General vs localized corrosion: what it means for the inspector

How they differ

General (uniform) corrosion spreads over the whole surface. The thickness is lower everywhere by about the same amount. Because the loss is even, a handful of thickness readings is a good picture of the whole, and a straight-line rate through the readings is a fair way to predict where the wall is going.

Localized corrosion is concentrated. Pitting makes small deep holes. Grooving follows a weld or a flow path. Patches or local thin areas form where something special happens, such as a leak, a deposit or a flow pattern. Most of the surface can be close to its original thickness while a small spot is nearly through.

Why it changes the inspection

• Where you read matters more. With general loss, a few readings are representative. With localized loss, readings that miss the bad spot give a false sense of safety. Grids, scans and visual examination of the surface are used to find the worst point.
• Which number you use. For general loss you can use the average thickness. For a localized area, the thinnest point in that area is what counts, and the code has separate rules for judging local thin areas and pitting that you read in your own copy.
• Rate prediction. Pits can grow much faster than the general loss rate, so a low average rate is not a promise about the deepest pit.
• The cause. General loss points to a broad cause such as a corrosive process fluid. Local loss points to something specific: a flow pattern, a deposit, an interface, a weld or microbial activity.

Mix-ups to avoid

• General versus atmospheric corrosion. Atmospheric corrosion is a cause (exposure to weather). It can look general or local. General versus local describes the shape of the damage, not the cause.
• Pitting versus cracking. A pit is a small hole with metal loss. A crack is a thin planar break with almost none. They need different methods.
• Local versus uniform erosion. Wear at a bend, as in erosion-corrosion, is local by design.

On the API exam

This is a way of thinking about damage, not one RP 571 paragraph. It shows up in how questions use the words thinning, pitting and local thin area, and in calculation stems that give you several readings and ask which one to use. The habit to build is to look at the pattern first (even or local), then choose the method and the thickness that match.

Practice it

Try a set of readings in the thickness survey analyzer and see how a single bad point changes the rate. Compare it with Erosion-corrosion and Microbiologically influenced corrosion, 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 RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd edition, March 2020. Read the matching paragraph in your own copy.