Pitting Corrosion: IR-Drop Pit Stability (2D)

Electrochemical Conditions

Applied potential E 0.20 V

Chloride [Cl⁻] 0.10 M

Temperature 25 °C

Inclusion / weak-site density 8 %

Live Readouts

Film breakdown potential E_bd

— V

Active pits

Self-arrested pits

Deepest active pit

0 µm

Mean IR-solved current i

0 µA/cm²

Intact passive oxide film Stable pit — cleared the Galvele X·i test Metastable / arrested pit (X·i test failed)

How it works

The 3D version of this simulator lets pits repassivate by rolling a scripted probability. This 2D cross-section instead solves the actual coupled electrochemistry that decides whether a real pit escapes or dies: Ohm's law for the ohmic (IR) potential drop across the pit cavity, combined with mixed activation/mass-transport-controlled anodic kinetics — Galvele's classic pit-stability (X·i) criterion, tested the moment each pit nucleates.

Film breakdown potential (chloride/temperature, same empirical form used
in real corrosion engineering):
  E_bd = E_bd0 − S·log10[Cl⁻] − α·(T − 25°C)

At any pit depth X, dissolution current is throttled by the pit's own
ohmic drop (Ohm's law) before it reaches the Tafel-kinetic surface:
  η(i) = E − E_active0 − ρ_IR(Cl)·i·(X + X_mouth)      [IR drop grows with i AND X]
  i_kin(η) = i0·10^(η/βa)                               [anodic Tafel law]
  i = i_kin·i_lim /(i_kin + i_lim)                       [mixed kinetic/mass-transport control]

Solved by bisection on i (the equation is implicit — i sets its own
throttling voltage), never by an assumed closed form.

Galvele stability test, run once at the embryonic pit depth X_emb right
after nucleation:
  X_emb · i(X_emb)  >  (X·i)_crit   →  pit escapes metastability, keeps growing
  X_emb · i(X_emb)  ≤  (X·i)_crit   →  pit dies immediately (metastable crater)

A surviving pit keeps growing by Faraday's law, dX/dt = i·M /(n·F·ρ_metal),
with i re-solved every frame — so its growth visibly slows as its own IR
drop deepens, and it can still stall out later if you pull E back down.

Applied potential E — the driving force for dissolution; only once E sits enough above E_bd does a freshly nucleated pit's embryonic X·i product clear the stability threshold, so the "stable pitting potential" this model produces sits visibly above the breakdown potential — exactly the gap seen on real polarization curves.
Chloride [Cl⁻] — both lowers the film breakdown potential (more nucleation attempts) and lowers the effective pit-cavity resistivity ρ_IR, which is why concentrated-chloride pits clear the stability threshold at far lower applied potentials than dilute ones.
Temperature — raises both the active-dissolution exchange current and the film breakdown susceptibility, the same two levers real corrosion engineers track via the "critical pitting temperature."
Inclusion / weak-site density — fraction of surface sites capable of attempting nucleation at all; most attempts still die at the embryo stage even where the site itself is weak.
Watch most nucleation events flash into a tiny brown crater and stop (they failed the X·i test) while the rare orange survivor keeps digging — and its growth visibly decelerates as its own ohmic drop deepens, even though it never fully arrests.

Pitted surface: 0.0% Simulated time: 0 s

■ deepest active pit (µm)■ mean solved current i (µA/cm², log)