Does Carbon Steel rust?

[Grim]

Originally posted by eddies32

The addition of chromium (> 12%) causes an inert film of chromic oxide to be formed over the metal surface preventing the formation of iron oxide (rust). They are only 'stainless' in enviroments that do not damage the passive film.

 

Spot on I'd add the oxide film very thin, and is quite adherant due to an increase in volume of the metal to metal oxide causing a compressive stress in the oxide film. The oxide that forms on normal steels is slightly lower in volume, causing tensile stress, and therefore craacks form, allowing further corrosion. (the oxides are strong in compression, but not in tension).

 

I've probably gone somewhat ott on the explanation there

[nick2]

So the film doesn't wear off if the metal is inside your body (I'm thinking of piercings or those rods they put in bones) ?

[ferret]

Originally posted by nick2

Go to Mortons on West Street (next to the tram stop), they do a brilliant job sharpening knives.

 

Cheers, I'll go and see them.

[Walkerdylan]

yes it does it has to be cared for,oiled etc,but its far superior to stainless in keeping a sharp edge,it turns black after time if you dont oil it,but a quick wipe with emery cloth will bring it back to its shiny original self,i have a knife over 30 years old made of carbon,a couple of strokes on a steel and you could shave with it,

 

is there a certain grain that works best if you use emery cloth?

[Skink]

So the film doesn't wear off if the metal is inside your body (I'm thinking of piercings or those rods they put in bones) ?

 

also having degree in materials and masters in corrosion science wanted to add a little here

 

the surface of a material is generally what affects how it interacts with its environment - so surface and environment are important

 

"carbon steel" will not rust without moisture - keep it dry it will stay shiny, keep out water with oil and it will stay shiny, paint it properly and it will stay shiny (under the paint)

 

Rust is a soft oxide that is not well adhered to the surface of the metal, so it keeps rusting away. Bare metal that is outside and subject to rubbing may appear not to be rusting but it will be (railway tracks are a good example of both these mechanisms)

 

Stainless steels have a highly adherent oxide coating, <12Chromium is a minimum but this will show rust marks with salts or organic acids (think cheap cutlery left in food). Decent cutlery will be 18%Cr and 8%Nickel (called 18 8 )and is much more corrosion resistant. Once you start adding Molybdenum you are talking industrial grade corrosion resistance.

 

The thermo dynamics of the film formation on stainless steels are such that if the surface is scratched the new surface will pretty much instantly be protected

 

In vitro corrosion (ie inside the body) is a complicated subject but the materials used for pearcings etc are at a minimum going to be 18 8 (hopefully). Where there is a high wear or strength need (eg joints) highly specialised stainless steels (with Mo and Cobalt etc) or titanium are used

 

hope that helps

[depoix]

is there a certain grain that works best if you use emery cloth?

to be honest,i dont know,i just reached into the tool box found a piece of emery cloth and worked the blade with it from its back to it's edge,it came up almost as good as new and once resharpened and oiled it was back to being a great knife

[BJan]

also having degree in materials and masters in corrosion science wanted to add a little here

 

the surface of a material is generally what affects how it interacts with its environment - so surface and environment are important

 

"carbon steel" will not rust without moisture - keep it dry it will stay shiny, keep out water with oil and it will stay shiny, paint it properly and it will stay shiny (under the paint)

 

Rust is a soft oxide that is not well adhered to the surface of the metal, so it keeps rusting away. Bare metal that is outside and subject to rubbing may appear not to be rusting but it will be (railway tracks are a good example of both these mechanisms)

 

Stainless steels have a highly adherent oxide coating, <12Chromium is a minimum but this will show rust marks with salts or organic acids (think cheap cutlery left in food). Decent cutlery will be 18%Cr and 8%Nickel (called 18 8 )and is much more corrosion resistant. Once you start adding Molybdenum you are talking industrial grade corrosion resistance.

 

The thermo dynamics of the film formation on stainless steels are such that if the surface is scratched the new surface will pretty much instantly be protected

 

In vitro corrosion (ie inside the body) is a complicated subject but the materials used for pearcings etc are at a minimum going to be 18 8 (hopefully). Where there is a high wear or strength need (eg joints) highly specialised stainless steels (with Mo and Cobalt etc) or titanium are used

 

hope that helps

 

Reading that I feel incredibly stupid

 

Under which conditions would stainless steel rust? Would heat influence it at all? Thanks

[Skink]

Reading that I feel incredibly stupid

 

Under which conditions would stainless steel rust? Would heat influence it at all? Thanks

 

why should anyone feel stupid for not knowing something they werent taught/told ?

 

stainless steels dont generally "rust" - that is form a red iron oxide. As others have already said, they form a very stable passive surface because of the Chromium etc and if the surface is going to corrode it usually does so by local breakdown of that coating, that can then lead to pitting because whatever started the local breakdown sticks around. The environment need to be aggressive (eg stuff like Chlorides, Organic Acids or even worse Sulphides). Sometimes impurties in the steel will act to initiate the surface breakdown and pitting, but you still need a bad environment too

 

Heat accelerates wet corrosion, something like a 10 degree increase in temperature gives roughly a doubling of corrosion rate. If you mean high temperature corrosion then its going to have to get pretty hot (600+?)before the surface starts to be damaged and it will depend a lot on the type of stainless steel, but sorry I dont know for sure as thats not my field

[Twiglet]

So the film doesn't wear off if the metal is inside your body (I'm thinking of piercings or those rods they put in bones) ?

 

They are usually titanium (and if they are steel it's 'surgical' stainless steel')

[Cyclone]

The steels are given different hardness ratings as well, which presumably depends on their exact alloy. I had a knife stamped stainless 440 (I think the 440 is the hardness), unfortunately I lost it when I left my rucksack unzipped.