patina vs. rust

[rayback]

1. What's the difference between patina and rust? which one of them harms the knife most?
2. Is there any risk of rust forming on a O1 tool steel, when sharpening it on a wet benchstone?

Thanks
Ray

 

[Bill DeShivs]

Patina is a gradual aging of a substance. In the case of steel this would include oxidation (rust.) Rust will ruin any carbon steel knife, by pitting the blade.
O-1 is a great blade steel, but it will rust very easily.
Bill

 

[Dog of War]

IIRC rust is mostly ferric oxide. The non-red patina that forms on steel could be from other oxides of iron, other iron compounds, with maybe other possibilities I'm not aware of. Compared to rust patinas aren't nearly so harmful to your blade and help prevent rust, though patina forming right at the edge would degrade it.

I've seen knives rust very quickly when exposed to water, so it's possible this could happen using water when sharpening .... however as long as you're sharpening, any rust on the edge is going to be removed rapidly, so I think as long as you dry the knife off as soon as you're done it's nothing to worry about.

 

[Cliff Stamp]

1. What's the difference between patina and rust?

Both of them are oxides of iron. They type which is commonly called rust is orange (Fe2O3) which is harmful because it is a volume expansion and thus is flaky and tends to eat into steels. There is also Fe302 which is blackish and what is called a patina. It tends to form under more acidic conditions and will form a sort of coating which will thicken over time. However the black oxide can be severely damaging as well, it just depends on the steel and type of exposure. I have seen deep pitting of black rust on D2 and typical high carbon stainless.

Is there any risk of rust forming on a O1 tool steel, when sharpening it on a wet benchstone?

No, the exposure time is too short. I have seen it on 5160 when sharpening on a belt sander because the heat will accelerate the reaction and I have seen it form in the few minutes from walking up from the basement with the still damp knife.

-Cliff

 

[rayback]

Thanks for the replies.
So as soon as I finish sharpening an O1 tool steel on a wet benchstone, what's the first thing I should do? washing the knife and then oiling it?

I have read many threads where people have talked about difference stuff to use, Flitz, mineral oil, olive oil..... etc. but I couldn't nail down which one is best to use to protect the knife after exposure to water.

Thanks
Ray

 

[sevenedges]

Sharpen, dry, lube. I use ezzox. Do a search for it and you will be quite impressed with it's corrosion resistance performance. I don't think it is food grade but for any other applications it's the best I've used so far.
Jeff

 

[Eager]

Rust is the reddish corrosion product formed by electrochemical
interaction between iron and atmospheric oxygen. It is Fe2O3 (ferric oxide); with
iron(Fe) with a valence of 3. I think that the black patina on steel is Fe3O4 (ferrous ferric oxide) which has two of the irons in Fe3O4 in the +3 valence state while one is +2. Rust (Fe2O3) always has waters of hydration (water molecules attached), which may cause that expansion that Cliff's talking about.