Input Tafel slopes and corrosion current density to instantly compute corrosion rate (mm/year), polarization resistance, and visualize the Evans polarization diagram with mixed potential theory.
Parameters
Corrosion Potential Ecorr
V
Corrosion Current icorr
0.01 — 1000 μA/cm² (log scale)
Anodic Tafel Slope βa
mV/dec
Cathodic Tafel Slope βc
mV/dec
Reference Temperature T
°C
Results (Steel Fe/H₂O system)
Results
10.0
i_corr (μA/cm²)
-0.50
E_corr (V)
0.117
CR (mm/year)
2.19
R_p (kΩ·cm²)
26.1
B / mV (Stern-Geary)
Evans Diagram (Polarization Curve) — log|i| vs E
Corrosion Rate vs Temperature (Arrhenius: doubles per 10°C)
What exactly is an Evans diagram? I see it's a graph on the simulator, but what does it actually show?
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Basically, it's a visual map of a metal corroding. The horizontal axis is the electrode potential (like how "driving" the reaction is), and the vertical axis is the current (the reaction speed). The point where the anodic (metal dissolving) and cathodic (e.g., oxygen reducing) lines cross is the corrosion point. Try moving the 'Corrosion Potential (E)' slider above—you'll see that intersection point shift, which directly changes the corrosion current.
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Wait, really? So the slopes of those lines matter too? What are the "Tafel slopes" I can adjust?
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Great question! The Tafel slopes (β_a and β_c) tell us how sensitive the reaction rate is to a change in voltage. A steeper slope means you need a big voltage push to get more current. In practice, for steel in seawater, β_a might be around 60 mV/decade. Play with the anodic and cathodic Tafel slope controls—you'll see making one line steeper dramatically changes where they intersect, and thus the predicted corrosion rate.
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So the corrosion current from the graph gets turned into "mm/year" in the result. How does a current become a physical loss of metal?
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Exactly! That's where Faraday's law comes in. The current (Amps) is a flow of electrical charge, which corresponds to ions leaving the metal surface. We use the metal's density and atomic weight to convert that ion loss into a thickness lost per year. Change the 'Corrosion Current (i)' parameter and watch the mm/year result update instantly—it shows why a tiny current density, like 1 µA/cm², can still lead to significant damage over time.
Physical Model & Key Equations
The core model is the Butler-Volmer equation, which describes the net current when an electrode is driven away from its equilibrium (corrosion) potential by an overpotential, η.
Here, $i_{corr}$ is the corrosion current density (A/m²), $\eta = E - E_{corr}$ is the overpotential (V), $\beta_a$ is the anodic Tafel slope (V/decade), and $\beta_c$ is the cathodic Tafel slope (V/decade). At η = 0, the net current is zero, which defines the corrosion point.
The corrosion current is converted into a practical engineering loss rate using Faraday's law.
$$\mathrm{CR}= \frac{i_{corr} \cdot M}{n \cdot F \cdot \rho}$$
Where CR is the corrosion rate (mm/year), $M$ is the molar mass of the metal (g/mol), $n$ is the number of electrons transferred per atom (e.g., 2 for Fe → Fe²⁺), $F$ is Faraday's constant (96485 C/mol), and $\rho$ is the density of the metal (g/cm³). This equation links the electrochemical measurement to tangible material loss.
Frequently Asked Questions
The basic method is to obtain it from measured polarization curves using Tafel extrapolation. It is recommended to use literature values (e.g., βa = 60 mV/dec for active dissolution of iron, βc = 120 mV/dec for oxygen reduction) as initial values and then check the impact on corrosion rate through sensitivity analysis.
Based on Faraday's law, the corrosion current density i_corr [A/cm²] is multiplied by a conversion factor calculated from the equivalent mass and density (e.g., approximately 0.0116 for iron) to obtain mm/year. This tool automatically applies the factor corresponding to the metal type.
According to mixed potential theory, the unique intersection point where the total anodic and cathodic currents balance is the corrosion potential. If multiple intersections appear on the diagram, check the input range and scale of the polarization curves, and select the intersection in a physically reasonable region (active region).
The polarization resistance Rp is used for rapid evaluation of corrosion rate. It is calculated as Rp = (βa × βc) / (2.303 × i_corr × (βa + βc)), and a smaller value indicates that corrosion is more likely to progress. It can be utilized in combination with on-site electrochemical measurements.
Real-World Applications
Pipeline Integrity Monitoring: Engineers use tools like this to interpret data from field probes. By measuring Tafel slopes and corrosion potential in soil, they can predict the wall thinning rate of buried oil and gas pipelines and schedule maintenance before leaks occur.
Material Selection for Marine Environments: When designing ship hulls or offshore platforms, different alloys are tested electrochemically. Comparing their corrosion currents and Tafel slopes from diagrams helps select the most cost-effective, durable material for saltwater exposure.
Evaluating Corrosion Inhibitors: A common test is to add an inhibitor chemical to a solution and re-measure the Tafel slopes. A good inhibitor dramatically increases the polarization resistance (flattens the Tafel lines), which the simulator shows as a lower corrosion current at the intersection point.
Cathodic Protection Design: For protecting underground tanks, the Evans diagram is used to find the minimum protective current needed. By shifting the potential (on the x-axis) to a more negative value, the anodic dissolution current is suppressed, which the Butler-Volmer equation quantifies.
Common Misunderstandings and Points to Note
When you start using this tool, there are a few common pitfalls to watch out for. First is the "Unit of the Tafel Slope β". We use mV/decade (millivolts per decade) here, but literature might use V/decade or a natural logarithm base in V. For example, β_a=60 mV/decade means the current increases tenfold for every 60mV increase in potential. Getting the units wrong will make your calculations completely off, so be careful.
Next is the "Interpretation of Polarization Resistance Rp". While it's true that a larger Rp value indicates a lower corrosion rate, don't forget this is an approximation valid only in the "micro-polarization region". When determining Rp from actual measurements, if the applied potential range is too large, it can deviate from the Tafel approximation, causing the calculated i_corr to be higher than the actual value. Try moving the "Polarization Resistance Rp" slider in the tool to see how the slope near the origin changes on the Evans diagram.
Finally, a fundamental understanding: "Reality Isn't a Straight Line". The Evans diagram is a powerful model to aid understanding, but actual measurement data often doesn't form perfect straight lines. For instance, if mass transport in the solution becomes rate-limiting, the cathodic reaction line can become horizontal (limiting current), or if a passive film forms, the anodic line can jump significantly upwards. Keep in mind that this tool's graph shows the textbook behavior of an "ideal case".