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Inspectioneering Journal

99 Diseases of Pressure Equipment: Ammonia Stress Corrosion Cracking

By John Reynolds at Intertek. This article appears in the November/December 2003 issue of Inspectioneering Journal

Ammonia stress corrosion cracking (SCC) has been around a long time. Most everyone has experienced it from time to time. It's not uncommon in brass tubes in cooling water service that is contaminated with ammonia due to biological growths or other contamination. Sometimes ammonia is added intentionally to process streams as a neutralizer by folks who do not know what it might do to brass tubes. Brass condenser tubes will fail brittlely when bent after they have significant ammonia stress corrosion cracking present. Eddy current inspection of brass tubulars is effective at finding ammonia cracking. Cupro-nickel alloys are usually not susceptible, and if necessary you can upgrade to austenitic stainless steels (which has it's own set of problems).

Another form of ammonia SCC afflicts carbon steel equipment. Unlike the cracking mechanism on brass, which occurs in an aqueous solution, the cracking of steel equipment occurs in anhydrous ammonia (just the opposite). Adding a very small amount of water (0.2%) to the anhydrous ammonia is enough to inhibit the cracking of steel. Systems that are not PWHT'd are much more susceptible (especially with hard welds), as well as systems that have air/oxygen contamination. Unlike the ammonia SCC of brass tubulars which usually results in some sort of economic or reliability impact, we must be much more careful to avoid ammonia cracking of steel, as the consequence of failure of ammonia systems can be deadly. Ammonia cracking of steel equipment can be readily detected with wet fluorescent magnetic particle examination where access to the surface is available, and shear wave UT from the opposite side where surface availability for examination is limited.

Do the right people in your facilities know about ammonia stress corrosion cracking of brass and steel and do they know their role in preventing it from occurring?


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