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Post by duckbill on Apr 28, 2008 11:46:35 GMT -8
When stainless bands are mounted on galvanized tanks a whitish-grey, powdery oxidation forms. I imagine it must be a product of a galvanic reaction of the zinc with something in the stainless steel.
Is this a self-limiting reaction? If I leave it alone will it not affect the galvanizing any further? Or, is it something that should be dealt with somehow? It is marking up my nice, clean cylinder.
I thought about lining the inside of the bands with thin, clear packaging tape to provide a barrier, but would rather not do that if the reaction only goes so far then stops.
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Post by scubadiverbob on Apr 29, 2008 8:13:12 GMT -8
Please see PM I'm sending you.
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Post by scubadiverbob on Apr 29, 2008 8:19:45 GMT -8
Unable to send PM .... I'll send a steel tank band with a plastic part to keep tank band from touching the tank (I always wondered why they made them) with Dave. It's off a bc I was going to restore; but, is sort of getting used for parts.
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Post by Captain on Apr 29, 2008 8:39:18 GMT -8
I think you will find that no matter what the bands are made of there will be some discoloration to the galvanize because of trapped salt and water. Even galvanized or nylon bands will do the same just as it does under a tank boot. The only solution is to remove the bands and rinse after diving.
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Post by duckbill on Apr 30, 2008 22:02:01 GMT -8
Thanks Robert and Tom.
I knew that mineral deposits might be part of it, but thought more might be going on. My set of doubles with galvanized bands don't seem to leave as much residue.
Realizing that stainless steel bands for cylinders have been in use for a long time now, I guess I should know that no real harm is done to the galvanizing or they wouldn't have used that combination.
Thanks again.
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Post by Broxton Carol on May 1, 2008 4:51:18 GMT -8
Captain is correct. I have some old tanks from the 50's and usually upon removing the bands, there is corrosion or decay, or stains/scars under them. I try to buy only freshwater vintage gear, but this corrosion seems to occur in fresh water too. The stains marks mnake it easy to place the bands back on after the tank is hydro'd! Dont get the marks on painted tanks.
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Post by luis on May 1, 2008 4:56:20 GMT -8
I agree with Captain that you will always get some of the zinc will show some corrosion due to the trapped salts and or minerals in the water. That normally is not a big deal, but I also think that the galvanic action between dissimilar metal can accelerate corrosion and/ or pit the zinc coating under the stainless steel bands.
In the galvanic tables stainless steel is so much noble than zinc; I just would not risk having them in direct contact. It is the same reason that I am planning on isolating aluminum tanks from stainless steel bands.
Both zinc and aluminum are very active in the galvanic tables (they are next to each other). That is part of the reason why zinc is used as a sacrificial coating
It is a very easy solution to isolate the two with a thin piece of rubber. I am probably going to order some rubber from McMaster-Carr with sticky back so I can easily bond it to the inside of the SS bands.
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Post by duckbill on May 1, 2008 21:08:08 GMT -8
When Captain said "discoloration", I thought he meant just deposits. I guess I understand now that salt and minerals themselves corrode zinc? I was wondering more about galvanic reaction.
I resorted to laying a layer of clear packing tape on the inside of the bands. I know that water will still get trapped, but I hope the barrier will at least slow down any galvanic reactions between the stainless and zinc. I'll have to see if it holds up well to submersion. I didn't really want to resort to black rubber straps as a barrier. The SS bands look sweet on that clean, galvanized tank with no rubber pieces to detract from "the look".
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