Plasma Nitriding Techniques for Enhanced Corrosion Resistance in Stainless Steels

Plasma nitriding has emerged as a versatile surface-engineering technique to improve the corrosion resistance and mechanical durability of stainless steels without compromising their inherent passivation behaviour. By generating a glow discharge in a nitrogen-rich atmosphere, nitrogen ions are driven into the steel surface, forming supersaturated interstitial phases or fine nitride precipitates. Low-temperature variants hinder chromium mobility, enabling the formation of a metastable expanded austenite (S-phase) or expanded ferrite and martensite, which deliver high hardness and wear resistance while preserving or enhancing pitting resistance. Alternative approaches, such as active screen plasma nitriding and combined nitrocarburising treatments, optimise ion flux and species distribution, yielding deeper, more uniform modified layers. Recent advances in thermodynamic modelling have elucidated the role of spinodal decomposition in achieving colossal nitrogen supersaturation, offering a unified explanation for the interplay between diffusion kinetics, microstructure evolution and long-term corrosion performance. The global significance of these methods spans oil and gas infrastructure, biomedical implants and high-end manufacturing, where extended service life and integrity in corrosive environments are paramount.

Research from Nature Portfolio

Innovative thermodynamic analysis has revealed that incorporation of spinodal decomposition into paraequilibrium models accurately predicts the exceptionally high nitrogen solubility achieved during low-temperature gaseous nitridation of both ferritic and austenitic stainless steels. This refinement explains the nucleation resistance to chromium nitride formation and the retention of a continuous supersaturated solid solution. Experimental validation across multiple alloy grades confirms that spinodal-driven nitrogen uptake leads to thicker modified layers with superior hardness and maintained passivation behaviour, bridging longstanding gaps between theoretical predictions and observed corrosion resistance enhancements.

Plasma Nitriding Techniques for Enhanced Corrosion Resistance in Stainless Steels publication trend

The graph below shows the total number of articles in plasma nitriding techniques for enhanced corrosion resistance in stainless steels across all publications each year (not limited to Nature Index journals).

Technical terms

Plasma nitriding: A surface treatment in which nitrogen ions generated in a low-pressure glow discharge diffuse into a metal substrate, increasing hardness and wear resistance while preserving corrosion performance.

Expanded austenite (S-phase): A supersaturated, metastable variant of austenitic stainless steel in which interstitial nitrogen or carbon atoms are retained in solid solution, producing high hardness without chromium nitride precipitation.

Active screen plasma nitriding: A technique that employs a perforated conductive screen between the cathode and workpiece to remotely generate plasma, improving uniformity and depth of nitriding on complex geometries.

Nitrocarburising: A combined low-temperature plasma process introducing both nitrogen and carbon into steel surfaces to form hard, wear-resistant and corrosion-resistant expanded phases.

Spinodal decomposition: A mechanism of spontaneous phase separation in supersaturated alloys, where compositional fluctuations grow without a nucleation barrier, influencing solubility limits and microstructure evolution.

References

1. From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase. Metals (2020).
2. The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review. Metals (2022).
3. Mechanism of ion nitriding of 316L austenitic steel by active screen method in a hydrogen-nitrogen atmosphere. The International Journal of Advanced Manufacturing Technology (2020).
4. Wear and Corrosion Properties of Cold-Sprayed AISI 316L Coatings Treated by Combined Plasma Carburizing and Nitriding at Low Temperature. Coatings (2018).
5. Thermodynamic reasoning for colossal N supersaturation in austenitic and ferritic stainless steels during low-temperature nitridation. Scientific Reports (2019).