An icorr to mm/year calculator converts a corrosion current density from an electrochemical test into a penetration rate by applying Faraday’s law. The inputs are the corrosion current density, the equivalent weight of the metal and its density, which makes it the electrochemical counterpart of the gravimetric corrosion rate weight-loss calculator. For the measurement side, including how icorr is obtained, see the tutorial on corrosion rate calculation.
icorr to mm/year Calculator (Faraday’s Law)
- Faraday’s law links electric charge to mass of metal dissolved, so icorr gives corrosion rate directly.
- CR in mm/year equals 3.27 x 10^-3 times icorr (µA/cm²) times EW divided by density.
- For carbon steel, 1 µA/cm² is about 0.0116 mm/year, or 0.46 mils per year.
- icorr can come from Tafel extrapolation or from polarisation resistance using the Stern-Geary relation.
- The result is an instantaneous uniform-corrosion rate and says nothing about pitting.
What Does an icorr to mm/year Conversion Do?
Faraday’s law equates corrosion current to metal dissolved, giving penetration rate from icorr.
Corrosion is an electrochemical reaction: metal atoms give up electrons at anodic sites while a cathodic reaction consumes them. The rate at which electrons flow is the corrosion current, and dividing by the area gives the corrosion current density icorr. Because each metal atom dissolved releases a known number of electrons, the current tells the amount of metal lost. Converting that mass loss to a thickness loss with the density gives the penetration rate that engineers use to judge wall loss and life.
The method is standardised in ASTM G102 and complements gravimetric methods. Electrochemical measurements give a rate in minutes or hours, which is why they are used for monitoring and screening, while coupons average over weeks or months. The gravimetric route and its arithmetic are covered by the weight-loss calculator linked earlier.
How Does Faraday’s Law Give a Corrosion Rate?
Faraday’s law states that the mass dissolved is proportional to the charge passed: m = Q M / (n F), where M is the atomic weight, n the number of electrons per atom and F is the Faraday constant, 96 485 C/mol. Dividing the charge rate by area and the mass by density gives a thickness loss rate. Collecting the unit conversions into one constant gives the working form.
Checking the constant from first principles for iron: 55.845 / (2 x 96 485 x 7.87) is 3.68 x 10^-5 cm of thickness per C/cm2 of charge. One year at 1 uA/cm2 passes 31.6 C/cm2, so the loss is 1.16 x 10^-3 cm, or 0.0116 mm, which matches the standard’s formula. Because the equation is linear in icorr, a ten-fold rise in current density gives a ten-fold rise in penetration rate.
How Do You Use the icorr to mm/year Calculator?
Start from a measured corrosion current density, or from a polarisation resistance if icorr has not been derived. Then:
- Choose whether to enter icorr directly or to calculate it from the polarisation resistance Rp and the Stern-Geary constant B.
- Enter icorr with its unit, or Rp in ohm.cm2 and B in mV.
- Select the metal to fill the equivalent weight and density, or enter your own values for an alloy.
- Optionally enter a wall allowance in mm to get the years needed to consume it.
- Press Calculate, or press Enter in any field, then press Reset to start a new case.
The result panel lists the penetration rate in mm/year, mils per year and micrometres per year, the mass loss rate and, if a wall allowance is entered, the years needed to consume it. For related tools, see the Calculators hub, which includes the fatigue life calculator for assessing thinned sections.
Worked Example: Carbon Steel at 10 µA/cm²
A linear polarisation test on carbon steel in a neutral electrolyte gives an Rp of 2600 Ω·cm². Taking the Stern-Geary constant B as 26 mV for active corrosion, the example then converts icorr to a penetration rate and asks how long a 1.5 mm wall allowance lasts.
The same current density gives broadly similar penetration rates for most common metals because a lower equivalent weight is offset by a lower density. Magnesium is the exception, with a rate about double that of iron, and the cleaner way to compare metals is therefore through the rate rather than the current.
How Is icorr Measured?
icorr is not measured directly; it is derived from the response of the metal to a small potential or current perturbation. The common routes are:
- Tafel extrapolation. Polarise well away from the corrosion potential and extrapolate the straight Tafel regions back to Ecorr; the intersection gives icorr.
- Linear polarisation resistance (LPR). Polarise within about 10 to 20 mV of Ecorr, measure Rp = dE/di, and calculate icorr = B / Rp.
- Electrochemical impedance spectroscopy. The low-frequency polarisation resistance is used in the same Stern-Geary relation.
The constant B equals ba bc / (2.303 (ba + bc)), where ba and bc are the anodic and cathodic Tafel slopes. Where the slopes are unknown, B is commonly taken as 26 mV for actively corroding steel and 52 mV for passive metals, which introduces an uncertainty of up to a factor of two. The potential-measurement practice is set out in ASTM G59, and the step-by-step data treatment is in the tutorial linked earlier. Coupled with the galvanic corrosion risk estimator, the same icorr can also be used to rank dissimilar-metal joints.
How Do You Find the Equivalent Weight of an Alloy?
For a pure metal, the equivalent weight is the atomic weight divided by the valence of the dissolution reaction. For an alloy, ASTM G102 weights each element by mass fraction and valence:
The valence assumed for each element matters: chromium can dissolve as Cr3+ or Cr6+, and iron as Fe2+ or Fe3+. The standard allows elements below about 1 mass per cent to be ignored. Using the lowest plausible valence gives the highest equivalent weight and thus the highest, most conservative, penetration rate. Densities for engineering alloys are available from the supplier or from handbooks, and the way grain boundary segregation and chemistry affect local behaviour is discussed in grain boundaries in steel.
What Are the Limits of the Conversion?
Common mistakes
- Mixing units. The constant 3.27 x 10^-3 requires icorr in uA/cm2. Convert A/m2 by multiplying by 100.
- Using the wrong equivalent weight. An alloy needs the weighted EW, not the atomic weight of the base metal.
- Using a default B without checking. The Stern-Geary constant can change the rate by a factor of two.
- Ignoring solution resistance. Uncompensated resistance inflates Rp and lowers the apparent rate.
- Extrapolating a single reading. One short test does not represent months of service.
Range of validity
The conversion assumes that all the current goes to uniform dissolution at one oxidation state. It does not describe pitting, crevice attack, stress corrosion cracking or erosion, where local rates greatly exceed the average. High-temperature scaling follows different kinetics, as covered in the guide to high-temperature oxidation. For pressure equipment, the measured thickness and t-min decide fitness, and the pressure vessel wall thickness calculator gives the minimum thickness itself. Where a structure is protected cathodically, the design questions are covered in cathodic protection for offshore structures and the sacrificial anode materials comparison.
Quick Reference
| Metal | Reaction | EW, g/eq | Density, g/cm³ | mm/year per 1 µA/cm² |
|---|---|---|---|---|
| Iron | Fe to Fe2+ | 27.92 | 7.87 | 0.0116 |
| Copper | Cu to Cu2+ | 31.77 | 8.96 | 0.0116 |
| Zinc | Zn to Zn2+ | 32.69 | 7.14 | 0.0150 |
| Aluminium | Al to Al3+ | 8.99 | 2.70 | 0.0109 |
| Nickel | Ni to Ni2+ | 29.35 | 8.91 | 0.0108 |
| Magnesium | Mg to Mg2+ | 12.15 | 1.74 | 0.0228 |
| µA/cm² | mA/m² | A/m² |
|---|---|---|
| 1 | 10 | 0.01 |
| 10 | 100 | 0.1 |
| 100 | 1000 | 1 |
Key Terms
- Corrosion current density (icorr)
- The anodic dissolution current per unit area at the corrosion potential, usually in uA/cm2.
- Faraday’s law
- The relationship stating that mass dissolved is proportional to the charge passed.
- Equivalent weight (EW)
- The mass of metal dissolved per mole of electrons, equal to atomic weight divided by valence for a pure metal.
- Penetration rate
- The rate at which the metal thickness is lost, usually in mm/year or mils per year.
- Polarisation resistance (Rp)
- The slope of the potential-current curve at the corrosion potential.
- Stern-Geary constant (B)
- The constant that converts polarisation resistance to corrosion current density, from the Tafel slopes.
- Tafel slope
- The change in potential per decade of current in the linear region of a polarisation curve.
Frequently Asked Questions
How do you convert icorr to mm/year?
Multiply icorr in microamps per square centimetre by the equivalent weight in grams per equivalent, divide by the density in grams per cubic centimetre, and multiply by 3.27 x 10^-3. The result is the penetration rate in millimetres per year.
What is the corrosion rate of steel at 1 uA/cm2?
For carbon steel dissolving as Fe2+, with an equivalent weight of 27.92 and a density of 7.87 g/cm3, 1 uA/cm2 corresponds to about 0.0116 mm/year, or about 0.46 mils per year.
What is equivalent weight in corrosion calculations?
It is the mass of metal dissolved per mole of electrons. For a pure metal it is the atomic weight divided by the valence of the dissolution reaction. For an alloy it is a mass-fraction-weighted value defined in ASTM G102.
How do you get icorr from polarisation resistance?
Use the Stern-Geary relation, icorr = B / Rp, where B = ba bc / (2.303 (ba + bc)) from the Tafel slopes. If the slopes are unknown, B is commonly taken as 26 mV for active and 52 mV for passive metals.
Why do different metals give similar mm/year at the same icorr?
The lower equivalent weight of some metals is offset by a lower density, so thickness loss per unit charge is similar for iron, copper, nickel and aluminium. Magnesium and zinc differ more noticeably.
Is an electrochemical corrosion rate the same as a coupon rate?
Not necessarily. Electrochemical methods give an instantaneous rate, while coupons give an average over the exposure time. Differences arise from changing conditions, non-uniform attack and measurement assumptions.
Can the calculator predict pitting rates?
No. The conversion assumes uniform dissolution. Pitting rates are far higher locally and require separate measurement of pit depth.
Conclusion
Faraday’s law turns a corrosion current into a penetration rate with one equation: multiply icorr in microamps per square centimetre by the equivalent weight, divide by the density and apply 3.27 x 10^-3 to get millimetres per year. The arithmetic is simple; the quality of the answer rests on the icorr measurement, the Stern-Geary constant and the equivalent weight chosen for an alloy. Treat the result as an instantaneous uniform-corrosion rate and confirm it against thickness or coupon data before using it to set life. For the wider set of tools and background, see the corrosion science category.
About This Guide
This guide is published by the MetallurgyZone editorial team. The formulae follow the standard Faraday’s-law relationships used in electrochemical corrosion testing, and the calculator has been tested against the worked example on this page. It is educational material and does not replace the relevant standard or a qualified corrosion engineer’s judgement.
Standards and References
- ASTM G102, Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, ASTM International (astm.org).
- ASTM G59, Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements, ASTM International.
- ASTM G3, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, ASTM International.
- ASM Handbook, Volume 13A: Corrosion: Fundamentals, Testing, and Protection, ASM International (asminternational.org).
- M. Stern and A. L. Geary, Electrochemical Polarization: I. A Theoretical Analysis of the Shape of Polarization Curves, Journal of the Electrochemical Society, 1957.