Hydrochloric acid corrosion: what it means for the inspector

How it works

Hydrogen chloride (HCl) gas can be present in refinery and chemical streams, for example from the breakdown of chloride salts. As a dry gas it is not very corrosive to carbon steel. Once liquid water is present, the gas dissolves in it and makes hydrochloric acid, a strong acid that attacks carbon steel quickly and attacks many stainless steels too.

The key idea is the first condensate. When a hot vapor stream cools, it reaches a temperature where liquid water first starts to form. The first drops are tiny in volume, so the dissolved HCl is concentrated and very acidic. Further downstream, more water condenses, the same amount of acid is spread through more liquid, and the solution is less aggressive. That is why the damage focuses on a narrow band at the start of condensation.

Where it shows and how it is found

Look at systems where a vapor stream carrying chlorides cools through the water dewpoint.

• Overhead lines, condensers and coolers in distillation and similar systems.
• Effluent coolers and the first cold surface a hot vapor meets.
• Spots where cold spots or poor insulation bring the wall temperature below the dewpoint locally.

Because the zone is narrow and the loss can be severe, thickness readings and radiography are focused on the condensation zone, taken at the same locations over time. Corrosion is often local and deep, so spot readings away from the zone can make the line look fine. Process monitoring, such as tracking water and chloride levels, is as important as inspection in controlling it.

Mix-ups to avoid

• HCl corrosion versus naphthenic acid corrosion. HCl needs liquid water and chlorides. Naphthenic acid is an organic acid in hot crude oil, with no water needed.
• HCl corrosion versus condensate corrosion. Condensate corrosion is driven by dissolved oxygen and carbon dioxide in steam condensate, not chlorides.
• The dry vapor versus the wet zone. A line can carry HCl vapor for years with little damage until it cools enough for liquid water to form. Where the wall is cold enough to condense water is where to look.

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. Hydrochloric acid corrosion is RP 571 paragraph 3.37 and is one of the 15. Typical stems describe an overhead system, a chloride source and the first condensing water, and ask for the mechanism or the best location for thickness monitoring. The location idea is the core: damage is controlled by where liquid water first appears, so that is where the readings belong.

Practice it

See all 15 sheet mechanisms side by side in the API 571 damage mechanisms list for API 510. Compare it with Naphthenic acid corrosion and Boiler water and condensate 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, paragraph 3.37. Cited by number only; read it in your own copy.