High wear resistance and better pitting corrosion resistance of AISI 316L stainless steel by a self-protective oxy-nitrocarburizing paste

Abstract

Simultaneous oxidation and nitrocarburizing of AISI 316 SS at 580 °C for 5, 10, 15, and 20 min were studied using self-protecting pastes of sodium cyanate and sodium carbonate. The analytical techniques of XRD and SEM/EDS allowed the characterization of surface modifications. The corrosion resistance was evaluated in a synthetic seawater solution using the cyclic potentiodynamic polarization technique. The cross-section hardness and wear resistance were assessed by Vickers indentation and pin-on-disc, respectively. This thermochemical process formed an oxidized compound layer above a solid nitrogen solution in austenite. The compound layer was composed of intermixed crystals of Fe3O4, ε-Fe3N, and expanded austenite. With increasing relative proportions of the first two phases and decreasing relative proportions of the third phase with increasing treatment time. The balance between the expanded austenite and the chemical composition of the compound layer improved the hardness, the wear-resistant, and the pitting corrosion resistance of the AISI 316L SS surface simultaneously. Oxy-nitrocarburizing at 580 °C for 10 min allowed the best-localized corrosion resistance. However, the sample oxy-nitrocarburized for 20 min reached a more stable passive zone with the highest wear resistance. Self-protective pastes based on sodium cyanates and carbonates allowed the oxy-nitrocarburizing process to consume less salt than the traditional molten salts.

Keywords

• Self-protective paste
• Oxy-nitrocarburizing
• Stainless steels
• Localized corrosion
• Wear
• Cyclic potentiodynamic polarization