Passivation of abrasive blasted stainless steel

A. Yup, your guys screwed up but I guess you already know that.

Any standard passivation procedure will work for you.

Soak 'em in 10% nitric acid solution for half an hour at 70 degrees or higher and you should be alright.

You could also use citric acid if you have a disposal problem.

See ASTM A967 for full details or search this website for passivation. There is lots of information here.

For the heavy contamination that you have I would prefer to use nitric. It seems to work a little better than citric in this particular circumstance. If you find that you still get some rusting after the passivation procedure it is because of the amount of iron that has been driven into the surface of your diamond plate by the blasting procedure. How deep it has been driven depends on the air pressure used and the state of the blasting material. Have faith and re-passivate. It will clear it all given a little patience.

Good luck,

A. Ramon:

Yes, you can easily get your deck plates passivated. A local jobshop should be able to handle the job.

A. Ramon:

You will find that the correct citric acid based formulas will give you faster and better removal of the rust than the nitric solutions.

A. If anything, I would think the blast would remove oxides and activate the surface. Anyway, we passivate stainless with a solution of 40-60% nitric acid. 30 min dip.

A. Dear Ross,

Yes, after you work stainless steel in any way , the "Passivation" must be renewed, as the New Olympic Swimming Stadium in Sydney will tell you. There are several supposed treatments but the only constantly reliable one (in my opinion) is ELECTROPOLISHING and this can be achieved by "Brush electropolishing" on site or by contracting the work out .

Regards

A. Dear Ross: After sandblasting or bead blasting it is necessary to re-passivate the metal. For a bright finish and superior corrosion resistance electropolishing is the answer. If the handrails are electropolished first, and then welded on-site, the weldments should also be passivated. It is generally more economical to passivate on-site than to electropolish.

Sincerely,

A. If certifying the passivation, you must passivate after the bead blasting. Alternating hot and cold water rinses help draw out trapped passivation solution from behind tack welds. Pressurized spray rinsing also helps. The final rinse should be hot DI water. Test the adequacy of the rinsing by wicking out residue from the crevices with pH paper.

A. Yes, you must passivate after blasting. If you are using nitric acid to passivate it certainly would be a problem to have that trapped in crevices. citric acid based passivation would make this less of a concern.

adv.
Let us know if we can help.

A. Hi Sam. As you see, we appended your inquiry to an earlier thread on similar subjects, and the answer is, no, blasting is not a substitute for passivation. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Any time you modify stainless by blasting, grinding, welding, etc., it needs to be re-passivated to get proper corrosion protection. For large parts a big pickling bath works best. For small local areas electrochemical cleaning systems work awesome. If you're doing a small job with local cleaning you might be able to get a demo model from a local dealer.

A. Muhammad,
In theory, a stainless surface that has been contaminated with iron needs to be passivated, probably for a longer time than the standards typically suggest.

However, contamination from blasting with steel shot is extra bad, due to the contamination being beat deep into the surface. As such, a heavy dose of pickling (or nitric acid passivation) may be the best remediation. Non-ferrous blasting followed by passivation may be viable also, though it is possible that the blasting may drive the contamination further into the surface rather than removing it.

A. I think the easiest way to fix the problem is to re-blast with a 70 grit crushed glass to remove all the iron contamination and then glass bead blast if you want it to be a smoother and shinier surface.

A. Hi Mike,

I think, like you say, the correct form is blasting, passivating, and then priming or direct painting.

Blasting exposes the inner side of the metal, and it is, as you say, "active". You then, inactivate it with passivation. Passivating makes a chrome oxide to appear between the inner part and the air, this chrome oxide has good properties like anchoring the paints or primers and giving the material good corrosion resistance.

You BLAST to get ride of scales, rust, etc., OR to get an uniform, rough surface. This rough surface makes paint to get more thickness, and to improve adhesion.

The only way I can think you would be getting some disadvantages passivating the surface prior to painting, is electrostatic paint. Chrome oxide is an insulator, and you may get less thickness passivating prior to paint. But, my recommendation, if you passivate-blast-paint, do not wait more than a few hours between blast and paint. And I think passivating is not necessary that way.

Hope you don't get more confused after this! Regards,

A. Mike,
It seems to depend on the coating. I think for most things, the surface texture matters far more than whether or not the surface was passivated after blasting. For some coatings it may work better with passivation, or without passivation, or with an activation (e.g. nickel strike) treatment.

The only conceivable argument to passivating before blasting is if there's a terrible amount of iron contamination on the surface and there's a concern that the blasting will drive some of it deeper into the surface rather than removing it. You would still want to passivate after blasting also, though.

A. Hi Mike,

As has been posted, it depends on the desired results. I see passivate called out all the time prior to post surface treatments which would destroy the "passive" surface. In many cases engineers have called out for this as a precleaning more than anything else. They know the grit blasting will roughen up the surface to allow for better grip of the primer into the roughness of the surface.

A. I've Worked in architectural metals for the last 20 years. We discovered some years ago that different types of wire wheel brushes, contaminated Scotch-Brite and sanding discs would embed carbon into the stainless steel.

Also, not sure if already mentioned but T304 does rust, and is magnetic. We use T316L stainless for applications prone to corrosion. The bead blasted surface will still provide an ideal "substrate" for rust, etc.