"rust" of a stainless steel tank 2

[rob768]

Hi all,

I am faced with a stainless steel (316L) tank having been used in the storage of Choline chloride. This tank shows corrosion, in particular in welds and heat affected zones. Strangely enough, the corrosion products are deep "rust" coloured (reddish-brown), suggesting formation of iron oxides. I had expected greyish products, black even. In surface areas, covered with these rust stains (dripping down the wall of the tank) serious pitting is present underneath. These pits are relatively deep, and completely covered by a thin wall of stainless steel. Inside the pits, there is no corrosion product and the surface is bright metallic.
Has anyone seen this before? Especially the rust colour puzzles me.

regards
rob

 

[skailup]

I think 316, even an L grade, is not compatible with chloride environment. Pitting/crevice would be expected. As protective layer breaks down, red rust will be visible.

 

[EdStainless]

Fe2O3 is red, you put a chemical that is too aggressive into the tank and started pitting it.
The pits are bright because of active corrosion, this is common.
The welds and HAZ have a lot lower corrosion resistance than the base metal. This is partially due to the heating and forming problems in the microstructure (chrome carbides in the HAZ and residual delta ferrite in the welds). The other part could be improper surface condition. If there were oxides formed during welding and they were just ground off then this won't work. They would need to actually be pickled to clean them.
And what the heck is Chlorine Chloride?
Was the tank sealed? Inert gas purged? Any moisture would be fatal.
My guess is that the wetted part of the tank was pitted but not discolored until you drained it and let humid air reach it, then the corrosion products turned orange.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

It is choline chloride. Look it up it is used for chicken feed supplement.

 

[EdStainless]

Sorry, I know it by a different name.
Were you storing it pure and dry or as a solution?

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P.E. Metallurgy, Plymouth Tube

 

[dik]

Ed... the material, as I know it is hygroscopic and absorbs moisture from the air.

Dik

 

[EdStainless]

Unless it was under purge.
But this is often sold as a solution ~70%.
Lets put it this way, at pH=4 316 will only tolerate about 20ppm Cl before you have pitting attack.
A lined CS steel tank might be a better option.
If you want metal look at 2205 for construction, and remember that it is a lot stronger so you can make it a lot thinner.

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P.E. Metallurgy, Plymouth Tube

 

[dik]

I'd have thought that a duplex 2205 would be a good alternative. Thinking of using this for some potash mine enclosures.

Dik

 

[rob768]

Sorry for the typo, it is indeed choline chloride. Apparently it is, or is related to, Vitamin B, for nutritious purposes. This product is supposed to be an non-corrosive aqueous solution. You guys helped me a lot so far, and certainly in conformation of some of my thoughts. I was very puzzled by the brown discoloration but EdStainless explanation makes very good sense.

 

[rwwmetweld]

I could have written your exact question a few weeks ago. And I do mean exact. Since then, what I've learned is that 316L can be used as a choline chloride container, but only within limits. Temperature and oxygen content are crucial.

It's been a few months since your original post, so I don't know whether you still need an answer. Is it still an issue?

 

[FIG Journey]

If that is what Choline Chloride does to 316L, I feel sorry for those chickens.

 

[EdStainless]

FIG, you should see what a lot of the ingredients in 'personal care products' do to 316L when they are concentrated.

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P.E. Metallurgy, Plymouth Tube