Short-term vs long-term corrosion rate: what it means for the inspector

How each rate is worked out

Both rates are the same calculation: thickness lost divided by time. The difference is which two readings you use.

• Long-term rate: (first or initial thickness minus current thickness) divided by the years between those two readings.
• Short-term rate: (previous thickness minus current thickness) divided by the years between the last two readings.

Use the same location for every reading and the same units throughout. Thickness readings taken at different points do not give a rate.

A worked example (invented numbers)

A shell location read 0.500 in. when new, 0.440 in. at the first inspection 7.0 years later, and 0.367 in. at the second inspection, 5.0 years after that. The required thickness is 0.300 in.

• Long-term rate: (0.500 - 0.367) / 12.0 = about 0.0111 in. per year.
• Short-term rate: (0.440 - 0.367) / 5.0 = 0.0146 in. per year.

The short-term rate is higher, which tells you the corrosion has sped up recently. The loss available before the required thickness is 0.367 - 0.300 = 0.067 in. At the short-term rate that is 0.067 / 0.0146 = about 4.6 years. At the long-term rate it is 0.067 / 0.0111 = about 6.0 years. Same wall, different answers, which is why the choice matters.

Which one to use

The usual reason for comparing the two is to see whether something has changed. A short-term rate well above the long-term rate suggests the damage is accelerating, perhaps from a process change. A short-term rate below the long-term rate may mean the damage has slowed, or that an old high rate no longer applies, and a lower recent rate may not be reliable if the readings are close together and measurement error is large. Choosing between them is a judgment call that needs process knowledge, not a fixed formula. API 510 paragraph 7.1 covers how the rate is chosen, so read it in your own copy of the code.

Be careful when a process change, a repair or a new coating splits the history. A reading from before the change may not describe the equipment after it.

Mix-ups to avoid

• Rate versus remaining life. The rate is a slope. Remaining life uses the slope to say how long the margin lasts.
• Short-term rate from too-close readings. Two readings a few months apart can swing wildly from measurement scatter alone.
• Mixing locations or units. Use one location and one unit system.

On the API exam

Corrosion rate is a calculation area in the Body of Knowledge, with API 510 paragraph 7.1 the starting point. Calculation stems often give you three readings and ask for one of the rates, or ask which rate best reflects the process. Read what the stem asks for, and work in the units it gives you. Practise on the corrosion rate calculator and the API 510 calculations guide.

Practice it

Check your arithmetic with the corrosion rate calculator. Compare it with Remaining life and Corrosion allowance vs minimum thickness, or browse every term in the damage mechanism glossary.

Know the mechanisms? Check the rest of the Body of Knowledge

Take the free 10-question check. It is spread across the Body of Knowledge, needs no signup and explains every answer, so you see which domains to open first. The Long Seam course then adds a lesson for each domain, timed practice exams and a study plan from your exam date.

Sources

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• API 510 Exam Publications Effectivity Sheet, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Body of Knowledge, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Pressure Vessel Inspector: exam information, American Petroleum Institute
• API 510, Pressure Vessel Inspection Code, section 7 (evaluation of inspection findings). Cited by number only; read it in your own copy.