Patina clarification

[akadave2]

So as I understand it from the comments I read here and there in this forum, patina helps prevent deep pitting and red rust? Or is it purely a cosmetic thing?

 

[Will Power]

Patina is rust It's just a less ferocious form of red-rust.

It certainly protects against pitting, and when it's even, looks good.

 

[akadave2]

Ah yes, knew it was a form of oxidation (rust) but wasnt sure that it helped to have an even patina as far a protecting from ugly pitting etc.

 

[Obsessed with Edges]

Red rust is oxidation of iron in the presence of water/moisture, which adds hydrogen to the reaction, and alters the structure of the oxide in such a way as to make it occupy more volume than the previously unoxidized steel. It also weakens the structure of the oxidized metal. This is what makes it crumble, flake and slough off, and why it causes pitting.

'Patina', or black iron oxide, is a reaction between iron and oxygen only, and doesn't alter the steel in the same way. It just forms a stable oxide layer on the exposed surface of the steel, and that's it. That stable oxide layer makes the iron less reactive to the other agents that cause rust. It's harder and more durable, and doesn't crumble or flake or slough off on it's own (without significant abrasion).

In other words, rust is literally destructive to steel, because mass is lost (by erosion) in the process. That doesn't happen with black oxide (patina).

Sometimes, the two oxides happen together, and this may lead some to believe that they're one in the same. When forcing a patina with vinegar or other acids, some red rust will usually form. But, when that part of the reaction ceases, and the red rust is cleaned away, the black/grey oxide remains. The fact that it remains, after the red rust is cleaned away, is the most obvious example of the difference in hardness and durability between the two.

Technically, any oxide of iron is often called 'rust', by 'experts' and others. I always cringe when I hear 'patina is rust' because, in terms of what it does (or doesn't do) to the steel, the 'rust' association with patina is completely misleading and usually draws the wrong conclusions in the minds of many, by the inference.


David

 

[akadave2]

Red rust is oxidation of iron in the presence of water/moisture, which adds hydrogen to the reaction, and alters the structure of the oxide in such a way as to make it occupy more volume than the previously unoxidized steel. It also weakens the structure of the oxidized metal. This is what makes it crumble, flake and slough off, and why it causes pitting.

'Patina', or black iron oxide, is a reaction between iron and oxygen only, and doesn't alter the steel in the same way. It just forms a stable oxide layer on the exposed surface of the steel, and that's it. That stable oxide layer makes the iron less reactive to the other agents that cause rust. It's harder and more durable, and doesn't crumble or flake or slough off on it's own (without significant abrasion).

In other words, rust is literally destructive to steel, because mass is lost (by erosion) in the process. That doesn't happen with black oxide (patina).

Sometimes, the two oxides happen together, and this may lead some to believe that they're one in the same. When forcing a patina with vinegar or other acids, some red rust will usually form. But, when that part of the reaction ceases, and the red rust is cleaned away, the black/grey oxide remains. The fact that it remains, after the red rust is cleaned away, is the most obvious example of the difference in hardness and durability between the two.

Technically, any oxide of iron is often called 'rust', by 'experts' and others. I always cringe when I hear 'patina is rust' because, in terms of what it does (or doesn't do) to the steel, the 'rust' association with patina is completely misleading and usually draws the wrong conclusions in the minds of many, by the inference.


David

Thanks David. That is the best explanation I have heard.

 

[shrapne1]

David:

thank you for that explanation, you said it 100 times better than I could.

 

[Obsessed with Edges]

Thanks David. That is the best explanation I have heard.

I've learned a ton from watching the difference between patina and 'rust' on my Schrade 8OT. A while back, I forced a patina on the clip blade, and left the spey and sheepsfoot blades 'as is'. I noticed later on, when testing a hot water/vinegar mix to change the clip's patina, the tang area that hadn't previously been 'forced' showed some visible red rust from the water/vinegar, and the previously patinated portion of the blade just turned BLACK, with no additional rust. That hammered the point home for me, that the patina really does afford some protection from rust.

In recent months here in NM, in our 'monsoon season' of higher humidity and some occasional heavy rain, the spey & sheepsfoot blades on that knife have been showing new spots developing (with pitting). The clip blade looks the same as it ever did, with a nice even grey patina. That was lesson #2, in drawing the same conclusion. I'm really wanting to go ahead and force a patina on those two blades as well, as I'm thoroughly convinced now.


David

 

[ol' smoke]

I recently ordered 4 Old Hickory 7" butcher knives from Ontario. I reconfigure the blades and put antler scales on, make a sheath. I sanded down past the grooves. I was having rust problems throughout the process. I searched the old posts here and finally did the vinegar forced patina. It came out like gun blue. I like the way it looks and believe this is going prevent future damage to the carbon steel blades. Been doing this for years - the learning never stops. Thanks and Adios!

 

[arizonaranchman]

David,

Thanks thats an outstanding lesson on the topic. I let my folders age naturally, I don't fight the process just let it happen over time. I give it a quick wipe of the blade after each use and oil the blades once a week.