Enter Tafel constants for anodic and cathodic half-reactions and watch the Evans diagram update in real time, with Ecorr, icorr and corrosion rate derived from the mixed-potential intersection.
Electrochemical Parameters
Metal (Anode) Selection
Equilibrium Potential and Exchange Current Density
Tafel Constant
Results
Corrosion Potential Ecorr
—
V vs SHE
Corrosion Current Density icorr
—
μA/cm²
Corrosion Rate CR
—
mm/year
Mass Loss Rate
—
g/(m²·day)
Evans
Change in corrosion current density as cathodic equilibrium potential E₀,c varies.
Tafel
Change in corrosion rate as βc varies from 20 to 200 mV/dec.
What is an "Evans diagram"? I see two lines drawn on the graph—why is their intersection so important?
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Roughly speaking, it's a diagram that shows both the metal dissolution (anode: oxidation) reaction and the oxygen/hydrogen reduction (cathode: reduction) reaction on the same graph. The horizontal axis is potential, and the vertical axis is the logarithm of current density (Tafel lines). The point where the two reactions balance is the "corrosion potential Ecorr," and the current density there is the "corrosion current density icorr." The larger this value, the faster the corrosion. Try moving the E₀ cathode slider to the right in the simulator. You should see the intersection of the two lines shift to the upper right, confirming that the corrosion rate increases.
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Wow, it really does! So raising the cathode potential increases the corrosion rate. What does this correspond to in real life?
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For example, if the outside of an iron pipe is in contact with oxygen-rich water (high dissolved oxygen), the equilibrium potential of the oxygen reduction reaction becomes higher (the cathode line shifts upward). That's why corrosion is often faster in open waterways than in sealed plumbing. Conversely, using deoxygenated water (e.g., in boilers) slows down corrosion—this is the principle behind it. You can confirm this by lowering E₀c in the "Sensitivity Analysis" tab and seeing the corrosion rate decrease.
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What happens to the corrosion rate when the "Tafel constant βc" increases? I tried it in the "Tafel Sensitivity" tab… the corrosion rate went down!
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Sharp observation! A large βc means the cathode reaction is "insensitive" to potential changes. The slope of the line becomes shallower, shifting the intersection to the lower left. In practice, a large βc (≈120 mV/dec) is typical for the hydrogen evolution reaction, which tends to promote corrosion less than oxygen reduction (≈60 mV/dec). This parameter is also important in designing corrosion inhibitors—some adsorb on the surface, alter the reaction mechanism, and intentionally increase β.
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It's interesting that corrosion inhibitors can change β! When I switch the metal type to "zinc," the corrosion potential drops quite a bit.
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That's the principle behind galvanized steel roofs and zinc plating. Zinc has a lower corrosion potential (standard potential) than iron, so when in contact with iron, zinc preferentially dissolves and protects the iron—acting as a "sacrificial anode." Even today, zinc blocks are attached to ship hulls and submarine pipelines for protection. By switching between iron and zinc with the slider, you can intuitively understand the difference in corrosion potential (galvanic potential difference).
Tafel Equation and Faraday's Law
The relationship between potential and current density is represented by the Tafel equation. The overpotentials $\eta$ for anodic oxidation and cathodic reduction are:
$\beta_a, \beta_c$: Tafel constants [V/decade], typically 0.04-0.12 V/decade. $i_{0,a}, i_{0,c}$: exchange current densities [A/cm²]. The corrosion potential $E_{corr}$ is the point where anodic and cathodic current densities are equal.
Corrosion rate is calculated from corrosion current density $i_{corr}$ using Faraday’s law:
$M$: metal atomic weight [g/mol], $n$: dissolution valence (for Fe→Fe²⁺, n=2), $F$: Faraday constant (96485 C/mol), and $\rho$: density [g/cm³].
Practical Applications in Corrosion Engineering
Life Prediction for Piping and Structures：This model estimates corrosion rates of steel structures in seawater and buried pipelines, supporting service-life design and inspection interval planning. Comparing with measured $i_{corr}$ helps build corrosion maps.
Corrosion-Protection Design：The same theory underlies designs that lower $E_{corr}$ with impressed current or sacrificial anodes such as zinc and aluminum, reducing corrosion rate toward zero. In the simulator, lowering E₀c shows corrosion stopping when Ecorr drops below the anodic equilibrium potential.
Corrosion Inhibitor Evaluation：The model can quantify how inhibitors change β values and exchange current density $i_0$, helping determine effective dosage when combined with experimental data such as electrochemical impedance spectroscopy (EIS).
Frequently Asked Questions
It is obtained by "polarization curve measurement," where the potential of the sample metal in an electrochemical cell is continuously varied while measuring the current. The slopes βa and βc are read from the region (Tafel region) where the log(i) vs. E plot is approximately linear, typically within ±50 to 300 mV of Ecorr. Using an automatic polarization device (potentiostat), the measurement time is usually 15 to 30 minutes.
Generally, the evaluation criteria for corrosion rate are: <0.1 mm/year: excellent (most materials rated as excellent), 0.1–0.5 mm/year: good, 0.5–1.0 mm/year: acceptable (thin parts require caution), >1.0 mm/year: dangerous. However, the judgment varies depending on the wall thickness of the structure and the required service life. For example, for an offshore structure with a plate thickness of 20 mm, even a rate of 0.1 mm/year would reduce the thickness by 2 mm over 20 years, so this must be considered in the design.
Paint corrosion protection physically isolates the metal from the corrosive environment, effectively bringing both current densities (i₀) in the Evans diagram close to zero. On the other hand, cathodic protection lowers Ecorr below the anodic equilibrium potential using an external current or sacrificial anode, thermodynamically prohibiting the anodic reaction. In practice, both methods are often combined (e.g., coating + impressed current cathodic protection for buried pipelines), offering the advantage that cathodic protection can supplement even where the coating is damaged.
This tool is a 1D model for uniform corrosion (general corrosion). In real corrosion, important phenomena include: ① pitting corrosion: localized corrosion due to chlorides, not shown in the Evans diagram; ② stress corrosion cracking (SCC): synergistic effect of stress and corrosion; ③ galvanic corrosion: contact between dissimilar metals; ④ crevice corrosion: oxygen concentration cell; ⑤ microbiologically influenced corrosion (MIC). The Evans diagram is used for understanding uniform corrosion trends and as a basis for corrosion protection design, but separate evaluation methods are needed for localized corrosion risks.
This is a non-destructive method that measures current at very small ΔE (around ±10–20 mV) from Ecorr. From the linear polarization resistance Rp and the relationship with corrosion current density (Stern-Geary equation) $i_{corr} = B/R_p$ (B = βaβc/(2.303(βa+βc))), icorr can be estimated quickly with minimal damage to the sample. It is also possible to calculate B from the Tafel constants βa and βc in this simulator.
For stainless steel and aluminum, when the potential is increased, the current first increases (active dissolution region), then suddenly drops sharply—a phenomenon called "passivation." In the Evans diagram, this appears as the anodic line transitioning from the active region to a "passive region" where the current decreases. This simulator assumes simple Tafel lines and cannot represent passivation, but in actual stainless steel corrosion evaluation, comparing the passivation potential (potential for maintaining the passive state) with the corrosion potential is important.
What is Corrosion Electrochemical?
Corrosion Electrochemical is a fundamental topic in engineering and applied physics. This interactive simulator lets you explore the key behaviors and relationships by directly manipulating parameters and observing real-time results.
By combining numerical computation with visual feedback, the simulator bridges the gap between abstract theory and physical intuition — making it an effective learning tool for students and a rapid-verification tool for practicing engineers.
Physical Model & Key Equations
The simulator is based on the governing equations behind Electrochemical Corrosion Analyzer. Understanding these equations is key to interpreting the results correctly.
Each parameter in the equations corresponds to a slider in the control panel. Moving a slider changes the equation's solution in real time, helping you build a direct connection between mathematical expressions and physical behavior.
Real-World Applications
Engineering Design: The concepts behind Electrochemical Corrosion Analyzer are applied across mechanical, structural, electrical, and fluid engineering disciplines. This tool provides a quick way to estimate design parameters and sensitivity before committing to full CAE analysis.
Education & Research: Widely used in engineering curricula to connect theory with numerical computation. Also serves as a first-pass validation tool in research settings.
CAE Workflow Integration: Before running finite element (FEM) or computational fluid dynamics (CFD) simulations, engineers use simplified models like this to establish physical scale, identify dominant parameters, and define realistic boundary conditions.
Common Misconceptions and Points of Caution
Model assumptions: The mathematical model used here relies on simplifying assumptions such as linearity, homogeneity, and isotropy. Always verify that your real system satisfies these assumptions before applying results directly to design decisions.
Units and scale: Many calculation errors arise from unit conversion mistakes or order-of-magnitude errors. Pay close attention to the units shown next to each parameter input.
Validating results: Always sanity-check simulator output against physical intuition or hand calculations. If a result seems unexpected, review your input parameters or verify with an independent method.
Enter anodic exchange current density (i0a) in A/cm² and cathodic exchange current density (i0c) in A/cm² as logarithmic values (e.g., i0a = -6 means 10⁻⁶ A/cm²)
Input standard electrode potentials: E0a (anode, typically -0.5 to 0 V vs. SHE) and E0c (cathode, typically 0.4 to 1.2 V vs. SHE) for your material system
The simulator computes the corrosion potential (Ecorr) at the Evans diagram intersection and derives current density (icorr), then converts to corrosion rate in mm/year using Faraday's law and material density
Worked Example
For mild steel in neutral aqueous solution: E0c = 0.6 V (cathodic reduction), E0a = -0.62 V (anodic iron oxidation), i0c = -5 (10⁻⁵ A/cm²), i0a = -4 (10⁻⁴ A/cm²). The Evans diagram intersection yields Ecorr ≈ -0.55 V and icorr ≈ 0.8 µA/cm². With Fe atomic mass 55.845 g/mol, density 7.87 g/cm³, and 2 electrons per Fe²⁺, this produces a corrosion rate of 0.094 mm/year—consistent with unprotected steel atmospheric exposure.
Practical Notes
For stainless steel (e.g., 316L), cathodic slopes are steeper and i0a drops dramatically below -7 A/cm² due to passive oxide films; expect mm/year rates near 0.001 or lower if passive
Verify Tafel coefficients empirically via potentiodynamic polarization testing; literature values vary ±30% with surface roughness, temperature, and electrolyte composition
If computed rate exceeds 1 mm/year, check for localized pitting initiation; Evans diagram assumes uniform kinetics and breaks down in crevices or under applied stress
Multiply result by safety factor 2–4 in design calculations for long-term subsea or buried carbon-steel structures