Corrosion Rate Calculator
Find metal corrosion rate in mpy and mm/yr from weight loss data or from electrochemical corrosion current density.
🦀 What is the Corrosion Rate Calculator?
This corrosion rate calculator finds how fast a metal loses thickness to corrosion, reported in mils per year (mpy) and millimetres per year (mm/yr). Use Weight Loss mode when you have coupon test data (mass lost, exposed area, and exposure time), following the ASTM G1 standard practice. Use Electrochemical mode when you have a measured corrosion current density from a Tafel extrapolation or linear polarization resistance test, following the ASTM G102 conversion.
Corrosion rate calculations are used constantly in pipeline integrity management, materials selection for chemical plants, marine and offshore structure design, and failure analysis. An engineer choosing between carbon steel and stainless steel for a tank, for instance, needs a real corrosion rate number, not just a qualitative "steel rusts" judgment, to size a corrosion allowance or set an inspection interval.
A common misconception is that weight-loss and electrochemical methods measure fundamentally different things. In practice they estimate the same physical quantity (average metal penetration rate) through different experimental routes: weight loss integrates mass loss over a real exposure period (days to months), while the electrochemical method infers the same rate instantaneously from measured reaction kinetics.
This calculator is useful for corrosion engineers, materials scientists, and students who need a fast, correctly-unit-converted corrosion rate without hand-calculating ASTM G1 or G102 conversion factors.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - Weight loss, mild steel coupon after 30 days
Example 2 - Weight loss, copper coupon after 90 days
Example 3 - Electrochemical, steel from linear polarization test
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate corrosion rate?
Corrosion rate can be calculated from weight loss data using CR (mpy) = 3.45x10^6 x W / (A x T x D), where W is mass lost in grams, A is exposed area in cm2, T is exposure time in hours, and D is metal density in g/cm3. This is the ASTM G1 weight-loss method.
What is mpy in corrosion?
Mpy stands for mils per year, a common corrosion rate unit meaning thousandths of an inch of metal thickness lost per year. It is widely used in North American corrosion engineering; the metric equivalent is mm/yr, where 1 mpy = 0.0254 mm/yr.
What is the ASTM G102 corrosion rate formula?
ASTM G102 converts electrochemical corrosion current density to a penetration rate: CR (mm/yr) = 3.27x10^-3 x i_corr x EW / D, where i_corr is in microamps per square centimetre, EW is equivalent weight in grams per equivalent, and D is density in g/cm3.
What is a good corrosion rate?
Per common NACE/ASTM severity ratings, a corrosion rate below 0.1 mm/yr (about 4 mpy) is considered excellent, 0.1 to 0.5 mm/yr is good, 0.5 to 1 mm/yr is fair, and above 1 mm/yr (about 40 mpy) is generally considered poor for most structural applications.
What is equivalent weight in corrosion calculations?
Equivalent weight (EW) is the mass of metal that reacts with one mole of electrons during corrosion, equal to atomic weight divided by valence for a pure metal (for example, iron corroding as Fe2+ has EW = 55.85/2 = 27.92 g/eq). For alloys, EW is a weighted average across all elements.
How is corrosion current density measured?
Corrosion current density (i_corr) is typically measured with a Tafel extrapolation, where the linear regions of an anodic and cathodic polarization curve are extrapolated back to the corrosion potential, or with linear polarization resistance (LPR), a faster, less destructive technique.
Why does temperature affect corrosion rate?
Higher temperature speeds up both the electrochemical reaction kinetics and diffusion of reactants and products, generally increasing corrosion rate. In some systems, however, reduced oxygen solubility at high temperature can partially offset this effect.
What is the difference between uniform and localized corrosion rate?
A calculated corrosion rate from weight loss or i_corr describes uniform, general attack averaged across the whole exposed surface. Localized corrosion, such as pitting or crevice corrosion, can penetrate far faster at specific points even when the average calculated rate looks low.
How do I convert mpy to mm per year?
Multiply mpy by 0.0254 to get mm/yr, since 1 mil equals 0.0254 mm. This calculator reports both units automatically from the same weight loss or electrochemical input, so no separate conversion step is needed.
Does exposure time affect the calculated corrosion rate?
In the weight-loss method, corrosion rate is inversely proportional to exposure time for a given amount of mass lost, so longer exposure with the same total mass loss gives a lower calculated rate. Real corrosion rates can also change over time as protective scales form or breakdown occurs.