I didnt mean for my earlier comments to sound too harsh, concerning my knifemaking brethren. They are serious crafters, earnestly seeking information on how to make the best and most beautiful blades possible. Their sincere desire for knowledge can leave them all too willing to accept information at face value, rather than questioning why.
The knife fella may be referring to AISI 13XX "Manganese Steel" or AISI 15XX Carbon Steels
I know the knifemaker discussions about high manganese steels etching darker, did not include the 1300 or 1500 series of steels. The considerations were most often between the use of 1095 and 1084 steels, in making damascus steel. Below is a link to one of these discussions.
Knifemaker Discussion Electrochemical Corrosion of Steel:
Electrochemical Corrosion The etching of steel is an electro chemical reaction. The etchant solution acts like countless microscopic batteries, which attach to the metal surface and conduct an electron exchange with the metal. The attachment points for these microscopic batteries, are the metals grain boundaries. The more numerous the grain boundaries, the more nucleation points for the chemical reaction. Steel with a finer grain structure, will etch much more quickly than a steel with larger grain structure.
As manganese promotes hardenability and fine grain structure, it is not surprising that steel with a high manganese content has the potential to etch darker. However it is not the manganese itself that affects the etch, it is simply the fine grain structure of the steel. If a steel with low manganese and a high manganese steel are heat treated to have the same grain structure, both would etch similarly.
In regards to harder steel etching darker than soft steel, it is not the Rockwell numbers that cause the difference. It is the fact that hardened steel has a finer grain structure than unhardened steel.
