The nitrocarburizing process improves corrosion resistance, and an even greater increase is seen in low alloyed and unalloyed steels after QPQ treatment. During post-oxidation, a thin FE3O4 layer is formed on the outer compound layer and, at the same time, the pores are sealed with oxide.
Often the corrosion resistance is better than obtainable with other surface-engineering processed, such as hard chrome or nickel plating.
The graph below compares the corrosion resistance of layers formed by galvanic and chemical plating processes with those imparted by nitrocarburized QPQ 1. This test measured the number of hours until the first rust spot developed.
Corrosion resistance, QPQ evaluation by means of salt spray test, DIN 50 021 SS