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Ammonia Stress Corrosion Cracking is a form of stress corrosion cracking that commonly occurs in brass tubes in cooling water service that has been contaminated with ammonia due to biological growths or other contamination. This cracking can also occur when ammonia is intentionally added to process streams as a neutralizer by someone unaware of its potential effect on brass tubes. Brass condenser tubes can undergo brittle fracture if bent when significant ammonia stress corrosion cracking is present.
Ammonia stress corrosion cracking can also affect carbon steel equipment, but unlike the cracking mechanism on brass which occurs in an aqueous solution, the cracking of steel equipment occurs in anhydrous ammonia. Systems with air/oxygen contamination also tend to be particularly vulnerable to this issue. Thankfully though, there are several ways in which ammonia stress corrosion cracking can be prevented in steel equipment. First and foremost, systems that have not undergone postweld heat treatment (PWHT) are much more susceptible, thus proper PWHT is essential. Adding a very small amount of water (0.2%) to the anhydrous ammonia can also inhibit the cracking of steel.
When inspecting for ammonia stress corrosion cracking in brass tubes, some of the best techniques to involve the use of eddy current, such as pulsed eddy current testing or eddy current array testing. If you want to inspect for ammonia cracking in steel tubes, wet fluorescent magnetic particle testing is best for where access to the surface is available; when it’s not, shear wave ultrasonic testing tends to work best.
Cupro-nickel alloys are usually not susceptible to ammonia stress corrosion cracking, so if all else fails, upgrading to a new material is another form of prevention. Austenitic stainless steels likewise are resistant to this form of corrosion, so using them is another option.
November/December 2003 Inspectioneering Journal
By John Reynolds at Intertek
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).