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pbetai
Participant

Join Date: Oct 2010
Posts: 2

SS316 - Prevention of Pitting

10/11/2010 2:07 AM

Please advise

1) what material coating should be done over the Stainless Steel (SS)-316 equipments (switchboards, external portion of reactors etc) to avoid rust coloured pitting marks formed over these materials...

2) We have a Rotomac Vaccuum Drier (RVD) made of SS-316, that moves hot water (80 - 90 degreec C & 720 to 760 mm/Hg vacuum application) within its outer jacket. Wet cake of Glucosamine HCL is kept inside the RVD alongwith 5% Ethanol to dry. Vapour temperature inside RVD=upto 60 degreec C. Ph=1. The RVD internal tefflon coating has peeled off in 1 year. Would internal coating for RVD as Polystyrene be successful instead of Tefflon as Polystyrene is far more economical ??

Thanks for your cooperation and assistance.
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Milo
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Join Date: Jul 2006
Location: "Dancing over the abyss."
Posts: 4884
Good Answers: 243
#1

Re: SS316 - Prevention of Pitting

10/11/2010 2:50 PM

None. It is exposure to oxygen in air that keep s stainless "stainless"

1) The pits are likely from placement/ contact of ferrous/steel materials at some time during the fabrication or operation.

This is why we never use steel wool on our stainless steel appliances at home or in restaurants.

2) The material may not have been properly passivated either.

Milo

__________________
 People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe
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pbetai
Participant

Join Date: Oct 2010
Posts: 2
#2
In reply to #1

Re: SS316 - Prevention of Pitting

10/12/2010 12:34 AM

The reasons may be true or otherwise....... but how do we solve the problem now... We have had BUFFING done over the stainless steel vessels but the pits were again observed after 6 months thereof.......
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Milo
Guru

Join Date: Jul 2006
Location: "Dancing over the abyss."
Posts: 4884
Good Answers: 243
#3
In reply to #2

Re: SS316 - Prevention of Pitting

10/12/2010 11:21 AM

Buffing is merely mechanical. It does nothing to chemically remove the free iron, and may in fact grind into the metal particles lossened utp from elsewhere...

Passivation is needed to restore the stainless to stainless condition.

Please see the links below.

http://www.delstar.com/passivating.html

http://www.houghtonglobal.com/downloads/Cleaning%20and%20passivation%20of%20stainless%20steel%20using%20Citrisurf.pdf

Citric acid is one method, and For work in situ, I would rather have my folks woprkiing with it than Nitric acid.

Milo

__________________
 People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe
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