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Liang

2010

Surface Engineering

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High nitrogen face centred cubic phase formed on the nitrogen modified Fe-Cr-Ni austenitic stainless steel, which is named by c N phase, has a combined wear and corrosion resistance. Xray diffraction (XRD) patterns of the c N phase on the stainless steel depict a set of broad and asymmetry diffraction peaks with a peak shift to low Bragg angles and a decrease in intensity, even disappearing of some peaks of high index planes, from each austenite peak for the c matrix. The peak shift of the XRD patterns was first explained by Warren's XRD theory by Blawert et al. (Surf. Coat. Technol., 2001, 136, 181). In this paper, a systematic study of XRD on the peak shift, peak asymmetry, peak broadening and peak intensity of the c N phase has been carried out, based on a fault induced scattering geometry in diffraction to imperfect crystals by Warren's theory and Wagner's method. Both the higher deformation faults density a in a range of 0?02-0?25 and the lower twin faults density b of 0?01-0?1 were successfully used to describe the line profiles in the XRD patterns of the c N phase formed on plasma source ion nitrided 1Cr18Ni9Ti (18-8 type) austenitic stainless steel. A novel stacking faults factor S ab dependent on a and b described, in good agreement, the peak intensity of the c N phase. The calculated XRD patterns of the c N phase using the imperfect crystals model of a face centred cubic phase with a higher deformation faults density and a lower twin faults density were associated with the experimentally structural characteristics.

Section: Resultsmentioning

confidence: 87%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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X-ray diffraction of high nitrogen face centred cubic phase formed on nitrogen modified austenitic stainless steel

Liang

2010

Surface Engineering

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“…The analysis results by X-ray diffraction ͑XRD͒ and transmission electron microscopy ͑TEM͒ showed that the nitrogen with high concentration in the ␥ N phase is disordered distribution in the fcc lattice of austenite. 16 The ␥ N phase has the weaker Cr-N ionic-type bonds and stronger Fe-N ionic-type bonds than that of the stoichiometric nitrides, as CrN and Cr 2 N, and Fe 4 N and Fe 2-3 N. 17 Compared with the original stainless steels, the ␥ N phase layer possesses superior pitting corrosion resistance and equivalent general corrosion resistance, as well as high hardness and wear resistance. 18,19 In this article, we investigated the passivating mechanism of the ␥ N phase in 3% NaCl solution by analyses of chemical composition and states in the passive film in order to explore the effect mechanisms of nitrogen on pitting corrosion resistance of austenitic stainless steels.…”

mentioning

confidence: 99%

Role of Nitrogen in Pitting Corrosion Resistance of a High-Nitrogen Face-Centered-Cubic Phase Formed on Austenitic Stainless Steel

Zhu

2005

J. Electrochem. Soc.

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The role of nitrogen in pitting corrosion resistance of Fe-Cr-Ni austenitic stainless steel was investigated using a high-nitrogen face-centered-cubic phase ͑␥ N ͒ formed on the plasma source ion-nitrided 1Cr18Ni9Ti ͑18-8 type͒ austenitic stainless steel. No pitting corrosion for the ␥ N phase layer was confirmed by electrochemical polarization measurement in 3% NaCl solution. In order to explore the mechanism of pitting corrosion resistance, the passive film on the ␥ N phase was analyzed by Auger electron spectroscopy and X-ray photoelectron spectroscopy in conjunction with ion-beam sputtering. The passive film with a duplex character was by two to three times thicker than that of original stainless steel. It was essentially composed of two regions: the iron hydroxide/oxides in the outer region and the chromium hydroxide/oxide and iron oxides accompanying the chromium and iron nitrides in the inner region. The conventional bilayer substructure, e.g., outer hydroxide sublayer and inner oxide sublayer, was also detected in each region. During anodic polarization a part of the chromium with weak Cr-N ionic-type bonds together with the iron in minor amount in the ␥ N phase was prone to chemical oxidation, releasing sufficient nitrogen to form stable oxides and ammonia on the passivating surface. The thick iron hydroxide/oxides region from preferential solution of the iron in metallic state was formed on the stable chromium and iron oxides due to increase of alkalinity in the aqueous solution buffered by ammonia formation. The protective passive film could completely barrier the penetration of localized attack of aggressive ions.Austenitic stainless steels are used in diverse applications for corrosion resistance. Although they have extremely good general corrosion resistance, they are nevertheless susceptible to pitting corrosion. Extensive studies of pitting corrosion properties of the nitrogen-containing stainless steels undoubtedly proved that nitrogen dissolved in the austenitic stainless steels improves pitting corrosion resistance in chloride aqueous solutions. In recent years, more than ten mechanisms of improvement in pitting corrosion were proposed, although some mechanisms were in succession excluded as explanations for effect observed by new experimental results. 1-3 Arguments for the effect of nitrogen on pitting corrosion resistance were focused on two mechanisms: ͑i͒ formation of ammonium ions consumes protons and increases the pH value in incipient pits and promotes repassivation; 4 and ͑ii͒ concentrated nitrogen in the passive film or at the interface between the passive film and stainless steel stabilizes the passive films. 5-8 The role of nitrogen in pitting corrosion resistance is not fully understood due to complexity of the pitting corrosion behavior. Furthermore, the bulk alloying of nitrogen in austenitic stainless steels is limited by its limited solubility in austenite matrix. The low nitrogen concentration in austenite leads to difficulty of trace in quantitative analysis for examining the ...

“…28 No nitrides, such as chromium nitrides, c9-Me 4 N, e-Me 223 N and a9 phases, were detected, besides a single c N phase on the nitrided Fe-Cr-Ni austenitic stainless steel. 29 The fcc structure of the c N phase was also confirmed unambiguously by the TEM observation with electron diffraction. 29 It can be seen that the nitrided layer formed on the AISI 316 stainless steel by plasma based low energy nitrogen ion implantation comprised a single c N phase with precipitation free.…”

Section: Resultsmentioning

confidence: 66%

Tribological behaviour of plasma based low energy nitrogen ion implanted AISI 316 austenitic stainless steel against same stainless steel counterface

Zhou

et al. 2010

Surface Engineering

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Tribological properties of a single high nitrogen face centred cubic phase c N layer formed on the plasma based low energy nitrogen ion implanted AISI 316 austenitic stainless steel were investigated on a ball on disc tribometer against the same stainless steel counterface under a normal load of 2 N with a sliding speed from 0?16 to 0?39 m s 21 . The microhardness of the c N phase layer with a thickness of ,13 mm and a nitrogen peak concentration of ,35 at-% was measured to be ,22 GPa. During the sliding for 1 h, the faint wear tracks of the c N phase layer against the same stainless steel ball were observed with all the sliding speeds. The friction coefficient of the c N phase layer changed in the range from 0?85 to 0?70 with increasing sliding speed. Formation of the c N phase layer on the stainless steel significantly improved the wear resistance of the stainless steel against the same stainless steel. An oxidative wear mechanism of the c N phase was found instead of an adhesive wear mechanism of the original stainless steel because the c N phase was prone to tribochemical oxidation on the worn surface due to the nitrogen supersaturation in austenite matrix.