Why does Chromium oxide form before ferrous oxide?

[HyperSniper]

It's been a while since I've had a Chemistry class, and I've been trying to get a more detailed answer as to how stainless steel works. Specifically, why does Chromium oxide form on stainless steel at a rate faster than that of rust (ferrous oxide)? What determines the rate at which these two reactions take place?

 

[mheslep]

It's been a while since I've had a Chemistry class, and I've been trying to get a more detailed answer as to how stainless steel works. Specifically, why does Chromium oxide form on stainless steel at a rate faster than that of rust (ferrous oxide)? What determines the rate at which these two reactions take place?

I don't know, but I suspect you have the right words in asking about http://en.wikipedia.org/wiki/Reaction_rate" explains the difference in the rate of the two reactions.

 

[HyperSniper]

Ah, looks like those two things look like good places to start investigating. Thanks.

If I find come to a good enough answer I'll come back and post it. If I haven't posted anything yet, that probably means I don't know. So if any kind soul would like to enlighten us go ahead an post an explanation in this thread.

 

[HyperSniper]

Okay, I think I know what the reason is now. The thing I overlooked was the fact that they are both oxidation reactions, so it would be little bit easier to compare them that way.

Chromium oxide forms faster than ferrous oxide is because of Chromium's postion on the electrochemical series. Chromium has an electrode potential of -0.744 while iron has an electrode potential of -0.037, which means Chromium is a better reducing agent. Part of the reason that the electrode potential is greater is because there are less electrons to remove from the outter most energy level. Therefore, the activation energy is far less, which means that the reaction is a lot faster.

Have I overlooked anything or made a mistake in my reasoning?

 

[lightarrow]

Chromium is also more reducing than Iron, so its oxidation is favorite energetically.

 

[Borek]

I don't think it is rate of the reaction that counts here. From what I know once chromium oxide layer covers the steel it isolates it from the atmospheric oxygen, this is called passivation.

 

[HyperSniper]

No, the rate of the reaction definitely counts for something. If the stainless steel is scratched lightly it still won't develop rust. If you scratch it too much it may rust because there is not enough chromium to prevent it though, depending on what type of stainless steel it is.

When it is first made no passivation layer is put on the metal, it develops on its own. It develops because the reaction is faster than that of ferrous oxide, therefore preventing the iron from oxidizing.

If I've got time later I'll try and get some math and citations up to prove this.

 

[Borek]

Intuition tells me it doesn't have to be faster to prevent iron oxidation, but if you will find something more reliable than intuition - that's OK with me