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Erosion-corrosion: what it means for the inspector

Long Seam editorial teamSources checked 3 min read

Erosion is metal loss caused by fluid or particles hitting a surface. Erosion-corrosion is the same wear combined with corrosion: moving fluid strips the protective film or scale, bare metal corrodes, a new film forms, and the cycle repeats. The result is local, often smooth wear where flow is fast or turbulent.

Pipe elbow in section with fluid turning from left to downward, and the inside face of the outer wall of the bend cut back where the flow strikes it, marked with a dashed outline
The flow is thrown against the outside of the bend, which is where the wall thins.

How it works

Pure erosion is mechanical. Liquid droplets, solid particles or high-speed flow knock metal off the surface. The effect grows with velocity, with particle content and with how sharply the flow is forced to change direction.

Erosion-corrosion adds chemistry. Many metals survive in a corrosive fluid because they build up a protective scale or film. Fast or turbulent flow keeps tearing that film away, so the fresh metal under it keeps corroding. The wall wears faster than either erosion or corrosion alone would manage. The surface is often smooth, with horseshoe-shaped or scalloped patterns, and the damage follows the flow direction.

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Where it shows and how it is found

Look for places where the flow speeds up, changes direction or becomes turbulent.

  • Bends and elbows, especially on the outside of the turn.
  • Inlet nozzles and the surface opposite them, where incoming flow strikes.
  • Downstream of valves, orifices and reducers, where turbulence is high.
  • Internals, such as baffles and tube ends in exchangers.

Because the loss is local, a single thickness reading can miss the worst spot. Inspectors use thickness grids or scanning, and visual examination of internals when the equipment is open. Trending the same grid points over time shows how fast the wall is moving. Our corrosion rate calculator and the thickness survey analyzer take a set of readings and give you a rate and remaining life.

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Mix-ups to avoid

  • Erosion versus erosion-corrosion. If the fluid has no corrosive effect on the metal and the loss is purely from impact, that is erosion. If removing the protective film is what lets the corrosion happen, it is erosion-corrosion. On the sheet the two share one paragraph.
  • Erosion-corrosion versus general corrosion. General corrosion is broad and slow-moving; erosion-corrosion is local and tied to a flow pattern.
  • Erosion-corrosion versus naphthenic acid corrosion. Naphthenic acid attack is also velocity-sensitive, but it needs a hot oil with the acid, while erosion-corrosion can occur in many services.

On the API exam

API's effectivity sheet for September 2026 to May 2027 limits RP 571 questions to Section 2 plus 15 mechanism paragraphs. Erosion and erosion-corrosion is RP 571 paragraph 3.27 and is one of the 15. A typical stem describes a thin spot at a nozzle, bend or downstream of a restriction and asks for the mechanism, or asks which examination approach suits a local loss pattern. The method point matters as much as the name: spot readings can understate the damage next to them, which is why grids and scans are the usual approach.

Practice it

Turn a set of readings into a rate with the thickness survey analyzer. Compare it with General vs localized corrosion and Naphthenic acid corrosion, or browse every term in the damage mechanism glossary.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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