Passivation VS Pickle

[ENEPPA]

I am passivating 316ss with 50%nitric and 50%water for 30 minutes at 72 degrees f and passing a copper sulftate just fine. But recently I saw a print that said pickel. Can & how do I pickle with nitric & water. My Question is what is the differance between Passivation & Pickle.

 

[mcguire]

Pickling involves removing by acid mixtures(such as nitric/hydrofloric)the low chromium surface layer which you get from prior oxidation. Passivation does not remove metal to any significant degree. If he surface has been significantly oxidized, i.e. dark scale, you need to pickle to restore normal corrosion resistance.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

Pickling is agressive enough to remove oxides from the surface, as well as metal for as long as you leave it in.
Passivation is an oxidizing environment that will only remove easily soluble metals like Fe, and only at the surface. It will also promote formation of a more solid Cr oxide layer.

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[kenvlach]

316 SS will nearly always pass the copper sulfate test w/o passivation, unless it's been contaminated by steel tooling.

Whoever specified pickling had some reason for doing so -- most commonly, to remove heat treatment scale or welding scale & heat tint. See ASTM A380 Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems for pickling solutions & instructions:

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/A380.htm?L+mystore+culy0605

Passivation in the USA is mostly specified per QQ-P-35C (old, but still common) or ASTM A967 or proprietary spec. Some use ASTM A380 although technically it's a guideline, not a spec. QQ-P-35C is a free download:

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=50793

A useful article "HOW TO PASSIVATE STAINLESS STEEL PARTS" from Carpenter Technology:

http://crswnew.cartech.com/wnew/techarticles/TA00042.html

 

[EdStainless]

ASTM A967 is the offical replacement for QQ-P-35.

In some high purity services passivation is critical. On a microscopic level you are forming a sruface layer that is very Cr rich. Instead of the standard Cr:Fe ratio of 0.25 on the bulk alloy, a well cleaned and passivated surface will have a ratio of 2 or higher. You will selectivly disolve Fe from the surface. You also form a chrome oxide layer that is denser, thicker and more chemically stable.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm