"SCC of AL6XN"

Question:

"Has anyone experienced stress corrosion cracking of AL6XN tubing. I know of a heat exchange (deionized water <100 psi shell side, seawater < 100 psi tube side) with transgranular cracking (axial) of the tube (7/8" OD, 0.028" wall) at all the baffle locations. Cracking is not located near the seam weld. There does appear some OD wall loss due to fretting (~20%). The baffles are carbon steel. The tubes have been in service for approximate 18 months. The tubes were GTAW seam weld without additional filler material, annealed and mechanically straightened. The cracking of the tubes is appearing directly at baffles locations. There is some OD damage at the baffle locations most likely due to vibrations. So far a testing lab says that the cracks (majority of which is axially oriented, some have intersecting transverse cracks also) initiate both from the ID and OD. The shell side is deionized water and the tube side is sea water. The tubes have been in service for 18 months. The cooler previously contained Al-Brass tubing (for 30 years). The cracking is transgranular and not associated with the seam weld. Most chloride SCC is intergranular (correct?).

My theory is that the tubing became extremely work hardening (due to flow induced vibration as evidenced by fretting) and an OD flaw such as a scratch or localized pit initiates the cracking.""

Answer:

You don't say whether the cracking was on the tube or the shell side. I do not have personal experience with chloride SCC of AL6XN; however, based on its nickel content, AL6XN is not immune to chloride stress corrosion cracking. It will crack if the exposure conditions are severe enough, i.e., chloride content, temperature and solution acidity. I would expect it to be resistant at a neutral pH. You might want to contact Allegeny Ludlum for their opinion. Yes, most chloride stress corrosion cracking is transgranular, although it can exhibit intergranular tendancies if the material is sensitized. I doubt that chloride SCC initiated in deionized water service, though, unless deposits high in chlorides built up at the baffle locations.

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