Corrosion Behavior of Plasma Nitrided and Nitrocarburised Supermartensitic Stainless Steel

Abstract

Supermartensitic stainles steels (SMSS's) are a new generation of martensitic steels that have been increasingly used in oil and gas applications due to their adequate corrosion resistance and mechanical properties. In the present study, SMSS specimens (UNS S41425) were solution heat treated and air cooled followed by plasma nitriding and nitrocarburising at 400, 450 and 500°C for 5h. The produced layers were characterized by optical microscopy, microhardness testing, X-ray diffraction and corrosion testing in NaCl 3.5% solution. Surface alloying with nitrogen or both nitrogen and carbon results in increased surface hardness and homogeneous layers in which layer thickness increases with temperature. However, plasma nitriding yields a slightly thicker case than nitrocarburising. X-ray diffraction indicates very broad and overlapped peaks for the treatments conducted at 400°C. Increasing treatment temperature appears to result in the formation of chromium nitrides and iron nitrides and carbides, depending on the treatment. It was also found that treatment temperature drastically affects the corrosion response of the steel. The untreated steel presented a pitting potential close to 250mV. Plasma nitriding at 400°C was the only condition in which significant improvement of corrosion is observed. For plasma nitriding and nitrocarburising at 450 and 500°C, some pitting was detected.

The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review

“…At a greater depth, the inner part of the modified layer becomes delineated by the chemical etching, and it does not show a strongly etched interface with the substrate. This characteristic was observed for modified layers consisting of expanded martensite only [49,[68][69][70][71]89], and layers in which both expanded martensite and ε-Fe 2-3 N nitride [49,67,93], or expanded martensite and expanded austenite [62,65,69], or expanded martensite, expanded austenite, and ε-Fe 2-3 N [83] were detected. It should be noted that this featureless appearance may be due to the chemical etchings used, such as Marble's reagent [33] or Vilella's reagent [49,65,70,71,83], which are unable to etch Fe-based nitrides as well as N-rich phases, as was observed for the expanded austenite [11].…”

“…This characteristic was observed for modified layers consisting of expanded martensite only [49,[68][69][70][71]89], and layers in which both expanded martensite and ε-Fe 2-3 N nitride [49,67,93], or expanded martensite and expanded austenite [62,65,69], or expanded martensite, expanded austenite, and ε-Fe 2-3 N [83] were detected. It should be noted that this featureless appearance may be due to the chemical etchings used, such as Marble's reagent [33] or Vilella's reagent [49,65,70,71,83], which are unable to etch Fe-based nitrides as well as N-rich phases, as was observed for the expanded austenite [11]. The microstructure of the modified layer can be better delin-eated, highlighting the martensite structure, by using acetic glyceregia [91], as showed in Figure 2b, or Kalling's reagent [92].…”

“…It was observed that, with nitrocarburizing treatments, thinner hardened layers were obtained as compared to those produced with nitriding at the same temperature and for the same treatment time [78,79,83], in contrast with the fact that C atoms diffuse faster than N atoms. As such, thicker hardened layers were expected for the nitrocarburized samples, as observed in austenitic stainless steels [84].…”

“…The micrographs of the cross-section of AISI 420 martensitic stainless steel samples, which were plasma nitrided at 250 and 350 • C, are shown in Figure 1, together with the X-ray diffraction patterns of samples, untreated and treated in the range of 200-400 • C. The surface morphology and the microstructure of the cross-section of an AISI 431 martensitic stainless steel sample, plasma nitrided at 400 • C for 5 h, together with its X-ray diffraction patterns before and after the treatment, are depicted in Figure 2. The surface of low-temperature treated stainless steels with a martensite microstructure shows an etched appearance when plasma-based treatments are used, with the grains well delineated and reliefs at grain boundaries [66,70,83,91,92], as depicted in Figure 2a.…”

Electrochemical study on the corrosion resistance of surface modified Cr-Mo-V steel by elevated temperature plasma nitriding

“…Plasma nitriding has been widely accepted as an efficient and eco friendly process for the enhancement of wear, fatigue and electrochemical properties of steels and Ti alloys. Formation of nitrides and compressive stress generation in the crystal lattice may be considered for the enhancement of these properties [1][2][3][4][5][6][7][8][9]. Other techniques such as PVD and CVD may also improve these properties; however delamination of the coated layer limits their applications [10][11][12][13].…”