Corrosion rate and remaining life
Enter your dated thickness readings for a single measurement point. The tool calculates both rates, the remaining margin and the due date. It refuses to conclude when the data does not allow it.
One measurement point at a time. Everything is calculated in your browser: no data is sent.
Readings
| Date | Thickness (mm) | Remove |
|---|
Why two rates and not one
The long-term rate is calculated between the first and the last reading: it describes the underlying behaviour and copes well with measurement uncertainty. The short-term rate is calculated between the last two readings: it reacts fast, including to noise.
The gap between the two is the useful signal. A markedly higher short-term rate signals an acceleration that the long average dilutes. That is why the tool keeps the more unfavourable of the two to estimate the due date.
What the tool refuses to do
It does not calculate a due date below three readings: two points always define a straight line, and the slope obtained may be no more than a measurement artefact. It flags rising thicknesses instead of smoothing them: metal does not grow back, a rise indicates a different point, a different technique or a data-entry error.
Finally, it replaces no decision: fitness for service and requalification remain acts that carry a signature. For the full method, see calculate a corrosion rate and estimate remaining useful life.
Definition
The corrosion rate is the loss of thickness over time at an identified measurement point, expressed in millimetres per year. Two rates are computed from the same readings: the long-term rate, between the first reliable reading and the latest one, which describes the underlying behaviour; the short-term rate, between the two latest campaigns, which captures the current regime and reveals an acceleration that the average dilutes. The rule is to keep the more unfavourable of the two when estimating a deadline, and to examine the equipment as soon as the short-term rate clearly exceeds the long-term one. A rate always applies to a point, never to a whole equipment.
Formula
Corrosion rate = (thickness1 - thickness2) / (date2 - date1), in mm per year, thickness1 being the older reading and thickness2 the most recent, at the same measurement point. Remaining life = (current thickness - minimum allowable thickness) / rate used. The minimum allowable thickness is not produced by the tool: it is the responsibility of the technical manager or of the approved body and is used as an input.
How to use this tool
- Gather dated readings taken at the same identified point, with its stable tag: comparing campaign minima that come from different points produces nothing usable.
- Have at least three campaigns: two values always define a straight line, and the slope obtained may be no more than a measurement artefact.
- Compute both rates, long term between the first and the last reading, short term between the two latest campaigns, and display them side by side.
- Keep the more unfavourable of the two, and verify before concluding when the gap is wide: a change of technique, a mismatched point or a data-entry error usually come before a real acceleration.
- Apply a margin to the result and set it against the inspection plan: the duration obtained sets a schedule, it replaces neither the regulatory interval nor the inspector's judgement.
Worked example
At point A of a pipe, the thickness read is 12.0 mm in 2019 and 10.5 mm in 2024. The long-term rate is (12.0 - 10.5) / (2024 - 2019) = 1.5 / 5 = 0.30 mm per year. With a minimum allowable thickness of 8.0 mm, the remaining margin is 10.5 - 8.0 = 2.5 mm, that is 2.5 / 0.30 = 8.3 years, about 8 years at this pace. If an intermediate campaign in 2022 gave 11.4 mm, the short-term rate would be (11.4 - 10.5) / 2 = 0.45 mm per year, and the same margin would cover only 2.5 / 0.45 = 5.6 years. It is this second value, the more unfavourable one, that must be kept.
References
- API 570, in-service piping inspection code: calculation of short-term and long-term corrosion rates from successive readings, and derivation of the remaining life.
- Order of 20 November 2017 on the in-service monitoring of pressure equipment: the regulatory deadlines apply independently of any remaining-life estimate (Legifrance).
Frequently asked questions
Should the long-term or the short-term rate be used?
The more unfavourable of the two. The long-term rate smooths out the equipment's history, the short-term rate captures what is happening right now, and it is the one that raises the alarm when a process has changed.
How many readings are needed?
Three at the very least for a defensible estimate. With two points, the measurement uncertainty is indistinguishable from the degradation.
Is the duration obtained a service life?
No. It is the time remaining before the minimum allowable thickness is reached if the degradation continues at the same rate. An extrapolation, not a forecast, and still less an authorisation to operate.