letter 52039

Mechanical polishing or passivation of Stainless Steel - which comes first?  

April 30, 2009 Hello, I know that if you electropolish it is not necessary to passivate stainless steel. But what if mechanical polishing is the process? Should you passivate then polish or polish then passivate. Thank-you, Micheline Forth Finishing Company - Rincon, Georgia May 1, 2009 Hi, Micheline. The mechanical polishing comes first, and then the passivation. The reason you don't need to passivate if you electropolish is because the electropolishing process (putting the component into a strong acid with an oxidizing electrical current at work) is pretty much the same thing as passivating (putting the component into an oxidizing acid) from the standpoint of de-activating the stainless steel. Regards, Ted Mooney, P.E.  finishing.com Brick, New Jersey May 7, 2009 Ted, If you polish first will passivation not ruin the beautiful finish? Micheline Micheline Forth - Rincon, Georgia

Not if it is done correctly. There will be virtually no change.

James Watts
- Navarre, Florida

Mechanical polishing will give you the surface finish you require. This could be enhanced by electropolishing, which will also provide you with a better passivation than normally obtained with simple polishing. For instance, with stainless steel the passivation film is normally an oxide that will give a dull finish; electropolishing gives a brighter finish that is passivated by phosphate

Trevor Crichton R&D practical scientist - UK

The answer to that question is no. A passivation will not change surface quality :)

Bill Grayson
Metal Finishing - Santa Clara, California

Thank-you for all the responses.

I did find a couple of scrap pieces that had been polished and was able to practice on them, and of coarse I found out that you are all correct. It did not change the appearance, but now I must be very careful of scratching.

I use Stainless Steel baskets and SS wire in my tanks. can I get any suggestions that would be less abrasive.

Sincerely,

Micheline Forth
- Rincon, Georgia

You can get plastic "pails" that have lots of small holes in them. Some places have different size holes. I would prefer polyethylene to polypropylene for resistance to the nitric acid, but PP will do. They are quite stout and will last for a couple years or more in normal use. Eventually, the oxidizing acids will make the pail brittle.
Also, they cost about the same as a good 316SS basket.

James Watts
- Navarre, Florida

Electropolishing and passivation are both good for corrosion resistance of stainless steel, for different reasons.

Electropolishing produces a very smooth surface, and corrosion is best able to occur on rough surfaces. Passivation produces an iron-free surface with high amounts of chromium and chrome oxide, which is resistance to chemical reaction and protects the underlying iron. A electropolished surface that is then passivated enhances the corrosion resistance even further.

Nitric passivation can change the finish if not done properly, i.e. parts are left in too long. Citric acid passivation does not present that risk.

Ray Kremer Stellar Solutions, Inc. McHenry, Illinois

September 18, 2009 Electropolishing or Passivation/Pickling comes after mechanical polishing. Passivation in Nitric/Hydrofluoric solutions will produce a dull grey matte surface finish if processed too long. Pickling pastes/gels (Nitric/Hydrofluoric/Jelling agent) will produce a dull grey matte surface finish if processed to long. Electropolishing produces a bright polished surface finish with the same benefits as ultra-passivation. You can use different combinations of the above to meet clients requests. Cliff Kusch electropolishing shop - North Vancouver, BC, Canada September 18, 2009 Hi, Cliff. I've never heard of passivating in nitric/hydrofluoric acid, but I've certainly heard of pickling in it. I think we're starting to confuse pickling with passivation. Regards, Ted Mooney finishing.com Brick, New Jersey October 8, 2009 Hey Ted, Yes you're right. My apologies, the Hydrofluoric in Nitric/HF solutions make it a pickling solution. The HF removes the passive layer, rust, heat scale, and just about everything else on the surface leaving a "white pickled" finish. Conditions being right in the area will lend the freshly pickled surface to autopassivation by reacting with oxygen in the air. Again my apologies to everyone. cliff kusch - north Vancouver, bc, Canada