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1.
A new type of Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al2O3 scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na2SO4 salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.  相似文献   

2.
Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na2SO4 and V2O5 salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO4 phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al2O3 coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.  相似文献   

3.
The blasting process generates a renewed surface on the surface of metallic biomaterials with a different topography and a different chemical composition. The impact of particles on the metallic surface increases both the surface roughness and susceptibility pitting corrosion. The aim of this work is to smoothen the sharp edges of blasted Ti6Al4V alloy surfaces by means of oxidation treatment and the evaluation of their susceptibility to pitting corrosion after this thermal oxidation. Oxidation treatments were performed at 500 and 700 °C for 1 h on samples blasted with SiO2/ZrO2 and Al2O3 particles. Compositional, microstructural and topographical characterization of the blasted surfaces were carried out by scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX), and atomic force microscopy (AFM). The surface reactivity and corrosion behaviour of the samples were assessed by scanning Kelvin probe (SKP) and by anodic polarization curves. The susceptibility to pitting corrosion of the Ti6Al4V blasted surfaces becomes higher as roughness increases. The oxidation treatment of the Ti6Al4V blasted surfaces causes the presence of nuclei of oxides that cover the area free of particles, especially in the samples treated at 700 °C, giving rise to a higher micro-nano roughness. The presence of the oxide, covering the blasted Ti6Al4V, decreases the surface reactivity leading to a lower passive current and wider pasivation region, decreasing the susceptibility to pitting corrosion.  相似文献   

4.
In the present work the corrosion resistance of micro-cracked hard chromium and Cr3C2-NiCr (HVOF) coatings applied on a steel substrate have been compared using open-circuit potential (EOC) measurements, electrochemical impedance spectroscopy (EIS) and polarization curves. The coatings surfaces and cross-section were characterized before and after corrosion tests using optical microscopy (OM) and scanning electron microscopy (SEM). After 18 h of immersion, the open-circuit potential values were around −0.50 and −0.25 V/(Ag∣AgCl∣KClsat) for hard chromium and Cr3C2-NiCr, respectively. The surface analysis done after 12 h of immersion showed iron on the hard chromium surface inside/near surface cracks, while iron was not detected on the Cr3C2-NiCr surface even after 18 h. For longer immersion time hard chromium was more degraded than thermal sprayed coating. For hard chromium coating a total resistance values between 50 and 80 kΩ cm2 were measured and two well-defined time constants were observed, without significant change with the immersion time. For Cr3C2-NiCr coating the total impedance diminished from around 750 to 25 kΩ cm2 as the immersion time increased from 17 up to 132 h and two overlapped time constants were also observed. Polarization curves recorded after 18 h of immersion showed a lower current and higher corrosion potential for Cr3C2-NiCr coating than other samples studied.  相似文献   

5.
An in situ hydrothermal deposition process is being developed to apply a film of zirconia (ZrO2) onto interior surface of 304 stainless steel and Alloy 600 as a potential method for mitigating intergranular stress corrosion cracking (IGSCC) in boiling water reactors (BWRs). The coating process is discussed. The obtained zirconia coatings are adherent. Monoclinic ZrO2 is the dominant phase when ZrO(ClO4)2 was the oxidant; tetragonal ZrO2 became the dominant phase when ZrO(NO3)2 was the oxidant. A preliminary experiment did not measure significantly lower values of the electrochemical potential (ECP), relative to the uncoated specimen over a wide range of dissolved oxygen in 265 °C water.  相似文献   

6.
In order to improve the corrosion resistance of metallic materials in molten zinc, ZrO2-Ni/Al gradient coatings were sprayed on the surface of the Fe-0.35-0.44 wt.% C steel. The corrosion behaviour and corrosion mechanism of the ZrO2-Ni/Al gradient coatings in molten zinc were studied. The ZrO2-Ni/Al gradient coatings on the surface of steels prolonged the lifetime of samples and changed the corrosion behaviour of the samples in molten zinc. The lifetime of the ZrO2-Ni/Al gradient coatings immersed in molten zinc at 620 °C is 28 days, which is 4 times as long as that of the general ZrO2 coatings. The ZrO2-Ni/Al gradient coatings were corroded in molten zinc at 620 °C, which was caused by zinc atom diffusing along the crystal boundary and pores of the ZrO2-Ni/Al gradient coatings, and reacting with Ni/Al particle in the ZrO2-Ni/Al gradient coatings. The corrosion mechanism of the coatings in molten zinc at 620 °C was crystal boundary corrosion, pitting corrosion and reaction corrosion.  相似文献   

7.
Phase pure zirconium oxide powders have been synthesized using the single step auto-ignition combustion method, the particles were nanometer sized (20 nm) and the size distribution was very narrow (3.4 nm). Systematic structural characterization revealed the t-ZrO2 and indexed for its tetragonal structure (a = 3.5975 Å and c = 5.1649 Å). Calculated microstrain in most of the plane indicated the presence of compressive stress (65-288 MPa) along various planes of the particles. Observed space group (P42/nmc) revealed the presence of cations in the 8e positions (0.75, 0.25, 0.75) and the anions in the 16 h positions (0.25, 0.25, 0.4534). The metal-oxide (Zr-O) band observed at the low wavenumber region further confirmed the phase purity of the as-prepared ZrO2 nanopowders. Peaks at the binding energy positions 2.042 and 0.525 keV in the energy dispersive X-ray spectrum revealed oxygen deficient zirconia. The particle size estimated by TEM was in good agreement with the results obtained through X-ray line broadening (20.81 nm) measurements. The nanopowders were sintered to above 98% of the theoretical density by using vacuum sintering technique at a relatively low temperature of 1300 °C. Stable tetragonal ZrO2 experimentally yield the permittivity value of about 28 at 10 MHz.  相似文献   

8.
Aluminum coating was plasma sprayed on Fe-0.14-0.22 wt.% C steel substrate, and heat diffusion treatment at 923 °C for 4 h was preformed to the aluminum coating to form Fe2Al5 inter-metallic compound coating. The corrosion mechanism of the Fe2Al5 coating in molten zinc was investigated. SEM and EDS analysis results show that the corrosion process of the Fe2Al5 layer in molten zinc is as follows: Fe2Al5 → Fe2Al5Znx (η) → η + L(liquid phase) → L + η + δ(FeZn7) → L + δ → L. The η phase and the eutectic structure (η + δ) prevent the diffusion of zinc atoms efficiently. Therefore the Fe2Al5 coating delays the reaction between the substrate and molten zinc, promoting the corrosion resistance of the substrate.  相似文献   

9.
Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H+ and HFeO2, thereby favouring the formation of Fe3O4. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.  相似文献   

10.
To study the effect of ZrO2 particles on corrosion behaviour of Cr coating, steel samples were plated in Cr(VI) baths without and with ZrO2. The corrosion behaviour of plated samples was studied at different exposure times in a solution containing 0.01 mol l−1 H2SO4 + 0.5 mol l−1 Na2SO4 using cyclic voltammetry and impedance spectroscopy. The equivalent circuit model Re(QcRpore)(Qs[ORs]) was proposed to fit the corrosion process and the parameters Y0(Qc),Y0(Qs) and Rpore reflecting corrosion behaviour of samples were evaluated. From the results, it was found that samples plated in bath containing ZrO2 exhibited improved protective properties as a result of the structural characteristics of the coatings obtained; namely, the size and shape of pores.  相似文献   

11.
Novel YSZ (6 wt.% yttria partially stabilized zirconia)-(Al2O3/YAG) (alumina-yttrium aluminum garnet, Y3Al5O12) double-layer ceramic coatings were fabricated using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies. The thin Al2O3/YAG layer had good adherence with substrate and thick YSZ top layer, which presented the structure of micro-sized YAG particles embedded in nano-sized α-Al2O3 film. Cyclic oxidation tests at 1000 °C indicated that they possessed superior properties to resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1000 °C and 1100 °C indicate that the 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. These beneficial effects should be mainly attributed to that, the oxidation rate of thermal grown oxides (TGO) scale is decreased by the “sealing effect” of α-Al2O3, the “reactive element effect”, and the reduced thermal stresses by means of nano/micro composite structure. This double-layer coating can be considered as a promising TBC.  相似文献   

12.
The behavior of dielectric and microwave properties against sintering temperature has been carried out on CaO-SiO2-B2O3 ceramic matrix composites with ZrO2 addition. The results indicated that ZrO2 addition was advantageous to improve the dielectric and microwave properties. X-ray diffraction (XRD) patterns show that the major crystalline β-CaSiO3 and a little SiO2 phase existed at the temperature ranging from 950 °C to 1050 °C. At 0.5 wt% ZrO2, CaO-SiO2-B2O3 ceramic matrix composites sintered at 1000 °C possess good dielectric properties: ?r = 5.85, tan δ = 1.59 × 10−4 (1 MHz) and excellent microwave properties: ?r = 5.52, Q · f = 28,487 GHz (11.11 GHz). The permittivity of Zr-doped CaO-SiO2-B2O3 ceramic matrix composites exhibited very little temperature dependence, which was less than ±2% over the temperature range of −50 to 150 °C. Moreover, the ZrO2-doped CaO-SiO2-B2O3 ceramic matrix composites have low permittivity below 5.5 over a wide frequency range from 20 Hz to 1 MHz.  相似文献   

13.
Alumina matrix composites containing 5 and 10 wt% of ZrO2 were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y2O3 and with a particle size of 40 nm were used as starting materials. The influence of ZrO2 content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO2 content, and the samples containing 10 wt% of ZrO2 had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m1/2.  相似文献   

14.
Poly(3-octyl thiophene) (P3OT) coatings have been chemically deposited by drop casting onto 304-type stainless steel. P3OT films were thermally annealed at 55, 80 and 100 °C in air during 30 h and their corrosion resistance was estimated by using polarization curves, linear polarization resistance (LPR), and electrochemical impedance spectroscopy measurements, EIS. P3OT films decreased the corrosion rate of the substrate by at least one order of magnitude, although the best corrosion protection was given by annealing it at 100 °C whereas the worst corrosion protection was given by annealing the coating at 80 °C.  相似文献   

15.
The coating Cr3C2 with 50 wt.% Ni20Cr deposited by high velocity oxy-fuel (HVOF) spray process was characterized in detail to investigate the effect of annealing on the solid particle erosion behaviour and understand the influence of the binder properties. Systematic characterization of the coating was carried out using electron microscopy (scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA)), X-ray diffraction (XRD), microindentation and nanoindentation techniques. The solid particle erosion tests were done on the as-sprayed coating and coatings annealed at 400 °C, 600 °C and 800 °C using silica erodent particles. The coefficient of restitution of the coated samples was also measured by WC ball impact tests to simulate dynamic impacts. The as-sprayed coating consisted of primary carbides and binder that was a mixture of amorphous and nanocrystalline phases. Annealing leads to recrystallisation of binder phase and precipitation of secondary carbides. The coating hardness and binder ductility change with annealing temperature. The erosion resistance improves with annealing up to 600 °C. In the as-sprayed coating, the amorphous phase, inter-splat boundaries and the elastic rebound characteristics affect the erosion response. While in the case of the coating annealed at 600 °C, the presence of ductile crystalline binder, fine carbide precipitates and embedment of erodent particles together improve solid particle erosion resistance.  相似文献   

16.
This paper concerns with the effect of nitrogen addition to 904 L stainless steel (SS) welds on their stress corrosion cracking (SCC) behavior in high temperature (288 °C) and high pressure (1050 psi) water of high oxygen content (100 ppb) and high conductivity (2.5 μS/cm). For this study, 316 L SS base plate TIG welded with 904 L SS filler wire and with nitrogen contents of 0.027, 0.058 and 0.095 wt.% were used. Flat pin-loaded tensile specimens were fabricated from transverse welds, with the weld in the gauge length. Slow strain rate tests (SSRT) were carried out at a strain rate of 2.2 × 10−6 s−1. The study shows that the samples, when tested in air, failed at the weld fusion zone for 0.027 and 0.058 wt.% N and at the base metal for 0.095 wt.% N. In the environment, the samples failed in the base metal except the one with least nitrogen content (0.027 wt.%). With nitrogen addition, as the failure location shifted to the base alloy, the weld seemed to acquire SCC resistance and became even more resistant than the base alloy.  相似文献   

17.
Cr2AlC coating was deposited at 370 and 500 °C by D.C. magnetron sputtering from an as-synthesized bulk Cr2AlC target. The phase composition and preferential orientation of the coating were investigated using XRD, and the microstructure of the coating was characterized by TEM. Results indicated that Cr2AlC coating with a strong (110) preferential orientation could be obtained. The coating microstructure was clearly affected by the deposition temperature. At 370 °C, the deposited coating possessed a triple-layered structure with an α-(Cr, Al)2O3 inner layer, an amorphous intermediate layer and a crystalline Cr2AlC outer layer. However, the coating deposited at 500 °C had a single-layered structure consisting of crystalline Cr2AlC layer. The growth mechanism of the Cr2AlC coating at different deposition temperatures is discussed.  相似文献   

18.
This study demonstrates that carbon tetrafluoride (CF4) plasma can result in relatively hydrophobic and hydrophilic surfaces formed in-situ on polyimide (PI) films using a mask and controlling the distance of the mask to the substrate. The surface properties of plasma-treated PI films are characterized by contact angle measurement, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Under specific modification conditions, contact angles for hydrophobic and hydrophilic surfaces reach values of 108.3 ± 0.6° and below 5°, respectively. The XPS analyses indicate that the “unshielded” surfaces contained a high proportion of the CF2-CF2 group and therefore decreased the wettability of the surface. On the other hand, the “shielded” surface contained hydrophilic groups such as carbonyl or carboxyl with few fluorinated groups, resulting in increased wettability of the surface.  相似文献   

19.
Polycrystalline Ti3SiC2 suffered from serious hot corrosion attack in the mixture of 75wt.%Na2SO4 + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO2 and an inner layer of a mixture of TiO2 and SiO2 was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na2SO4 + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO2 would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti3SiC2 corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.  相似文献   

20.
The corrosion protection performance of electroless deposited nickel-phosphorus (Ni-P) alloy coatings containing tungsten (Ni-P-W) or nano-scattered alumina (Ni-P-Al2O3) composite coatings on low carbon steel was studied. The effect of heat treatment on the coating performance was also studied. The optimum conditions under which such coatings can provide good corrosion protection to the substrate were determined after two weeks of immersion in 3.5% NaCl solution. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance before and after heat treatment. The Ni-P-W coatings showed the highest surface resistance compared with Ni-P-Al2O3 and Ni-P. The surface resistance of Ni-P-W coatings was 12.0 × 104 Ω cm2 which is about the double of the resistance showed by Ni-P-Al2O3 (7.00 × 104 Ω cm2) and twenty times greater than the surface resistance of Ni-P (0.78 × 104 Ω cm2). XRD analysis of non-heat-treated samples revealed formation of a protective tungsten phosphide phase. Heat treatment has an adverse effect on the corrosion protection performance of tungsten and alumina composite coatings. The surface resistance decreased sharply after heat treatment.  相似文献   

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