I really had not thought a lot about it until you put this post up and then this appeared...
Gonna have to get one of these!
Nitrate finish on the outside of a GM SS barrel.
I have several parts that have been Nitrate and I really like them. The finish is extremely durable... and wow! does it clean easy as well as protect.
This is a Nitrate treated bolt that I have been using for 3 years now... It shows no sign of wear and tear...
Black nitride coating
A black nitride surface is very corrosion resistant, it doesn't corrode underneath as chroming or rust through as parkerizing does and is very durable. It is NOT black paint or a surface coating.
This link will provide information about the hardening process - using the Nitrate system...
https://en.wikipedia.org/wiki/Nitriding