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.
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.
Next: remaining-life and inspection-interval calculator → Enter the required thickness from your approved engineering basis; these tools do not calculate ASME minimum thickness.