (Cr₂O₃ on stainless steel with ≥ 10.5% Cr, Al₂O₃ on aluminium, TiO₂ on titanium). Chlorides attack passive films; molybdenum and nitrogen improve pitting resistance (PREN: 304 ≈ 19, 316 ≈ 25, duplex 2205 ≈ 35). Low-carbon grades (304L/316L) or stabilised grades (321, 347) avoid sensitisation. Pourbaix (potential–pH) diagrams show immunity, corrosion and passivity regions.

Protection strategies

Materials selection (stainless, duplex, Ni alloys, polymers, FRP); coatings (paint systems, epoxy, polyurethane, fusion-bonded epoxy on pipelines, powder coating); metallic coatings (hot-dip galvanising, electroplating); cathodic protection (sacrificial Zn/Al/Mg anodes or impressed current) for pipelines, ships, jetties and rebar; inhibitors in cooling water and oil wells; design (avoid crevices, drain water, isolate dissimilar metals). High-temperature oxidation follows parabolic kinetics; polymers degrade by UV, hydrolysis and oxidation; concrete rebar corrodes by chlorides and carbonation.

Formulas

Worked example: Corrosion rate

Given: i_corr = 25 µA/cm² on steel

Find: CR.

Answer: 0.29 mm/year

Worked example: PREN

Given: 316: 17% Cr, 2.2% Mo, 0.05% N

Find: PREN.

Answer: 25.1

Worked example: Corrosion allowance

Given: CR 0.1 mm/yr, 20-year design life

Find: Allowance.

Answer: 2 mm (plus margin)

Potentials, corrosion rate and anode life

import math
# Corrosion: Nernst potential, galvanic cell and corrosion rate from current density (Faraday)
E0_Zn, E0_Fe = -0.763, -0.440                  # V vs SHE
R, T, F = 8.314, 298.15, 96485
for c in (1.0, 0.01):
    E = E0_Fe + (R * T / (2 * F)) * math.log(c)
    print(f"Fe2+ at {c} M: electrode potential {E:.3f} V")
print(f"Zn-Fe galvanic couple emf = {E0_Fe - E0_Zn:.3f} V -> zinc corrodes, protecting steel")
# Corrosion rate of steel with i_corr = 10 uA/cm2
i = 10e-6                                      # A/cm2
M, n, rho = 55.85, 2, 7.87
rate_cm_s = i * M / (n * F * rho)
print(f"corrosion rate {rate_cm_s*3.15576e7*10:.3f} mm/year")
# Sacrificial zinc anode life: 20 kg anode delivering 0.5 A, utilisation 85%, Zn capacity 780 Ah/kg
print(f"anode life {20*780*0.85/0.5/8766:.1f} years")
# Fe2+ at 1.0 M: electrode potential -0.440 V
# Fe2+ at 0.01 M: electrode potential -0.499 V
# Zn-Fe galvanic couple emf = 0.323 V -> zinc corrodes, protecting steel
# corrosion rate 0.116 mm/year
# anode life 3.0 years

Zinc is more active than iron, so in a Zn–Fe couple zinc becomes the anode — the principle of galvanising and sacrificial anodes. A modest corrosion current of 10 µA/cm² eats 0.12 mm of steel per year: over 25 years that's 3 mm — often the corrosion allowance designers add to pipe walls. A 20 kg zinc anode delivering 0.5 A lasts about 3 years before replacement.

Common mistakes

Applications

Key takeaways

Going further