Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: chris on 23/01/2016 12:37:28
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"Stainless" steel is made by adding small amounts of chromium or nickel to the iron. The resulting alloy does not corrode so easily.
But why is this? How does the admixture of chromium or nickel atoms retard the oxidation process?
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All metals will form an oxide layer on the surface in our atmosphere. But some metals form a "passivating" oxide layer, which protects from further oxidation. Iron and copper do not--as apparent by how quickly they corrode. Metals like aluminum, despite being much more easily oxidized than iron, are much more resistant to corrosion because the oxide layer is impervious.
Adding some nickel or chromium to steel allows the surface oxide layer to be much stronger, and prevents further oxidation.
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The crucial mechanical characteristic of the passivating layer is that the metal oxide takes up slightly more volume than the original volume of metal, forming a solid barrier which is tightly bonded to the underlying metal surface, inhibiting further access by oxygen. Chromium, Titanium and Aluminum do this.
If the oxide shrinks away from the metal surface, more oxygen can reach the metal surface, allowing further oxidation. This is why iron rusts or corrodes.
A cosmetic benefit occurs if the passivating layer is transparent, giving the object a fresh metallic sheen, rather than patchy black, as in tarnished silver. Various colored treatments are also available aluminum.
For more corrosive environments or longer-term protection, chemical treatment is often used to form a more robust passivating layer. Painting is also an option on most metals.
See: http://en.wikipedia.org/wiki/Passivation_(chemistry)
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So does the chromium form an oxide on the surface, or does it merely sacrificially protect the iron that contains it? I note from the reactivity series (https://en.wikipedia.org/wiki/Reactivity_series) that chromium is a bit more reactive than iron and therefore could act as a sacrificial anode, could it not?
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So does the chromium form an oxide on the surface, or does it merely sacrificially protect the iron that contains it? I note from the reactivity series (https://en.wikipedia.org/wiki/Reactivity_series) that chromium is a bit more reactive than iron and therefore could act as a sacrificial anode, could it not?
My understanding is that it (the Cr or Ni) actually concentrates to the surface to form the passive layer.
It should also be pointed out that some alloys are much LESS corrosion resistant than either of their components. For instance, mercury will react with aluminum to form an alloy that will oxidize before your very eyes!
(the white solid being formed here is aluminum oxide, but it cannot bind to the alloy as well as it does to pure aluminum, so the corrosion continues...)
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does the chromium form an oxide on the surface?
Stainless steel alloys consist of 13%-26% chromium.
- The iron oxidises, shrinks and falls off. In bulk, iron oxides are the familiar rust-red color.
- The chromium oxidizes and remains bonded to the metal, producing a thin, clear layer which protects the metal underneath.
I presume that the initial oxide layer would be enhanced by chemical means in the factory.
But scratches occurring during normal use are self-healing.
See: http://en.wikipedia.org/wiki/Stainless_steel#Oxidation
mercury will react with aluminum to form an alloy that will oxidize before your very eyes!
This is why mercury is banned on planes (planes are mostly built from aluminum alloys).
Barometers and thermometers are banned - old barometers and thermometers were filled with mercury.
Modern smartphones often have a barometer and thermometer, but these are built out of silicon, and aren't a threat to planes.
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My guess is iron will oxidize to form iron +2 and iron +3. The oxygen will gain two or three iron electrons to form rust. The most common oxidation states of chromium are +2, +3, +6. This means the oxygen can get more electrons from chromium since it will release up to six electrons to oxygen. The chromium acts like a firewall, does for a computer, taking the bulk of the oxidation hit to protect the iron.
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My guess is iron will oxidize to form iron +2 and iron +3. The oxygen will gain two or three iron electrons to form rust. The most common oxidation states of chromium are +2, +3, +6. This means the oxygen can get more electrons from chromium since it will release up to six electrons to oxygen. The chromium acts like a firewall, does for a computer, taking the bulk of the oxidation hit to protect the iron.
No, it's not just an issue of the chromium being more easily oxidized. If it were that simple, magnesium, aluminum or zinc could be used for the same purpose. Sometimes we do use one of those more easily oxidized metals as a sacrificial metal to protect iron, but stainless steel is a different animal.
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The most common oxidation states of chromium are +2, +3, +6.
The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen.
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No, it's not just an issue of the chromium being more easily oxidized. If it were that simple, magnesium, aluminum or zinc could be used for the same purpose. Sometimes we do use one of those more easily oxidized metals as a sacrificial metal to protect iron, but stainless steel is a different animal.
So why doesn't the chromium act in a sacrificial manner?
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The Cr may may well get oxidized before the Fe, but if it did not also form the passivating layer on the surface it wouldn't really matter. The Cr is still part of the steel, so if it corrodes, the steel corrodes. By forming a passive oxide surface, the Cr content actually prevents the oxidation reaction from occurring (or at least slows it down by a few orders of magnitude)