Corrosion resistance evaluation of highly dispersed MgO-MgAl2O4-ZrO2 composite and analysis of its corrosion mechanism: A chromium-free refractory for RH refining kilns

The corrosion resistance behavior of a highly dispersed MgO-MgAl₂O₄-ZrO₂ composite refractory material is examined by testing with high-basicity and low-basicity RH (Ruhrstahl-Hereaeus) slags. The composite material exhibits greater resistance to the RH slags than the traditional MgO-Cr₂O₃ composite, MgO-ZrO₂ composite, and MgO-MgAl₂O₄-ZrO₂ composite. On the basis of the microstructural analysis and mechanisms calculations, the corrosion resistance behavior of the MgO-MgAl₂O₄-ZrO₂ composite is attributable to its highly dispersed structure, which helps protect the high activity of ZrO₂. When in contact with the slag, ZrO₂ reacts with CaO to form the stable phase CaZrO₃, which protects MgAl₂O₄ against corrosion, thereby enhancing the corrosion resistance of the composite.     

Columnar grains-covered small grains Cu–Sn alloy prepared by two-phase zone continuous casting

A new theory of two-phase zone continuous casting(TZCC) has been established in order to improve mechanical properties,corrosion resistance and conductivity properties of metals with wide solid-liquid two-phase zone.A Cu-Sn alloy with continuous columnar grains-covered non-columnar small grains of same phase microstructure containing many self-closed grain boundaries were produced by the self-developed TZCC process.Compared with water-cooled mold continuous casting Cu-Sn alloy,the tensile strength and ductility of the TZCC alloy are greatly improved,the corrosion resistance is improved up to fifteenfold,and the conductivity is improved by 12.2%.The excellent high strength may be due to the effective blockage of dislocation motion by numerous self-closed grain boundaries,which suppress the propagation of grain boundary corrosion,and the extremely low electrical resistivity and high ductility may be attributed to continuous columnar grains.     

Influence of Rare Earth Elements on Mechanical Properties and Corrosion Resistance of Cu-15Ni Alloy

In order to improve the mechanical property and Cl-+S2-corrosion resistance of B15 copper-nickel alloy,Cu-15Ni-xRE(x:0-0.1%by weight)alloy was prepared by adding rare earth(RE)in melted Cu-15Ni alloy using metal mould casting method.Optical microscopy(OM),electronic tensile testing machine,X-ray diffraction(XRD),scanning electron microscope(SEM),and electrochemical testing system were used to analyze mechanical property,corrosion resistance property,and surface microstructure of different treatment samples.The results of OM and tensile testing show that the RE addition can effectively deoxidize the alloy melt and the microstructure of the alloy changes from coarse dendrite to small equiaxed grain.By addition of 0.05%RE,the tensile strength and elongation of Cu-15Ni alloys are improved from 294 MPa to 340MPa,and 8%to 33.5%respectively.The results of electrochemical testing show that the corrosion resistance of Cu-15Ni alloy is greatly improved by adding proper amount of RE,whereas excess addition of RE worsens the corrosion resistance.The optimum RE content was about 0.05%by weight.In comparison with the alloy without RE,the corrosion potential and corrosion current density of Cu-15Ni alloy containing proper RE decreased by about-0.28 V and 70A/cm2,respectively.     

放电等离子烧结制备石墨烯纳米片增强7075铝合金复合材料的力学性能

A 0.3wt% graphene nanoplatelets (GNPs) reinforced 7075 aluminum alloy matrix (7075 Al) composite was fabricated by spark plasma sintering and its strength and wear resistance were investigated. The microstructures of the internal structure, the friction surface, and the wear debris were characterized by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. Compared with the original 7075 aluminum alloy, the hardness and elastic modulus of the 7075 Al/GNPs composite were found to have increased by 29% and 36%, respectively. The results of tribological experiments indicated that the composite also exhibited a lower wear rate than the original 7075 aluminum alloy.     

Fabrication and properties of aluminum silicate fibrous materials with <Emphasis Type="Italic">in situ</Emphasis> synthesized K<Subscript>2</Subscript>Ti<Subscript>6</Subscript>O<Subscript>13</Subscript> whiskers

To improve their mechanical and thermal insulation properties, aluminum silicate fibrous materials with in situ synthesized K₂Ti₆O₁₃ whiskers were prepared by firing a mixture of short aluminum silicate fibers and gel powders obtained from a sol-gel process. During the preparation process, the fiber surface was coated with K₂Ti₆O₁₃ whiskers after the fibers were subjected to a heat treatment carried out at various temperatures. The effects of process parameters on the microstructure, compressive strength, and thermal conductivity were analyzed systematically. The results show that higher treatment temperatures and longer treatment durations promoted the development of K₂Ti₆O₁₃ whiskers on the surface of aluminum silicate fibers; in addition, the intersection structure between whiskers modulated the morphology and volume of the multi-aperture structure among fibers, substantially increasing the fibers' compressive strength and reducing their heat conduction and convective heat transfer at high temperatures.     

Corrosion behavior of ultrafine-grained AA2024 aluminum alloy produced by cryorolling

The objectives of this study were to produce ultrafine-grained (UFG) AA2024 aluminum alloy by cryorolling followed by aging and to evaluate its corrosion behavior. Solutionized samples were cryorolled to ~85% reduction in thickness. Subsequent aging resulted in a UFG structure with finer precipitates of Al₂CuMg in the cryorolled alloy. The (1) solutionized and (2) solutionized and cryorolled samples were uniformly aged at 160℃/24 h and were designated as CGPA and CRPA, respectively; these samples were subsequently subjected to corrosion studies. Potentiodynamic polarization studies in 3.5wt% NaCl solution indicated an increase in corrosion potential and a decrease in corrosion current density for CRPA compared to CGPA. In the case of CRPA, electrochemical impedance spectroscopic studies indicated the presence of two complex passive oxide layers with a higher charge transfer resistance and lower mass loss during intergranular corrosion tests. The improved corrosion resistance of CRPA was mainly attributed to its UFG structure, uniform distribution of fine precipitates, and absence of coarse grain-boundary precipitation and associated precipitate-free zones as compared with the CGPA alloy.     

Effect of heat treatment on microstructures and properties of a novel Al-Zn-Mg-Cu alloy for oil drilling

In view of the special requirements for strength, heat resistance and corrosion resistance of Al-Zn-Mg-Cu alloy for oil drilling, the Al-6.2 Zn-2.5 Mg-1.6 Cu alloy was prepared by increasing Cu content on basis of Russian Series 1953 alloy. The effect of heat treatment on the microstructures and properties of the alloy was characterized by optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscope(TEM), and investigated by tensile test at room temperature, thermal exposure test and corrosion test. The results show that the strength after T6 aging treatment exhibit a decrease trend as an increase of the solution temperature from465 °C to 480 °C. After the solution treated by the rate of 470 °C/1 h, second phases dissolve into the matrix very well and the strength property reaches optimum. The alloy has better comprehensive properties treated by a solution treatment of 470 °C/1 h and then followed by an aging treatment of 120 °C/24 h + 170 °C/1 h + 120 °C/24 h. Under the aging state, the precipitated phases inside the grains are suitable in size, while on the grain boundary distribute discontinuously and the precipitate-free zone is obvious. Besides, the alloy still maintain high tensile properties. The yield strength, tensile strength and elongation are 650 MPa, 686 MPa,12.0%, respectively. The yield strength retention after heat exposure is 92%. The alloy has good corrosion resistance and the exfoliation corrosion degree. The average corrosion rate in the H_2S and CO_2 environment is 0.0024 mm/a, which is far less than the required 0.12 mm/a. It is insensitive to H_2S and CO_2 environments.     

Prediction model for atmospheric corrosion of 7005-T4 aluminum alloy in industrial and marine environments

Accelerated corrosion tests of the 7005-T4 aluminum alloy were conducted to determine a suitable service life prediction method by using alternating wet-dry cycles in three kinds of solutions. The morphology and composition analysis of the corrosion product revealed that slight corrosion occurred on the surfaces of the samples immersed in a 0.25wt% Na₂S₂O₈ solution. However, pitting corrosion occurred on the surfaces of the samples immersed in a 3.5wt% NaCl solution, whereas exfoliation corrosion occurred on the surfaces of the samples immersed in a mixture of 0.25wt% Na₂S₂O₈ and 3.5wt% NaCl solutions. A power exponent relationship was observed between the mass loss and exposure time of the 7005-T4 aluminum alloy immersed in the three kinds of solutions. In the mixture of 0.25wt% Na₂S₂O₈ and 3.5wt% NaCl solutions, the mass loss of the aluminum alloy yielded the maximum value. Based on the calculation of the correlation coefficients, the alternating wet-dry procedure in a 3.5wt% NaCl solution could be used to predict the corrosion behavior of 7005-T4 aluminum alloy exposed in the atmosphere of Qingdao, China. The prediction model is as follows:T=104.28·t0.91, where T is the equivalent time and t is the exposure time.     

Development of high-strength,low-cost wrought Mg–2.0 mass% Zn alloy with high Mn content

Mg–Zn–Mn-based alloys have received considerable attention because of their high creep resistance, strength,and good corrosion resistance. The alloying element Mn in Mg–Zn-based alloys is commonly less than 1 wt%. In the present study, the effect of high Mn content(1 wt% and 2 wt%) on the microstructures and mechanical properties of Mg–2Zn–0.3Sr extruded alloy was investigated. The results revealed that the high Mn content significantly increased the ultimate tensile strength, tensile yield strength, compress yield strength, and yield asymmetry of the alloy without affecting its ductility. The dynamically recrystallized(DRXed) grains of Mg–2Zn–0.3Sr were remarkably refined because of the large amount of fine Mn precipitates in the homogenized alloy. The improved strengths were mainly attributed to the fine DRXed grains according to the Hall–Petch effect and to the large amount of spherical and 0001 Mn precipitates through the precipitation and dispersion strengthening. The fine DRXed grains and numerous Mn precipitates effectively suppressed the extension twining, substantially enhanced the compress yield strength, and resulted in improved anisotropy.     

HOT CORROSION BEHAVIOR OF Ni-BASED SUPERALLOY WITH HIGH Cr CONTENTS -PART 1.EXPERIMENTAL STUDY

A systematic study of the effects of Ti, Ta and Nb on the hot corrosion behavior of alloy system Ni-16Cr-gAl-2W-1Mo-4Co-(0~4)Ti-(0~4)Ta-(0~4)Nb (at%) was conducted. The results showed that, in certain compositional regions, the hot corrosion resistance in 75% Na₂SO₄+25%NaCl molten salt of the expermental alloys was similar to that of IN738LC alloy. The hot corrosion property balances with other properties, which provides the basis for selecting alloy compositions to develop single crystal superalloys of high performance hot corrosion resistance.     

Study on the strength of cold-bonded high-phosphorus oolitic hematite-coal composite briquettes

Composite briquettes containing high-phosphorus oolitic hematite and coal were produced with a twin-roller briquette machine using sodium carboxymethyl cellulose, molasses, starch, sodium silicate, and bentonite as binders. The effect of these binders on the strength of the composite briquettes, including cold strength and high-temperature strength, was investigated by drop testing and compression testing. It was found the addition of Ca(OH)₂ and Na₂CO₃ not only improved the reduction of iron oxides and promoted dephosphorization during the reduction-separation process but also provided strength to the composite briquettes during the briquetting process; a compressive strength of 152.8 N per briquette was obtained when no binders were used. On this basis, the addition of molasses, sodium silicate, starch, and bentonite improved the cold strength of the composite briquettes, and a maximum compressive strength of 404.6 N per briquette was obtained by using starch. When subjected to a thermal treatment at 1200℃, all of the composite briquettes suffered from a sharp decrease in compressive strength during the initial reduction process. This decrease in strength was related to an increase in porosity of the composite briquettes. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses showed that the decrease in strength of the composite briquettes could be caused by four factors: decomposition of bonding materials, gasification of coal, transportation of byproduct gases in the composite briquettes, and thermal stress.     

Microstructure and corrosion resistance of Fe/Mo composite amorphous coatings prepared by air plasma spraying

Fe/Mo composite coatings were prepared by air plasma spraying (APS) using Fe-based and Mo-based amorphous and nanocrystalline mixed powders. Microstructural studies show that the composite coatings present a layered structure with low porosity due to adding the self-bonded Mo-based alloy. Corrosion behaviors of the composite coatings, the Fe-based coatings and the Mo-based coatings were investigated by electrochemical measurements and salt spray tests. Electrochemical results show that the composite coatings exhibit a lower polarization current density and higher corrosion potentials than the Fe-based coating when tested in 3.5wt% NaCl solutions, indicating superior corrosion resistance compared with the Fe-based coating. Also with the increase in addition of the Mo-based alloy, a raised corrosion resistance, inferred by an increase in corrosion potential and a decrease in polarization current density, can be found. The results of salt spray tests again show that the corrosion resistance is enhanced by adding the Mo-based alloy, which helps to reduce the porosity of the composite coatings and enhance the stability of the passive films.     

Electrochemical behavior of 304 stainless steel with electrodeposited niobium as PEMFC bipolar plates

In order to enhance the corrosion resistance of 304 stainless steel, niobium was electrodeposited on its surface in air- and water-stable ionic liquids. The electrochemical behaviors of bare and niobium-coated 304 stainless steel were evaluated by electrochemical tests in a simulated proton exchange membrane fuel cell (PEMFC) environment. The results showed that niobium could be electrodeposited on the surface of 304 stainless steel from ionic liquids, and a smooth and strong chemical inert compound film was obtained on the surface of 304 stainless steel, which was mainly composed of NbO and Nb₂O₅. The thin composite film acted as a barrier and remarkably improved the corrosion resistance of 304 stainless steel in the PEMFC environment.     

Graphene-reinforced aluminum matrix composites prepared by spark plasma sintering

Graphene-reinforced 7055 aluminum alloy composites with different contents of graphene were prepared by spark plasma sintering (SPS). The structure and mechanical properties of the composites were investigated. Testing results show that the hardness, compressive strength, and yield strength of the composites are improved with the addition of 1wt% graphene. A clean, strong interface is formed between the metal matrix and graphene via metallurgical bonding on atomic scale. Harmful aluminum carbide (Al₄C₃) is not formed during SPS processing. Further addition of graphene (above 1wt%) results in the deterioration in mechanical properties of the composites. The agglomeration of graphene plates is exacerbated with increasing graphene content, which is the main reason for this deterioration.     

Investigation on microstructural, mechanical and electrochemical properties of aluminum composites reinforced with graphene nanoplatelets

In present study, the microstructure, mechanical and electrochemical properties of aluminum–graphene nanoplatelets (GNPs) composites were investigated before and after extrusion. The contents of graphene nanoplatelets (GNPs) were varied from 0.25 to 1.0 wt.% in aluminum matrix. The composites were fabricated thorough powder metallurgy method, and the experimental results revealed that Al-0.25%GNPs composite showed better mechanical properties compared with pure Al, Al-0.50%GNPs and Al-0.1.0%GNPs composites. Before extrusion, the Al-0.25%GNPs composite showed ~13.5% improvement in ultimate tensile strength (UTS) and ~50% enhancement in failure strain over monolithic matrix. On the other hand, Al-0.50%GNPs and Al-0.1.0%GNPs composites showed the tensile strength lower than monolithic matrix. No significant change was observed in 0.2% yield strength (YS) of the composites. However, the extruded materials showed different trends. The 0.2%YS of composites increased with increase in GNPs filler weight fractions. Surprisingly, UTS of composites with 0.25 and 0.50% GNPs was lower than monolithic matrix. The failure strain of the baseline matrix was enhanced by ~46% with 0.25% graphene nanoplatelets. The superior mechanical properties (in terms of failure strain) of the Al-0.25%GNPs composite maybe attributed to 2-D structure, high surface area and curled nature of graphene. In addition, the corrosion resistance of pure Al and its composites reinforced with 0.5 and 1.0 wt% GNPs was also investigated. It was found that the corrosion rate increased considerably by the presence of GNPs.     

Effect of TiO<Subscript>2</Subscript> pigment gradation on the properties of thermal insulation coatings

This study was designed to evaluate the thermal performance and mechanical properties of coatings with different gradations of TiO₂ pigments. The solar reflectance, cooling performance, wash resistance, and film adhesion strength of the coatings were investigated. The influence of TiO₂ powder gradation on the final properties of the coatings was studed. The solar reflectance and the thermal insulation were observed to increase with increasing content of nanosized TiO₂. The mechanical properties of the coatings, such as their wash resistance and film adhesion strength, were observed to increase with increased incorporation of nanosized TiO₂. Such improvements in the properties of the coatings were attributed to the greater specific surface area and lower thermal conductivity of nanosized TiO₂ particles compared to normal TiO₂ particles.     

Wear properties of NiAl based materials

The NiAl based materials including NiAl-TiC-Al2O3 composite,NiAl-Cr(Mo)-Hf-Ho eutectic alloy and NiAl-Cr(Mo)-CrxSy in situ composite were fabricated and their wear properties were tested at different temperatures.The results revealed that the NiAl-TiC-Al2O3 composite,NiAl-Cr(Mo)-Hf-Ho eutectic alloy and NiAl-Cr(Mo)-CrxSy in situ composite exhibited the excellent wear properties between 700℃ and 900℃.The microstructure observations exhibited that the self-lubricant films formed on the worn surfaces during the dry sliding test at high temperature,which decreased the wear rate and friction coefficient significantly.TEM observation on the self-lubricant film revealed that it was mainly comprised by ceramic amorphous and nanocrystalline.Compared with the NiAl-TiC-Al2O3 composite,the NiAl-Cr(Mo)-CrxSy in situ composite has lower friction coefficient at low temperature.Such phenomena may be ascribed to the addition of sulfide which contributes much to the formation of self-lubricant,and moreover the TiC addition increase the strength of NiAl based material and its wear resistance.     

Microstructural and mechanical properties of Al-4.5wt% Cu reinforced with alumina nanoparticles by stir casting method

The microstructure and mechanical properties of Al-4.5wt% Cu alloy reinforced with different volume fractions (1.5vol%, 3vol%, and 5vol%) of alumina nanoparticles, fabricated using stir casting method, were investigated. Calculated amounts of alumina nanoparticles (about ?50 nm in size) were ball-milled with aluminum powders in a planetary ball mill for 5 h, and then the packets of milled powders were incorporated into molten Al-4.5wt% Cu alloy. Microstructural studies of the nanocomposites reveal a uniform distribution of alumina nanoparticles in the Al-4.5wt% Cu matrix. The results indicate an outstanding improvement in compression strength and hardness due to the effect of nanoparticle addition. The aging behavior of the composite is also evaluated, indicating that the addition of alumina nanoparticles can accelerate the aging process of the alloy, resulting in higher peak hardness values.     

Machining studies of die cast aluminum alloy-silicon carbide composites

Metal matrix composites (MMCs) with high specific stiffness, high strength, improved wear resistance, and thermal properties are being increasingly used in advanced structural, aerospace, automotive, electronics, and wear applications. Aluminum alloy-silicon carbide composites were developed using a new combination of the vortex method and the pressure die-casting technique in the present work. Machining studies were conducted on the aluminum alloy-silicon carbide (SiC) composite work pieces using high speed steel (HSS) end-mill tools in a milling machine at different speeds and feeds. The quantitative studies on the machined work piece show that the surface finish is better for higher speeds and lower feeds. The surface roughness of the plain aluminum alloy is better than that of the aluminum alloy-silicon carbide composites. The studies on tool wear show that flank wear increases with speed and feed. The end-mill tool wear is higher on machining the aluminum alloy-silicon carbide composites than on machining the plain aluminum alloy.     

Effect of magnesium and nickel coatings on the wetting behavior of alumina toughened zirconia by molten Al-Mg alloy

The wettability of alumina toughened zirconia (ZTA) by Al-Mg alloy was investigated using the sessile drop technique. The effects of nickel coating, magnesium content, nitrogen atmosphere, and processing temperature on the contact angle between the molten alloy and the substrate were determined. Likewise, the effect of these factors on the wetting properties was studied. The results showed that the nickel coating on the ceramic substrate caused a significant reduction in solid/liquid surface energy and the contact angle decreased obviously. The presence of magnesium in the molten aluminum alloy in nitrogen atmosphere reduced the contact angle effectively. The presence of magnesium in the alloy must be at a minimum amount of 2wt%-3wt%. Moreover, it was suggested that some chemical reactions in the Al-Mg-N system led to the production of Mg₃N₂ and AlN compositions. These compositions improved the wetting properties of the systems by reducing the surface energy of the molten. It was shown that increasing the temperature is also an effective factor for the enhancement of wetting properties.