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Author Topic: Why Stainless Steels are corrosion resistant????  (Read 3601 times)

Offline Femi

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Why Stainless Steels are corrosion resistant????
« on: 22/10/2010 10:13:38 »
 Stainless newbielink:http://www.regentsteel.com/stainless-steel.html [nonactive] are corrosion resistant as it has minimum chromium content. Therefore it doesn't corrode or stain and neither it get rusted.

The above information I got from an article but question is whether chromium is element which leads to rusting and why it is used even if in a minimum quantity if it leads to rusting???
Any one can help me out.... ???


 

Offline lightarrow

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Why Stainless Steels are corrosion resistant????
« Reply #1 on: 22/10/2010 11:23:24 »
Chromium forms a thin layer of very resistant chromium oxide Cr2O3 at its surface, immediately after contact with air oxygen. This oxide is what protect it and stainless steel from rust.
Since it is required just a thin layer of this oxide, 10 percent of chromium in the alloy is enough.
« Last Edit: 22/10/2010 11:26:30 by lightarrow »
 

Offline Geezer

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Why Stainless Steels are corrosion resistant????
« Reply #2 on: 22/10/2010 19:41:08 »
Chromium forms a thin layer of very resistant chromium oxide Cr2O3 at its surface, immediately after contact with air oxygen. This oxide is what protect it and stainless steel from rust.
Since it is required just a thin layer of this oxide, 10 percent of chromium in the alloy is enough.

Aluminun/aluminium is similar. It's so highly reactive, it very rapidly forms a layer of oxide on it's surface that protects the underlying metal.

I seem to remember seeing steel barriers along roads that were made intentionally "rusty" (at least it looked like rust to me). Any ideas about what they maybe did to them Lightarrow?
 

Offline Vereava

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Why Stainless Steels are corrosion resistant????
« Reply #3 on: 22/10/2010 23:43:30 »
Aluminun/aluminium is similar. It's so highly reactive, it very rapidly forms a layer of oxide on it's surface that protects the underlying metal.

Is this why aluminum cools so quickly that you can pick it up after it has been in an oven? (Don't try this, I will say I do it all the time and I don't get burned from the aluminum, but the food in the aluminum or the oven itself may burn you)
 

Offline Geezer

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Why Stainless Steels are corrosion resistant????
« Reply #4 on: 23/10/2010 02:56:44 »
Aluminun/aluminium is similar. It's so highly reactive, it very rapidly forms a layer of oxide on it's surface that protects the underlying metal.

Is this why aluminum cools so quickly that you can pick it up after it has been in an oven? (Don't try this, I will say I do it all the time and I don't get burned from the aluminum, but the food in the aluminum or the oven itself may burn you)

I don't think so. I think you are talking about aluminum foil? If so, I think the reason it cools down quickly is because it has a large surface area relative to it's mass, so the heat transfers rapidly to the surrounding air. Aluminum is also a very good conductor, so that also helps the heat to transfer quickly.

Different materials have different heat capacities, but Al does not seem to be too different from other common metals, so I don't think the heat capacity (aka Specific Heat) has too much to do with it.
 

Offline Geezer

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Why Stainless Steels are corrosion resistant????
« Reply #5 on: 23/10/2010 04:48:38 »
BTW - I should have added that the anti-corrosion properties of stainless steel and aluminum are chemical processes. There is a chemical reaction between the metal and the oxygen in air.

The heating effect with aluminium, or anything else for that matter, is a physical process. The chemical composition of the metal was not changed, although it is possible that heat will alter the internal structure of the metal (it could be softer, or tougher etc.)

Of course, heat may help to speed up a chemical reaction too!
 

Offline lightarrow

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Why Stainless Steels are corrosion resistant????
« Reply #6 on: 23/10/2010 13:14:08 »

I seem to remember seeing steel barriers along roads that were made intentionally "rusty" (at least it looked like rust to me). Any ideas about what they maybe did to them Lightarrow?
Don't know, maybe it was not intentional, maybe it was with the aim to avoid light reflecting off to the drivers' eyes.
 

Offline imatfaal

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Why Stainless Steels are corrosion resistant????
« Reply #7 on: 23/10/2010 14:38:49 »
Quote
I seem to remember seeing steel barriers along roads that were made intentionally "rusty" (at least it looked like rust to me).
Crash investigation would be made a lot easy - even the slightest graze would show up , but no way they would build for that reason.  I reckon lightarrow got it right off the bat
 

Offline Geezer

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Why Stainless Steels are corrosion resistant????
« Reply #8 on: 23/10/2010 18:45:30 »
Thanks guys. Good ideas. Any thoughts on what process they used? I have to think it was some special chemical treatment that created some sort of corrosion barrier. Rust on regular steel does not do that, or does it?
 

Offline imatfaal

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Why Stainless Steels are corrosion resistant????
« Reply #9 on: 23/10/2010 20:54:40 »
You sure it wasn't red lead paint?  I have an inkling that that's what made the forth bridge red - and it's a fine corrosion resistant for steel structures
 

Offline Geezer

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Why Stainless Steels are corrosion resistant????
« Reply #10 on: 23/10/2010 21:29:38 »
You sure it wasn't red lead paint?  I have an inkling that that's what made the forth bridge red - and it's a fine corrosion resistant for steel structures

I'm sure it was not paint of any sort. It just looked like rust, but it was completely uniform as if it had been created chemically. It was on a bridge - it was the crash barriers along the Erskine Bridge. Mind you, this was shortly after it was opened, so it must be coming up on fifty years ago.
 

Offline Bored chemist

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Why Stainless Steels are corrosion resistant????
« Reply #11 on: 24/10/2010 10:26:16 »
50 years ago it would have been red lead paint.
 

Offline Geezer

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Why Stainless Steels are corrosion resistant????
« Reply #12 on: 24/10/2010 17:31:06 »
50 years ago it would have been red lead paint.

Well, of course, they hadn't invented galvanizing that far back, so they had no other choice!

Nope - definitely not red lead paint.
 

Offline simplified

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Why Stainless Steels are corrosion resistant????
« Reply #13 on: 24/10/2010 18:27:46 »
Forged cooled down steel can be red.You can look in a smithy.
 

Offline Geezer

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Why Stainless Steels are corrosion resistant????
« Reply #14 on: 24/10/2010 21:31:56 »
Forged cooled down steel can be red.You can look in a smithy.

Ah yes. I know what you mean.

These barriers simply looked as if they had a layer of brown rust on them, although it was very uniform. This was around the time the bridge opened, so maybe they were in a hurry to complete the project and had to use them before they could be galvanised. I always assumed it was some clever anti-corrosion technique, but maybe it was not so clever.
 

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Why Stainless Steels are corrosion resistant????
« Reply #14 on: 24/10/2010 21:31:56 »

 

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