OK, in order to confuse things a little further I looked up Spydercos steel chart and the element information. From that available information I understood that it's mainly the chrome (Joe Talmadge says in the Steel FAQ that it's the free chrome thats not tied by carbon or something...) that increases corrosion resistance.
...but also
copper increases corrosion resistance,
molybdenum increases corrosion resistance,
nitrogen increases corrosion resistance.
I found no further information as to their contribution/effect to corrosion resistance other than that they increase it.
So from there I made a list of steels and amount of elements according to Spyderco steel chart:
Steel..........C%.........Cr%.........Cr/C.........Mo%.........Cu%.........N%
420J2.........0.45.........13.........29.............0................-............-
440A..........0.67.........17.........25.............0.75............-............-
AUS6..........0.60.........13.7......23.............0................-............-
440B..........0.85.........17.........20.............0.75............-............-
AUS8..........0.72.........13.7......19.............0.20............-............-
440C..........1.07.........17.........16.............0.75............-............-
154CM........1.05.........14.........13.............4.00............-............-
From here I can conclude that according to carbon and chrome 420J2 does indeed seem the most corrosion resistant and the 154CM the least. But 154CM seems to have quite a bit of the molystuff. Apparently according to better informed people 154CM is not at the top of this list so it can't help too much with corrosion resistance so I'm left with chrome and carbon.
am I even close?
Well, you're certainly in the ballpark.
What you are trying to do is put a barrier between the steel and the environment (oxygen and water). All steel oxidizes (corrodes). The difference between stainless steel and carbon steel is what happens once it corrodes. With carbon steel, the oxide layer is imperfect and has "holes" in it. Water and oxygen can get through the layer, react with the iron and the corrosion continues. With stainless steel, that first oxide layer that is only a few atoms thick has less imperfections, and it adheres better to the steel. Thus you have a layer of protection between the steel and the outside environment.
It is chromium in the oxide matrix that makes the difference. When you have carbon and chromium together, the carbon can react with the chromium to form chromium carbides. Chromium in that form is not free to form part of the oxide matrix, and the corrosion resistance drops.
Molybdinum opperates somewhat differently. It has the greatest impact on reducing pitting corrosion, as opposed to surface corrosion.
This is the first I have heard of copper reducing corrosion in steel. If you have a chunk of copper in contact with the knife it could act as a saccrificial anode. But that ain't happening within a chunk of steel.
It's not that nitrogen increases corrosion resistance, it's that nitrogen replaces carbon. And carbon has a negative impact on corrosion resistance. So it is the lack of carbon, not the presence of nitrogen that helps in a nitrogen alloy steel.
As a materials engineer who often works with corrosion problems in aircraft, I usually stop at Chromium and carbon content. But I give extra credit if the alloy has Moly. No points for copper. But the honest truth is that I look in a table and determine the corrosion resistance of the alloy at that temper and make my recommendations from there.
I should also add that corrosion is a funny beastie. It is not uncommon to put two identical samples into identical conditioins and get different results. Corrosion is one of those things where you always have to say YMMV. Gotta remember that corrosion is a very complex chemical reaction. So even when you think everything is the same, it ain't.
Hope that helps.
Knarf the engineer
