Why does Chromium oxide form before ferrous oxide?

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Discussion Overview

The discussion centers on the comparative formation rates of chromium oxide and ferrous oxide on stainless steel, exploring the underlying chemical principles and factors influencing these rates. Participants delve into oxidation reactions, electrochemical properties, and the concept of passivation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about the reasons for the faster formation of chromium oxide compared to ferrous oxide and seek to understand the factors determining these reaction rates.
  • One participant suggests that the difference in reaction rates may be explained by the electrochemical series, noting that chromium has a lower electrode potential than iron, which could imply it is a better reducing agent.
  • Another participant emphasizes that chromium's oxidation is favored energetically due to its reducing properties.
  • Concerns are raised about the role of the chromium oxide layer in passivation, which isolates the steel from atmospheric oxygen, potentially affecting oxidation rates.
  • Some participants argue that the rate of reaction is significant, particularly in the context of scratches on stainless steel and the subsequent risk of rust formation.
  • One participant expresses skepticism about the necessity of faster reaction rates for preventing iron oxidation, suggesting that intuition may not suffice without empirical evidence.

Areas of Agreement / Disagreement

Participants express differing views on the importance of reaction rates in the context of oxidation prevention, with some emphasizing the significance of the chromium oxide layer and others questioning whether speed is the determining factor. The discussion remains unresolved regarding the relative importance of these factors.

Contextual Notes

Participants reference the electrochemical series and activation energy but do not provide detailed mathematical or empirical support for their claims. The discussion reflects varying levels of certainty and understanding regarding the chemical processes involved.

HyperSniper
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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?
 
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HyperSniper said:
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.
 
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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.
 
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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?
 
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Chromium is also more reducing than Iron, so its oxidation is favorite energetically.
 
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.
 
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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.
 
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 :wink:
 
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