Dr. Spine,
In evaluating our trademarked Tiger Stripe finish, we discovered the findings that you are speaking of.
The black area you are referring as carburization is more correctly referred to as "heat tintinting". It is actually oxidation (a stage of rust). I am not trying to sound condescending, I just wish to answer your excellent question completely.
The blackening of the surface occurs in tempering when oxygen is present at high temperatures. This surface oxidation is normal and uniform. (good thing)
If surface interuption were to happen during our hardening cycle, or a non-uniform oxidizing of the tool during our tempering, this would indicate a problem known as decarburization. (bad thing)
The factors that come to play when evaluating corrosion resistance are not that many, however they are complex.
For example:
The corrosion resistance of ATS-34 is partly dependant upon the temperatures used in the tempering procedures. Higher temperatures yield lower corrosion resistance. The reason for this is that at the higher temperature, more of the Chromium is formed into carbides and less is available for corrosion reisitance.
Therefore, the decision to lower the temperature and achieve better corrosion resistance must be weighed against the desired outcome of the tool. By lowering the temperatures and affecting hardness, toughness and wear resistance, you may be making the wrong move for the intended purpose of the tool.
This is an example of just one factor of corrosion resistance and is identified here to point out STRIDER KNIVES Standard Operating Procedures. If we have to sacrifice one facet of our product in order to better accomplish another, we will choose the procedure that will give us the chest pounding, man killing, war tool useage requirements that we demand.
So, to end this horrific, long reply... yep, thats rust.
Wipe the knife down with some oil and drive on.
Duane Dwyer