Tafel & Corrosion Rate Guide

Electrochemistry/Tafel & Corrosion Rate GuideMethod guide

Tafel Extrapolation and How to Calculate Corrosion Rate

The Tafel polarisation curve is the most common way to calculate a metal's corrosion rate with a potentiostat. This guide explains the theory, fitting steps, a worked example and when to use LPR or EIS instead.

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Short theory

At the corrosion potential Ecorr, metal oxidation and oxidant reduction proceed at the same rate, so the net current is zero. When the potential is shifted far enough (usually more than ±50 mV), one reaction dominates and the logarithm of current is linear with potential. This straight region is the Tafel region.

The straight anodic and cathodic branches are extrapolated back to Ecorr; their intersection gives the corrosion current density icorr. Their slopes are the Tafel constants βa and βc (mV/decade). The corrosion rate is then calculated from icorr using Faraday's law.

Formulas and a worked example

Corrosion rate (ASTM G102)CR (mm/tahun) = 3,27 × 10⁻³ · icorr · EW / ρicorr in µA/cm², EW = equivalent weight, ρ = density (g/cm³).
Example: carbon steel in 3.5% NaClicorr = 10 µA/cm² → CR = 3,27×10⁻³ × 10 × 27,92 / 7,87 ≈ 0,116 mm/tahunCarbon steel EW ≈ 27.92 (Fe → Fe²⁺), ρ ≈ 7.87 g/cm³. The icorr value is illustrative.
Stern–Geary (from LPR)icorr = B / Rp B = βa · βc / [2,303 (βa + βc)]Rp from the E–i slope around Ecorr (±10–30 mV). Nearly non-destructive.
Penetration rate in mpyCR (mpy) = 0,1288 · icorr · EW / ρ1 mm/y ≈ 39.4 mpy.

Tafel fitting steps

  1. Stabilise the OCPWait for a stable corrosion potential before scanning.
  2. Scan from cathodic to anodicFor example −250 mV to +250 mV vs OCP at 0.1667 mV/s (ASTM G5).
  3. Pick the linear regionTake a straight segment spanning at least one current decade, starting about 50 mV from Ecorr.
  4. ExtrapolateExtend the Tafel lines to Ecorr; the software calculates icorr, βa and βc.
  5. Normalise by areaDivide the current by the exposed area to get current density.
  6. Calculate the corrosion rateUse the ASTM G102 formula and compare with immersion tests where possible.

Tafel, LPR or EIS?

Method Strengths Limitations
Tafel Gives icorr, βa and βc at once; intuitive Destructive; needs a clear Tafel region
LPR Fast, nearly non-destructive, repeatable for monitoring Needs a B value (from Tafel or assumed)
EIS Separates solution, film and charge-transfer resistances; mechanistic insight Interpretation needs a circuit model

Common mistakes

  • Fitting too close to Ecorr, which is not the Tafel region.
  • Scanning too fast so capacitive current is included.
  • Forcing a fit on a cathodic branch that is oxygen-diffusion-limited (not straight).
  • Forgetting iR correction in high-resistance electrolytes.
  • Comparing corrosion rates without matching specimen area and immersion time.

Frequently asked questions

What is the difference between Ecorr and icorr?

Ecorr is the free-corrosion potential, indicating thermodynamic tendency. icorr is the corrosion current density, a direct measure of corrosion rate.

Does a more positive Ecorr mean better corrosion resistance?

Not necessarily. Corrosion rate is set by icorr. Ecorr can rise because of a passive film but also because of a stronger cathodic reaction.

What are typical βa and βc values?

Often 60–120 mV/decade for many metals in aqueous solutions, but the actual values must be measured as they depend on the system.

Setting up a corrosion lab?

See our corrosion testing packages or ask our team about the right potentiostat, cell and electrodes.

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