Corrosion Rate and Remaining Life Calculator (API 510 / 570)
Inspection engineers repeat the same calculation after every thickness survey: corrosion rate, remaining life and the date of the next inspection. This tool does it in one pass and lets you test how readings change the answer.
Your numbers
From the code calculation plus any structural minimum.
Often 10 years for pressure vessels and 5 years for some piping classes, per your inspection code and RBI plan.
Your results
Simple linear corrosion model. Short-term and long-term rates, localised pitting and measurement error must be assessed by an authorised inspector under the applicable inspection code.
Remaining life (years) vs measured thickness
Sweeps Latest measured thickness from 7.5 to 12.7, holding your other inputs fixed.
Trend inspection readings in the ERP
Optional review of how to store thickness survey data against asset records.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the earlier (or original) thickness, the latest ultrasonic reading, and the time in years between them. Add the retirement thickness, which is the greater of the code minimum for pressure design and any structural minimum, plus corrosion allowance policy. The primary result is the corrosion rate in millimetres per year and the remaining life until the wall reaches the minimum. The next inspection output uses the common rule of half the remaining life, capped by a maximum interval from your inspection plan.
Method
Corrosion rate is CR = (t_previous - t_actual) / years between readings. Remaining life is RL = (t_actual - t_required) / CR. Inspection codes such as API 510 for vessels and API 570 for piping set the interval at no more than half the remaining life, subject to a maximum time. Where several measurement pairs exist, inspectors compare the short-term rate with the long-term rate from original thickness and use the one that best represents expected behaviour.
- Use the same thickness measurement location each time.
- Short intervals amplify gauge error, so use long baselines.
- A rate above 0.125 mm per year (5 mpy) is usually a flag for review.
Worked example
A shell course was 12.7 mm when new and reads 10.9 mm six years later, so the long-term rate is 0.30 mm per year, 11.8 mpy. With a retirement thickness of 7.5 mm the margin is 3.4 mm, and the remaining life is about 11.3 years. Half of that is 5.7 years, so the next inspection is due within 5.7 years even though the vessel code allows up to 10.
Common mistakes
A single ultrasonic reading has an error of a few tenths of a millimetre, which can swing a short-interval rate wildly. Do not mix grid points. Do not extrapolate a localised pit as if it were general corrosion. Check that the retirement thickness uses the current design conditions and not the original ones. Where inspection data is stored in the ERP or maintenance system, keep the location ID, date and instrument calibration with each reading so trends are defensible.
Frequently Asked Questions
What is a high corrosion rate?
Rates under 0.05 mm per year (2 mpy) are generally considered low. Rates above 0.25 mm per year (10 mpy) are usually treated as high and call for review of the cause, material or operating conditions.
Why use half the remaining life?
Inspection codes such as API 510 and API 570 limit the interval to half the calculated remaining life, within a maximum time cap, so that measurement error and rate change are caught before the wall reaches its retirement thickness.
How do I convert mils per year to mm per year?
One mil is one thousandth of an inch, which is 0.0254 mm. Multiply mpy by 0.0254 to get mm per year, or divide mm per year by 0.0254 to get mpy. The calculator shows both.
What if the thickness grew between readings?
A negative loss is measurement scatter or a different location, not real growth. The calculator then reports a zero or negative rate and a very large remaining life. Re-take readings at the exact grid point before accepting it.
If your thickness readings sit in spreadsheets or a CMMS, we can help you pull them into the ERP asset record and trend corrosion by location.
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