Passivation. 5

[mechantaeus]

Group:

What is the advantage of passivation; are there any cheaper alternatives? Will using corrosion resistant SS help?
Thank in advance for all your help.

Regards,

-mechantaeus

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Work Hard and Work Smart.

 

[israelkk]

Passivation is a treatment for stainless steels to enhance their natural resistance.

 

[tomwalz]

Passivation
We use it to mean making the surface of a part selectively non-reactive.

We prepare ceramics for brazing though an aqueous plating process. The goal is to make a surface that will bond readily and well with standard braze alloys. However we may prepare parts here and ship them elsewhere.

Brazing is both a physical and a chemical joining so you have a surface ready to react chemically. If that surface reacts with oxygen you can get an oxide layer that will hinder a good bond between the braze alloy and the ceramic underlying the oxide layer. It is like painting over rust. It will look good but will not hold well.

To prevent this we do a final, quick dip that will passivate the surface and make it less reactive or selectively non-reactive.

Often passivation is to protect a finish and prevent discoloration.

One way to passivate is to do a final rinse in deionized water. In our experience this can help prevent discoloration of final parts.

I suppose the final rise could also be considered passivation.

Tom

 

[Kenneth]

Passivation of stainless steels involves the use of an acid solution to remove iron particles that become embedded in the surface as a result of handling, machining, grinding, etc. Stainless steel that is contaminated with iron in this manner (and that is not given a passivation treatment) will develop surface corrosion.

http://www.azom.com/details.asp?ArticleID=1142#_Passivation

 

[yates]

Passivation is a term used to describe dipping stainless steel components in an acid bath (often nitric, often with bichromate added). It affords some degree of additional corrosion protection when applied to martensitic (400 series) stainless steels, but when applied to austenitic stainless steels (300 series), as Kenneth says, it should really be reffered to as a 'decontamination' since it removes any embedded ferritic particles (e.g.from grinding wheels used for carbon steels) from the surface. Strictly speaking, passivation as such does not improve corrosion resistance of austenitic stainless steels, but it does avoid corrosion of any surface-embedded ferritic particles by removing them, which really amounts to the same thing.

 

[mcguire]

Passivation has two meanings. One is, as stated above, the process of acid cleaning which removes foreign matter, especially iron, which hinders the formation of a sound passive film. The other more fundamental meaning relates to the formation of the passive film itself, which on stainless steel forms spontaneously in air or water. It consists of several atomic layers of oxygen and hydrogen bonded to the chromium-rich surface in such a way that ionic conductivity through the layer is negligible, hence precluding dissolution of the alloy.

The only problem with the first meaning is that it leads some people to believe that the acid wash process creates the passive film rather than simply removing contamination which hinders its formation; a small but important distinction.

Incidentally, a acid wash can also pickle. This process is the removal by dissolution of surface layers of the alloy. After oxidation the surface can have lower levels of chromium than the bulk composition and if this layer isn't removed then the lower chromium surface produces a passive film which is "weaker" (actually thicker, but less effective in preventing corrosion). Some passivation is done with strong enough acid than some pickling occurs, which is why some people think passivation creates the corrosion resistance. After welding when scale forms and is removed abrasively, pickling must be done to remove the chromium-depleted surface. At the same time oxide particles are also removed, so two important things are done in one process. As if this isn't confusing enough, the weld's corrosion resistance is still inferior because there can remain some microscopic chromium-depleted regions around manganese sulfide inclusions in the weld zone. Electropolishing or chemical polishing can remove these pit-initiating zones and restore full corrosion resistance.

Too much information?

 

[mechantaeus]

THANK YOU all, this info was really very useful in understanding the concept.

-mechantaeus

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Work Hard and Work Smart.