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Ammonia Stress Corrosion Cracking

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Ammonia Stress Corrosion Cracking is a form of stress corrosion cracking (SCC) caused by the combined effects of an ammonia environment and the presence of either applied or residual tensile stress. Ammonia SCC is most prevalent in carbon steel (especially high-strength steel) equipment in anhydrous ammonia service or in copper-zinc alloys, such as admiralty brass or aluminum brass, in aqueous ammonia environments (or process conditions that contain ammoniated chemical compounds). Ammonia SCC is generally either transgranular or intergranular in copper alloys, and more intergranular in nature for carbon steels.

Areas Susceptible to Ammonia SCC

In general, ammonia storage tanks, piping, and related components in ammonia refrigeration units are particularly susceptible to ammonia SCC damage. Some lube-oil refining processes may also introduce risk for ammonia SCC. Additionally, ammonia may be present in cooling water or steam condensate in boiler feedwater systems due to chemical treatments, or be present in certain process streams either as a contaminant or an acid neutralizer, thereby introducing the risk for ammonia SCC.

Ammonia SCC commonly occurs in brass tubes in cooling water service that have 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 SCC is present. In carbon steel equipment, systems with air/oxygen contamination tend to be particularly vulnerable to ammonia SCC.

Ammonia SCC Prevention/Mitigation

There are several ways in which ammonia SCC can be prevented in steel equipment. First and foremost, systems that have not undergone post weld 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. 

Cupro-nickel alloys are usually not susceptible to ammonia SCC, so if all else fails, upgrading to a new material is another preventative option. Similarly, austenitic stainless steels are generally resistant to ammonia SCC, so using them is another option.

Ammonia SCC Inspection

When inspecting for ammonia SCC in brass tubes, some of the best techniques involve the use of eddy current, such as pulsed eddy current testing or eddy current array testing. If you’re inspecting for ammonia SCC in steel tubes, wet fluorescent magnetic particle testing is an effective inspection technique when access to the surface is available; when it’s not, shear wave ultrasonic testing has proven effective.

References

  1. Prueter, P., 2022, “Damage Control: Stress Corrosion Cracking Detection,” Inspectioneering Journal, 28(2), pp. 45-52.

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Articles about Ammonia Stress Corrosion Cracking
July/August 2022 Inspectioneering Journal

This final installment of a 3-part series on SCC mitigation will provide practical, actionable steps to improve long-term equipment reliability through design, fabrication/welding, heat treatment, maintenance/repair, and process operating practices.

May/June 2022 Inspectioneering Journal

Part 2 of the three-part series on stress corrosion cracking (SCC) that examines common engineering and FFS methods that can be leveraged to understand and evaluate the propensity for an existing form of SCC to lead to loss of containment.

March/April 2022 Inspectioneering Journal

This article provides an overview of some of the more common forms of stress corrosion cracking (SCC) with a focus on effective inspection methods for detecting these forms of cracking.

November/December 2003 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...

Authors: John Reynolds
Partner Content

Pro-Surve recently invested in the latest portable Higher Order Modes Clusters (HOMC) Guided Wave Testing tool, a new concept for the inspection of inaccessible regions using short-range guided waves. Inspection is carried out from the accessible...

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