**1. Introduction**

Since the beginning of the twentieth century, stainless steel has been developed to improve the corrosion resistance of parts in contact with corrosive and oxidative media. These corrosion-resistant alloys have been used in the chemical, petrochemical, automotive, aeronautical, food, medical, and construction industries. Chromium, above 11 wt.%, grants corrosion resistance by forming a nanometric thin and adherent Cr2O3 passive layer. When exposed to oxygen, whether in the air or water, this layer prevents corrosion by isolating the alloy from contact with the oxidizing media.

However, chlorine and chlorine ions may damage the passive layer favoring stainless steel's crevice, pitting, and stress corrosion cracking. Mo additions are very effective in improving the resistance to damage of the passive layer by chlorine, although it negatively influences the final price of the stainless steel. On the other

hand, N has been thoroughly investigated, since the 1980s, as an alloying element with great potential for protecting the passive layer against damage, being abundant in nature (21 wt.% in the atmosphere), and giving a cost-effective solution for surface protection against corrosion.

Controlled addition of nitrogen to stainless steel has been encouraged over the last three decades due to the possibility of improving the surface properties (not only the corrosion but also the tribological and mechanical properties). Highpressure and powder metallurgy techniques were developed for medium and largescale fabrication of high nitrogen steels (HNS). Still, in general, these procedures are costly and require sophisticated equipment. Nitrogen-bearing stainless steel is a new class of corrosion-resistant alloys, exhibiting much better surface properties, better corrosion, and wear resistance associated with good bulk mechanical properties: very high strength, good ductility, and toughness. Therefore, considerable emphasis has been placed on liquid and solid-state routes to produce highperformance, low-cost nitrogen-alloyed stainless steels. The liquid state processing routes demand high-pressure metallurgy, which is laborious, demands special equipment, and is costly. In the solid-state production routes, the steel surface and near-surface regions are nitrogen alloyed through thermochemical, implantation, plasma, or laser techniques.

Diffusion surface treatments have been extensively studied and have become, for many applications, current industrial practice. The diffusion of nitrogen and carbon toward the core increases the surface hardness and wear resistance. However, nitrogen and carbon must remain in solid solution. Precipitation of chromiumrich carbides or nitrides reduces the chromium content in the metal matrix, preventing the formation of a continuous passive layer and harming the corrosion resistance of the steel.
