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
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"
R&D practical scientist
Chesham, Bucks, UK
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 scally. 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.
- Mumbai, India
March 27, 2012
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.
- Bjornevatn, Finnmark, Norway
A. Hi Lars.
It's tough to say precisely what is happening. I think chlorine gas is released from the process, with the reaction being:
2NaCl + 2H2O => Cl2 + 2NaOH + H2
But I would note that this reaction does not run cleanly to completion; the formula only suggests overall a lot of what can happen. To get it to run to completion requires diaphragms, recycling, and other complications. You can look up chlor-alkali for the industrial processes of obtaining the resulting chemicals from salt brine.
But I also believe that as the alkalinity rises from the generation of NaOH, that some chrome does start dissolving into the solution. When chrome-bearing items are placed in a strong alkali electrocleaning tank, the chrome can dissolve as a yellow chromate.
Ted Mooney, P.E.
Pine Beach, New Jersey
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