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Old 03-13-2009, 07:12 PM   #1
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Default Phosphoric Acid does not re-passivate Stainless Steel.

I was just told this.

I had technical help on the phone as I was asking about chemical compatibility with various materials regarding 5 Star Acid #5 and in the conversation this statement was made. ".....phosphoric acid does not re-passivate SS it's the Nitric Acid that re-passivates...."

I mentioned that I was taking this info from Palmers Metallurgy appendice and was told that "...Palmer is making corrections...."

I have also read that Oxalic Acid (BKF) re-passivates and now I wonder if that is true or not.

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Old 03-13-2009, 07:59 PM   #2
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Well I think BKF does re-passivate. I cut the handle and 'ears' off a stainless steel strainer (that sits in a pot) and it is now my hop-stopper. I cut them off with a dremel, scrubbed it with BKF, waited 10 minutes then washed it off, put it in the pot and did a 60 minute boil. I didn't see any rust in post-cleaning.

It should also be noted that I used a grinding disk on a dremel, not a saw. The saw might leave regular steel in the cut and promote rusting of the stainless.

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Old 03-13-2009, 09:32 PM   #3
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50-50 mix of citric and ascorbic acid in water, heated to 180 deg. F and circulated, this is the method of choice these days in the food and pharmaceutical piping system passivation. Solution is circulated until soluble iron content remains constant which is the signal that the passivation is finished.
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Old 03-13-2009, 11:31 PM   #4
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Yes, oxalic acid will work. Take a listen to this: Brew Strong, Metals. Palmer spends a bit of time talking about how to handle stainless.
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