We have installed U-Tubes of SS-304 material in our gas water cooler. The heat exchanger was put into service in 2009 at our ammonia plant.
Service Condition Of Exchanger:
Fluid: Process Gas
Composition: Hydrogen 44%, Nitrogen 15.5%, CH4 18.4%, NH3 18%, Argon 3.7%
Shell Side is Cooling Water.
We have experienced catastrophic leakage in this exchanger within one year of service. We re-examined the chemical composition results of the tubes purchased and found 1% Cu in the composition, which is not a standard element in ASTM specified Code.
My questions are: 1. What will be the effect of 18% ammonia in process gas on the SS-304 tube having 1% copper? 2. What is the effect of 1% Cu on SS-304 material while annealing process during U-Bends stress relieving? 3. Is there any kind of leeching phenomena associated with the presence of copper in SS-304 material?
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Re: Is stress corrosion cracking the cause of leaks?
4 August 2010 at 1:29pmStandard austenitic stainless steel grades such as type 304 are widely used in ammonia service since they are resistant to general corrosion and ammonia stress corrosion cracking (SCC). However, these stainless steel grades can fail by SCC that is initiated by sensitization formed during heating/welding. It is common to find low levels of elements in alloys that are not in the specification. Also, copper is intentionally added at a level of 3%-4% to produce type 302Cu (UNS 30430) stainless steel. The copper in 302Cu reduces the cold work hardening rate and 302Cu is often used as a material for fasteners. The general corrosion resistance of 302Cu is similar to that of 304 stainless steel so I suspect that the low copper levels you have found in your 304 stainless steel is not the source of your failures. You do not mention what type of failure is occurring. The type of failure may give you some hints as to the cause. For example, standard austenitic grades can suffer from chloride SCC if either the gas stream or cooling water contain chlorides.
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