Effect of plasma nitriding parameters on corrosion performance of 17-4 PH stainless steel

ABSTRACT

This study investigates the effect of plasma nitriding parameters on corrosion susceptibility of 17-4 PH stainless steel in 3.5 wt-% NaCl solution. In this regard, 17-4 PH stainless steel was plasma nitrided at 400°C for 5 and 10 h, 450°C for 5 h and 500°C for 5 h. Cross-sectional images after nitriding process showed that a uniform nitrided layer has been formed on steel substrate. Depending on the temperature and time of the nitriding process, different phases were formed in the nitrided layer. This affected general corrosion and pitting corrosion performance of 17-4 PH stainless steel in 3.5 wt-% NaCl solution. While precipitation of chromium nitrides for nitrided specimens at 450°C and higher increased the susceptibility to pitting and general corrosion, formation of expanded martensite (EM) in nitriding at 400°C improved the pitting corrosion resistance of 17-4 PH stainless steel. This is believed to be due to the release of nitrogen atoms from EM phase to form ammonium ions and increase the pH of the solution, supressing pit growth.

RÉSUMÉ

Cette étude examine l’effet des paramètres de nitruration par bombardement atomique sur la susceptibilité à la corrosion de l’acier inoxydable 17-4 PH dans une solution de 3.5% en poids de NaCl. À cet effet, on a nitruré par bombardement atomique l’acier inoxydable 17-4 PH à 400°C pendant 5 et 10 heures, à 450°C pendant 5 heures, et à 500°C pendant 5 heures. Les images de coupe transversale après le procédé de nitruration montraient qu’une couche uniforme nitrurée était formée sur le substrat d’acier. Dépendant de la température et de la durée du procédé de nitruration, différentes phases étaient formées dans la couche nitrurée. Ceci affectait la performance à la corrosion générale et à la corrosion par piqûre de l’acier inoxydable 17-4PH dans la solution de 3.5% en poids de NaCl. Alors que la précipitation de nitrures de chrome des échantillons nitrurés à 450°C et plus augmentait la susceptibilité à la corrosion par piqûre et à la corrosion générale, la formation de martensite expansée lors de la nitruration à 400°C améliorait la résistance à la corrosion par piqûre de l’acier inoxydable 17-4 PH. On croit que cela est attribuable au relâchement d’atomes d’azote par la phase de martensite expansée, formant des ions ammonium et augmentant le pH de la solution, supprimant ainsi la croissance des cavités de corrosion.

KEYWORDS:

• 17-4 PH stainless steel
• general corrosion
• pitting
• plasma nitriding
• precipitation hardening

Acknowledgements

The authors thank Isfahan University of Technology for its financial support and provision of research facilities.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Hamidreza Riazi (PhD's student, Isfahan University of Technology) is a last year Ph.D. student working in the field of corrosion and surface engineering. His area of study is on plasma nitriding of 17-4PH stainless steel with particular focus on its fatigue and corrosion behaviour.

Fakhreddin Ashrafizadeh (Professor, Isfahan University of Technology) is a professor of materials engineering with deep experience in surface and innovating coating technologies. He has over last 30 years of experience working as an academia.

Abdoulmajid Eslami (Assistant professor, Isfahan University of Technology) has recently joined university working in the field of corrosion and coatings. His previous experience was as a materials engineer working for one of the world leading pipeline companies.

ORCID

Hamidreza Riazi http://orcid.org/0000-0002-8252-7628

Notes

1. The drop in pitting resistance for nitrided specimen at 450 and 500°C in comparison to nitrided specimen at 400°C seems inconsistent with that reported by Esfandiari and Dong. [17] They reported that pitting resistance for specimens nitrided at 420 and 500°C for 10 h improved in comparison to that of un-nitrided specimens and specimen nitrided at 350°C. This inconsistency could be due to the longer treatment time (10 h) for the nitriding process used in their study. The authors have claimed that improvement in pitting resistant is due to the formation of uniform Fe₄N layer, acting as a passive layer. The passivation behaviour was not observed in this study for specimens nitride at 450 and 500°C possibly due to shorter treatment time (only 5 h).