It sounds like you might be getting heat tint on the tube interior. Do
you purge the inside?
There is a detailed 52 page reference book on stainless welding you
can get for free from the Nickel Institute. It is Nickel Development
Institute publication No. 11007 titled "Guidelines for the welded
fabrication of nickel-containing stainless steels for corrosion
resistant services". There also is a newer 20 page book also, No 11026
on "Fabricating stainless steels for the water industry"
Go to: www.nidi.org
On the top of the page is a box labeled "Nickel & Its Uses". Click on
it. On the right side of the page is a column with a box labeled
"Technical Literature. Click on it. Change the search box under
Technical Literature Locator to be by Catalogue No., and put in 11007
or 11026. Then you can download the book.
(You may have to register if this is the first time at the site).
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Or it could also be something as simple as a porous weld. if you
leave pinholes, and then passivate it, passivation solution gets
trapped in the cavities.
bleedout is a common issue in plated items like cast iron where
porosity is just the nature of the material
I always get a kick out of telling our engineers here in the shipyard these
alloys are "stainless" not "stainfree." I didn't check the NIDI article,
but rather than a simple passivation, they might need a pickel to get rid of
any free iron that might plate out on the cooler base material. There's a
vapor above the liquid weld puddle that's iron rich and that iron will
condense on the nearest cool surface. We had a case where our customer
noticed rusty streaks paralelling the seam weld inside the refrigerator. I
suggested they continue to clean the piece with a green scrub pad and a
common tub and tile cleaner that contained citric acid.
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