Corrosion Rate Calculator
Find how fast a metal is corroding — from a weight-loss coupon, a measured thickness loss, or an electrochemical current — in mpy, mm/year and µm/year. Then get the answer that matters: how long until the wall reaches its corrosion allowance, with a pass/fail against your required service life.
From Corrosion Rate to Remaining Life
Measuring how fast a metal corrodes is only half the job. A rate of 4 mils per year means nothing on its own — what an engineer needs to know is when a 0.25-inch pipe wall drops below its safe minimum. This calculator computes the rate three ways, then turns it into the answer that matters: how long until the corrosion allowance is gone, with a pass or fail against your required service life.
The Corrosion Rate Formula
The weight-loss method weighs a coupon before and after exposure; with mass loss in grams, area in cm², time in hours and density in g/cm³, K = 87,600 gives mm/year and 3.45 million gives mpy. The thickness-loss method simply divides a measured wall loss by the time over which it happened. The electrochemical method converts a measured corrosion current density to a rate through Faraday’s law: CR (mm/yr) = 0.00327 × icorr × EW / density, with icorr in microamps per cm² and EW the equivalent weight.
MPY vs mm/year Conversion
Mils per year (mpy, thousandths of an inch) is the traditional unit in oil, gas and process industries; mm/year is the metric standard. They convert directly: 1 mpy = 0.0254 mm/year, and 1 mm/year = 39.37 mpy. Micrometres per year (1000 times mm/year) suits very slow corrosion. This tool reports all three at once.
| mpy | mm/yr | Rating |
|---|---|---|
| < 1 | < 0.025 | Outstanding |
| 1 – 5 | 0.025 – 0.13 | Excellent |
| 5 – 20 | 0.13 – 0.5 | Good |
| 20 – 50 | 0.5 – 1.3 | Fair |
| 50 – 200 | 1.3 – 5 | Poor |
| > 200 | > 5 | Unacceptable |
Corrosion Allowance Explained
A corrosion allowance is the extra wall thickness a designer adds on purpose — sacrificial metal meant to be eaten away over the design life while the load-bearing minimum stays intact. A vessel might carry a 3 mm allowance on top of the thickness the pressure actually requires. The part is considered to reach end of life not when it perforates, but when corrosion consumes that allowance and starts into the structural minimum. Divide the allowance by the corrosion rate and you get the service life; compare it to the life you need, and you have a straight pass or fail.
Reference Notes
These calculations follow the spirit of ASTM G1 and G31 for coupon testing and the usual NACE/AMPP relative-resistance bands. They give an average uniform rate. Localized forms — pitting, crevice corrosion, stress-corrosion cracking and erosion-corrosion — can perforate a wall while the average rate still looks mild, so uniform-rate life is a best case. The temperature factor reflects the rule of thumb that reaction rates roughly double for every 10 °C rise; enter your own multiplier where data exists.
Frequently Asked Questions
How do you calculate remaining life from corrosion rate?
Divide the remaining corrosion allowance (or wall above the minimum) by the corrosion rate. At 0.4 mm/year, a 3 mm allowance lasts about 7.5 years. Enter a required life and the tool flags pass or fail.
What is a good corrosion rate?
Under about 5 mpy (0.13 mm/year) is good to excellent for carbon steel; under 1 mpy is outstanding. But a low rate can still fail if the wall is thin or the required life is long – which is why remaining life matters more than rate alone.
How do I convert mpy to mm/year?
Multiply mpy by 0.0254. So 10 mpy equals 0.254 mm/year. The calculator shows mpy, mm/year and micrometres per year together.
Does this account for pitting?
No. All three methods give an average uniform rate. Pitting and crevice corrosion can penetrate far faster, so treat the life estimate as optimistic and rely on inspection for localized attack.
What is equivalent weight in the electrochemical method?
The atomic weight divided by the number of electrons exchanged – about 27.9 for iron, 9.0 for aluminium. It sets how much metal a given current dissolves.
Related calculators
- Material Weight Calculator — mass of the section being lost.
- Factor of Safety Calculator — margin once the wall has thinned.
- Thermal Stress Calculator — stress from temperature change.
- Hardness Conversion Calculator — hardness and strength of the alloy.