Stainless Steel --> Iron Oxide? 2

[32chevy]

From my understanding, stainless steel is an iron alloy consisting of carbon, chromium, nickel. Chromium and nickel atoms form oxides that seal iron off from oxidizing.

How do the oxides seal the iron, does it have something to due with the crystal structure?

How exactly does does iron oxide occur from abrasion with a steel object?



Field Test Engineer w/BSME

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[berkshire]

'''How exactly does does iron oxide occur from abrasion with a steel object? '''


Non stainless particles ( free iron ) are transferred to the surface of the stainless part from the steel object.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[32chevy]

Then oxidation is of the steel object particle thats creates a stain on the stainless steel object? If true then it shouldn't be a problem removing the stain from the stainless.


Why/how does a stainless steel weldment some times form iron oxides at its welds

Field Test Engineer w/BSME

I heart Beer Brewing & Traditional Hotrods

 

[Rich2001]

The Chromium forms an thin hard passive film on the exposed surface, protecting the substrate. If the Chromium oxide is remove , via erosion etc, the corrosion will continue. If by welding of other mechanisms if carbon is present Chromium carbides are formed in lieu of the chromium oxides. These carbide do not protect the surface and one have a localized corrosion issue in the chromium depleted zone, weld decay.

Richard

http://www.6sproductivitycom.com

 

[redpicker]

Then oxidation is of the steel object particle thats creates a stain on the stainless steel object? If true then it shouldn't be a problem removing the stain from the stainless.

As Rich2001 alluded to, the "stainless" property of Stainless Steel results from a thin protective layer of Chromium Oxide forming on the surface. If abrasion of this protective layer occurs as a result of, say, rubbing against a carbon steel component, you will get some of that carbon steel embedded in the surface, as mentioned by berkshire. This will oxidize to form an oxide that does not have enough chromium in it to be protective, but has a higher chromium oxide content than is normally found in carbon steels.

You can clean the surface, but unless you completely remove the "stain", it will remain (obviously). This "stain" is a often a mixture of iron and chromium oxides and it can be very hard. It usually takes abrasive methods to remove it. Of course, you are also removing the protective chromium oxide layer adjacent to the "stain", so you would want to allow the surface to "passivate" prior to putting such a cleaned part back into service.

rp

 

[berkshire]

You can test for free iron on the surface with a feroxyl test.
If the tested area turns blue you have free iron present.

Some juristictions are now limiting this test, because it uses cyanide compounds.
It is also possible to remove the stain by electro passivation.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[EdStainless]

If there is enough Fe on the surface then as it corrodes (rusts) to local chemistry may be aggressive enough to break down the passive Cr oxide layer. This would lead to pitting of the stainless itself.

There are other tests for free Fe on the surface.

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Plymouth Tube

 

[abramb]

Why/how does a stainless steel weldment some times form iron oxides at its welds

32chevy,
As suggested by berkshire, Feroxyl test can be used to detect free iron on surface of Stainless Steel (SS).

A possible cause of iron oxides at welds could be also the welding consumable used, if not appropriate the local Chromium content could fall down and avoid the formation of Chromium Oxide.

Other possible causes for oxidation of stainles steel can be:

- uncorrect sandblasting consumable (e.g. abrasive for carbon steel material grades used on SS material),
- pickling of SS material not sufficient,

If present, the carbon steel embedded in the surface is very difficult to remove and it usually requires long/aggressive pickling.

From my experience, the best way to remove free iron/Fe is to re-perform sandblasting with appropriate abrasive for Stainless Steel materials, followed by an accurate pickling.

Hope this helps, regards.

Attilio Brambilla
Material Engineer

http://it.linkedin.com/pub/attilio-brambilla/25/465/544