Effect of nitrogen alloying on the semiconducting properties of passive films and metastable pitting susceptibility of 316L and 316LN stainless steels

The beneficial effect of nitrogen alloying on the corrosion resistance of stainless steels has been attributed to the increase of the local pH within the active sites and the enhanced repassivation of the metastable pits. In order to better understand the effect of nitrogen alloying, in situ capacitance measurements and potentiostatic polarization were conducted for 316L and 316LN stainless steels with different nitrogen contents in deaerated 0.1 M Na₂SO₄ and 0.1 M NaCl aqueous solutions. The Mott–Schottky plots obtained from the in situ capacitance measurements offered information on the donor concentration and the thickness of the space charge region within the passive film. The metastable pitting susceptibility was investigated by performing potentiostatic polarization tests. The results showed that nitrogen alloying decreased the donor densities and the number of metastable pits, while the absorption of chloride ions on the passive film had the opposite effect. Auger electron spectroscopy (AES) analysis demonstrated that nitrogen alloying enriched the chromium within the passive film. The relationship between the semiconducting properties of the passive film and the metastable pitting susceptibility was elucidated.     

Electrochemical and analytical investigation of passive films formed on stainless steels in alkaline media

Passive films were grown in potentiodynamic mode, by cyclic voltammetry on AISI 316 and AISI 304 stainless steels. The composition of these films was investigated by X-ray photoelectron spectroscopy (XPS). The electrochemical behaviour and the chemical composition of the passive films formed by cyclic voltammetry were compared to those of films grown under natural conditions (by immersion at open circuit potential, OCP) in alkaline solutions simulating concrete. The study included the effect of pH of the electrolyte and the effect of the presence of chloride ions.The XPS results revealed important changes in the passive film composition, which becomes enriched in chromium and depleted in magnetite as the pH decreases. On the other hand, the presence of chlorides promotes a more oxidised passive layer. The XPS results also showed relevant differences on the composition of the oxide layers for the films formed under cyclic voltammetry and/or under OCP.     

Viscoplastic behaviour of stainless steels AISI 316L and 316H

Summary The theory of viscoplasticity based on total strain and overstress is used in order to simulate the sensitivity to the rate of loading of two commonly used stainless steels, namely AISI 316L and 316H. The consitutive model has been implemented within a transient finite element computer code using a stress update algorithm based on the elastic predictor-return mapping concept. Both monotonic and cyclic loading conditions are considered in one or more space dimensions. Experimental results showing strain-rate dependence at room temperature are reported for both types of steel and used for calibrating the viscoplastic numerical model. An explicit dependence of the nonlinear viscosity function on the strain rate has been obtained and the calibrated model is found to yield results which are in excellent agreement with the experimental data. Finally the calibrated viscoplastic model is applied to predict the response of two representative structures subjected to impulsive loading. The results indicate a significant effect of the rate of loading on the internal stress distribution. With 21 Figures     

Synthesis of nanostructured zirconia electrodeposited films on AISI 316L stainless steel and its behaviour in corrosion resistance assessment

This work deals with the study of AISI 316L stainless steel samples coated with nanostructured zirconia thin films, using electrodeposition methods. The chemical composition and compounds formed were determined by X-ray photoelectron spectroscopy (XPS). The morphology of zirconia films was analysed by scanning electron microscopy (SEM) and atomic force microscopy (AFM).Corrosion resistance of the coated steel was tested in a chloride environment. XPS analysis results show zirconium element on the metal surface, bound to oxygen-forming zirconia. The anodic polarization curves obtained in Hank's solution show that zirconia coating can be used as protective coating against pitting corrosion of AISI 316L stainless steels.     

Semi-conductive properties of passive films formed on copper in chromate solutions

The semi-conductive properties of passive films formed on copper in chromate solutions were investigated by means of capacitance measurements. The effects of the formation potential and temperature, chloride and chromate concentration in electrolyte solution, as well as measured frequency on the semi-conductive properties of passive films were studied. The results show that the passive films formed on copper display n-type semi-conductive characteristics. The slopes of the straight lines in Mott-Schottky plots decrease with moving the formation potential toward positive, increasing the chloride ion concentration, decreasing the formation temperature and the chromate solutions concentration. Moreover, the initial grain size of copper surface impacts remarkably on the semi-conductive properties. The donor density of the passive films formed on ultra-fine grained (UFG) copper is higher than that of the passive films on coarse-grained (CG) copper.     

Pitting corrosion resistance of La added 316L stainless steel in simulated body fluids

Lanthanum (La), a rare earth element with anticoagulative and antiphlogistic function, was added into the medical grade 316L stainless steel in order to improve its biocompatibility. The corrosion resistance of the La added 316L steel in two different simulated body fluids, simulated blood plasma and Hank's solution, was evaluated. The result showed that the addition of La in the steel could largely affect the corrosion behavior of the steel. The steel with 0.01% La showed the widest passive region and the best resistance to pitting attack, within the addition range of La from 0.01% to 0.08%. The corrosion resistance improvement of La added 316L stainless steel is probably due to the effect of La on the purification of the steel, the modification of inclusions, and the passive film formation in the simulated body fluids.     

Characterization of sputter-deposited 316L stainless steel films

Sputter-deposited 316L stainless steel films deposited on various substrates were characterized using transmission electron microscopy and X-ray diffractometry. The deposits were found to be fine-grained and the phases present in the films depended on the nature of the substrate. Films of various thicknesses deposited on microscope slides or oxidized stainless steel substrates contained a mixture of two phases: a body centre cubic (b c c) and a modified hexagonal -phase. The hexagonal phase appeared to be an ordered phase, as suggested by the a _O value of the structure, which is twice that for the -martensite found in many deformed stainless steels. These films were hard and brittle, as indicated by microhardness measurements. Films deposited on oxide-free austenitic stainless steel substrates, on the other hand, were mostly b c c and exhibited a dominant 2 00 texture. These films were softer and less brittle than those deposited on oxidized substrates. In situ high-temperature X-ray diffractometry revealed that the -phase transformed to b c c when the films were annealed at 773 K. On annealing at 873 K, the b c c phase transformed to face centre cubic, which remained stable on cooling to room temperature. These results agree with published data which suggest stability of the b c c phase up to 840 K. Some discrepancies from earlier published reports are discussed in the light of the present results.     

Influence of silver additions to type 316 stainless steels on bacterial inhibition,mechanical properties,and corrosion resistance

Bacterial contamination is a major concern in many areas. In this study, silver was added to type 316 stainless steels in order to obtain an expected bacteria inhibiting property to reduce the occurrence of bacterial contamination. Silver-bearing 316 stainless steels were prepared by vacuum melting techniques. The microstructure of these 316 stainless steels was examined, and the influences of silver additions to 316 stainless steels on bacterial inhibition, mechanical properties, and corrosion resistance were investigated. This study suggested that silver-bearing 316 stainless steels could be used in areas where hygiene is a major requirement. The possible mechanisms of silver dissolution from the surfaces of silver-bearing 316 stainless steels were also discussed in this report.     

Sintered duplex stainless steels from premixes of 316L and 434L powders

Duplex austenite–ferrite stainless steels were prepared from the premixes of 316L and 434L stainless steel atomized powders. Pronounced densification was observed after 1350°C sintering in hydrogen. 316L-60w/o 434L steel composition exhibited maximum transverse rupture strength, while 40 and 60w/o 434L containing compositions showed total immunity in 1N H₂SO₄ even after a exposure time of 360 h. Anodic polarization curves also suggest high-corrosion resistance of those two compositions. Magnetic coercivity decreased with increase in sintering temperature while magnetic saturation follows the reverse trend. Wear resistance of the duplex stainless steels under sliding condition was in between the straight steels.     

Effect of TIG welding on corrosion behavior of 316L stainless steel

316L stainless steel (SS) is one of the most consumable materials in orthopedic implants. Certain types of orthopedic implants such as mono-bloc hip stems are often made of two elements welded together. In this study, effect of TIG welding on corrosion behavior of 316L stainless steel in physiological solution was investigated. In this method, filler metal wasn't used due to the small thickness of samples and it was welded to lap form. Corrosion behavior in physiological solution at 37 °C was investigated with potentiodynamic polarization curves. Microstructure of base metal (BM) and weld metal (WM) was studied with scanning electronic microscopy (SEM). The corrosion behavior of weld metal, base metal and couple (BM and WM together) was compared together. For detecting microstructure and phases in BM and WM, X-ray diffraction analysis was done. Finally, post-weld heat treatment (PWHT) was performed on as-welded samples. Results indicated that corrosion behavior of WM was better than the BM. This phenomenon was attributed to secondary phases that were present in the BM. Secondary phases in the weld metal are dissolved when the base metal is melting due to the welding process. Based on the results of electrochemical analysis, it was determined that the corrosion rate of a couple was more than of other parts. Heat affected zone (HAZ) is responsible for this phenomenon. The adjacent zones of the weld metal are classically less corrosion resistant, thereby being attacked preferentially when the steel is exposed to corrosive environments. PWHT decreased the corrosion rate of the couple.     

In vitro corrosion resistance of Lotus-type porous Ni-free stainless steels

The corrosion behavior of three kinds of austenitic high nitrogen Lotus-type porous Ni-free stainless steels was examined in acellular simulated body fluid solutions and compared with type AISI 316L stainless steel. The corrosion resistance was evaluated by electrochemical techniques, the analysis of released metal ions was performed by inductively coupled plasma mass spectrometry (ICP-MS) and the cytotoxicity was investigated in a culture of murine osteoblasts cells. Total immunity to localized corrosion in simulated body fluid (SBF) solutions was exhibited by Lotus-type porous Ni-free stainless steels, while Lotus-type porous AISI 316L showed very low pitting corrosion resistance evidenced by pitting corrosion at a very low breakdown potential. Additionally, Lotus-type porous Ni-free stainless steels showed a quite low metal ion release in SBF solutions. Furthermore, cell culture studies showed that the fabricated materials were non-cytotoxic to mouse osteoblasts cell line. On the basis of these results, it can be concluded that the investigated alloys are biocompatible and corrosion resistant and a promising material for biomedical applications.     

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.     

轧制工艺对316L不锈钢组织和耐蚀性能的影响

为探索改善不锈钢耐腐蚀性能的途径,对316L不锈钢施加相同变形量的同步轧制和异步轧制,利用X射线衍射、透射电镜观察、电化学测量和扫描电镜表面观察研究了轧制工艺对钢的显微组织和腐蚀性能的影响.结果表明,经过异步轧制后显微组织中出现大量孪晶界,优化了晶界结构,在酸性介质中的晶间腐蚀敏感性明显减轻;而经过同步轧制后,样品呈现出高位错密度的显微组织,在酸性介质中的耐腐蚀性能降低.异步轧制后耐蚀性能得到改善是由于大量孪晶界的形成优化了晶界结构.     

Experimental investigation on corrosion and hardness of ion implanted AISI 316L stainless steel

The AISI 316L stainless steel has been widely used both in artificial knee and hip joints in bio-medical applications. In the present study AISI 316L SS was implanted with two different ions: nitrogen and helium at 100 keV with a dose of 1 × 10¹⁷ ions/cm² at room temperature. The crystallographic orientation and surface morphology were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM). The effects of ion implantation on the corrosion performance of AISI 316L stainless steel was evaluated in 0.9% NaCl solution using electro chemical test both on the virgin and implanted samples. The subsequent Tafel analysis shows that the ion implanted specimens were more corrosion resistant when compared to the bare specimens. Microhardness was also measured by Vickers method by varying the loads. The results of the studies indicated that there was a significant improvement in both corrosion resistance and hardness of implanted samples.     

An automatic pitting corrosion detection approach for 316L stainless steel

Corrosion is considered a critical problem in many engineering structures and materials. Electrochemical techniques are the most popular techniques to study the corrosion behaviour of the materials. These techniques entail a surface study of the material to analyse its passive state. Therefore, the evaluation using these techniques requires visual interpretation steps which may lead to subjectivity in the results. In this work, different models based on artificial neural networks (ANNs), support vector machines (SVMs), classification tree (CT) and k-nearest neighbour (kNN) were presented to develop an automatic way to predict pitting corrosion behaviour of stainless steel in different environmental conditions. In addition, the influence of two different feature selection methods on the classification performance was considered. Receiver operating characteristics (ROC) space was applied to analyse the classification performance. The results, based on different statistic metrics considering 5-fold cross validation (93.1% of precision, 95.8% of sensitivity and 91.5% of accuracy), demonstrated the effectiveness of the proposed technique based on ANNs to predict corrosion behaviour by automatic way, not requiring the use of electrochemical techniques.     

The effects of saline aqueous corrosion on fatigue crack growth rates in 316 grade stainless steels

A study of fatigue crack propagation rates of 316 grade stainless steels in air and in an aqueous saline environment was carried out in an attempt to assess the fatigue properties encountered when such materials are used as surgical implants. The effects of variables such as temperature, pH, oxygenation level, bulk electrode potential, mean stress, frequency and stress waveform on the Paris crack growth law parameters were determined. Corrosion fatigue effects were observed in the aqueous saline environment, and a mechanism to describe this effect is proposed.     

Influence of high power diode laser surface melting on the pitting corrosion resistance of type 316L stainless steel

High Power Diode Lasers (HPDL) are becoming more and more attractive for industrial materials processing because of their high efficiency, low running costs, small sizes and low weight. Surface melting experiments have been carried out on 316L steel with a 1 kW HPDL, with application to modify its pitting corrosion resistance in NaCl 0.05 M. Surface modifications have been investigated with optical microscopy for the microstructure, microprobe analysis for the chemical content and X Ray Diffraction for phase transformations and residual stresses. Heat conduction characteristics, analysed with a 2D Finite element code, have driven to a 28% calculated absorption of the laser light generating nearly 400 m melted depth. A refinement and homogenization of structure together with -ferrite transformation and the dissolution of inclusions was found in the melted thickness, driving to enhanced pitting resistance (nearly + 0.2 V on the pitting potential values, and factor 2 decrease of the passive current density). This pitting resistance, investigated at different depths below the surface, was found to be little affected by the ferrite content (6% estimated value), and the fineness of the microstructure, but depreciated by the surface state without post-polishing. Therefore, it is believed that localized corrosion improvements can be mainly attributed to the dissolution of Al-base and Mn-base detrimental inclusions, despite the generation of up to 6% ferrite susceptible to drive to enhanced galvanic couplings with phase.