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Boiler water and condensate corrosion: what it means for the inspector

Long Seam editorial teamSources checked 3 min read

Boiler water and steam condensate corrosion is metal loss in steam and condensate systems caused by dissolved gases and impurities in the water. Dissolved oxygen pits carbon steel, and dissolved carbon dioxide forms carbonic acid in condensate and thins the wall. It shows up in condensate return lines, feedwater equipment and similar parts.

Cross-section of a horizontal condensate return pipe with water in the bottom containing gas bubbles, and small pits on the bottom wall under the water
Dissolved gases in the water attack the wall under it, often as pits on the low side.

How it works

Boiler feedwater is treated to remove dissolved oxygen and to control chemistry. When treatment fails or is missing, two things go wrong.

  • Dissolved oxygen attacks carbon steel and produces pits. Oxygen is most aggressive where fresh feedwater, with its dissolved gas, meets hot metal, such as in feedwater heaters and economizers, and it causes localized attack rather than an even loss.
  • Dissolved carbon dioxide dissolves in condensate to make carbonic acid, which lowers the water's pH. Condensate that is acidic thins the wall of condensate return piping, often as a general thinning or as grooving along the bottom of the pipe, where the liquid flows.

Boiler water chemistry can also matter in other ways, such as deposits and carryover of impurities, but the oxygen and carbon dioxide pair is the one to hold on to.

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Where it shows and how it is found

  • Condensate return lines, especially low points and the bottom of horizontal runs.
  • Deaerators and feedwater equipment.
  • Steam traps and places where steam condenses, because condensate forms there.

Inspection usually combines several tools. Internal visual examination looks at the surface where access allows. UT thickness readings and profile radiography from outside examine the wall, with extra readings at low points and the bottom of the pipe. Water chemistry records, such as oxygen and pH measurements, are as useful as the metal readings, because they show whether the conditions that cause the damage exist.

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Mix-ups to avoid

  • Condensate corrosion versus hydrochloric acid corrosion. Both occur where vapor condenses. Condensate corrosion comes from dissolved oxygen and carbon dioxide in steam condensate. HCl corrosion comes from chlorides in a process vapor.
  • Condensate corrosion versus atmospheric corrosion. Condensate corrosion attacks from inside, caused by the water. Atmospheric corrosion attacks the outside, caused by weather.
  • Oxygen pitting versus CO2 thinning. Both can be present. Pits point to oxygen. A wide thinning or a groove along the pipe bottom points to carbonic acid.

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. Boiler water and steam condensate corrosion is RP 571 paragraph 3.9. API prints it in red on the current sheet, and it was added to the RP 571 list at the September 2025 cycle. A stem tends to describe a condensate or feedwater system, dissolved gases or a pH issue, and pitting or thinning at low points. Know the two causes and what each looks like.

Practice it

This is one of the red-marked entries in the API 571 damage mechanisms list for API 510. Compare it with Hydrochloric acid corrosion and Atmospheric 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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