Passivation—anyone tried it on their stainless steel knives or multitools?

[SgtTowser]

Just read a bit about passivation. Passivation is the dipping of stainless steel products immediately after manufacture in citric acid to remove any traces of free iron on the surface of the stainless steel. This results in a thin outer layer of pure chromium, which does not rust/tarnish. If I understand correctly, this outer layer cleansed of free iron is called a chromic oxide layer. Stainless steel treated by the chemical process of passivation is nearly impervious to rust, unless the chromic oxide layer is breached, or unless the citric acid bath did not remove all the free iron.

It occurred to me that some old stainless steel knife blades have rust pits, or stains.

Question: Has anyone tried to repeat the passivation process on old stainless steel pocket knife blades, as part of rehabilitating them?

I wondered if re-passivation might remove the rust, stains, etc., and restore a blade?

[Poncho65]

Never heard of it but it sounds neat Be good to hear an expert's thoughts on this

 

[David]

No. How strong is the solution of citric acid. Without proper SOP's and PPE this may be something the average person shouldnt be messing with.

[Rotten66]

I have excellent results cleaning brass for reloading by means of a citric acid hot water bath. Never tried it on stainless steel.

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[Vidar]

On some previous project we would typically passivate stainless steel parts. Partly because there are dust of other metals present which might contaminate the surface and cause rust, and partly to thicken the chromium layer that makes stainless stainless. Either with immersion in a bath of weak acid, or more common with welds, by using a somewhat gel like substance that is rinsed off later. The latter can be found easily enough found as pickling paste, but as many of those contain hydrofluoric acid I stay well away - that one is no joke. (And today there are better options like electrolytic brushes for welds).

If your actual goal is to remove rust then a rust remover product is going to do a better job. As for rehabilitation that shouldn't be necessary as stainless steel is stainless because the chromium in it forms chromium oxide upon contact with oxygen which stops further oxidation. Thus if you make a clean scratch it will repair and form a new layer of chromium oxide and you'll have a rust proof surface again.

If you mess up the surface with introduction of other metals or ordinary rusty steel then passivation might be enough - or not if it is bad enough. The usual suspects is using the same grinder/ sand paper/ file/ stone/ etc on normal rusty steel and then on some poor innocent stainless.

[SgtTowser]

I have excellent results cleaning brass for reloading by means of a citric acid hot water bath. Never tried it on stainless steel.

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That is so terrific to learn!. I collect old brass kerosene sailboat lanterns.

What percentage citric acid do you use?

[SgtTowser]

No. How strong is the solution of citric acid. Without proper SOP's and PPE this may be something the average person shouldnt be messing with.

Good points. I will have to do some research for sure.

[Rotten66]

That is so terrific to learn!. I collect old brass kerosene sailboat lanterns.

What percentage citric acid do you use?

I use roughly a teaspoon to a gallon of HOT wate
The following is FYI based on my experience. As to the water temp you want it just shy of boiling and do not add the citric acid until the water is heated.
After adding the CA stir it in a little. Add the item to be cleaned and circulate the water around the item either by stirring or direct agitation of the item. You will see that mixture will do 99% of its work in just seconds but personally I leave my brass in for about a minute stirring occassionally. This will not remove all the dirt and tarnish but you will be impressed. I like the fact that it does leave a bit of whatever age the brass has visible. I don't remember what the pink spots and black spots neglected brass can get are called and the CA mix will not remove the discoloration.

All that said I have been using CA for a long time and have brass that I CA washed 5+ years ago and it still looks good with no signs of deterioration.

Before I forget, the best place to find CA is at a grocery store that carries canning supplies. A few ounces will last a very long time.

Hope that all makes sense, it's been a long day and I'm very tired.

[SgtTowser]

I use roughly a teaspoon to a gallon of HOT wate
The following is FYI based on my experience. As to the water temp you want it just shy of boiling and do not add the citric acid until the water is heated.
After adding the CA stir it in a little. Add the item to be cleaned and circulate the water around the item either by stirring or direct agitation of the item. You will see that mixture will do 99% of its work in just seconds but personally I leave my brass in for about a minute stirring occassionally. This will not remove all the dirt and tarnish but you will be impressed. I like the fact that it does leave a bit of whatever age the brass has visible. I don't remember what the pink spots and black spots neglected brass can get are called and the CA mix will not remove the discoloration.

All that said I have been using CA for a long time and have brass that I CA washed 5+ years ago and it still looks good with no signs of deterioration.

Before I forget, the best place to find CA is at a grocery store that carries canning supplies. A few ounces will last a very long time.

Hope that all makes sense, it's been a long day and I'm very tired.

Crystal clear.

Thank you.

Hope you get a well deserved rest.