corrosion of stainless steel through iron contact

[200966]

I am using iron tooling to bend stainless steel pipes, and even though i am using a metal foil for protection, on some spots the tooling is making friction contact with the stainless steel, there has been a lot of contact.

Will a nitric acid treatment repassivate the stainless steel, i.e, will the acid take care of areas where particles from iron tooling are deeply embedded in the stainless steel?

Will a 2500 psi water pressure wash be enough?

I'm trying to find the most reliable method to repassivate the stainless steel without having to sand all parts which had contact with the iron tooling.

Thank you for any suggestions.

 

[rustbuster1]

Passivating with about 10% nitric acid (warmer is better)will remove the iron contamination. It also leaves a stronger passive film, modestly increasing the time before piting by chlorides occurs (compared to a non-passivated surface, even if it does not have iron to remove.)
2500 psi (or even 4000 psi) water blasting will not reliably remove iron contamination.
Rust stain from wetting the SS will reveal if the iron is still present.

 

Dear Rustbuster1,
Thank you for your answer.
I am having some trouble to find a supplier for 10% nitric acid pickling or paste. Either companies provide a service to do passivation or chemical suppliers offer Nitric acid in liquid, undiluted form. I did find some suppliers in Europe and India but non in the States. (Im located in Brooklyn, NY)
Do I have to produce the pickling myself, and if how?
I would be very grateful for further suggestions.

 

[rustbuster]

Roxy,

You could mix the solution yourself, using demineralized water.

If you are looking at localized areas, Copper supfate solution will identify the areas of contamination. the copper sulfate will turn any areas of contamination pink. These areas could be sanded with a flapper wheel and the test repeated until the contamination is removed.

Good luck!

 

Though it's been some time, I seem to recall some companies beginning to use Citric based acids (claiming that they're more environmentally friendly). My apologies for lack of further information, but I might suggest that you contact the Nickel Institute who has been (in the past) a wealth of information on current stainless trends and technologies.

 

[rustbuster1]

Last I knew, one could buy "pickling" paste from AVESTA-SHeffield. This contains HF and HNO3 acids and therefore requires special care in handling. They may also have a HNO3-only, passivating paste.
As suggested, other acids besides HNO3 are also effective. Phosphoric acid is a common alternative. FYI- Coca Cola has enough phosphoric acid to give it a pH about 2.2 and has been used to remove rust from chromed bumpers on cars. Warm the Coke to make it more aggressive ;<)

Checking with Nickel Development Institute (NiDI) will explain the technical aspects but is unlikely to locate a commercial source, but cannot hurt to try. They are at

http://www.nidi.org