The corrosion of INCONEL alloy 740 in simulated environments for pulverized coal-fired boiler

The corrosion of a new nickel base superalloy, INCONEL alloy 740, has been studied at 550 and 700 °C on exposure to the synthetic coal ash/flue gas environments by means of XRD, SEM, and EDX. Low temperature hot corrosion of the new alloy occurred at two temperatures. The corrosion started to form the thin Cr₂O₃ scale on the alloy at 550 °C and developed as pitting attack resulted from sulfidation. The frontal attack at 700 °C consisted of two successive stages in which the corrosion mechanism started from the sulfidation and ended up in the fluxing of oxide. The compact and protective Cr₂O₃ scale formed and the internal sulfidation took place during the initial stage. The severe hot corrosion occurred due to the presence of the molten CoSO₄ during the propagation stage. The loose and porous outer layer and the compact inner layer consisted of spinels and oxides, respectively. The sulfides of Cr, Ti, and Nb formed on the front of oxide scale and in Cr-depletion zone. The rapid degradation of corrosion resistance of the alloy can be attributed to the dissolution of both cobalt and cobalt oxide on the surface. The alloy of 25% Cr exhibited better resistance to coal ash/flue gas corrosion as compared to the alloy of 23% Cr in the present case.     

一种新型燃煤锅炉管材镍基高温合金740的发展与研究

简要介绍了一种将要在超级超临界参数为37.5MPa/700℃条件下使用的过热器管材新型镍基高温合金在力学性能、组织稳定性和耐腐蚀等方面的研究成果.研究结果表明:合金在750℃时的持久强度大于100MPa/105h;合金在700～750 ℃的主要析出相为MC、M23C6、γ'和η相,且具有优异的组织稳定性;合金在700℃具有较好的耐煤灰/烟气腐蚀性能.实验室为了提高合金的组织稳定性,对合金740的成分进行了调整,通过增加Al/Ti比,即适当提高Al含量、降低Ti含量,并压低Si含量,并且Al和Ti与Cr一起协同作用会使抗腐蚀性更好.     

Improvement of mechanical property and corrosion resistance of Mg-Zn-Nd alloy by bi-direction drawing

Mg-Zn-Nd alloy is a promising biodegradable metal material for surgical staples during the reconstruc-tion of digestive tract due to its good biocompatibility and suitable mechanical properties.However,its deformation property and corrosion resistance should be improved to make better safety and effective-ness of staples.In the present study,bi-direction drawing was adopted to maintain the initial texture characteristics,and improve mechanical property and corrosion resistance of Mg-2Zn-0.5Nd alloy.The results showed that the microstructure after bi-direction did not change too much,but the texture could maintain its initial characteristics.The ductility of the alloy with 60 ％ accumulative area reduction after bi-direction drawing was increased by 70 ％,indicating that an outstanding deformation property of Mg-Zn-Nd alloy can be obtained by bi-direction drawing.The corrosion resistance was also improved after bi-direction drawing compared with that under single direction drawing.     

An additively manufactured Al-14.1Mg-0.47Si-0.31Sc-0.17Zr alloy with high specific strength,good thermal stability and excellent corrosion resistance

A novel Al-14.1 Mg-0.47 Si-0.31 Sc-0.17 Zr alloy was applied in the printing process of selective laser melting(SLM),and the corresponding microstructural feature,phase identification,tensile properties and corrosion behavior of the Al Mg Si Sc Zr alloy were studied in detail.As fabricated at 160 W and 200 mm/s,the Mg content of bulk sample decreased to 11.7 wt%due to the element vaporization at high energy density,and the density of this additively manufactured Al Mg Si Sc Zr alloy was 2.538 g/cm³,which is4.2%8.5%lighter than that of other SLM-processed Al alloys.After heat-treated(HT)at 325℃and 6 h,the microstructure was almost unchanged with an alternate distribution of fine equiaxed crystals and coarse columnar crystals.Nano-sized Al3(Sc,Zr)and Mg₂Si phases precipitated dispersedly in the Al matrix,and the tensile strength increased from 487.6 MPa to 578.4 MPa for precipitation strengthening and fine grain strengthening.With a fine grain size of 2.53μm,an excellent corrosion resistance was obtained for the as-printed(AP)Al Mg Si Sc Zr alloy.While the corrosion resistance of HT sample decreased slightly for the formation of non-dense oxide layer and pitting corrosion induced by diffuse precipitation distribution.This SLM-printed Al Mg Si Sc Zr alloy with high specific strength,good thermal stability and excellent corrosion resistance has broad prospects for the aerospace and automotive applications.     

Effects of deformation processes on morphology,microstructure and corrosion resistance of LDHs films on magnesium alloy AZ31

Highly oriented Mg-Al layered double hydroxide(LDHs)films were deposited on magnesium alloy AZ31 with different deformation processes by an easy in-situ growth method.The characteristics of the films were investigated by optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM),and electrochemical,immersion and hydrogen evolution tests.The corrosion protection performance ranked the LDHs films as the increasing series:CS-LDHs(as-cast sample with LDHs)相似文献   

Distinct beneficial effect of Sn on the corrosion resistance of Cr-Mo low alloy steel

In this work,the beneficial effect of Sn addition on the corrosion resistance mechanism of Cr-Mo low alloy steel was studied.Results demonstrated that Sn improves the corrosion resistance of the steel matrix mainly by influencing the microstructural transformation.Sn addition and the synergistic effect of Sn,Cr,and Mo promote the formation of α-FeOOH,SnO2,SnO,Cr(OH)3,and molybdates,lead to the improved protection and stability of the rust layer.This synergistic effect also endows the inner rust layer with cation selectivity,preventing the further penetration of Cl-and inhibiting the localized corrosion of steel.     

Influence of remelting on microstructure,hardness and corrosion behaviour of AlCoCrFeNiTi high entropy alloy

Abstract

High entropy alloys are a newly developed class of alloys, which tend to form a single solid solution or a mixture of solid solutions with simple crystal structures. These alloys possess excellent mechanical properties, thermal stability and corrosion resistance. In the present paper, an AlCoCrFeNiTi high entropy alloy was obtained by induction melting, and the influence of the remelting process on the mechanical and corrosion resistance characteristics of the alloy was investigated. Thus, optical and scanning electron microscopy revealed less phase segregation and a fine dendritic structure for the remelted alloy, while corrosion tests indicated that present alloy, in remelted state, has better corrosion resistance than as cast alloy and stainless steel. The Vickers microhardness measurements demonstrated an improvement of the alloy microhardness by remelting process due to the decrease in phase segregation and the increase in dendrite refinement level.     

Research on the microstructure,fatigue and corrosion behavior of permanent mold and die cast aluminum alloy

Permanent mold (PM) and high pressure die cast (HPDC) AlMg5Si2Mn are employed to investigate the microstructure, fatigue strength and corrosion resistance. Results indicated that the mechanical properties (R_m, R_0.2 and δ) of HPDC specimens (314 MPa, 189 MPa and 7.3%) are significantly better than those of PM specimens (160 MPa, 111 MPa and 2.5%) due to the finer grain size and less cast defects. Fatigue cracks of PM samples dominantly initiated from shrinkage pores and obscure fatigue striations are observed in crack growth region. Corrosion and pitting potentials of PM and HPDC AlMg5Si2Mn alloy are around −1250 mV, −760 mV and −1220 mV, −690 mV respectively. Numerous pits are observed around the grain boundaries because the corrosion potential of Mg₂Si is more anodic than that of α-Al matrix. In addition, the superior corrosion resistance of HPDC samples can be attributed to the fine grain size and the high boundary density which improved the formation of oxide layer on the surface and prevented further corrosion.     

A novel polymer critical re-melting treatment for improving corrosion resistance of magnesium alloy stent

Polymer coating was widely used as a protective coating on Mg alloy stent due to its excellent deformability. However, the polymer coating with lots of macro- and micro-holes after solvent evaporation during forming process would make corrosion medium permeate easier and decrease the corrosion resistance of Mg alloy stent. In this study, a novel critical re-melting method was adopted to improve the polymer coating densification, which was evaluated by the surface morphology of coating. The corrosion resistance of Mg alloy stent after critical re-melting treatment was examined by the electrochemical and immersion tests. The results indicated that the corrosion resistance of Mg alloy stent with polymer coating was improved significantly by polymer critical re-melting treatment.     

Effect of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y magnesium alloy

The influence of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y (wt.%) magnesium alloy was investigated. The grain size of alloys decreased with Zr content from 0% to 0.93% (wt.%). The addition of Zr greatly improved the ultimate tensile strength (UTS) and the elongation (EL), while slightly improved the tensile yield strength (TYS). The UTS and the EL of the alloy containing 0.93% Zr increased by 125.8 MPa and 6.96% compared with base alloy, respectively. The corrosion resistances were found to decrease with Zr content from 0% to 0.42% and then increase from 0.42% to 0.93%. The differences in the sizes and distributions of the Zr-rich particles have significant effects on the corrosion behaviors. The alloy with 0.42% Zr addition revealed the optimum combination of mechanical properties and corrosion resistance.     

高强度铸造钛合金的耐空蚀性能

采用不同的试验方法对高强度铸造钛合金的耐空蚀性能进行了探讨研究。结果表明，该合金具有优良的耐空蚀性能，是处于空蚀条件下工作的高速旋转件的理想材料。     

Effect of hydrogen charging on microstructural evolution and corrosion behavior of Ti-4Al-2V-1Mo-1Fe alloy

The effect of hydro gen charging on microstructural evolution and corrosion behavior of a Ti-4Al-2V-1Mo-1Fe alloy in a 3.5 wt.% NaCl solution was investigated.The results showed that the hydrogen charging induced the formation and growth of γ-TiH and δ-TiH₂ phases,leading to the initiation and propagation of hydrogen-induced cracks.It was also found that hydrogen charging can change the passivity of this alloy and increase its pitting corrosion susceptibility.The main reason for these was attributed to the fo rmation of hydrides in α phase in the Ti-4Al-2V-1Mo-1Fe alloy,leading to the preferential dissolution of the α phase and thus the deterioration in the protective ability of passive film.     

Hierarchical microstructure and two-stage corrosion behavior of a high-performance near-eutectic Zn-Li alloy

In order to improve mechanical and corrosion properties of biodegradable pure Zn,a knowledge-based microstructure design is performed on Zn-Li alloy system composed of hard β-LiZn4 and soft Zn phases.Precipitation and multi-modal grain structure are designed to toughen β-LiZn4 while strengthen Zn,resulting in high strength and high ductility for both the phases.Needle-like secondary Zn precipi-tates form in β-LiZn4,while fine-scale networks of string-like β-LiZn4 precipitates form in Zn with a tri-modal grain structure.As a result,near-eutectic Zn-0.48Li alloy with an outstanding combination of high strength and high ductility has been fabricated through hot-warm rolling,a novel fabrication process to realize the microstructure design.The as-rolled alloy has yield strength (YS) of 246 MPa,the ultimate tensile strength (UTS) of 395 MPa and elongation to failure (EL) of 47 ％.Immersion test in simulated body fluid (SBF) for 30 days reveals that Li-rich products form preferentially at initial stage,followed by Zn-rich products with prolonged time.Aqueous insoluble Li2CO3 forms a protective passivation film on the alloy surface,which suppresses the average corrosion rate from 81.2 μm/year at day one down dramatically to 18.2 μm/year at day five.Afterwards,the average corrosion rate increases slightly with decrease of Li2CO3 content,which undulates around the clinical requirements on corrosion resistance(i.e.,20 μm/year) claimed for biodegradable metal stents.     

Improved corrosion resistance of Mg alloy AZ31B induced by selective evaporation of Mg using large pulsed electron beam irradiation

Large pulsed electron beam (LPEB) irradiation was employed as a surface treatment of magnesium (Mg) alloy AZ31B to enhance its corrosion and wear resistance. Selective evaporation of Mg induced by LPEB irradiation at an energy density of 5 J/cm² for 40 cycles has led to the formation of an Al-enriched re-solidified layer with nano-grained structure consisting of Mg_3.1Al_0.9 metastable phase. The formation of such a re-solidified layer after LPEB irradiation has enabled a decrease in corrosion rate of Mg alloy AZ31B in 3.5% NaCl solution. Different equivalent electrical circuit models were proposed to account for the corrosion behavior of untreated Mg alloy AZ31B and those subjected to LPEB irradiation. A decrease in wear depth when compared to that of the untreated alloy suggests an increase in wear resistance of LPEB-irradiated Mg alloy AZ31B. Adhesive wear is the predominant mechanism of untreated Mg alloy AZ31B while abrasive wear mechanism dominates for LPEB-irradiated Mg alloy AZ31B.     

Review on corrosion resistance of mild steels in liquid aluminum

The corrosion of mild steels by liquid aluminum is an intractable issue in aluminum industry.This review aims to provide an overview of the corrosion behavior of mild steels in the static liquid aluminum with an emphasis on the thermodynamic and kinetic aspects.The corrosion mechanisms of mild steels in liquid aluminum are discussed systematically,based on which four corrosion control approaches including alloying,introducing secondary phase,matrix microstructure control and surface treatment are introduced.Currently,a combination of improvement approaches may have a great potential for further enhancement in corrosion resistance.     

Effect of aging treatment on microstructure and corrosion behavior of Al-Zn-Mg aluminum alloy in aqueous solutions with different aggressive ions

The microstructure and corrosion behavior of Al-Zn-Mg alloy(namely 7×××)after natural aging treatment(NAT)and artificial aging treatment(AAT)in aqueous NaCl solutions containing different aggressive ions have been investigated in current work.Results of microstructure characterization demonstrate that the aging treatment has a great influence on the grain size and precipitates.The grain size is relatively sizeable and no evident precipitates are observed in alloy after NAT comparable with that after AAT.The electrochemical corrosion behavior of alloy was studied by polarization curve and electrochemical impedance spectroscopy(EIS).The corrosion potential(E_corr)of the aluminum alloy is more negative in 3.5 wt.%NaCl containing 0.052 wt.%NaHSO₃solution than that in 3.5 wt.%NaCl solutions with or without 0.907 wt.%NaHCO₃.Charge transfer resistance(Rct)results reveal that alloy after AAT has an enhancement of corrosion resistance compare with that after NAT.With the immersion time increasing,mostly pitting spreads over the surface of the alloy only in NaCl solution,whereas exfoliation corrosion mainly occurs in NaCl solutions containing NaHSO₃or NaHCO₃.     

Homogeneous corrosion of high pressure torsion treated Mg-Zn-Ca alloy in simulated body fluid

Magnesium alloys have got extensive attention as biodegradable implant materials due to their biodegradability in the physiological environment and similar elastic modulus to natural bone. But their poor corrosion resistance is a dominant problem that limits their clinical application due to the inhomogeneous distribution of the second phase. Nevertheless, after high pressure torsion (HPT) treatment, the second phase became nano-sized particles and distributed uniformly in grain interiors instead of along grain boundaries. The immersion tests indicated that the HPT-treated sample exhibited homogeneous corrosion resulting from the uniform distribution of the second phase. The results of the potentiodynamic polarization experiments showed that, compared with the as-cast alloy, the corrosion current density of the HPT-treated alloy decreased from 5.3 × 10^− 4 A/cm² to 3.3 × 10^− 6 A/cm².     

Optimization of mechanical property,antibacterial property and corrosion resistance of Ti-Cu alloy for dental implant

Ti-Cu alloys with different Cu contents (3, 5 and 7 wt%) were fabricated and studied as novel antibacterial biomaterials for dental application. The Ti-Cu alloys were annealing treated at different temperatures (740 °C, 830 °C and 910 °C) in order to obtain three typical microstructures, α-Ti + Ti₂Cu, α-Ti + transformed β-Ti, and transformed β-Ti. Mechanical, antibacterial and biocorrosion properties of Ti-Cu alloys with different microstructures were well analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), tensile test, electrochemical test and antibacterial test. The results indicated that the Ti-Cu alloys with microstructure of α-Ti + Ti₂Cu showed the best ductility compared with other Ti-Cu alloys with microstructures of α-Ti + transformed β-Ti and complete transformed β-Ti, and meanwhile, increase of the Cu content significantly contributed to the decreased ductility due to the increasing amount of Ti₂Cu, which brought both solid solution strengthening and precipitation strengthening. Finally, the Ti-5Cu alloy with microstructure of α-Ti + Ti₂Cu exhibited excellent ductility, antibacterial property and corrosion resistance, providing a great potential in clinical application for dental implants.     

Anisotropic corrosion resistance of TiC reinforced Ni-based composites fabricated by selective laser melting

Electrochemical measurements on three planes of TiC/Inconel 718 composites fabricated by selective laser melting (SLM) were performed to study the corrosion property. The results showed that the YZ-plane with dense and fine columnar structures possessed high microhardness and superior corrosion resistance in 3.5 wt% NaCl solution. For the XZ-plane, a decreased anti-corrosion property was observed owing to its inhomogeneous microstructures. While the XY-plane with large irregular pores and clustered ring-like structures was more susceptible to corrosion compared with the other two planes. Comparative analysis suggested that the anisotropic corrosion behaviors were significantly dependent on the surface defects, microstructure characteristics and added reinforcements.     

Growth and corrosion of aluminum PVD-coating on AZ31 magnesium alloy

Magnetron sputtering was applied to prepare aluminum coating on a mechanically polished AZ31 magnesium alloy. A loose oxide film was spontaneously formed on the surface of AZ31 magnesium alloy during polishing process. The aluminum coating, which was subsequently deposited on this oxide layer, presented a developed columnar microstructure. Attributed to the barrier effect of Al coating, the Al coated AZ31 showed a higher corrosion resistance than bare AZ31 in corrosion tests. Generally, Al coating is cathodically protected by magnesium alloy substrate. But it is interesting in this study that Al coating still suffered from severe corrosion due to the occurrence of the alkalization effect.