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Old May 8, 2019, 09:31 AM   #26
Unclenick
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Yes. RH is key. Here is a graph of surface rusting activity in steel:

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File Type: gif Steel Corrosion Rate.gif (33.9 KB, 151 views)
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Old May 8, 2019, 04:43 PM   #27
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Unclenick, the corrosion resistance/rate of steels varies considerably, both among the various stainless steels and among the various non-stainless steels. Do you know what steel alloy was used to develop the graph you posted?
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Old May 8, 2019, 05:27 PM   #28
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IIRC, it was A992 hot rolled I-beam steel, as it was in the context of dehumidifying sandblasting air on its way toward taking rust off bridge steel.

You are correct the rate will vary considerably, but the shape of the graph should apply proportionally. The important point is low humidity stops rust from initiating. This was also discovered by the U.S. Bureau of mines when they worked out that potassium salt in primer residue is what caused rust in gun barrels back in the 1920s (well, worked it out for Americans; the Swiss had figured it out nearly 20 years earlier, but back in those days publications didn't circulate worldwide very well).
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Old May 9, 2019, 07:40 PM   #29
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Quote:
Originally Posted by Unclenick View Post
You are correct the rate will vary considerably, but the shape of the graph should apply proportionally.
As far as common firearm steels, that might be true. 416 seems to be the most common stainless steel used in firearms and it is not among the most stain (rust) resistant stainless steels. But I'm not sure the graph would apply proportionally for all steels.

Some stainless steels will fend off significant rust for a very long time, even under condensing or submerged conditions. Those highly rust resistant stainless steels may be suitable for some firearm parts, but may not be practical or appropriate for others like barrels or actions.
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Old May 10, 2019, 10:19 AM   #30
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Oh, I meant rustable steels. I wouldn't expect it to apply to stainless steels at all. I've never seen uniform rust on stainless of any kind. To rust, stainless has to have iron without chromium oxide shielding exposed or else iron at the surface embedded from an outside source. There's no way to predict how much area that will constitute from one piece to the next as it will vary with its machining and handling history.
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Old May 10, 2019, 06:31 PM   #31
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Quote:
I've never seen uniform rust on stainless of any kind.
The welding of certain stainless steels will cause the phenomenon called, ".
chromium carbide precipitation." I learned that in welding class many years ago.
https://www.finishing.com/03/36.shtml
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Old May 10, 2019, 08:21 PM   #32
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The welding of certain stainless steels will cause the phenomenon called, "chromium carbide precipitation." I learned that in welding class many years ago.
When this happens, it changes the properties of the steel. If enough of the chrome in the alloy converts to carbides then there is no longer enough left to provide rust-resistance. One could even say that it is no longer stainless steel when that happens.
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Old May 11, 2019, 04:03 PM   #33
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I think that's right. I read the thread and they were hoping passivation would fix it, but I don't know there'd be enough chromium left or not. My Parkerizing tank was welded up by a fab shop that did some medical work. The welds have never shown a trace of rust, but I don't know if they did anything special to control it beyond just not getting too hot for too long, I don't know. I imagine medical equipment that includes welded stainless pretty much has to be as stainless as possible.
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