Comment on this corrosion (Link to picture)

The manufacturer of an electrical water still has said that the failure of the heating tube elements in their unit is due to the 3ppm of
chloride measured in the RO/DI feed water.
I have heard of stress corrosion cracking but a 40 mm x 5mm inch gash after only 24 to 36 hours running time post installation seems extreme. The manufacturer claims that while operating, the heating bundle was fully submerged and at a max of 150C . The heating bundle was installed upright in a 12 inch column. The tubes are about 10 mm in diameter. Unit was electro polished and passivated (citric type) before use.
Take a look at a picture of the corrosion using the following link. What do you think the mechanism was for that kind of corrosion?
http://community.webshots.com/myphotos?action=showPhoto&albumIDH7646690&photoIDH7652793&security=fhfDWG
Allan MacDonald
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Please use corrected link to picture
http://community.webshots.com/photo/487646690/487652793fhfDWG
My apologies.
Liz MacDonald wrote:

http://community.webshots.com/myphotos?action=showPhoto&albumIDH7646690&photoIDH7652793&security=fhfDWG

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Hard to be sure, but that kind of cracking is consistent with SCC. Which material is the heating elements made from? What is the tube sheet made from? Could contribute to galvanic corrosion.

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Liz MacDonald wrote:

Alan:
Corrosion requires a combination of a material and an environment. What is the material?
Some stainless steels like Types 304 and 316 are relatively susceptible to chloride stress corrosion cracking at quite modest levels of chloride (5 ppm or so).
Go to www.nickelinstitute.org, and download publication 11021 (Corrosion and heat resistant alloys, aka High-Performance Stainless Steels). It is a 95 page booklet as a 1Mb Acrobat file. See Figure 65.
Pittsburgh Pete
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