Effect of Cu on the surface hardness of Ni−B coating for the strengthening of the Cu surface

In this paper, the effect of Cu on the decrease of the surface hardness of a Ni−B layer electrolessly coated was investigated. Ni₃B crystallized at approximately 300 °C was decomposed to metallic Ni and free B after annealing above 500 °C. The metallic Ni formed a solid solution with Cu and the free B diffused to the surface. The free B diffusing to the surface formed the oxide B₂O₃ after it was annealed in an Ar atmosphere and was volatilized after it was annealed in an H₂ atmosphere. However, decomposition of the Ni₃B was not observed in Ni−B alloy powders without Cu. Therefore, it was concluded that the decomposition of Ni₃B by Cu decreased the surface hardness of the Ni−B coating and strengthened the surface of Cu.     

Microscopic Features in the Transition from External to Internal Oxidation in an Fe–6 mol.% Si Alloy Annealed Under Various H2O–H2 Atmospheres

The occurrence of external or internal oxidation in Fe–Si alloys is strongly affected by oxidation conditions. In the present study, X-ray diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary-ion mass spectrometry, and glow-discharge optical-emission spectrometry were used for characterizing the microscopic features of oxide layers formed on the (011) surface of an Fe–6 mol.% Si alloy. The starting materials were annealed at 1473 K under dry hydrogen gas and were subsequently annealed at 1123 K under a 75% H₂–25% N₂ atmosphere with various partial pressures of water vapor. The results show that the microscopic morphology and elemental distribution in oxide layers strongly depend on oxidation conditions. The surface was found to become rough by annealing in higher partial pressures of water vapor. This phenomenon may be induced by internal oxidation. Corresponding to the morphological changes of the surface, changes in the distribution of alloying elements have systematically been characterized in surface layers. These experimental results are discussed in conjunction with thermodynamic data on oxidation of elements.     

Effect of ferric ions in AISI 316L stainless steel pickling using an environmentally‐friendly H2SO4‐HF‐H2O2 mixture

A mixture of hydrogen peroxide, sulphuric and hydrofluoric acids has been used as pickling solution at pH 2.0 for AISI 316L austenitic stainless steel (SS). The stability of the H₂SO₄‐HF‐H₂O₂ mixture is assessed varying the ferric ions content from 0 to 40 g/L, the temperature from 25 to 60°C, and with and without stirring of the pickling solution. The AISI 316L SS pickling rate at 50°C was 2.6 and 0.2 mg/dm² day (mdd) in the absence and presence of 40 g/L ferric ions, respectively. p‐toluene sulphonic acid (PTSA) has been used as stabiliser of hydrogen peroxide.     

Corrosion of Three Commercial Steels Under ZnCl2 –KCl Deposits in a Reducing Atmosphere Containing HCl and H2S at 400–500°C

The corrosion of three commercial steels in a reducing atmosphere containing HCl plus H₂S in the presence of ZnCl₂–KCl deposits has been investigated at 400–500°C and compared with the corrosion of the same materials in a similar gas mixture free from H₂S. The presence of H₂S in the gas accelerated the corrosion of the three commercial steels beneath ZnCl₂–KCl deposits. All materials suffered from severe corrosion with partial detachment and spalling of the external scales. Degradation of the steels resulted from the penetration of chlorine-containing species through the scale formed initially down to the metal matrix, because chorine-rich species were detected close to the alloy/scale interface. Although the corrosion resistance generally increased with increasing Cr content, even the high-Cr stainless steel SS304 is still unable to provide good corrosion resistance against the ZnCl₂–KCl deposits in the presence of H₂S due to the bad adherence of the scales to the alloy. The mechanisms of attack are discussed on the basis of thermodynamic considerations and of the active-oxidation model.     

Effect of Plasma Nitriding and Nitrocarburizing on HVOF-Sprayed Stainless Steel Coatings

In this work, the effects of plasma nitriding (PN) and nitrocarburizing on HVOF-sprayed stainless steel nitride layers were investigated. 316 (austenitic), 17-4PH (precipitation hardening), and 410 (martensitic) stainless steels were plasma-nitrided and nitrocarburized using a N₂ + H₂ gas mixture and the gas mixture containing C₂H₂, respectively, at 550 °C. The results showed that the PN and nitrocarburizing produced a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer depending on the crystal structures of the HVOF-sprayed stainless steel coatings. Also, the diffusion depth of nitrogen increased when a small amount of C₂H₂ (plasma nitrocarburizing process) was added. The PN and nitrocarburizing resulted in not only an increase of the surface hardness, but also improvement of the load bearing capacity of the HVOF-sprayed stainless steel coatings because of the formation of CrN, Fe₃N, and Fe₄N phases. Also, the plasma-nitrocarburized HVOF-sprayed 410 stainless steel had a superior surface microhardness and load bearing capacity due to the formation of Cr₂₃C₆ on the surface.     

Electrochemical performance of magnetron sputter deposited LiFePO4-Ag composite thin film cathodes

LiFePO₄ thin films have been sputtered from a pure LiFePO₄ target onto Ag/SS, Ag/Si₃N₄/Si and Si₃N₄/Si substrates. All of the deposited films were annealed at 973 K for 1 hr in H₂/Ar (5 %) atmosphere. Substrate induced microstructural and crystallographic evolutions have been observed by a scanning electron microscope and X-ray diffraction. Energy dispersion spectra and X-ray photoelectron spectra revealed that Ag was mixed in the LiFePO₄ films deposited on Ag under layers. Ceramic metal composite thin films were obtained. The film conductivity (1 × 10^− 3 Scm^− 1) is therefore elevated by an order of six, compared with pure LiFePO₄ (10^− 9 Scm^− 1). The electrochemical measurements of the LiFePO₄-Ag films showed a flat plateau at 3.4 V (v.s. Li/Li⁺) and a reversible capacity of 80 mAh/g. Optimization of Ag contents may further improve the discharge capacity.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

Stress corrosion cracking behaviour of 316L stainless steel exposed to CO2–H2S–Cl− environments in the absence and presence of methyldiethanolamine (MDEA)

Stress corrosion cracking (SCC) behaviour of 316L stainless steel in CO₂–H₂S–Cl^? environments with and without methyldiethanolamine (MDEA) was investigated by slow strain rate testing and scanning electron microscopy (SEM). The results show that elongation ratio, reduction in area ratio (RAR) and time to failure ratio (TTFR) of 316L stainless steel were low in CO₂–H₂S–Cl^? environments. The corresponding fractography exhibited flat brittle fracture with quasi-cleavage pattern, indicative of high SCC susceptibility. Hydrogen penetration and corrosion pits could be responsible for the high SCC susceptibility of 316L stainless steel in this condition. For the CO₂–H₂S–Cl^? environments in the presence of MDEA, 316L stainless steel possessed high ER, RAR and TTFR (nearly 100%). High SCC resistance of 316L stainless steel could be associated with MDEA induced removal of H₂S/CO₂ and absorption on the steel surface.     

Effects of Mo and Al Additions on the Sulfidation Behavior of 310 Stainless Steel

The high-temperature sulfidation behavior of 310stainless steel (310SS) with Mo and Al additions (up to10 at.%) was studied over the temperature range700-900°C in pure-sulfur vapor over the range of 10^-3 to 10^-1 atm. Thecorrosion kinetics followed the parabolic rate law inall cases and the sulfidation rates increased withincreasing temperature and sulfur pressure. Thesulfidation rates decreased with increasing Mo and Al contents and it wasfound that the addition of 10 at.% Mo resulted in themost pronounced reduction among the alloys studied. Thescales formed on 310SS with Mo additions were complex, consisting of an outer layer of ironsulfide (with dissolved Cr), (Fe,Ni)₉S₈, andCr₂S₃/Cr₃S₄(with dissolved Fe), and an inner heterophasic layer ofFe_1-xS,Cr₂S₃/Cr₃S₄,NiCr₂S₄,Fe_1.25Mo₆S_7.7, FeMo₂S₄, andMoS₂. The scales formed on 310SS with Mo andAl additions had a similar mixture as above, except thatAl_0.55Mo₂S₄ was alsoobserved in the inner layer. The formation ofMoS₂ andAl_0.55Mo₂S₄ partly blocked the transport of cations throughthe inner scale, resulting in the reduction of thesulfidation rates compared to 310SS.     

Spinel coatings for UNS 430 stainless steel interconnects

At the usual temperature of solid oxide fuel cell (SOFC) operation, ferritic stainless steels form electrically insulating or poorly conducting oxide scales, which can cause high internal resistance losses and chromium poisoning. In an effort to avoid this problem, we applied conductive copper manganite and cobalt manganite spinel coatings, with nominal composition MnCo₂O₄ and Cu_1.4Mn_1.6O₄, which were deposited on the surface of UNS 430 stainless steel by electroplating and subsequent air annealing. Microstructural evaluation indicated that the spinel layers inhibited outward diffusion of chromium. Moreover, excellent structural and thermal stability were observed after several thermal cycles at 750 °C and for up to 28 days, and the coating layers showed good adhesion to the substrate.     

Spectroscopic properties of vacancies and trap levels in Lu3Al5O12：Ce^3＋ crystals

In order to study the spectroscopic properties of vacancies and trap levels in Lu₃Al₅O₁₂:Ce³⁺ (LuAG:Ce³⁺) crystal, the <111>-oriented LuAG:Ce³⁺ crystal grown in pure nitrogen atmosphere by Czochralski method was annealed in oxidizing atmosphere (air) and reducing atmosphere (H₂+N₂), respectively. The excitation and emission spectra of LuAG:Ce³⁺ crystal after different thermal annealing treatments were measured in the temperature range of 8?450 K, and the thermally stimulated luminescence curves of LuAG:Ce³⁺ crystal were characterized. It is found that the oxygen vacancies in LuAG:Ce³⁺ crystal are effectively eliminated through the annealing treatment in air and four trap levels are observed in as-grown LuAG:Ce³⁺ crystal with temperature position peaking at 110, 210, 325 and 475 °C, respectively.     

Surface modification of austenitic stainless steel with plasma nitriding for biomedical applications

In the present study, plasma nitriding of AISI type 303 austenitic stainless steel (SS) specimens was performed using a microwave system. The nitrided layers were characterized by performing scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and a Vickers microhardness test. The antibacterial activities of the nitrided layers were evaluated. XRD and TEM showed that a single γ_N phase was formed by plasma nitriding at the plasma power of 700 W and 450 °C. The analytical results demonstrated that the hardness of type 303 specimens could be enhanced by plasma nitriding because of the formation of the γ_N phase. A bacterial test also demonstrated that the nitrided layer exhibited excellent antibacterial properties.     

Hemocompatibility evaluation of plasma-nitrided austenitic stainless steels at low temperature

In this study, plasma-nitrided austenitic stainless steel was prepared using a microwave system as a function of the nitriding duration. The nitride layers were characterized via scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy, atomic force microscopy and a Vickers micro-hardness tester. The effects of plasma nitriding on the compatibility of blood with the austenitic stainless steel were also investigated. Based on the XRD and TEM observations, it was inferred that only a single γ_N phase was detected in the whole process of experiments. The γ_N layer, which was formed on austenitic stainless steel, exhibited lower platelet adhesion and activation than the untreated specimens. The clotting time of the PN specimens was prolonged and increased with the treatment time. This work suggested that the blood-compatible manner of plasma nitriding by controlling surface characteristic on austenitic stainless steel improved the anticoagulant properties, and made austenitic stainless steel suitable candidates in the field of surgical and medical instruments.     

Fabrication of Al alloys reinforced with AlN particles formed byin-situ reaction

The tensile properties and microstructures of Al alloys reinforced with AlN particles formed byin-situ reaction under a nitrogen atmosphere were analyzed. It was found that AlN particle layers formed on the surface of the Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the reaction, Mg₃N₂+2A1 → 2AlN+3Mg. The tensile strength and 0.2% offset yield strength in the control alloys were significantly greater than those in commercial alloys. This increase was due to the fine AlN particles formed by the abovein-situ reactions of the Mg₃N₂ formation and its decomposition into AlN. This article is based on a presentation made in the symposium “The l^st KIM-JIM Joint Symposium: High Strength Ratio Aluminum Alloys”, held at Inha University, Inchon, Korea, October 22, 1999 under the auspices of The Korean Institute of Metals and Materials and The Japanese Institute of Metals.     

Effect of Al and Ce oxide layers electrodeposited on OC4004 stainless steel on its corrosion characteristics in acid media

The changes in the corrosion characteristics of stainless steel OC4004 in 0.1 M HNO₃ after electrodeposition of thin Al and Ce oxide films on it has been investigated. The Ce₂O₃–CeO₂ layers have been found to possess a pronounced stabilizing effect on the steel passive state and on its corrosion resistance, respectively, whereas the Al₂O₃ layers do not improve considerably the corrosion behaviour of the SS/Al₂O₃ system. A twice-lower corrosion current was observed with a ternary SS/Al₂O₃/Ce₂O₃–CeO₂ system in the passive region, while the zones of potentials, where the steel is in a stable passive state, are not changed. The obtained results permit the assumption that the cerium oxides layer acts as an effective cathode playing a determining role with respect to the improvement of the corrosion behavior of the steel. It has been concluded that when the SS/Al₂O₃/Ce₂O₃–CeO₂ system is used in media containing nitric acid, the corrosion will proceed at potentials where the passive state of steel would not be disturbed.     

Comparison of corrosion scales formed on KO8OSS and N8O steels in CO2/H2S environment

Abstract

Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were applied to analyse the microstructure and composition of the corrosion scale formed on KO80SS and N80 tubes with carbon dioxide (CO₂) and hydrogen sulphide (SO₂). The corrosion scales of both KO80SS and N80 tubes were of the double layer structure, and not only uniform corrosion but also localised corrosion was observed. The crystal of the surface layer is laminar. The main phase in the outer layer is calcium carbonate (CaCO₃), and the inner scale consisted of iron carbonate (FeCO₃) for KO80SS steel and FeS_0·9 with a little amount of FeCO₃ for N80 steel respectively. Additionally, the electrochemical techniques were used to investigate the characteristics of the corrosion scales. The results indicated that the polarisation resistance R_p of KO80SS steel film was nobler than that of N80 steel film. Finally, the corrosion current I_corr of KO80SS steels was lower than that of N80 steels. Corrosion scale of KO80SS tube steels is more protective to the matrix than that of N80 tube steels.     

Electrochemical study of corrosion behaviour of carbon steel A106 and stainless steel 304 in aqueous monoethanolamine

Abstract

Corrosion behaviour of carbon steel A106 and stainless steel 304 (SS304) in aqueous monoethanolamine was studied by performing electrochemical polarisation experiments. Potentiodynamic curves were studied and compared under conditions with different temperatures, carbon loading and O₂ percentage in purging gases. It was found that corrosion of A106 and SS304 was promoted under conditions with higher temperature. While the presence of O₂ speeds the corrosion of A106, it has a negligible impact on SS304 at 80°C and lowers the corrosion rate at 40°C. Corrosion rates and other important parameters were calculated based on the electrochemical curves for A106. Sample surfaces after tests were examined by scanning electron microscopy and energy dispersive spectroscopy. Mechanisms involved in iron dissolution and passivation from oxide films were discussed.     

Protective effect in sulfuric acid media of alumina and ceria oxide layers electrodeposited on stainless steel

The influence of thin layers of Al₂O₃ and Ce₂O₃-CeO₂, electrodeposited on stainless steel OC4004, on the corrosion behaviour of the systems Al₂O₃/SS, Ce₂O₃-CeO₂/SS and Al₂O₃-Ce₂O₃-CeO₂/SS has been studied in sulfuric acid medium. A pronounced stabilizing effect on the passive state of steel and enhancement of its corrosion resistance has been established both for the samples as deposited and for the thermally treated Ce₂O₃-CeO₂/SS systems. In comparison to them the layers of Al₂O₃ have a substantial impact on the corrosion resistance of the Al₂O₃/SS system only in the cases when the system is not subjected to thermal treatment. The consecutive deposition of Al₂O₃ and Ce₂O₃-CeO₂ films on SS gives as a result an outstanding corrosion-protective effect, whereupon the corrosion potential of the system Al₂O₃/Ce₂O₃-CeO₂/SS is shifted in positive direction with ∼ 0.3 V for the samples as deposited and with ∼ 1 V—for the thermally treated samples. The so established favourable effect has been explained by the increased concentration of chromium oxides in the surface passive film, caused by the presence of cerium oxides, as well as by their action as cathode, effective with respect to the reduction corrosion reaction, shifting strongly the potential (at which this reaction is occurring) in positive direction.     

In situ IR pyrometric analysis during thermal treatment in air of TiOxNy coatings

IR pyrometry is an original diagnostic tool for in situ analysis of surface transformations of coatings or bulk materials during the thermal treatment under reactive atmosphere such as high temperature oxidation. Significant oscillations of the pyrometric signal were observed during annealing in air of TiO_1.5N_0.5 coatings in the temperature range 673-823 K. This is due to interferences resulting from multi-reflections at the interfaces of a transparent growing film. This reveals the formation of a TiO₂ thin film on the top of the TiO_1.5N_0.5 coating. Modeling of the time dependence of the IR pyrometric signal allows the determination of the oxide layer thickness, transformation rate and optical properties of the films under the growth conditions. The progressive oxidation of a compact amorphous TiO_1.5N_0.5 coating from the external surface to the substrate interface was supported by SIMS, XRD and reflectance analyses.     

Oxidation of hydrocarbon films and metal materials in oxygen-containing low temperature plasma

Glow discharge in oxygen-containing gases (air, a mixture of O₂/N₂, N₂O) was used for selective removal of amorphous hydrocarbon (a-C:H) films predeposited on stainless steel, W, and Mo. The gasification rate of films was maximal in N₂O. H₂, H₂O, CO, and CO₂ were plasmolysis products. Thin (3?C30 nm) oxide layers, the thicknesses of which depended on the location of samples relative to plasma, formed on the metal after removal of the film.