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.     

低碳钢表面激光熔覆层及其高温磨损行为

为改善低碳钢材料的耐高温磨损性能,采用激光熔覆法,在低碳钢表面制备出Ni60合金、Ni60＋Y2O3的熔覆涂层和Ni60熔覆-重熔涂层,利用X射线衍射仪、磨擦磨损实验机、扫描和透射电镜分析了熔覆层相组成、高温耐磨性能和熔覆层显微形貌.结果表明：所制得熔覆层与熔覆-重熔层组织均一、致密,与基体形成了良好的冶金结合;熔覆-...     

Identification of the nitrides formed during the annealing of a low-carbon low-aluminium steel

The precipitation of a low carbon low aluminium steel with a sub-stoichiometric [Al]/[N] atomic ratio was investigated during an annealing at 600°C by Transmission Electron Microscopy (TEM) and chemical analyses by X-ray Energy Dispersive Spectroscopy (EDX) on thin foils and carbon extraction replicas. These studies showed that in this steel, the precipitates which form have a platelet-like morphology associated with a cubic structure (with a = 0.412 ± 0.005 nm) and the following orientation relationship with iron: (001)_p // (001)_Fe and [110]_p // [010]_Fe. The EDX microanalyses of these precipitates revealed that they contain, in addition to aluminium and nitrogen, chromium and/or manganese. It was suggested that in the early stages of annealing, the initial precipitates are of CrN or (Cr,Mn)N type. During a prolonged annealing, they serve as nucleation sites for the final precipitates of (Al,Cr)N and (Al,Cr,Mn)N type.     

Chemical-state information obtained by AES and XPS from thin oxide layers on duplex stainless steel surfaces

Thin oxide films were produced by the exposure of polished, sputter-cleaned metallic surfaces kept in UHV to several thousands of Langmuirs at 10⁻⁵ mbar oxygen yielding oxide layers of several nanometers. Metallic substrates used were iron, chromium and duplex stainless steel (DSS 2205). AES and XPS profiling analyses were performed. An attempt was made to use certain features observed in the AES spectra, i.e. to correlate Fe and Cr MNN peak shapes with the chemical state of the corresponding element. A similar approach has been tried before and may, when combined with high-lateral-resolution AES, provide small-area chemical-state information. Localized chemical-state information derived from the MNN peak shapes by a Linear Least Squares Fit (LLSF) procedure appeared to match reasonably well with that provided by XPS, which is averaged over approximately 2 mm². This is a plausible result for a thin homogeneous layer on a polished substrate.     

Chemical-state information obtained by AES and XPS from thin oxide layers on duplex stainless steel surfaces

Thin oxide films were produced by the exposure of polished, sputter-cleaned metallic surfaces kept in UHV to several thousands of Langmuirs at 10⁻⁵ mbar oxygen yielding oxide layers of several nanometers. Metallic substrates used were iron, chromium and duplex stainless steel (DSS 2205). AES and XPS profiling analyses were performed. An attempt was made to use certain features observed in the AES spectra, i.e. to correlate Fe and Cr MNN peak shapes with the chemical state of the corresponding element. A similar approach has been tried before and may, when combined with high-lateral-resolution AES, provide small-area chemical-state information. Localized chemical-state information derived from the MNN peak shapes by a Linear Least Squares Fit (LLSF) procedure appeared to match reasonably well with that provided by XPS, which is averaged over approximately 2 mm². This is a plausible result for a thin homogeneous layer on a polished substrate.     

Microstructure and properties of composite (B + C) diffusion layers on low-carbon steel

Combined surface hardening with boron and carbon was used for low-carbon 5120 steel. The microstructure, carbon profiles and chosen properties of borided layers produced on the carburized 5120 steel have been examined. These composite (B + C) layers are termed borocarburized layers. The microhardness profiles and wear resistance of these layers have been studied. In the microstructure of the borocarburized layer two zones have been observed: iron borides (FeB + Fe₂B) and a carburized layer. It has been found the depth (100–125 m) and microhardness (1500–1900 HV) of iron borides zone. The carbon content (0.83–1.46 wt pct) and microhardness (950 HV) beneath iron borides zone have been determined. The microhardness gradient in borocarburized layer has been reduced in comparison with the only borided layer. An increase of distance from the surface is accompanied by a decrease of carbon content and microhardness in the carburized zone. The carbon and microhardness profiles of borided, carburized and borocarburized layers have been presented. A positive influence of composite layers (B + C) on the wear resistance was determined. The wear resistance of the borocarburized layer was determined to be greater in comparison with that for only borided or only carburized layers.     

XPS characterisations of passive films formed on martensitic stainless steel: qualitative and quantitative investigations

HNO₃ passivation treatments on martensitic stainless steels used for surgical instrumentation were studied. The pitting corrosion resistance was determined by electropotentiodynamic experiments. The composition and the thickness of the passive films were investigated by XPS. The pitting corrosion potential was more noble for HNO₃-passivated sample than for air-passivated sample. The different methods proposed in the literature to estimate the thickness of passive layers were used and compared. Passive films obtained by HNO₃-passivation were thinner (3 nm) than air-passivated films (4–5 nm). The composition of these passive films also differs. HNO₃-passivated films were enriched in oxidised chromium which represent half of the metallic elements in the passive layer.     

Characterization of surface layers formed under natural environmental conditions on medieval glass from Siponto (Southern Italy)

In this paper a low-vacuum scanning electron microscope (SEM) coupled with an energy-dispersive X-ray spectrometer (EDX) was used to investigate the alteration processes that occur on silica–soda–lime glass exposed to soil materials and dated from XI to second half of XIII sec. The chemical data were collected for altered glass gel and fresh glass. In order to study the influence of chemical composition on weathering process, 16 glasses have been selected on the basis of the chemical characterization and on the basis of the different corrosion processes present on the fragments. Six selected samples had been produced with the use of natron as fluxer and 10 samples with the use of plant ash as fluxer. The analysed pieces come from Siponto excavations (Foggia, Italy) and they include feet and rims of chalices, fragments of lamps and of globular bottles.     

XPS characterization of surface films formed on surface-modified implant materials after cell culture

Nitrogen ion-implanted Ti–6Al–4V, Ti–5Al–2.5Fe and 316 L stainless steel and nitrogen or carbon sputter-coated samples were inoculated with rat bone marrow. The interface between the cell layer and the substrata was studied by X-ray photo-electron spectrometry and observed by scanning electron microscopy (SEM). Ca and P were detected on all materials after in vitro cell culture. Titanium appears to be present mainly in the form of TiO2.     

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.     

Microstructural characterization of surface layers formed on a low alloy steel during erosion – oxidation in a fluidized bed

Low alloy steel specimens were subjected to erosion-oxidation in a simulated fluidized bed environment in the range 100–600° C. Transmission electron microscopy was carried out on the surfaces which experienced particle impacts, with specimens prepared by backthinning. At low temperatures (≤ 200° C) a dense, protective layer of bed particle fragments formed on the steel surface due to fragmentation of particle asperities and their subsequent comminution to a very fine size (< 10 nm). At intermediate temperatures thin oxide films developed on the wear scar surfaces; these were predominantly magnetite with a fine grain size. There was rapid material loss with particle impacts removing the oxide and some metal below, but the thin oxide rapidly regrew due to the fine grain size, absence of a haematite layer and the mechanical damage during particle impact. At high temperatures the oxide became sufficiently thick to be mechanically protective. Erosion occurred within the fine grained surface haematite layer, while there was grain growth in the lower magnetite layer. This revised version was published online in November 2006 with corrections to the Cover Date.     

AES investigation of the stainless steel surface oxidized in plasma

Auger electron spectroscopy (AES) depth profiling was used to study the oxidation phenomena of AISI316L stainless steel during treatment with oxygen plasma. Samples were exposed to low-pressure RF plasma with a high dissociation degree, so that the flux of oxygen atoms onto the sample surface exceeded 10²⁴ m⁻² s⁻¹. A set of samples was oxidized 4 min at different temperatures up to 1300 K during plasma treatment. AES measurements showed that the oxide film thickness increased with the increasing temperature. The thickness of the oxide film on the samples oxidized in plasma at 300 K was nearly the same as for the untreated sample. The thickness of the oxide film of the samples which were oxidized at 1000 K was about 170 nm and it consisted of iron oxide. The thickest oxide film of about 350 nm was found on the samples heated in oxygen plasma to 1300 K. Depth profiling showed the uppermost layer of manganese oxide, followed by a mixture of chromium oxide and iron oxide. The scanning electron microscope analyses showed a dramatic increase of the surface roughness.     

Formation of zinc ferrite by solid-state reaction and its characterization by XRD and XPS

A dry mixture of ZnO and -Fe₂O₃was annealed at 1200°C; the progress of the formation of the ferrite was monitored by XRD and XPS analyses at different time intervals. The presence of octahedral zinc cation was observed along with the regular tetrahedral Zn in the sample that had undergone 30 minute heat treatment. After three hours of heating, pure normal zinc ferrite was formed. The Zn 2p_3/2peak binding energy, intensity and line shape were analyzed extensively to show the diffusion of ZnO and the growth of ferrite at different stages of heat treatment. Analysis of the Fe 2p_3/2line-shape supported the substitution of Fe²⁺by zinc cations during ferrite formation. The binding energy values of the Zn 2p levels for stoichiometric and non-stoichiometric ferrites were also determined and surface segregation of the zinc was observed by XPS in the non-stoichiometric ferrites.     

Characterization of converted tin phosphate coating on iron and steel by conversion electron Mössbauer spectrometry (CEMS)

The chemical states of tin in converted tin phosphate coatings on iron and steel and in tin phosphate powders were analysed by conversion electron Mössbauer spectrometry and transmission Mössbauer spectrometry. The tin species in stannous phosphate, stannic oxide and metallic tin were recognized in the converted tin phosphate coatings. Mössbauer parameters of Sn(II) included in the phosphate coatings were slightly different from those of Sn(II) incorporated in powders of zinc and manganese phosphate compounds. The tin atoms mainly occupied two different sites in three sites of stannous phosphate structure, depending on the preparations.     

Preparation and characterization of thin oxides films formed at a brass surface oxidized by plasma treatment

The surface of a 90Cu10Zn alloy exposed to a low pressure inductively coupled oxygen plasma gives rise to an oxide layer, the composition of which is determined by FTIR spectroscopy and X-ray diffraction techniques. The thickness of the oxide layer depends on the time of exposure to the plasma and on the distance between the sample and the first high voltage (HV) coil for given primary treatment parameters and on the preparation technique of the samples.     

An XPS and thermogravimetric study of oxidized AlN and AlN–Si3N4 layers deposited by liquid-phase chemical vapour deposition

The results of a comparative study of the resistance to oxidation of AlN and a codeposit of AlN–Si3N4 are presented. The oxidation of both types of layer was performed at 1200°C in an oxygen gas flow (pO2∼1 atm). A thermogravimetric analysis was made, recording the weight gain throughout the oxidation process, and revealed a dramatic improvement in the resistance to oxidation of the codeposit layer when compared with the aluminium nitride layer. An X-ray photoelectron spectroscopy depth-profile analysis was made by performing a series of successive abrasive polishes on the oxidized layers. After each abrasive polish the sample surface was cleaned by ionic bombardment under ultrahigh vacuum and the photoemission spectra recorded. This process of polishing, etching and analysis was continued until the oxygen content decreased to a level comparable with that observed in the layers before oxidation. The changes in chemical bonding throughout the various depth profiles were examined by deconvolution of the Al 2p and Si 2p photoelectron peaks and by analysis of the Auger K L L lines. It was concluded that the slower rate of oxidation of the codeposit could be attributed to either the formation of silica or a mixed oxide phase of the mullite type (3 Al2O3–2SiO2). This revised version was published online in November 2006 with corrections to the Cover Date.     

Study of oxidized layer formed on aluminium alloy by plasma oxidation

This paper presents results of plasma oxidizing of aluminium alloy which was carried out in a low-frequency pulse excited plasma. The processing temperature was about 450 °C and the processing time was 3 h. The modified layer was characterized by X-ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy. Atomic force microscopy and scanning electron microscopy observations indicated that no evident defects appeared on the surface of the oxidation layer. Immersion experiments in 3.5% NaCl solution were carried out to test the effect of plasma oxidation on the corrosion resistance of surface oxidized aluminium alloys. After immersion for 200 h, the severe corrosion occurred on the untreated sample surface while no evident corrosion was observed for the treated sample.