How the corrosion rate calculator works
Enter one row per CML: the initial thickness and date, the previous reading and date, and the actual (latest) reading and date. The initial value can be a measured baseline or the nominal wall with its in-service date. The calculator divides each thickness loss by the exact time between the dates and gives you two rates. The long-term rate runs from the initial reading to the actual one and averages the whole history. The short-term rate uses only the last two readings and shows what the wall did lately. The higher nonnegative rate is flagged as the candidate governing rate, a negative or unusually high rate gets a warning, and the CSV keeps the whole survey.
The rate is half the job. Remaining life is the metal left above the required thickness divided by the rate you choose, and the remaining life calculator picks up with your rate filled in. For the reasoning behind each step, read short-term vs long-term corrosion rate, remaining life and corrosion allowance vs minimum thickness.
Print a corrosion rate record
Press Print CML record under the results to get one page with every CML: the three readings and their dates, the short-term and long-term rates, the candidate rate and any warnings. It has lines for the equipment or circuit, the inspector, the date and the rate you chose with your reason, plus a link that reopens the same readings. Save it as a PDF from the print dialog, file it with the inspection or work a practice set on paper for API 510 and API 570.
Corrosion rate calculation formula
Average corrosion rate is thickness lost divided by elapsed time. Treat each CML separately and compare readings from the same physical location using a consistent measurement procedure.
- Short-term rate = (t_previous − t_actual) / years between previous and actual readings.
- Long-term rate = (t_initial − t_actual) / years between initial and actual readings.
- Elapsed years = UTC calendar days / 365.25, the explicit date convention used by this tool.
- 1 mil = 0.001 in; mils/yr = in/yr × 1,000; mm/yr = mils/yr × 0.0254.
These are average thickness-loss slopes from the thickness-versus-time monitoring method discussed in AMPP / CORROSION, High-Accuracy Ultrasonic Corrosion Rate Monitoring. They measure past average loss; projecting a rate into the future assumes it remains representative. The long-term input may be a measured initial thickness or an explicitly assumed nominal thickness with an in-service date. Nominal wall variation can bias the estimate.
Worked example: inches and mils per year
For one fictional CML, the initial reading is 0.500 in on 1 January 2016, the previous reading is 0.430 in on 1 January 2022, and the actual reading is 0.400 in on 1 January 2026.
The short interval is 1,461 days / 365.25 = 4 years. Short-term rate = (0.430 − 0.400) / 4 = 0.0075 in/yr = 7.5 mils/yr = 0.1905 mm/yr.
The long interval is 3,653 days / 365.25 ≈ 10.001369 years. Long-term rate = (0.500 − 0.400) / 10.001369 ≈ 9.99863 mils/yr. The long-term rate is higher in this example. The inspector must decide whether that rate represents future service.
Which rate governs? API 510 and API 570 pointers
The higher nonnegative rate gives the shorter predicted remaining life for the same thickness margin. This tool identifies it as a candidate governing rate; the responsible inspector chooses the appropriate rate per the applicable code and service history.
API's public Body of Knowledge documents point to API 510 §7.1 and API 570 §7.1.2 for rate calculations, and §7.2 in both for remaining life. Use your own applicable edition. See the API 510 and API 570 section pointers; no code text or tables are reproduced.
Worked example in millimeters: rate to remaining life
A fictional shell CML read 12.70 mm at a measured baseline on 1 June 2012, 11.60 mm on 1 June 2020 and 10.90 mm on 1 June 2024. The required thickness from the design calculation is 9.00 mm. On the days ÷ 365.25 convention the intervals come out at exactly 12 and 4 years.
- Long-term rate = (12.70 − 10.90) / 12 = 0.150 mm/yr (5.91 mils/yr).
- Short-term rate = (11.60 − 10.90) / 4 = 0.175 mm/yr (6.89 mils/yr).
- Metal above the required thickness = 10.90 − 9.00 = 1.90 mm.
- Remaining life at the short-term rate = 1.90 / 0.175 ≈ 10.9 years. At the long-term rate it is 1.90 / 0.150 ≈ 12.7 years.
The short-term rate is higher, so the wall has sped up since 2020 and gives the shorter life. That is the conservative pick, and the inspector confirms it against the service history before planning around it. Enter the three readings above in millimeters and the calculator returns the same two rates.
Common corrosion rate mistakes
- Using the wrong pair of readings. Long-term starts at the initial reading and short-term starts at the previous one. Mixing a start thickness from one with a start date from the other gives a rate that matches neither.
- Mixing locations. A rate only means something at one CML. Subtracting a reading at one spot from a reading at another, or averaging CMLs together, hides the worst one.
- Unit slips. 0.0075 in/yr is 7.5 mils/yr and 0.1905 mm/yr. Typing mils into an inch field is 1,000 times off, and millimeters into an inch field is 25.4 times off.
- Rounding the time. Readings on 1 March 2022 and 1 November 2025 are 3.67 years apart, not 3. Calling it 3 years makes the rate about a fifth too high.
- Trusting a short-term rate from close readings. Measurement scatter of 0.1 mm over six months reads as 0.2 mm/yr. Give the short interval enough time to beat the scatter.
- Putting nominal where required belongs. Remaining life runs down to the required thickness, not the nominal or original wall. The corrosion allowance guide shows how the two stack up.
Negative and implausible rates
Corrosion does not create new base metal. Apparent metal growth produces a negative rate and is a measurement-error flag: check calibration, CML relocation, surface preparation, temperature, coating and repairs. The tool keeps the raw rate visible and withholds the candidate governing rate until the discrepancy is resolved.
An unusually high rate can be a unit or date error, or real accelerated loss. The editable review threshold is only a screening aid and has no code authority. The AMPP study discusses temperature effects and measurement repeatability; a small thickness difference over a short interval can magnify measurement error.
This is thickness-loss arithmetic for general thinning. It does not assess localized pitting, cracking, erosion profiles, interacting damage, creep or fitness for service. Zero observed loss does not establish zero future corrosion.
Once you have a rate, the inspection interval calculator turns it into a next inspection date and shows which limit governs.
Have a whole survey to work? The thickness survey analyzer runs every point at once and finds the governing one.
Next: remaining-life and inspection-interval calculator → Enter the required thickness from your approved engineering basis; these tools do not calculate ASME minimum thickness.
Need the required thickness for a straight pipe? Work it with the pipe minimum thickness calculator.
Corrosion rate FAQ
How do you calculate corrosion rate from thickness readings?
Divide the thickness lost by the years between two readings at the same CML. A wall that went from 11.60 mm to 10.90 mm over 4 years lost 0.70 mm, so it is corroding at 0.175 mm/yr, about 6.89 mils/yr. Work from the actual reading dates, not whole calendar years.
What is the difference between short-term and long-term corrosion rate?
The long-term rate runs from the initial thickness (or the nominal wall at the in-service date) to the latest reading, so it averages the whole service history. The short-term rate runs from the previous reading to the latest one, so it shows what the wall did in the last interval. A short-term rate well above the long-term rate says the corrosion has sped up.
Which corrosion rate do you use for remaining life?
Start from the higher of the two, because it gives the shorter, more conservative life. The inspector then picks the rate that best reflects how the equipment runs now, since a process change, a repair or a new lining can make one of the rates unrepresentative. The rate calculation sits in API 510 §7.1 and API 570 §7.1.2; read them in your own edition.
How do you calculate remaining life from corrosion rate?
Subtract the required thickness from the actual thickness and divide by the chosen rate. With 10.90 mm actual, 9.00 mm required and 0.175 mm/yr, that is 1.90 / 0.175, about 10.9 years. API 510 and API 570 both cover remaining life in §7.2.
How do you convert mils per year to mm per year?
Multiply by 0.0254. One mil is 0.001 in, so 1 mil/yr is 0.0254 mm/yr. Going the other way, divide by 0.0254: 0.2 mm/yr is about 7.87 mils/yr.
Is corrosion rate on the API 510 exam?
Yes. The API 510 Body of Knowledge lists corrosion rates and remaining corrosion allowance under §7.1 and remaining service life under §7.2, and the API 570 Body of Knowledge lists rates under §7.1.2 and remaining life under §7.2. Expect to be handed readings and dates and asked for a rate, the governing rate or the remaining life.
Does this calculator work for coupon weight-loss corrosion rates?
No, the ASTM weight-loss coupon method is a different lab method based on the mass a test specimen loses, while this calculator works from wall thickness readings taken on the equipment itself.