Many engineering alloys (stainless steels, aluminum, titanium) survive in aggressive environments because a nanometres-thin passive oxide film forms spontaneously and blocks further dissolution. That film can fail locally — at inclusions, grain-boundary defects or scratches — triggering runaway pitting even while the rest of the surface stays fully protected.
Passive film growth (high-field / Cabrera–Mott):
dL/dt = (L_eq(E) − L) / τ L_eq(E) = L0 + k·max(0, E)
i_pass ∝ exp(−L / L_c) thinner film → higher leakage current
Pitting (breakdown) potential vs. chloride activity:
E_pit = E_pit,0 − S·log10[Cl⁻] − α·(T − 25°C)
Stochastic nucleation at a weak (inclusion) site:
P_nucleate·dt = P0·exp[(E − E_pit)/E_a]·dt
Autocatalytic pit growth once active (anodic Tafel kinetics
inside the acidified, occluded pit cavity):
dx/dt = k_pit·10^[(E − E_pit)/βa]
Metastable repassivation (pit reseals if the local
driving force is too weak to sustain the occluded cell):
P_repassivate·dt = Pr0·exp[−(E − E_pit)/E_a]·dt
- Applied potential E — the electrode potential vs. a reference electrode. Below Epit the surface stays passive; well above it, nucleated pits reliably grow instead of resealing.
- Chloride [Cl⁻] — aggressive anions adsorb into the oxide and thin it locally; Epit drops roughly linearly with log[Cl⁻], a relation measured on real stainless steels and aluminum alloys.
- Temperature — raises ionic mobility and lowers Epit slightly; above a material's "critical pitting temperature" stable pitting becomes almost unavoidable.
- Inclusion / weak-site density — the fraction of the surface (typically MnS inclusions or grain-boundary carbides) capable of nucleating a pit at all — most of a real passive surface never breaks down.
- Every red site is a stable, autocatalytically growing pit; brown craters are metastable pits that nucleated but resealed before doing serious damage — both behaviors are observed on real corroding alloys.
The aggregate corrosion rate readout uses Faraday's law, CR(mm/yr) = 3.27×10⁻³·icorr(µA/cm²)·EW/ρ, converting the average passive-film leakage current into an equivalent uniform metal-loss rate — real pitting damage, however, concentrates almost entirely in the few active sites rather than spreading evenly.