🦷 Electrochemical Corrosion (Tafel)

Metal corrosion via Evans diagram: anodic dissolution and cathodic reduction meet at corrosion potential E_corr. Butler-Volmer kinetics with Tafel slopes βa, βc determine corrosion rate.

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Evans diagram: log(i) vs E · Intersection = E_corr, i_corr

How it Works

The Evans diagram is a graphical representation of corrosion kinetics. It plots electrode potential (E) on the y-axis against the logarithm of current density (log i) on the x-axis. Two polarization lines are drawn: the anodic line (metal dissolution) and the cathodic line (oxygen reduction or hydrogen evolution). Where they intersect defines the corrosion potential E_corr and corrosion current density i_corr.

Butler-Volmer kinetics governs each electrode reaction. In the Tafel region (large overpotential), the exponential terms simplify to straight lines on a log scale with slopes βa (anodic) and βc (cathodic) in mV per decade of current.

i = i₀ · [exp(2.303·η/βa) − exp(−2.303·η/βc)]
Anodic Tafel: E = E_eq + βa · log(i/i₀)
Cathodic Tafel: E = E_eq − βc · log(i/i₀)
Stern-Geary: i_corr = B/Rp, B = (βa·βc)/(2.303·(βa+βc))

Frequently Asked Questions

What is the Evans diagram?

The Evans diagram plots electrode potential (E) vs. log of current density (log i). It shows anodic and cathodic polarization curves intersecting at the corrosion potential E_corr and corrosion current i_corr.

What are Tafel slopes?

Tafel slopes (βa for anodic, βc for cathodic) describe how the electrode potential changes with log current density in the Tafel region. Typical values are 60–120 mV/decade for many metal/electrolyte systems.

What is Butler-Volmer kinetics?

Butler-Volmer equation describes the relationship between electrode current and overpotential: i = i0[exp(αaFη/RT) − exp(−αcFη/RT)], where i0 is exchange current density, η is overpotential, and αa, αc are transfer coefficients.

How is corrosion rate determined from E_corr and i_corr?

The corrosion rate is proportional to i_corr determined at the intersection of anodic and cathodic Tafel lines. Using Faraday's law: CR = (i_corr × M)/(n × F × ρ), where M is molar mass, n is electrons, F is Faraday constant, ρ is density.

What is the exchange current density?

Exchange current density (i0) is the rate of anodic and cathodic reactions at equilibrium (zero net current). Higher i0 means faster electrode kinetics. It appears at the reversible potential of each half-reaction.

What is linear polarization resistance?

Near E_corr, current varies linearly with potential. The Stern-Geary equation relates polarization resistance Rp to corrosion current: i_corr = B/Rp, where B = (βa·βc)/(2.303·(βa+βc)).

Why do Tafel slopes vary between systems?

Tafel slopes reflect the reaction mechanism. For a simple 1-electron transfer, βa = RT/(αaF) ≈ 60 mV/decade at 25°C with α=0.5. Multi-step reactions, adsorption processes, and temperature all affect the observed slope.

What is passivation in corrosion?

Passivation occurs when an oxide film forms on the metal surface at high anodic potentials, drastically reducing the corrosion current. The anodic curve shows a peak (active peak) followed by a passive region with low, nearly constant current.

How does pH affect corrosion potential?

pH affects equilibrium potentials of both half-reactions via the Nernst equation. For hydrogen evolution: E = E0 − 0.0592·pH at 25°C. Higher pH generally shifts cathodic curve to more negative potentials, potentially lowering corrosion rate for some metals.

What is galvanic corrosion?

Galvanic corrosion occurs when two dissimilar metals are electrically connected in an electrolyte. The less noble metal (anode) corrodes preferentially. The mixed potential of the coupled system lies between the individual corrosion potentials of each metal.

About this simulation

This simulator builds a live Evans diagram for four real metals (Fe, Zn, Cu, Al), plotting anodic and cathodic Tafel lines on a log-current axis and solving for their intersection numerically. Drag the exchange current i₀, Tafel slopes βa/βc, or equilibrium potential E_eq and watch E_corr and i_corr shift in real time, then read off an estimated iron corrosion rate in mm/yr via Faraday's law.

🔬 What it shows

An Evans diagram: a red anodic dissolution line and a blue cathodic reduction line, each a straight Tafel segment on a log(i) vs E plot. Where they cross marks the corrosion potential (E_corr) and corrosion current density (i_corr), highlighted with a dashed crosshair.

🎮 How to use

Adjust the i₀, βa, βc, and E_eq sliders to see the lines rotate and shift, or pick a metal preset (Fe, Zn, Cu, Al) to load realistic textbook values instantly. Press "Recalculate" or hit R to redraw, and watch the stats panel update E_corr, i_corr, and the derived corrosion rate.

💡 Did you know?

Zinc's much higher exchange current and more negative equilibrium potential is exactly why it's used to galvanically protect steel — it happily corrodes first, sacrificing itself so iron doesn't have to.

Frequently asked questions

What exactly does the intersection point on the graph represent?

The point where the anodic and cathodic Tafel lines cross is the mixed potential of the corroding metal, E_corr, and its associated current density, i_corr. Because both curves must carry equal and opposite charge at steady state, this is the only self-consistent operating point of the corroding electrode, and i_corr is what actually drives metal loss.

Why does changing the metal preset move both lines at once?

Each metal preset loads a matched set of i₀, βa, βc, and E_eq values pulled from real electrochemical data (for example iron uses i0=10 μA/cm², E_eq=-500 mV, while zinc uses i0=50, E_eq=-760 mV). Since the cathodic equilibrium potential in this model is offset from the anodic one, switching metals reshapes the whole diagram, not just one line.

How is the corrosion rate in mm/yr actually calculated?

The simulator applies Faraday's law using iron's molar mass (56 g/mol), 2 electrons transferred per Fe atom, Faraday's constant (96485 C/mol), and iron's density (7.87 g/cm3), converting i_corr into a linear penetration rate. It's a simplified single-metal conversion, so treat it as illustrative rather than a certified corrosion-engineering result.

What do the Tafel slope sliders actually control?

βa and βc set how steeply the anodic and cathodic lines rise or fall per decade of current, in mV/decade. A shallow slope means the reaction rate is very sensitive to potential changes, while a steep slope means the current barely responds to overpotential — both shapes come straight from the Butler-Volmer equation in the Tafel-region approximation.

Why is the exchange current density i0 shown as a separate marker on the plot?

i0 marks where each Tafel line would sit at zero overpotential, i.e. the natural equilibrium rate of that half-reaction before any net corrosion current flows. The purple dots on the anodic and cathodic equilibrium potentials show visually how far the actual corrosion point (E_corr, i_corr) has been pushed away from equilibrium.