Pickling and passivation treatment 3

[shahdadf]

dear friends,
is it possible to eliminate the pickling and passivation treatment in reactors? why?
thanks and regards,

 

[0707]

To get an answer you should be more detailed in your question

 

[moltenmetal]

If the stainless steel has been selected for corrosion resistance to internal corrodents, then the answer is probably no. If the stainless has been selected because it's pretty and shiny, then yes.

 

[0707]

Nobody selects a stainless reactor only to be pretty and shiny.

 

[SnTMan]

shahdadf, we have built at least one 304 vessel to ASTM-G93 (Oxygen Clean, although not acutally in O2 service) by repeated glass grit blasting, steam cleaning and washing. I'd have just P&P if I could have.

Regards,

Mike

 

[moltenmetal]

0707- we see it all the time. In fact, we see stainless selected sometimes when carbon steel would be superior- merely because the stainless steel is shiny. We're not talking about large tonnage vessels here, granted.

 

[UKCats]

My experience as I tour mills and fabrication shops is after hot working SS components (plate or dished heads), its black and you have to shot blast it remove the scale. It is then a little dull so the Mill will pickled it with a farily strong acid to restore its Cr oxide (protective layer). At that point, it's ready to sell. Passivation is typically done after the fabrication process (welding, rolling) to remove residual oils, damage done by welding and continmination (iron picked up off rollers). It is a weaker acid (nitric).

I have seen SS uniformly pit because of containmined iron picked up in the fabrication process and we had to go back and passivate it.

 

[HEC]

UKCats, just need to clarify some terminology here. Pickling is generally required where there is free iron on the surface, ie there is chemical stripping of the metal. Passivation on is providing oxygen in a low pH environment to restore or build a "pasive" layer of chrom oxide, hence the use of HNO3 (nitric acid) for this. In your closing statement, for removal of embedded iron pickling followed by passivation is the correct treatment.

Mark Hutton

 

[UKCats]

It is my experience to remove free iron from the surface, only passivation is required. It is not as deep of oxidizing process as pickling.

I've performed a light nitric acid wash (passivation) before in place on a vessel because of what we thought was contaiminated free iron on the inside surface. We had a lot of dark spots forming about 1/8 to 1/4 inch apart... our vessel had the measles.

You can't see free iron initally but you can sure see scale if you buy a head hot formed and don't ask for it to be blasted.

 

[kenvlach]

Some further clarification:
Pickling is the chemical removal of surface oxides (e.g., heat treat & welding scale) and the chromium-depleted layer of metal immediately beneath heat-treated stainless. Often used are reducing acids (sulfuric, hydrochloric & hydrofluoric) which can cause hydrogen embritttlement. Definitely not a 'deep oxidation.'
Nitric acid, an oxidizing acid, is used to protect stainless steel from excessive attack by the hydrofluoric acid used to attack oxides. Typical pickling solutions are given in 'TABLE A1.1 Acid Descaling (Pickling) of Stainless Steel' of ASTM A380.

Pickling should only be done once or sometimes not at all in the fabrication of a stainless steel vessel (except for any subsequent welds).

Passivation per ASTM A967 uses nitric or citric acid solutions to selectively leach iron and nickel (and remove contaminants, e.g., zinc from contact with galvanized material) whilst creating a more protective, Cr₂O₃-rich oxide film. Cf. pickling, which removes all metals including Cr.

There are additional passivation processes used in maintainance procedures, most notably, an ammoniated citric acid solution (aka ammonium citrate).

Passivation can be repeated as necessary, with little harm to the substrate.

Hope this helps,
Ken

 

[UKCats]

I agree... well explained. I am US working in Malaysia.. where are you...