I posted a while back about the heat staining and heat corrosion of stainless steel boilers. I'm seeing distinct evidence of oxidation in my boiler in the areas which are directly exposed to the heat source. It occurs only on the interior of my boiler and in spots definitely appears to be rust. Stainless Steel is NOT rust resistant kids. air+moisture+extended heating will corrode many varieties of stainless. I went looking for answers to see if this situation could be remedied and I found this:
https://www.mmsonline.com/articles/how- ... teel-parts" onclick="window.open(this.href);return false;" rel="nofollow
It sounds like citric acid solutions are the best way to go for home distillers and brewers. Passivation restores the "stainless" quality to steel and it may be something some boilers will require periodically. And here I thought I was just learning about making hooch, I'll soon be an adept metallurge.
Passivation of stainless steel
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Alchemist75
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Passivation of stainless steel
SOLVE ET COAGULA, ET HABEBIS MAGISTERIUM
Re: Passivation of stainless steel
I have a bit to add here I think.
Stainless steel is in fact corrosion resistant, however at higher temperatures the chemical structure, and at even higher temps the composition, of steel changes. The 300 series of stainless is notorious for this... rather heat sensitive.
Aluminum is a perfect example to use here, have you ever seen aluminum rust? no? wrong... any Aluminum you have seen unless it was freshly cut and shiny is oxidized, aluminum oxidizes so incredibly fast and so completely it forms a layer of ceramic (Al2O3 = sapphire glass) that protects the metal from further oxidation. In some/most stainless steels, the nickel does this for the iron, creating a sacrificial oxidation layer. This layer when exposed to heat can be broken down, and more metal can burn (oxidize/rust) and this is usually combated with the addition of chromium of cobalt, sharply raising the heat resistance of this sacrificial layer.
330 stainless, because of it's up to 30% chromium content, if memory serves has excellent stability and oxidation resistance at temperature of up to 1000F in highly corrosive environments and in-flame temperatures of up to 1900F.
welded stainless that will be exposed to wet environments should also be welded with very high chromium/cobalt content rods, regardless of the kind of stainless you are welding or your welds will rust out well before your sheet will.
I'm not sure about the food safety of his particular stainless though, some leach harmful quantities of nickel into the wash. Which, I would be willing to bet that there's no way in hell nickle is going to make it over the head of the still, but I'm not sure at this particular moment. I know that 316 and 416 are considered food safe, with the exception of highly acidic foods for some people who are highly allergic to nickle.
Just a quick blurb about nickle toxicity, it's now the international standard across the jewelry industry to avoid any alloys containing nickel, and in the EU it's actually banned for use in wearables. The only exception is stainless steel watches, and even with those, there is a progressive switch over to alloys with less and less nickel due to the simple fact that some people can't wear their $15,000 rolex made with 416 because it makes their skin boil up.
Stainless steel is in fact corrosion resistant, however at higher temperatures the chemical structure, and at even higher temps the composition, of steel changes. The 300 series of stainless is notorious for this... rather heat sensitive.
Aluminum is a perfect example to use here, have you ever seen aluminum rust? no? wrong... any Aluminum you have seen unless it was freshly cut and shiny is oxidized, aluminum oxidizes so incredibly fast and so completely it forms a layer of ceramic (Al2O3 = sapphire glass) that protects the metal from further oxidation. In some/most stainless steels, the nickel does this for the iron, creating a sacrificial oxidation layer. This layer when exposed to heat can be broken down, and more metal can burn (oxidize/rust) and this is usually combated with the addition of chromium of cobalt, sharply raising the heat resistance of this sacrificial layer.
330 stainless, because of it's up to 30% chromium content, if memory serves has excellent stability and oxidation resistance at temperature of up to 1000F in highly corrosive environments and in-flame temperatures of up to 1900F.
welded stainless that will be exposed to wet environments should also be welded with very high chromium/cobalt content rods, regardless of the kind of stainless you are welding or your welds will rust out well before your sheet will.
I'm not sure about the food safety of his particular stainless though, some leach harmful quantities of nickel into the wash. Which, I would be willing to bet that there's no way in hell nickle is going to make it over the head of the still, but I'm not sure at this particular moment. I know that 316 and 416 are considered food safe, with the exception of highly acidic foods for some people who are highly allergic to nickle.
Just a quick blurb about nickle toxicity, it's now the international standard across the jewelry industry to avoid any alloys containing nickel, and in the EU it's actually banned for use in wearables. The only exception is stainless steel watches, and even with those, there is a progressive switch over to alloys with less and less nickel due to the simple fact that some people can't wear their $15,000 rolex made with 416 because it makes their skin boil up.
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Alchemist75
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Re: Passivation of stainless steel
The boiler is an ss milk can that may have been fabricated as early as the 1940's. Ironically it's a hand me down from a grand mother who adamantly opposed drinking. 
the quality of the steel isn't likely to be as good as a modern version which is probably why I'm seeing heat damage occurring. Not terribly concerned about any leaching metals making it in to my product but I'd like to slow or stop the process of I can. I picked up some citric acid and distilled water to apply to the bottom of the pot. Should I sand off the oxidized layer before I apply the heated solution?
the quality of the steel isn't likely to be as good as a modern version which is probably why I'm seeing heat damage occurring. Not terribly concerned about any leaching metals making it in to my product but I'd like to slow or stop the process of I can. I picked up some citric acid and distilled water to apply to the bottom of the pot. Should I sand off the oxidized layer before I apply the heated solution?SOLVE ET COAGULA, ET HABEBIS MAGISTERIUM
Re: Passivation of stainless steel
Another benefit of running citric besides passivation, is it will break up deposits of beer stone, and clean some things caustic or PBW won't.
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The Baker
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Re: Passivation of stainless steel
I know dairy farmers use special cleaning liquids to clean their stainless pipes.bluedog wrote:Another benefit of running citric besides passivation, is it will break up deposits of beer stone, and clean some things caustic or PBW won't.
I think there is a 'morning' liquid and an 'evening' liquid for the full cleaning regime.
Don't know the details.
Geoff
The Baker