Refractory Oxide Coatings on Titanium for Nitric Acid Applications

Tantalum and Niobium have good corrosion resistance in nitric acid as well as in molten chloride salt medium encountered in spent fuel nuclear reprocessing plants. Commercially, pure Ti (Cp-Ti) exhibits good corrosion resistance in nitric acid medium; however, in vapor condensates of nitric acid, significant corrosion was observed. In the present study, a thermochemical diffusion method was pursued to coat Ta₂O₅, Nb₂O₅, and Ta₂O₅ + Nb₂O₅ on Ti to improve the corrosion resistance and enhance the life of critical components in reprocessing plants. The coated samples were characterized by XRD, SEM, EDX, profilometry, micro-scratch test, and ASTM A262 Practice-C test in 65 pct boiling nitric acid. The SEM micrograph of the coated samples showed that uniform dense coating containing Ta₂O₅ and/or Nb₂O₅ was formed. XRD patterns indicated the formation of TiO₂, Ta₂O₅/Nb₂O₅, and mixed oxide/solid solution phase on coated Ti samples. ASTM A262 Practice-C test revealed reproducible outstanding corrosion resistance of Ta₂O₅-coated sample in comparison to Nb₂O₅- and Ta₂O₅ + Nb₂O₅-coated sample. The hardness of the Ta₂O₅-coated Cp-Ti sample was found to be twice that of uncoated Cp-Ti. The SEM and XRD results confirmed the presence of protective oxide layer (Ta₂O₅, rutile TiO₂, and mixed phase) on coated sample which improved the corrosion resistance remarkably in boiling liquid phase of nitric acid compared to uncoated Cp-Ti and Ti-5Ta-1.8Nb alloy. Three phase corrosion test conducted on Ta₂O₅-coated samples in boiling 11.5 M nitric acid showed poor corrosion resistance in vapor and condensate phases of nitric acid due to poor adhesion of the coating. The adhesive strength of the coated samples needs to be optimized in order to improve the corrosion resistance in vapor and condensate phases of nitric acid.     

Effect of Heat Treatment on Corrosion Behaviour of Amorphous Metal Fibers

Amorphous metal fiber has high corrosion resistance and excellent mechanical properties, making it a kind of good material for reinforcing concrete matrix. The effect of heat treatment on the corrosion behaviour of Fe73- Cr6C9Si1P11 amorphous metal fibers in 0.5 mol/L He SO4 solution was investigated by electrochemical polarization analysis. Differential scanning calorimetry （DSC） was used to measure the thermal properties. The evolution of the crystallization process after heat treatment was identified by X-ray diffraction （XRD）. The results show that the α- Fe, Fe2 P and Fea P crystalline phases individually precipitate in the amorphous matrix with increasing annealing temperature. The as prepared amorphous sample shows high corrosion resistance with a lower passivation current density and a wider passive region. The corrosion resistance dramatically decreases after the annealing temperature is higher than 400 ℃.     

Effect of Heat Treatment on Microstructure and Properties of High Boron-High Speed Steel

A new type of high boron-high speed steel (HB-HSS) with different boron content was selected for oil quenching at 1050 °C, and different temperature of tempering treatment was chosen. By using optical microscopy, scanning electron microscopy, X-ray diffraction, Rockwell hardness tester, red hard treatment and wear test, the effects of heat treatment on microstructure and properties of HB-HSS were investigated. The experimental results indicate that the quenching microstructure of HB-HSS consists of α-Fe, M₂(B, C), M₇(B, C)₃ and a few of M₂₃(C, B)₆. When the tempering temperature is lower than 500 °C, the shape of carboborides will change from discontinuous sheet to continuous net, and the uniformity in microstructure is improved, and the hardness is not changed during the process. When the tempering temperature is higher than 500 °C, the continuous net of M₂(B, C) is recovered. When the tempering temperature is higher than 600 °C, the microstructure of HB-HSS get thickened because of overheating, and the hardness get significantly reduced. With the increase of tempering temperature, the weight loss of the sample is decreased, and the wear resistance of the sample is increased. When tempering temperature exceeds 500 °C, the weight loss of the sample has an obvious increase and its wear resistance decreases. The wear resistance of the sample decreases after the red-hardness treatment. The wear loss is about 8.4 mg when the boron content is 2.0% and the tempering temperature is 500 °C, which is the best of test samples.     

Oxidation Resistance of α‐Al2O3 infiltrated Open Porous Cellular Iron

Open porous cellular iron exhibits low oxidation resistance which is limiting their use for application at room and elevated temperatures. Here we deposit α‐Al₂O₃ by Chemical Vapor Infiltration (CVI) to enhance oxidation resistance. The oxidation resistance before and after CVI is investigated by thermo gravimetrical analysis (TGA). The reaction products, surface morphologies and cross sections are investigated by X‐ray diffraction, chemical analysis and electron microscopy. α‐Al₂O₃ infiltrated cellular iron samples exhibit up to 6 orders of magnitude lower oxidation rate at an oxidation temperature of 600 °C, compared to the unprotected cellular iron. In addition, the infiltrated sample shows good resistance during thermal cycling up to this temperature.     

Effects of Surface Morphology on the Wear and Corrosion Resistance of Post-Treated Nitrided and Nitrocarburized 42CrMo4 Steel

The surface of alloyed carbon steel was subjected to thermochemical modification by nitrocarburizing and nitriding with or without postoxidation in order to improve its mechanical properties, corrosion, and wear resistance. Treated samples were characterized by testing their basic properties (compound layer thickness, nitriding, nitrocarburizing depth, and surface hardness) according to standards. Detailed estimation of the modified metal surface was performed by additional testing: X-ray diffraction, microstructure, surface roughness and topography, and wear and corrosion resistance. The surface layer obtained after nitrocarburizing treatment consists mainly of ε-Fe_2-3(N,C) and γ’-Fe₄(N,C); similarly, the nitrided surface is formed by ε-Fe_2-3N and γ’-Fe₄N iron nitrides. The surface layer after postoxidation contains additionally Fe₃O₄. The results obtained show that nitrocarburization, nitridation, and postoxidation result in better mechanical, wear, and corrosion resistance of 42CrMo4 steel, and postoxidized sample properties are influenced by surface morphology.     

The Fabrication and Characterization of a TiB<Subscript>2</Subscript>/Ni Composite Using Spark Plasma Sintering

Ni matrix composites synergistically reinforced by TiB₂ particles were prepared by spark plasma sintering. According to the Nelsone Riley method and Debye–Scherrer formula, the driving force for the growth of crystallite in sintered TiB₂/Ni composites was discussed by using X-ray diffraction technique to analyse the dislocation density and lattice strain of composite powders. TiB₂/Ni composites not only showed great increase in tensile strength but also possessed perfect ductility. Tensile results showed that Ni–3TiB₂ (3 vol% TiB₂) showed an increase in tensile strength, furthermore, contained an acceptable elongation of ~?45%. Besides, the strengthening mechanism was discussed later. The corrosion resistance test of TiB₂/Ni composites was performed by electrochemical method in 3.5 wt% NaCl solution. Results showed that the corrosion resistance of pure Ni sample was superior to that of other specimens. SEM of tensile fracture revealed that debonding phenomenon between TiB₂ and Ni was hindered.     

Development of Silica Glass Coatings on 316L SS and Evaluation of its Corrosion Resistance Behavior in Ringer’s Solution

Sol-gel derived silica glasses have many promising features, including low-temperature preparation as well as chemical and physical stability. Two silica glasses with Si₁₀₀ and Si₈₀ composition were prepared to understand the factors contributing to the rate of bioactivity. The effects of pH, solution aging temperature, and molar ratio of H₂O/tetraethyl orthosilicate (TEOS) were studied, and the obtained powder sample was characterized by Fourier transform infrared spectroscopy, X-ray diffraction studies, and scanning electron microscopy. The synthesized silica glasses were deposited on 316L SS by the spin coating method at the optimized speed of 2000?revolutions per minute. The corrosion resistance behavior of the coatings was determined by (1) open-circuit potential vs time of exposure, (2) electrochemical impedance spectroscopy, and (3) cyclic polarization in Ringer??s solution. A higher breakdown potential (E _b) and repassivation potential (E _p) value with lower current density was obtained from cyclic polarization. Similar results were observed from impedance analysis with higher charge transfer resistance (R _ct) and lower double layer capacitance (C _dl) indicating the corrosion resistance behavior of the coatings compared with the uncoated 316L stainless steel. From the results, it was observed that both Si₁₀₀ and Si₈₀ glass coatings had a positive effect on the corrosion resistance behavior. An adhesive strength of 46?MPa and 45?MPa was obtained for the Si₁₀₀ and Si₈₀ coatings, respectively. An accelerated leach out study was carried out by impressing the potential at their breakdown potential to determine the effect of glass coating for long-term contact between the implant and a normal biological medium.     

Effects of incorporation of micro and nano Al2O3 layers on thermal shock behaviour of YSZ thermal barrier coatings

In the present paper, thermal shock behaviour of usual YSZ and layered composite TBCs was evaluated. The layered composite TBCs used in this study were YSZ/micro Al₂O₃ and YSZ/nano Al₂O₃ in which Al₂O₃ was as a topcoat on the YSZ layer. In addition, coating microstructures were investigated using scanning electron microscope; EDS and X-ray diffraction techniques before and after thermal shock test. The results showed that incorporation of a nano Al₂O₃ oxygen barrier layer caused better thermal shock resistance, while micro Al₂O₃ layer on the top of YSZ sample resulted in sooner coating failure, compared to the usual TBC coating. Also, the thermal shock behaviour and failure mechanism of mentioned coatings were investigated.     

Improved wear resistance of magnesium alloys AZ91 by high current pulsed electron beam treatment

Surface modification of magnesium alloy AZ91 by high current pulsed electron beam (HCPEB) was investigated in the present work. After irradiated by HCPEB, crater-like defects were found scattering on the modified surface, when increasing pulse number, the density of craters sharply reduced. According to XRD analysis, hard intermetallic phase Mg₁₇Al₁₂ would sappear after several pulse of HCPEB treatment that is accordance with the improvement of wear resistance, but more pulses will lead to the solution of Mg₁₇Al₁₂ phase. Ultimately, due to the thickest depth of modified layer, sample treated with 30 pulses and accelerating voltage of 27 kV exhibited the best wear resistance, the depth of wear track decreased from 12.75 µm in initial state to 7.82 μm.     

Infiltration of Open Porous Cellular Iron with α‐Al2O3

Open porous cellular iron may be used as structural material but also in filters and heat exchangers due to its large specific surface area, low density, and good thermal conductivity. Currently, the low oxidation resistance of iron limits its use. One way to enhance the oxidation resistance is to deposit a protective α‐Al₂O₃ coating onto the cellular iron surface using chemical vapour infiltration. Here, we investigate the influence of deposition temperature on the α‐Al₂O₃ coating thickness homogeneity. Furthermore, the oxidation resistance of the infiltrated cellular iron is studied by thermogravimetric analysis. X‐ray diffraction results show that phase‐pure α‐Al₂O₃ coatings grow at 950～1100 °C. Homogeneous coating thickness is favoured at infiltration temperatures below 1000 °C, which is a prerequisite for efficient oxidation protection. Based on the thermogravimetric analysis at 600 °C, the parabolic rate constant of the as received cellular iron is 56% larger than its three hour‐infiltrated counterpart, indicating an improved oxidation resistance due to the protective film. In‐line processing integrating annealing on green cellular iron sample and TiN infiltration treatment steps in one process was successfully carried out.     

Electrochemical Studies and XPS Analysis of the Surface of Zirconium-702 in Concentrated Nitric Acid With and Without Fluoride Ions

Zirconium exhibited pseudo-passive behavior in fluorinated nitric acid (11.5 M HNO₃ + 0.05 M NaF) as the current density measured from the electrochemical studies was several orders higher than the value in fluoride free nitric acid. Impedance studies on zirconium sample exposed in 11.5 M HNO₃ for 240 h confirmed the formation of the passive film with high polarization resistance value and the calculated thickness of the film based on the capacitance value was about ~4.5 nm. On the other hand, in fluorinated nitric acid, the charge transfer resistance value associated with the zirconium dissolution process was dominant when compared to that of the film formation. Results of X-ray photoelectron spectroscopic investigations upheld the presence of ZrOF₂ and ZrF₄ and indicated that the protective oxide layer growth was restricted by the presence of fluoride ions.     

Effect of solid-solution treatment on corrosion and electrochemical behaviors of Mg-15Y alloy in 3.5 wt.% NaCl solution

The effect of solid-solution treatment on corrosion and electrochemical mechanisms of Mg-15Y alloy in 3.5 wt.% NaCl solution was investigated by electrochemical testing,immersion testing and SEM observation.The results indicated that the corrosion resistance of Mg-15Y sample gradually deteriorated with immersion time increasing,which was consistent with the observation of corrosion morphologies.The solid-solution treatment decreased the amounts of second phase Mg24Y5.The Ecorr and corrosion rate of as-cast samples were both lower than those of solid solution-treated samples,and both increased with increment of solid solution-treated time.The corrosion mechanism was proposed for the galvanic,pitting and filiform corrosion which varied with the immersion time and solid-solution treatment.     

Study of heat transfer through layers of casting flux: experiments with a laboratory set-up simulating the conditions in continuous casting

The heat transfer through layers of casting flux has been studied with a laboratory set-up which consists of a water-cooled heat flux probe made of copper and an electrically heated steel plate with a trough to hold the flux sample. The measured heat flux density values were converted to system conductivities k_sys which contain the radiation/conduction conductivity in the flux layer and the contact resistance at the probe side. It was found that k_sys is almost independent from the layer thickness. Ten commercial casting fluxes were investigated and the data for k_sys are given as functions of composition and strand surface temperature.     

Evaluation of Thermal Spray Alumina Coatings on Nickel Electrode Connector for Reprocessing Applications

Electro-oxidative dissolution of the spent mixed oxide nuclear fuels with high plutonium content from prototype fast breeder reactor need to be carried out in boiling 11.5 M HNO₃. Nickel electrode connectors employed in the electrolyser of the dissolver should possess good corrosion resistance as well as good electrical conductivity. Alumina coating deposited on Ni by plasma spraying was evaluated by electrochemical polarization and impedance experiments in 11.5 M HNO₃ at room temperature. In order to improve corrosion resistance, alumina coating relatively denser than plasma spray coating was deposited over Ni by detonation gun (D-gun) spray coating. This alumina coating exhibited a high insulation resistance and the weight loss of alumina coated Ni disc was only 3% compared to 29% for bare Ni disc sample when exposed to the vapour of 11.5 M boiling HNO₃ for 12 h. However, coating delamination observed at the alumina/bond coat interface was attributed to the penetration of HNO₃ vapour through the pores in the coating. Since alumina coating deposited by D-gun technique over Ni was also found to offer only short-term protection against corrosion due to HNO₃ vapour, monolithic dense alumina sleeve fabricated through powder metallurgy route was recommended instead of coating, for better corrosion protection in HNO₃ vapour compared to thermal spray coating.     

Correlating the microstructure of the die-chill skin and the corrosion properties for a hot-chamber die-cast AZ91D magnesium alloy

The corrosion of a hot-chamber die-cast AZ91D thin plate (1.4 mm in thickness) was investigated in terms of its microstructure, to elucidate the role of die-chill skin in corrosion. The die-chill skin was composed of a thin layer of chill zone and a thick layer of an interdendritic Al-rich α-Mg/Al₁₂Mg₁₇ β-phase particle/α-Mg grain composite microstructures. The chill zone (4±1 μm in thickness) had fine columnar and equiaxed grains and contained a distribution of submicron Mg-Al-Zn intermetallic particles. Beneath the chill zone, Al₁₂Mg₁₇ β particles were irregularly shaped but did not have an interdendritic network morphology. Furthermore, Al-rich α phase (also known as eutectic α) was in the interdendritic network, which occupied a higher volume fraction than the β phase in the die-skin layer. Corrosion characteristics were studied via constant-immersion and electrochemical tests. Although previous studies have ascribed the fine microstructure to good corrosion resistance for the AZ91D alloy, the present study showed severe corrosion of the sample with a die skin in chloride solution. Moreover, the sample without the die skin on the surface corroded more slowly. The inferior corrosion performance of the die skin was considered to be related to the high volume fraction of the interdendritic network of Al-rich α phase contained in the die skin, owing to the high cooling rate during solidification. The Al-rich α phase does not increase the corrosion resistance of the AZ91D alloy.     

Al2O3/TiCN-0.2% Y2O3 Composite Prepared by HP and Its Cutting Performance

Al₂O₃/TiCN-0.2% Y₂O₃ composites were fabricated by hot pressing sintering. The effect on mechanical property and microstructure of the sample composition and HP temperate was investigated. The results of Al₂O₃/TiCN-0.2% Y₂O₃ were satisfied. The bending strength, fracture toughness, Vickers hardness was respectively 1015 MPa, 6.89 MPa·m^1/2 20.82 MPa at 1650 °C for 20 min. Good wear resistance was found for the kind of ceramic material when used as cutting tools in the machining of the hardened carbon steel. By the compared experiment for the cutting performances, it could be seen that the performance of this composite material was better than that of the ceramic tool material YT15 for continuously cutting quenched steel. This kind of composite tool material is suitable for continuously cutting quenched steel No.45, especially intermittently cutting quenched steel.     

Microstructure and Properties of an Advanced Nickel-base PM Superalloy

The need for nickel-base powder metallurgy （PM） superalloy turbine discs is becoming increasingly evi dent. With the eventual aim of improving thrust-to-weight ratio of aeroengines for power generation, well integration of significantly high strength, high damage tolerance and high-temperature capability would be reasonably required. An advanced PM superalloy, which was designed for applications up to 815- 8 5 0 ℃, was experimentally investigated. Emphasis was primarily put on microstructure and mechanical properties. The results indicated the measured phases in the sample were composed of γ,γ＇, MC, and Ma B2. With uniform coarse grain microstruc ture （ASTM 5-6）, the sample appeared to exhibit overwhelming superiority over the prior art materials FGH95, FGH96, FGH97 and FGH98. The dominant embodiments consisted of high tensile strength （Rm = 1000 MPa and Rp0.2 800 MPa at 850℃）, strong creep resistance （ξp 0.12% at 815 ℃/400 MPa/50 h）, and considerable stressrupture life （τ=457.4 h at 815 ℃/450 MPa）. The technical practicability of applications up to 815-850 ℃ of this alloy was conclusively proved.     

Effects of the Treating Time on Microstructure and Erosion Corrosion Behavior of Salt-Bath-Nitrided 17-4PH Stainless Steel

The effects of salt-bath nitriding time on the microstructure, microhardness, and erosion-corrosion behavior of nitrided 17-4PH stainless steel at 703 K (430 °C) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and erosion-corrosion testing. The experimental results revealed that the microstructure and phase constituents of the nitrided surface alloy are highly process condition dependent. When 17-4PH stainless steel was subjected to complex salt-bathing nitriding, the main phase of the nitrided layer was expanded martensite (α`), expanded austenite (S), CrN, Fe₄N, and Fe₂N. The thickness of nitrided layers increased with the treating time. The salt-bath nitriding improves effectively the surface hardness. The maximum values measured from the treated surface are observed to be 1100 HV_0.1 for 40 hours approximately, which is about 3.5 times as hard as the untreated material (309 HV_0.1). Low-temperature nitriding can improve the erosion-corrosion resistance against two-phase flow. The sample nitrided for 4 hours has the best corrosion resistance.     

Physical,mechanical and dry sliding wear properties of hybrid and non-hybrid Al–V nanocomposites produced by powder metallurgy

The present work aims to characterise the sliding wear behaviour of non-hybrid Al–Al₃V and hybrid Al–(Al₃V, Al₂O₃) nanocomposites. Wear rates were calculated from mass loss measurements. X-Ray diffraction (XRD) pattern was utilised to evaluate the microstructural changes during milling hot-pressed samples. It was found that the wear resistance of hybrid nanocomposites was enhanced by increasing Al₃V–Al₂O₃ percentage due to an increase in hardness. The mass loss measurement showed that not only was the wear rate of hybrid samples lower than that of Al–Al₃V, but also it had lower friction coefficients in comparison to the non-hybrid sample. The worn surface evaluation in hybrid samples indicated that the formed darker layer possesses the features of the mechanically mixed layer (MML), which inhibits mass loss intensification. Moreover, formation of MML as a lubricant layer promotes the friction characteristic of the hybrid nanocomposite.