Originally Posted by
andrew375
It is actually EROSION not corrosion, that gets linked to double base powders. This comes from use of Cordite which contained large amounts, up to 40% depending on type.
Single base powders are oxygen poor. That means that there is not enough oxygen present to burn all the fuel. The result is a Nitrogen rich gas pushing the bullet out. Some of this Nitrogen links itself to the Iron in the barrel to produce Iron Nitride, a very hard (but brittle) inter-metallic compound which then flakes of on subsequent shots; this is the mechanism of barrel erosion. This only occurs near the chamber due to the heat required.
One of the reasons for adding NG is that it increases the oxygen content of the propellent. In cordite the high NG content resulted in very oxygen rich propellent gases. In this situation the free oxygen combined with the Iron in the barrel steel to make Iron Oxide, AKA rust. This too is an inter-metallic but the amount of barrel affected is much more pronounced. Coupled with the fact that cordite loaded ammo was only ever encountered loaded with corrosive primers and we get the stigma of barrel erosion linked to NG containing propellants.
The amounts of NG added to modern double base propellants for use in small arms is considerably less, rarely more than 5%.
Superb post, thank you. 
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