Corrosion properties of S-phase layers formed on medical grade austenitic stainless steel

The corrosion properties of S-phase surface layers formed in AISI 316LVM (ASTM F138) and High-N (ASTM F1586) medical grade austenitic stainless steels by plasma surface alloying with nitrogen (at 430°C), carbon (at 500°C) and both carbon and nitrogen (at 430°C) has been investigated. The corrosion behaviour of the S-phase layers in Ringer’s solutions was evaluated using potentiodynamic and immersion corrosion tests. The corrosion damage was evaluated using microscopy, hardness testing, inductive coupled plasma mass spectroscopy and X-ray diffraction. The experimental results have demonstrated that low-temperature nitriding, carburising and carbonitriding can improve the localised corrosion resistance of both industrial and medical grade austenitic stainless steels as long as the threshold sensitisation temperature is not reached. Carburising at 500°C has proved to be the best hardening treatment with the least effect on the corrosion resistance of the parent alloy.     

Structure and properties of ion-nitrided stainless steels

The structure and properties of ion-nitrided layers on several stainless steels, 410 martensitic stainless steel, 430 ferritic stainless steel and 321 austenitic stainless steel, has been studied under varying process conditions with microhardness-depth correlations, optical microscopy and transmission electron microscopy. The process variables studied include time (2 to 10 h) and temperature (400 to 600° C). The highest case depth values and hardness levels were observed in martensitic stainless steels. The lowest case depths were observed in austenitic stainless steel. In general, the behaviour of matensitic and ferritic stainless steels were similar. All three steels showed increasing case depths and decreasing surface hardnesses with increasing ion-nitriding temperatures and times. Nitriding depth was found to be parabolic with ion nitriding time in all three steels at all ion-nitriding temperatures investigated, the nitriding reaction being faster in martensitic stainless steel than the others. Electron microscopy showed that almost no structural difference arises in the core of ferritic and austenitic stainless steels whereas recrystallization of the martensitic structure was observed in the core of martensitic steel following ion nitriding. Electron microscopy results also showed that ion nitriding produces platelets or disc-shaped precipitates on {001} matrix planes, coherent with the matrix. These platelets showed a striated morphology which is thought to be the result of the elastic strain in the matrix.     

Einfluss von Werkstoffzustand und chemischer Zusammensetzung auf die Eigenschaften plasmanitrierter austenitischer Stähle

Plasma nitriding offers great potenzial for improving the wear properties of austenitic steels. Here, the austenitic standard grades 1.4307 and 1.4404, as well as the titanium-stabilized grades 1.4541 and 1.4571 were investigated regarding the influence of the material condition on the nitriding result and corrosion behavior. Special focus was put on the influence of the Ti-stabilisation. In addition, it was investigated to what extent corrosion properties are influenced by cold-forming induced defect structures. In comparison to 1.4307 and 1.4404, less nitrogen is incorporated in areas with forming martensite in titanium-stabilized austenitic steels and lower nitriding temperatures, while an increased diffusion of nitrogen is observed, when only slip bands are present. The corrosion resistance is generally improved by the plasma nitriding parameters used for this study. In general, a higher thickness of the S-phase, which forms during the plasma nitriding, results in better corrosion resistance and higher surface hardness. The titanium stabilization inhibits nitrogen diffusion in the presence of deformation induced martensite at lower nitriding temperatures and promotes diffusion in the presence of deformation induced slip bands.     

Influence of the microstructure on the corrosion resistance of plasma‐nitrided austenitic stainless steel 304L and 316L

Austenitic stainless steels are widely used in medical and food industries because of their excellent corrosion resistance. However, they suffer from weak wear resistance due to their low hardness. To improve this, plasma nitriding processes have been successfully applied to austenitic stainless steels, thereby forming a thin and very hard diffusion layer, the so‐called S‐phase. In the present study, the austenitic stainless steels AISI 304L and AISI 316L with different microstructures and surface modifications were used to examine the influence of the steel microstructure on the plasma nitriding behavior and corrosion properties. In a first step, solution annealed steel plates were cold‐rolled with 38% deformation degree. Then, the samples were prepared with three kinds of mechanical surface treatments. The specimens were plasma nitrided for 360 min in a H₂–N₂ atmosphere at 420 °C. X‐ray diffraction measurements confirmed the presence of the S‐phase at the sample surface, austenite and body centered cubic (bcc)‐iron. The specimens were comprehensively characterized by means of optical microscopy, scanning electron microscopy, glow discharge optical emission spectroscopy, X‐ray diffraction, surface roughness and nano‐indentation measurements to provide the formulation of dependencies between microstructure and nitriding behavior. The corrosion behavior was examined by potentio‐dynamic polarization measurements in 0.05 M and 0.5 M sulfuric acid and by salt spray testing.     

Influence of nitriding on corrosion resistance of martensitic X17CrNi16‐2 stainless steel

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440 °C and 525 °C and salt bath nitrocarburizing were carried out on X17CrNi16‐2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X‐ray diffraction (XRD) and glow‐discharge optical emission spectrometry (GD‐OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16‐2 steel.     

Aktivierte Randaufstickung nichtrostender Stähle

Activated Solution Nitriding of Stainless Steels The solution nitriding of the stainless steels can be optimized by a two stage process. The first stage involves an surface activation and an enrichment of nitrogen in the case due to internal nitriding. After this step at temperatures between 1070 °C and 1150 °C follows the dissolution of the chromium nitrides and a solution nitriding. Investigations of ferritic, martensitic and austenitic steels showed that this technology is superior compared to the one stage technology. The treatment time for an given layer thickness in the high temperature stage is cut in halve. The case concentration of nitrogen can be controlled by a material specific choice of the treatment temperature and the partial pressure of nitrogen. For the investigated steels the desired microstructure of the case could be achieved by partial pressures of nitrogen between 0,35 an 1 bar. The solution nitriding of ferritic-martensitic steels eneables the production of martensitic cases with a hardnesses up to 700 HV 0,3. An austenitic case with higher hardness and stability of the austenit can be produced by enrichment the surface of austenitic and ferritic-austenitic stainless steels with nitrogen.     

The Effect of Pulsed Magnetic Treatment of Nitrided Steels for Wear Resistance

Surface engineering approach to increase surface properties such as wear resistance performed by pulsed magnetic treatment of nitrided steels was studied. Selected steel surfaces were treated by pulsed magnetic treatment and plasma ion nitriding with different optimized process parameters. The obtained microstructures were examined to study the influence of magnetic treatment on ion nitriding. SEM, AES, microhardness measurements, and image analyser were used to characterize the surface and interface. The results of this study show that pulsed magnetic treatment reduces residual stresses on the surface, improves the bonding of deposited nitride layers to substrate, influences the nitride layers, case depth, and surface hardness formation and increases the wear resistance.     

Modifying the properties of AISI 316L steel by glow discharge assisted low-temperature nitriding and oxynitriding

Apart from titanium, its alloys and CoCrMo alloys, austenitic steels are widely used in medical applications. In order to improve the frictional wear resistance of these steels, they are subjected to various surface treatments such that the good corrosion resistance of the steels is preserved.The paper analyzes the structure and phase composition of AISI 316L steel after subjecting it to low-temperature nitriding and oxynitriding under glow discharge conditions. The treatments produced diffusion-type surface layers composed of nitrogen-expanded austenite (known as the phase S, i.e. supersaturated solution of nitrogen in austenite) with a thin surface layer of chromium nitride (CrN) zone (after nitriding) or chromium oxide (Cr₂O₃) zone (after oxynitriding). It has been shown that the treatments substantially increase the hardness and frictional wear resistance of the steel without degrading its good corrosion resistance (examined in the Ringer physiological solution at a temperature of 37 °C).     

含Cu抗菌不锈钢的工艺与耐蚀性能

与普通0Cr17铁素体不锈钢和0Cr18Ni9奥氏体不锈钢相比，含铜铁素体和奥氏体抗菌不锈钢均具有良好的冷热加工性能和焊接性能．通过提高浇铸温度，抗菌不锈钢能保持良好的铸造性能．奥氏体抗菌不锈钢的抗应力腐蚀性能比0Cr18Ni9不锈钢有很大的提高，而铁素体抗菌不锈钢比0Cr17有明显的下降．与相应的普通不锈钢相比，两种类型抗菌不锈钢的耐点蚀性能均略有下降．     

Ionen- und plasmagestützte Verfahren der Oberflächentechnik für die Randschichtbehandlung von Leichtmetallwerkstoffen

Surface engineering of light weight materials with ion- and plasma-assisted methods Increasing applications of light weight materials are expected in the future. Pursuing this trend surface engineering of these materials – especially ion- and plasma-assisted methods – swill be of increasing interest to enhance their wear and corrosion resistance. In a research co-operation some promising methods were examined on different aluminium and titanium alloys to assess their potential to increase the surface properties. Among these were magnetron sputtering of chromium nitride, ion beam assisted deposition of Cr/CrN and Al/A₂O₃ layers, ion implantation and ion beam assisted nitriding. Compared to the steel substrates the assessment of the mechanical properties such as the critical load of the scratch test of the coated light weight materials is different. Furthermore, it could be shown that both spherical section and glow discharge optical spectroscopy are useful methods to characterize the near-surface zone influenced by ion implantation.     

Zum Verhalten rost‐ und säurebeständiger Stähle unter Komplexbeanspruchung. Teil 1 Passivität und Lochkorrosionsbeständigkeit

Some Aspects on Corrosion Fatique of Stainless Steels. Part 1 Passivity and Pitting Corrosion Susceptibility Iron‐Chromium‐Nickel alloys are of special interest for many applications because of their excellent resistance to corrosion. The nature and composition of passive films formed on stainless steels depend on the prevailing conditions, viz. steel‐composition, passivation potential, aging, pH, electrolyt composition and temperature. Passive films may be damaged by local breakdown. At least two mechanisms are possible for this localisation: mechanical breakdown by slip steps and electrochemical breakdown (for e.g. by the effects of chloride ions). Because of this, steels suffer a degradation of their fatique properties when exposed to an aqueous environment. Passivation of austenitic, ferritic‐austenitic and martensitic stainless steels has been studied in different solutions using electrochemical techniques. The results clarified that for two of the investigated alloys the prediction of fracture initiation based on pitting corrosion in chlorid containing solutions is possible. (To be continued.)     

Verschleißschutz und Korrosionsbeständigkeit von austenitischem Stahl durch Plasmadiffusions behandlung

In this paper, we report on a series of experiments designed to study the influence of plasma nitriding on the mechanical properties and the corrosion resistance of austenitic stainless steel. Plasma nitriding experiments were conducted on AISI 304L steel in a temperature range of 375‐475°C using pulsed‐DC plasma with different N ₂‐H ₂ gas mixtures and treatment times. First of all, the formation and the microstructure of the modified layer will be highlighted followed by the results of hardness measurement, adhesion testing, wear resistance and fatigue life tests. In addition the corrosion resistance of the modified layer is described. The microhardness after plasma nitriding is increased by a factor of five compared to the untreated material. The adhesion is examined by Rockwell indentation and scratch test. No delamination of the treated layer could be observed. The wear rate after plasma nitriding is significantly reduced compared to the untreated material. Plasma nitriding produces compressive stress within the modified layer. This treatment improves the fatigue life which can be raised by a factor of ten at a low stress level. The results show that plasma nitriding of austenitic stainless steel is a suitable process for improving the mechanical and the technological properties without significantly effecting the excellent corrosion resistance of this material.     

Surface Alloying of Nitrogen to Improve Corrosion Resistance of Steels and Stainless Steels

It is well known that the addition of nitrogen to steels and stainless steels enhances the passivity and localized corrosion resistance, in addition to improving the mechanical properties. Selective alloying of surfaces of steels and stainless steels with nitrogen could also enhance the corrosion resistance and improve the mechanical properties without affecting the bulk properties. Techniques like ion implantation, laser alloying, nitriding, etc. can be effectively used to introduce very high levels of nitrogen. In addition, these techniques can also produce modified surfaces with novel microstructures to further improve the properties. The surface alloying methods also provide an opportunity to selectively nitrogenate the surface of finished components in order to obtain better properties. The review highlights the techniques, modifications and the properties obtained further.     

Stainless steels suited for solution nitriding

Solution nitriding is a new heat treatment to yield a high nitrogen case on stainless steels at 1100 ± 50°C. Combining experimental results and thermodynamic calculation steels are selected to give a hard martensitic or high strength austenitic case. Especially developed steels are discussed as well as the suitability of standard grades. A martensitic case is combined with a martensitic core in steel Cr13C0.2 and with a softer ferritic‐martensitic core in steel Cr13C0.1. The nitrogen content of an austenitic case increases with the Cr/Ni ratio, e.g. in the order of Cr17Ni12Mo2, Cr18Ni10, Cr22Ni5Mo3N0.2. The duplex microstructure of the latter provides the highest yield strength in the core. It is essential to stay clear of the austenite/austenite + M₂N boundary and avoid precipitates which deteriorate the fatigue and corrosion resistance. Seventeen steels are assessed in this report.     

Determination of the s-phase formation coefficient of plasma nitrided austenitic steel

Plasma nitriding is an effective surface hardening treatment for austenitic stainless steels. During plasma nitriding, s-phase formation takes place which is not only responsible for high hardness and wear resistance but also for good corrosion resistance. In order to estimate the thickness of the s-phase for austenitic stainless steel in a plasma nitriding process, an empirical model is devised. A number of plasma nitriding processes of austenitic stainless steel (304 L) were carried out with varying treatment temperature from 360 °C to 450 °C and process duration ranging from 10 hours to 24 hours with constant pressure, voltage, pulse-to-pause-ratio and gas mixture. A time-temperature dependent s-phase formation coefficient is determined by measuring the thickness of the s-phase using a scanning electron microscope (SEM) and glow discharge optical emission spectroscopy (GDOES). The developed model is verified by three controlled experiments. This model fits the thickness of the s-phase with an error of less than 6 %.     

Corrosion resistance of the layers formed on the surface of plasma-nitrided AISI 304L steel

Austenitic stainless steels have good corrosion resistance, but their low hardness and low wear resistance limit their use whenever surface hardness is required. Nitriding treatments have been successfully applied to stainless steels to improve their mechanical and tribological properties; however, at temperatures above 723 K, gas or salt bath nitriding processes decrease the corrosion resistance due to the formation of CrN and other phases within the modified layer. Chromium compounds draw chromium and nitrogen from the adjacent regions, degrading the corrosion resistance. The plasma nitriding technique permits the use of treatment temperatures as low as 623 K without promoting degradation in the corrosion resistance of stainless steel. In this work, the pulsed glow discharge (PGD) technique was used for nitriding steel (AISI304L) in order to investigate the effect of the temperature of this treatment in the morphology and, as a consequence, in the anodic behavior of the formed layers, in solution with and without chloride ions. Four different temperatures were employed (623, 673, 723, and 773 K). The samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), microhardness measurements, and electrochemical tests with potentiodynamic anodic polarization curves. The nitriding temperature alters the anodic behavior due to a displacement of the polarization curve towards higher currents, in a solution free of chloride ions. In a chloride solution, the nitriding temperature increases the pitting potential up to the oxygen evolution region.     

Entwicklung,Eigenschaften, Verarbeitung und Einsatz des hochsiliciumhaltigen austenitischen Stahls X2 CrNiSi 18 15

Development, Properties, Processing and Applications of High-Silicon Steel Grade X2 CrNiSi 18 15 Production, storage and transportation of highly concentrated nitric acid (approximately 98%) frequently occur in containers and vessels made of pure aluminium. In many cases, however, their service life is restricted by the insufficient corrosion resistance of the welds. Though tantalum exhibits a superior resistance to corrosion, it is only used in very specific occasions for cost considerations. Commercial grade austenitic chromium-nickel steels as well as ferritic chromium steels assume a transpassive state under such service conditions, and suffer from intergranular attack even if the structure is free of precipitates. A significant corrosion resistance to highly concentrated nitric acid in combination with good workability and weldability can be achieved by an austenitic chromium-nickel steel alloyed with silicon. For corrosion considerations a silicon contents of at least 3,7 wt. % has to be aimed at. Since the silicon is held in solid solution in the austenitic matrix, the mechanical properties of the special grade X 2 CrNiSi 18 15 are not very different from those of commercial chromium-nickel steel grades. Welding materials of the same kind are available for manual are welding as well as for TIG welding. The corrosion resultance of the weld deposit is similar to that of the base metal. However, the tendency of this steel to precipitate intermetallic phases is increased by the silicon addition. There, a proper heat control during welding is a necessary prerequisite in order to avoid intercrystalline attack in the heat-affected zones on both sides of the weld. A silicon contents of approximately 4 wt. % not only improves the corrosion resistance against highly concentrated nitric acid but also results in a considerable improvement when this special steel is used in chromic acid solutions and hot concentrated sulfuric acid. Also the high temperature corrosion, resistance of this material is remarkable. Several piping systems, chemical equipment, pumps, and fittings have been in successfull service for several years and proved the excellent properties of the X 2 CrNiSi 18 15 specially steel.     

Development of high nitrogen, low nickel, 18%Cr austenitic stainless steels

Two high nitrogen stainless steels are studied through metallographic, mechanical and corrosionistic tests and the results are compared with those shown by a standard AISI 304. These high nitrogen steels show a significantly higher mechanical strength than usual AISI 304 while their corrosion resistance lie among that of standard austenitic and that of standard ferritic stainless steels.     

Effect of silicon-ion implantation upon the corrosion properties of austenitic stainless steels

The structure of the surface layers and the corrosion resistance of austenitic stainless steels after silicon-ion implantation, were examined. The implanted silicon doses were 1.5×1017, 3×1017 and 4.5×1017 Si+ cm-2. Implantation with all these doses gave an amorphous surface layer. When samples implanted with 1.5×1017 Si+ cm-2 were annealed at temperatures of 300 and 500 °C, their surface structure remained unchanged. After annealing at 650 °C, the amorphous layer vanished. It was determined how, in terms of corrosion resistance, the amount of implanted silicon, subsequent heat treatment and long time exposure, affect highly corrosion-resistant austenitic stainless steel (18/17/8) in comparison to the 316L austenitic stainless steel subjected to the same treatment. Corrosion examinations were carried out in 0.9% NaCl at a temperature of 37 °C. After silicon-ion implantation the corrosion resistance of the 316L steel increased while that of highly resistant (18/17/8) did not. The corrosion resistance of the investigated steels, both implanted and non-implanted, increased with the exposure time of the samples in the test environment. © 1998 Kluwer Academic Publishers