Corrosion Resistance Index⚠ unverified
Mechanical / Materials · Compute a simple corrosion resistance index from pitting potential
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| pitting_potential | pittingpotential | V | 1.0 | Pitting potential of the material |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | CRI | V | Corrosion resistance index, in volts (V); negative potentials are clamped to zero |
The science & history
Understanding the Parameters
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Pitting potential $E_{pit}$ — the electrode potential (versus a reference) at which the protective passive film breaks down locally and pits initiate. Measured by cyclic potentiodynamic polarisation. A higher $E_{pit}$ means the film survives to more oxidising conditions.
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The $\max(0,\cdot)$ clamp — negative pitting potentials (active materials that corrode readily) are set to zero, giving them the worst index. It is a floor, not physics.
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Why it is only a proxy — pitting resistance is conditional: the same alloy has a different $E_{pit}$ in seawater versus fresh water, at $5\,{}^\circ$C versus $60\,{}^\circ$C, or as chloride rises. A single number cannot capture that, nor general corrosion, crevice corrosion, or stress-corrosion cracking.
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Better metrics — for stainless steels the PREN $= \%\text{Cr} + 3.3\,\%\text{Mo} + 16\,\%\text{N}$ ranks pitting resistance from composition; galvanic-series position ranks general corrosion in a couple.
Derivation (Approaching a Proof)
There is no first-principles derivation — the index is a definitional heuristic. Its only rationale is electrochemical: in a potentiodynamic scan, current stays low while the passive film protects the surface, then rises sharply once the potential reaches $E_{pit}$ and pits grow. Taking $E_{pit}$ as a resistance proxy assumes (i) pitting is the governing failure mode, (ii) a higher breakdown potential always means better field performance, and (iii) the environment is held fixed. The clamp $\max(0, E_{pit})$ merely prevents negative "resistance" values. All three assumptions are approximations, which is why the page flags it as coarse.
Dimensional check. $CRI = \max(0, E_{pit})$ carries the units of $E_{pit}$ (volts); the clamp is dimensionless.
History and Development
Pitting potential as a corrosion-resistance measure comes from electrochemical corrosion science and the potentiodynamic polarisation test (mid-20th century). It underlies alloy development for marine, chemical, and biomedical use, where localised pitting — not uniform thinning — is often the life-limiting mechanism. Practical selection blends $E_{pit}$ with composition indices (PREN), galvanic-series data, and service experience rather than any single number.
Related Concepts: Statistical Mechanics for Property Estimation, Material Selection Index Strength, Ashby Charts, Creep Resistance Index
Notes: Heuristic single-parameter proxy (pitting potential, floored at 0) — not a rigorous corrosion metric. Environment-dependent (chloride/pH/temperature); ignores general/crevice/SCC attack. For stainless, prefer PREN $=\%\text{Cr}+3.3\%\text{Mo}+16\%\text{N}$.