Influences of Si on corrosion of Fe–B alloy in liquid zinc

Abstract

Influences of silicon content on the microstructure and corrosion resistance of a Fe–2·5 wt-%B alloy have been investigated by using scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. Si can change the microstructure from hypereutectic to eutectic and furthermore, enhance the corrosion resistance of the alloy in molten zinc. The high corrosion resistance of the alloy was mainly attributed to the eutectic phase increase and solubility of Si in α phase enhancement. The corrosion of these alloys in liquid zinc was controlled by the diffusion mechanism. The reaction products are FeZn_6·67, Fe₅SiB₂ and FeSi. The reaction layer further prevents the diffusion of zinc atoms into the base material and delays the reaction between the substrate and the molten zinc efficiently.     

The corrosion of Fe3Al alloy in liquid zinc

A systematic study of the isothermal corrosion testing and microscopic examination of Fe₃Al alloy in liquid zinc containing small amounts of aluminum (less than 0.2 wt.%) at 450 °C was carried out in this work. The results showed the corrosion of Fe₃Al alloy in molten zinc was controlled by the dissolution mechanism. The alloy exhibited a regular corrosion layer, constituted of small metallic particles (diameter: 2-5 μm) separated by channels filled with liquid zinc, which represented a porosity of about 29%. The XRD result of the corrosion layer formed at the interface confirmed the presence of Zn and FeZn_6.67. The corrosion rate of Fe₃Al alloy in molten zinc was calculated to be approximately 1.5 × 10⁻⁷ g cm⁻² s⁻¹. Three steps could occur in the whole process: the superficial dissolution of metallic Cr in the corrosion layer, the new phase formation of FeZn_6.67 and the diffusion of the dissolved species in the channels of the corrosion layer.     

Solid-liquid phase equilibria at 727 °C in the ternary system Fe-Mg-Si

The solid-liquid phase equilibria in the Fe-Mg-Si ternary system were experimentally investigated at 727 °C by two complementary approaches: reaction to equilibrium of Fe-Mg-Si powder mixtures and growth of reaction zones at the interface of diffusion couples. X-ray powder diffraction, optical metallography (OM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) were used to characterize the phases formed in these experiments. It has been shown that except silicon and βFeSi₂, all the compounds and solid solutions stable at 727 °C in the Mg-Si and Fe-Si binary subsystems (Mg₂Si, εFeSi, α₁Fe₃Si, α₂Fe₃Si, and αFe) are in equilibrium at that temperature with a magnesium-rich Mg-Si liquid phase. The liquid simultaneously in equilibrium with FeSi and Mg₂Si contains 3.1±0.2 at.% Si; that involved in the three-phased equilibrium with εFeSi (50 at.% Si) and α₁Fe₃Si (27 at.%Si) contains 1.0±0.05 at.%Si. For lower silicon contents in the liquid, the conjugate solid phases are successively α₁Fe₃Si (DO₃ structure, homogeneity range 27 to 14 at.%Si), α₂Fe₃Si (B2 structure, homogeneity range 14 to 11.5 at.%Si), and αFe (A2 structure, homogeneity range 11.5 to 0 at.% Si). It is however to note that for a silicon content in the liquid as low as about 0.05 at.%, the conjugate solid phase is still quasistoichiometric α₁Fe₃Si with 25 at.% Si.     

Solid-liquid phase equilibria at 727 °C in the ternary system Fe-Mg-Si

The solid-liquid phase equilibria in the Fe-Mg-Si ternary system were experimentally investigated at 727 °C by two complementary approaches: reaction to equilibrium of Fe-Mg-Si powder mixtures and growth of reaction zones at the interface of diffusion couples. X-ray powder diffraction, optical metallography (OM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) were used to characterize the phases formed in these experiments. It has been shown that except silicon and βFeSi₂, all the compounds and solid solutions stable at 727 °C in the Mg-Si and Fe-Si binary subsystems (Mg₂Si, εFeSi, α₁Fe₃Si, α₂Fe₃Si, and αFe) are in equilibrium at that temperature with a magnesium-rich Mg-Si liquid phase. The liquid simultaneously in equilibrium with FeSi and Mg₂Si contains 3.1±0.2 at.% Si; that involved in the three-phased equilibrium with εFeSi (50 at.% Si) and α₁Fe₃Si (27 at.%Si) contains 1.0±0.05 at.%Si. For lower silicon contents in the liquid, the conjugate solid phases are successively α₁Fe₃Si (DO₃ structure, homogeneity range 27 to 14 at.%Si), α₂Fe₃Si (B2 structure, homogeneity range 14 to 11.5 at.%Si), and αFe (A2 structure, homogeneity range 11.5 to 0 at.% Si). It is however to note that for a silicon content in the liquid as low as about 0.05 at.%, the conjugate solid phase is still quasistoichiometric α₁Fe₃Si with 25 at.% Si.     

Interface characteristics and corrosion behaviour of oriented bulk Fe2B alloy in liquid zinc

The interface characteristics and corrosion behaviour of oriented Fe₂B in liquid zinc have been investigated. The results indicate that Fe₂B with preferential growth direction parallel to corrosion interface displays better corrosion resistance to liquid zinc. The Fe₂B/FeB phase transition occurs due to gradient of chemical potential in solid Fe₂B–liuqid zinc system. The liquid zinc corrosion is competition process of dissolution, Fe₂B/FeB transition and fracture–spalling. The fracture–spalling dominates corrosion process when preferred direction of Fe₂B is perpendicular to corrosion interface while Fe₂B/FeB transition plays a role in liquid zinc corrosion as preferred direction of Fe₂B is parallel to corrosion interface.     

The corrosion behavior of plasma sprayed Fe2Al5 coating in molten Zn

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 → Fe₂Al₅Zn_x (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.     

Identification of corrosion products resulting from accelerated oxidation process

Abstract

Scanning electron microscopy analysis, X-ray powder diffraction and room temperature ⁵⁷Fe Mössbauer spectroscopy were used to identify the corrosion products of uncoated and coated low alloy steels (LAS) and low carbon steels (LCS) resulting from an accelerated steam oxidation test for 180 h at 660°C. From the Mössbauer spectral analysis, it was shown that in all cases, a series of iron compounds such as α-Fe₂O₃, Fe₃O₄, γ-Fe₂O₃, δ-FeOOH, α-FeOOH, Fe(OH)₂ and Fe(OH)₃ were formed, while XRD measurements revealed only the α-Fe₂O₃ and/or Fe₃O₄/γ-Fe₂O₃ phases. In the LAS uncoated sample, an amorphous phase with magnetic features is found. In the spectra of the borided samples and of the uncoated LCS, an additional doublet was observed, which reveals the presence of a superparamagnetic phase. From the relative areas of the subspectra, it is concluded that the boron aluminised sample underwent the lowest degradation. The mechanism proposed for corrosion products formation is based on the dissociation process.     

The corrosion behavior of amorphous and nanocrystalline Fe73.5Cu1Nb3Si15.5B7 alloy

The potentiodynamic polarization curves in 0.5 M NaCl solution before and after crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy have been studied in relation to the microstructure and alloy composition. It was shown that the corrosion resistance of the alloy strongly depending on these two factors. The observed decrease in corrosion resistance of the alloy after the heat treatment up to 480 °C in comparison to the corrosion resistance of the alloy in the as prepared state is attributed to the increased inhomogeneity of the alloy that coincides with the first appearance of Fe₃Si phase. Further heating (up to 600 °C) resulted in an increase in the number of Fe₃Si nanocrystallites and the appearance of a FeCu₄ phase. After annealing at 600 °C the lowest corrosion rate, 0.004 mm a⁻¹, was observed. Annealing of the samples at higher temperatures (>600 °C) induced formation of six crystalline phases which proved detrimental to the corrosion resistance of the Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy. Solid corrosion products were identified on the surface of the samples after anodic polarization.     

Metallurgical and mechanical investigations of aluminium-steel butt joint made by tungsten inert gas welding–brazing

Abstract

Dissimilar metals of 5A06 aluminium alloy and SUS321 stainless steel were butt joined by tungsten inert gas welding–brazing with BJ380A filler metal and modified non-corrosive flux. The interface in seam/steel is made up of two kinds of intermetallic phase layers, τ ₅-Al₇Fe₂Si phase in the seam side and θ-FeAl₃ phase in the steel side. The granular phase in welded seam is CuAl₂, and the lath shaped phase is τ ₆-Al₅FeSi. In the fusion area, the chainlike phase is Al–Si eutectic structure, and the block-like phase is Al₆(Fe,Mn). The tensile strength of the butt joint reaches 125 MPa, and fracture occurs at τ ₅-layer, with highest hardness value of 950 HV. The present joint without coated layer can reach the same level to those with coated layer.     

Iron slicide formation in bulk iron-silicon diffusion couples

Silicide formation in bulk Fe-Si diffusion couples was studied. Couples were annealed in evacuated quartz capsules at temperatures ranging from 700 to 800 °C and for times as long as about 2 months. Microstructures were subsequently analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy. Three silicide phases were found in all couples examined— FeSi₂, FeSi, and Fe₃Si. Results concerning Fe₃Si disagreed with the present Fe-Si phase diagram. The silicide is stoichiometric with almost no compositional range, whereas the phase diagram predicts a wide range of homogeneity. Growth kinetics for FeSi₂ were quantified, and the activation energy for diffusion controlled βFeSi₂ layer growth was calculated to be 0.83 eV or 80.4 kJ/mol.     

Effect of TiB2 on univariant eutectic growth in a directionally solidified Al-alloy

Abstract

During directional solidification of a near-eutectic Al–Mn–Si alloy the univariant eutectic is formed, following the reaction: L→α-Al + α(AlMnSi) + L′ and reveals a non-faceted α-Al/faceted α(AlMnSi) structure. The addition of titanium diboride (TiB₂) promotes the nucleation of intermetallic α(AlMnSi) silicide phase at solid/liquid interface as particles, which are pushed and periodically engulfed by the advancing planar interface. This type of growth, called symbiotic growth, leads to a layered microstructure. To study the influence of TiB₂ upon the morphology of α(AlMnSi) phase various amounts (from 0·05 up to 2·00 wt-%) of TiB₂ were added to alloys with identical composition. The results show that the behaviour and the influence of the inoculant is not trivial. Both the presence of titanium diboride at the solid/liquid interface and its inoculation effect determine the final morphology of the silicide phase.     

活性元素Ti和SiO2界面结合机制的第一性原理计算

Ti元素是钎焊SiO_2f/SiO₂复合材料重要的活性元素,因此,使用第一性原理计算研究了Ti和SiO₂的界面结合机制. 分别建立了两种不同的终止面和化学计量比的界面,使用界面分离功、电子行为和界面能研究了界面原子间的结合. 结果表明,在O终止界面中,界面处Ti和O形成很强的离子-共价键,界面分离功最大可达到8.99 J/m2. 在Si终止面界面中,Ti和Si形成共价-离子键,界面分离功为2.65 J/m2. 在温度为1 173 K时,当Si的活度大于e?35时,富Si界面的界面能更低,界面倾向于形成Ti-Si化合物. 当Si的活度小于e?35时,富O界面在热力学上更加稳定,界面倾向于形成Ti-O化合物. SiO₂中的Si被Ti置换出后,Si扩散进入钎料,活度升高,与钎料中的Ti反应生成Ti-Si化合物,所以界面结构为SiO₂/Ti-O化合物/Ti-Si化合物/钎料.     

Phase evolution in AlSi20/8009 aluminum alloy during high temperature heating near melting point and cooling processes

The AlSi20/8009 aluminum alloy was heated to high temperatures near the melting point and cooled to investigate the effect of external Si addition on the phase evolution of Al₁₂(Fe,V)₃Si dispersion. Differential scanning calorimeter, scanning electron microscope, energy dispersive spectrometer and X-ray diffractometer were employed. The results showed that Al₁₂(Fe,V)₃Si and Si phases evolved into a needle-like Al_4.5FeSi phase and a nano-sized V-rich phase during holding the alloy at 580−600 °C. With increasing holding temperature to 620−640 °C, Al_4.5FeSi and nano-sized V-rich phases evolved reversibly into Al₁₂(Fe,V)₃Si and Si phases, of which Al₁₂(Fe,V)₃Si occupied a coarse and hexagonal morphology. During the alloy (after holding at 640 °C) furnace cooling to 570 °C or lower, Si and Al₁₂(Fe,V)₃Si phases evolved into strip-like Al_4.5FeSi and the V-rich phases, which is a novel formation route for Al_4.5FeSi phase different from Al−Fe−Si ternary system.     

Evolution of corrosion in cast Al alloy in antifreeze radiator coolant

Corrosion is an important issue for cast Al alloy in an engine cooling system, but how the microstructural features affect the coolant‐related corrosion behaviour is not well understood. In this research, the evolution of corrosion in an ISO 2379 cast Al alloy was studied in an antifreeze radiator coolant under heat‐rejecting conditions. Extensive analyses of microstructures and corroded surfaces were carried out using an optical microscope, scanning electron microscope equipped with energy dispersive spectroscopy and X‐ray diffractometer. Intergranular cavitation corrosion was observed to occur at interfaces between α‐Al matrix and intermetallics (Al₂Cu and Al₅FeSi) or to a less degree at interfaces between α‐Al matrix and Si phase. The large area fraction of the cathodic phases (Al₂Cu, Al₅FeSi and Si) led to the galvanic coupling between them and the adjacent anodic α‐Al matrix. The heat‐rejecting condition in antifreeze radiator coolant was favourable condition to cavitation process while severe crevice corrosion was predominant at oxygen‐depleted regions in the heat‐transfer corrosion cell.     

Effects of boron concentration on the corrosion resistance of Fe-B alloys immersed in 460 °C molten zinc bath

In order to investigate the effects of boron concentration on the corrosion resistance of Fe-B alloys in molten zinc, Fe-B alloys, with the boron concentrations of 1.5 wt.%, 3.5 wt.% and 6.0 wt.% respectively, were dipped into a pure molten zinc bath at 460 °C and kept in different time intervals. The results show that, in comparison with 1Cr18Ni9Ti stainless steel, Fe-B alloy with 3.5 wt.%B exhibits excellent corrosion resistance, due to the dense continuous network or parallel Fe₂B phase which hinders the Fe/Zn interface reaction in Fe-B alloys. The energy dispersive spectrum (EDS) results indicate that the coarse and compact δ phase with the length about 40 μm generates near the matrix of Fe-B alloy and massive ζ phase occurs close to the liquid zinc. The corrosion process includes Fe/Zn reaction and the isolation and fracture of Fe₂B. The failure of boride is mainly caused by the microcrack.     

Effect of arc re-melting on microstructure,mechanical and tribological properties of commercial 390A alloy

To investigate the effect of the arc re-melting on the microstructure, mechanical and tribological properties of the 390A alloy, its ingot produced by the conventional induction melting method was subjected to the arc re-melting process. The microstructure of the 390A alloy was examined by OM and SEM. Mechanical properties of the 390A alloy were determined by the Brinell method and tensile tests. Tribological properties were investigated with a ball-on-disc type tester. It was observed that the microstructure of both conventional induction melted and arc re-melted 390A alloys consisted of α(Al), eutectic Al–12Si, primary silicon particles, θ-CuAl₂, β-Al₅FeSi, δ-Al₄FeSi₂, and α-Al₁₅(FeMnCu)₃Si₂ phases. Re-melting with the arc process caused grain refinement in these phases. In addition, after this process, the α(Al) phase and primary silicon particles were dispersed more uniformly, and sharp edges of primary silicon particles became round. The arc re-melting process resulted in an increase in the hardness of the 390A alloy produced by the conventional method from 102 HB to 118 HB and the tensile strength from 130 to 240 MPa. It also caused an increase in the wear resistance of the 390A alloy and a decrease in the friction coefficient.     

Corrosion processes and their influence on the magnetic flux density of FeNbCuSiB alloys

This paper describes the effect of corrosion process in the magnetic flux density of FeNbCuSiB alloys and correlates with the Si content in the alloys and in the surface oxide layer after a corrosion process. The corrosion resistance of FeNbCuSiB alloys was studied by ordinary weight loss tests, the potentiodynamic polarization technique and spectroscopy impedance. The photoelectron spectroscopy was used in order to identify the compositional changes in the surface oxide layer. Two alloy compositions, Fe_77.5Cu₁Nb₃Si_2.5B₁₆ and Fe₇₃Cu₁Nb₃Si_16.5B_6.5, with different Si content were analyzed in two different conditions, amorphous and crystalline states, respectively. The Fe_77.5Cu₁Nb₃Si_2.5B₁₆ alloy displayed lower corrosion resistance than the Fe₇₃Cu₁Nb₃Si_16.5B_6.5 alloy. The increase of the Si amount in the alloy composition results in an improvement in the corrosion resistance. The enrichment of the Si content on the surface oxide layer was found to be responsible for the improved corrosion resistance. Losses in magnetic properties depend not only on the Si content but also on the structural state.     

Effect of Fe additions on microstructure and mechanical properties of a multi-component Nb–16Si–22Ti–2Hf–2Al–2Cr alloy at room and high temperatures

The phase constitutions, microstructural evolutions, and mechanical properties of Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe alloys (where x = 1, 2, 4, 6 at.%, hereafter referred to as 1Fe, 2Fe, 4Fe and 6Fe alloys, respectively) prepared by arc-melting were investigated. It was observed that the nominal Fe content affected the solidification path of the multi-component alloy. The as-cast 1Fe alloy primarily consisted of a dendritic-like Nb_SS phase and (α+γ)-Nb₅Si₃ silicide, and the as-cast 2Fe and 4Fe alloys primarily consisted of an Nb_SS phase, (α+γ)-Nb₅Si₃ silicide and (Fe + Ti)-rich region. In addition to the Nb_SS phase, a multi-component Nb₄FeSi silicide was present in the as-cast 6Fe alloy. When heat-treated at 1350 °C for 100 h, the 1Fe and 6Fe alloys almost exhibited the same microstructures as the corresponding as-cast samples; for the 2Fe and 4Fe alloys, the (Fe + Ti)-rich region decomposed, and Nb₄FeSi silicide formed. The fracture toughness of the as-cast and heat-treated Nb–16Si–22Ti–2Hf–2Al–2Cr–xFe samples monolithically decreased with the nominal Fe contents. It is interesting that at room temperature, the strength of the heat-treated samples was improved by the Fe additions, whereas at 1250 °C and above, the strength decreased, suggesting the weakening role of the Nb₄FeSi silicide on the high-temperature strength. As the nominal Fe content increased from 1 at.% to 6 at.%, for example, the 0.2% yield strength increased from 1675 MPa to 1820 MPa at room temperature; also, the strength decreased from 183 MPa to 78 MPa at 1350 °C.     

The effect of fe on high temperature oxidation of NiAl

A study was conducted to observe the oxidation of NiAl+3.5at.%Fe alloy inβ-NiAl phase field at air temperatures of 1000, 1200 and 1400°C, and these results were compared with those of pure NiAl alloys. The primary effects of the Fe-addition in NiAl were found to be: 1) decrease in oxidation resistance and adherence of scales during both isothermal and cyclic oxidation tests, 2) enhancement of phase transformation rate from θ toα-AL₂O₃, 3) more rapid formation of characteristically ridgedα-A1₂O₃ scales during initial oxidation stages, and 4) partial sealing of voids formed at the scale-substrate interface and dissolution of Fe inside the alumina scale by the outward diffusion of Fe from the substrate alloy.