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What a passivated oxide layer on stainless does

Could someone explain to me the role of a passivated oxide layer in influencing corrosion resistance of stainless steels for corrosion like uniform, pitting and stress corrosion cracking?

Bran C.
student - Limerick, Ireland
2005

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The oxide on stainless steel is chromium oxide. It is a thin, tenacious, opaque oxide that is difficult to see, so it doesn't give the appearance of being "rusty". Furthermore, it forms a mixture of oxide and hydroxides of different (chromium) oxidation states. As with many hydroxides, it is formed by the reaction between the oxide and water, but when the oxide is converted to hydroxide, it swells, making the layer more impervious to the corrosive media. In this sense, the passivating layer is "self sealing"

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2005

Pickling & Passivation process removes free iron contamination left behind on the surface of the stainless steel from machining and fabricating. These contaminants are potential corrosion sites that result in premature corrosion and ultimately result in deterioration of the component if not removed. In addition, the passivation process facilitates the formation of a thin, transparent oxide film that protects the stainless steel from selective oxidation (corrosion).
The failure of stainless steel components due to corrosion can reach catastrophic proportions. The cost in time, materials, rectification and lost production can be tremendous. If passivation is not carried out stainless steel can rust due to surface free iron, and since chlorides are also often absorbed from the atmosphere, some ferric chloride will be produced. Prolonged contact with ferric chloride will eventually initiate pitting and crevice corrosion on the stainless steel surface. Proper passivation will assist in the optimal restoration of the chrome oxide passive layer.
Any mechanical treatment damages top protective chromium oxide layer of stainless steel components. It damages by:
· Ferritic contamination matter.
· Structural changes in top layers of stainless steel.
· Inbuilt stress development.
· Chromium content reduction on top layer of stainless steel.
Even high temperature applications like welding & annealing makes surface of stainless steel discoloured and oxide scaley. This makes value of stainless steel to reduce not only by appearance but also with lower corrosion resistance due to ferric oxide. Due to these contaminations protective layer of chromium oxide can not form at that spot and corrosive stain builds up on insufficiently passivated stainless steel.

Properly pickled & passivated stainless steel and welding seams offers following qualities,
1. Smooth surface, which is metallically pure & free from any discolouration and scale.
2. High level of corrosion resistance of the S.S. component.
3. Good appearance.

I hope this will answer your question.

Vinay Golwala
- Mumbai, India
2005

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Q. Hi. I was watching a video on YouTube where a kid electrolyzes salt water in a glass using two stainless steel spoons as electrodes: The link is here:

If you read the publisher's notes below the video, he says that after a long period he sees a greenish algae-like substance in the glass. My guess is that this is chromium oxide (green) from the coating on the spoons. Am I right? If so - or otherwise, what are Your thoughts on this phenomena, why does it occur, and why first after a long while? Is it perhaps because 'all' the Cl- ions in the solution is 'used up'?

Thanks in advance.

Lars Tuff
- Bjornevatn, Finnmark, Norway
March 27, 2012