API 570 Calculations: Corrosion Rate and Remaining Life

Where these calculations sit in the exam

API's Body of Knowledge says calculation questions are about existing, in-service piping and that you must work in both US customary and SI units. It points to API 570 7.1.2 for corrosion rates, 7.2 for remaining life, 6.3.3 and Table 1 for intervals, and 7.5 for the MAWP of corroded pipe. It adds that the formulas and interval rules for rate, remaining life and intervals may be closed-book, so expect to run them at 1.5 minutes a question in the first part. See the Body of Knowledge article for the full list.

We give you methods, not code text. Keep your own copies of API 570 and B31.3 open when you check the paragraph numbers.

Move 1: corrosion rate, long-term and short-term

A corrosion rate is thickness lost divided by time between readings. Two versions matter:

• Long-term rate: from the first reading to the latest.
• Short-term rate: from the previous reading to the latest.

Worked example (invented). A piping thickness monitoring location read 0.280 in. in 2019, 0.262 in. in 2022 and 0.249 in. in 2025.

• Long-term: (0.280 minus 0.249) divided by 6 years = 0.0052 in./yr (0.005167 before rounding).
• Short-term: (0.262 minus 0.249) divided by 3 years = 0.0043 in./yr (0.004333).

Two rates, and the stem or your judgment picks one. If the process changed in 2022, the short-term rate better reflects the pipe today. If conditions have been steady, the long-term rate is the safer average of a longer history. The exam tells you what to use or gives you enough to decide. Read the stem for the words "long-term" and "short-term" before you touch the calculator. The corrosion rate calculator checks your hand work.

Move 2: remaining life

Remaining life is the margin above the minimum required thickness, divided by the rate.

Continuing the example. The required thickness is 0.190 in.

• Margin: 0.249 minus 0.190 = 0.059 in.
• With the long-term rate: 0.059 divided by 0.005167 = 11.4 years.
• With the short-term rate: 0.059 divided by 0.004333 = 13.6 years.

The gap between 11.4 and 13.6 years is the cost of choosing the rate carelessly. The inspection interval is then set from remaining life under the rules in API 570 (6.3.3 and Table 1). Look the interval rule up in your own copy, and note which version you reached for. The remaining life calculator will confirm the arithmetic, and our vessel calculations guide shows the same setup written for pressure vessels.

Watch for three traps:

• Units. A stem may give millimeters. 0.249 in. is 6.32 mm. Convert once, at the start, and stay in one system.
• Required thickness versus nominal. Use the minimum required thickness the stem names, not the nominal wall.
• Dates. Count calendar years between readings, not the number of readings.

Move 3: project the wall to find the MAWP

API 570 7.5 deals with the MAWP of corroded pipe, and the Body of Knowledge says to compensate for the corrosion expected by the next inspection. The idea: the pipe has to stay safe until you look at it again, so you rate it on the wall it will have then, not the wall it has today.

Continuing the example. The next inspection is planned in 5 years.

• Expected further loss: 0.0052 times 5 = 0.026 in.
• Projected thickness: 0.249 minus 0.026 = 0.223 in.

Now you take 0.223 in. into the pressure formula for straight pipe in B31.3 (the BOK points to 304.1.1 and 304.1.2). The formula is in your copy of the code, and it also needs the allowable stress, the weld joint quality factor and the pipe diameter. The step the exam is checking is that you used the projected thickness, and any corrosion allowance the stem tells you to add or subtract. If a question says "at the next inspection", this is the move.

If your projected thickness had fallen below 0.190 in. you would not have a pipe that reaches the next inspection, which is a different answer from "the pipe is fine today."

Move 4: the supporting factors

Other BOK items show up inside these same problems rather than alone:

• Weld joint quality factor. It enters the required thickness formula, so a longitudinal seam with more NDE can change the answer.
• Thermal expansion. The total expansion between two temperatures comes from a table lookup, then a multiplication by length. Do the lookup first and carry units.
• Test pressure. Hydrostatic and pneumatic test pressures use different factors. Name which test the stem asks for before you compute.

Our API 570 practice questions article has original items that use these.

A routine that sticks

Write ten fresh versions of the corrosion example each week, changing one input each time: a different year, a different required thickness, a switch to millimeters. Time yourself at 1.5 minutes each and log any miss as a method gap or an arithmetic slip. Ten different problems beat one repeated twenty times. Our study plan before 26 November schedules this block.

Next step

Take the free 10-question check at /api-570#q1 to see whether your calculation misses are method or speed. Then see the plans at /api-570/offer#plans, and ask the study buddy at /study-assistant for a plan built around your exam date.

FAQ

Can I use a calculator on the API 570 exam?

API's tutorial and policies say what is allowed at the test center, so read them before exam day. Practice with a basic calculator and no notes, since notes and books are not allowed in the room.

Do I need to memorize the corrosion rate formula?

Yes. The Body of Knowledge says the formulas for corrosion rate, remaining life and intervals may be closed-book, so plan to know them without a book.

Which corrosion rate should I use, long-term or short-term?

Use the one the stem names, or the one that best reflects current conditions if you are left to choose. Check whether the process changed between readings before deciding.

Is the MAWP formula on the closed-book part?

The Body of Knowledge does not say. Treat it as a lookup in B31.3 and practice finding it in your own copy quickly, while knowing the projected-thickness step by heart.