Investigation of steel corrosion in MX80 bentonite at 120°C

Corrosion experiments were performed on two metallic substrates, a ferritic–pearlitic steel (P285NH) and a ferritic one (Armco), in silicate environment during 30 days at 120°C. Corrosion products were characterized in terms of morphology (scanning electron microscope and transmission electron microscopy), composition (energydispersive X-ray spectroscopy) and structure (µ-Raman, selected area electron diffraction, X-ray absorption near edge structure). Results show a nanometric inner layer made of compact and adherent nanocrystallized magnetite with, locally a thickness up to several micrometers due to the metal microstructure. An outer layer of Fe-rich phyllosilicate, smectite and serpentine, more porous than the inner one and poorly adherent is observed around both the samples.     

Layer growth kinetics and wear resistance of martensitic precipitation hardening stainless steel plasma nitrocarburized at 460°C with rare earth addition

To study the effect of rare earth (RE) addition on low temperature plasma nitrocarburizing of martensitic precipitation hardening stainless steel, 17-4PH stainless steel was plasma nitrocarburized at 460 °C for different times with RE addition. The modified layers were tested by optical microscope, scanning electron microscope, X-ray diffraction, microhardness tester and pin-on-disc tribometer. The experimental results show that the layer depth of plasma RE nitrocarburized layer can be increased up to 56% compared with plasma nitrocarburizing without RE addition. Incorporation of RE element is beneficial to the formation of nitrogen and carbon expanded martensite (α′_N). The surface microhardness of plasma RE nitrocarburized layer can be increased to 1286 HV and higher up to 80 HV than that obtained from the conventional treated one. The friction coefficient of martensitic stainless steel can be dramatically decreased by low temperature plasma nitrocarburizing with RE addition, and the friction coefficient of the modified specimens decrease gradually with increasing process time in the present test condition.     

Stress corrosion cracking at constant load of 304L stainless steel in molten NaCl-CaCl2 at 570°C

The incubation and propagation times of cracks in 304L in molten NaCl-CaCl₂ at 570°C were related to the applied stress value, from creep and creep rate curves. Rest potential versus time curves were recorded simultaneously. The results showed intergranular stress corrosion cracking. When the temperature was kept at 570°C, precipitation of chromium carbide M₂₃C₆ which promoted cracking propagation, was induced. Determination of the crack rate shows that anodic dissolution at the bottom of the cracks is the main process during the stress corrosion crack propagation of 304L stainless steel in the stress range used.     

Ar ion irradiation hardening of high-Cr ferritic/martensitic steels at 700 °C

High-Cr ferritic/martensitic (FM) steels are being considered for applications as fuel cladding or core structures for Generation-IV reactors. Because high temperatures approaching 923-973 K (650-700 °C) are envisioned in the designs of Generation IV reactors, irradiation response of high-Cr FM steels at the high temperatures requires investigations. Response of two high-Cr FM steels P92 and 11Cr to irradiation at 973 K (700 °C) was investigated through Ar ion irradiation in combination with damage simulations, nanoindentation measurements and microstructure analyses. Irradiation hardening occurred in both steels after Ar ion irradiation at 973 K (700 °C) to 10 dpa, providing the first evidence that irradiation hardening can occur at a high irradiation temperature of 973 K (700 °C) in high-Cr FM steels. Argon bubbles with a very high number density and an average diameter of about 2.6-3 nm formed in the two steels after the irradiation. The irradiation hardening occurred in the two steels is attributed to the formation of these high-number-density fine argon bubbles produced by the irradiation homogeneously distributed in the matrix. Difference in the magnitude of irradiation hardening between the two steels was also discussed.     

Microstructural changes and mechanical behaviour of a near lamellar γ-TiAl alloy during long-term exposure at 700 °C

Ti-44Al-5Nb-1W-1B with a near lamellar microstructure was exposed at 700 °C for up to 10000 h in air. The changes in microstructure were investigated using scanning and transmission electron microscopy. It has been found that the combined addition of Nb and W can restrict parallel decomposition of α₂ lamellae into ultrafine γ lamellae, but causes prevalent precipitation of fine β(B2+ω) particles from α₂ lamellae and precipitation/growth of ω particles from β(B2) grains. However, although 3/4 of α₂ lamellae dissolved and majority of them transformed to β(B2+ω), tensile ductility is reduced only by 30% while the strengths remain essentially unchanged for the thermally exposed alloy. This is attributed to the widespread distribution of β(B2+ω) particles. On the other hand, fatigue limit was found to decrease during the first 5000-h exposure but finally increased by 11% after 10000-h exposure. The reasons for the decrease and increase of fatigue strength at different exposure stages are discussed by considering two contradictory effects on the exposed alloy: 1) exposure-induced embrittlement due to microstructural changes (harmful); 2) annealing of fatigue samples in a warm air environment for prolonged time (beneficial).     

Oxidation protection of 20Cr-25Ni-Nb stainless steel at 1300°C

Enhanced environmental protection of chromia-forming advanced metallic alloys at normal operating temperature, typically 900°C, may be provided by two well-established approaches—incorporation of reactive elements into the protective oxide scale or an amorphous ceramic coating acting as a diffusion barrier. The continued effectiveness of such approaches, namely by cerium and yttrium ion implantation and with a vapor deposited amorphous silica coating, in reducing oxidation of 20Cr-25Ni-Nb stainless steel in a carbon-dioxide-based environment has been examined during 0.5 and 1 hr transients to 1300°C. The influence of pre-oxidation of the ion-implanted, silica-coated, and uncoated steel for extended periods in the same environment at 825–900°C has also been established.     

Application of nanoparticles in cast steel:An overview

The innovative and environmentally friendly methodologies for comprehensively enhancing the performances of high-strength steels without damage to plasticity,toughness and heat/corrosion/fatigue resistance are being developed.In recent years,nanoparticles elevate the field of high-strength steel.It is proposed that nanoparticles have the potential to replace conventional semi-coherent intermetallic compounds,carbides and alloying to optimize the steel.The fabrication process is simplified and the cost is lower compared with the traditional methods.Considerable research effort has been directed towards high-performance cast steels reinforced with nanoparticles due to potential application in major engineering.Nanoparticles are found to be capable of notably optimizing the nucleation behavior and precipitate process.The prominently optimized microstructure configuration and performances of cast steel can be acquired synchronously.In this review,the lattice matching and valence electron criterion between diverse nanoparticles and steel are summarized,and the existing various preparation methods are compared and analyzed.At present,there are four main methods to introduce nanoparticles into steel:external nanoparticle method,internal nanoparticle method,in-situ reaction method,and additive manufacturing method.These four methods have their own advantages and limitations,respectively.In this review,the synthesis,selection principle and strengthening mechanism of nanoparticles in cast steels for the above four methods are discussed in detail.Moreover,the main preparation methods and microstructure manipulation mechanism of the steel reinforced with different nanoparticles have been systematically expatiated.Finally,the development and future potential research directions of the application of nanoparticles in cast steel are prospected.     

The influence of dissolved oxygen concentration on the corrosion of grey cast iron in water at 50°C

The formation of corrosion scales has been studied on grey cast iron in flowing water at 50°C as a function of O₂ concentrations from 0.1 to 3.95 ppm O₂. Below 1.0 ppm O₂, nodular scales form containing Fe₃O₄ and a green rust, GR. At higher O₂ concentrations, a continuous scale eventually forms, consisting of a porous subscale of Fe₂O₄ + GR overlaid with a compact crust of Fe₃O₄ + GR and a thin surface layer of γ-FeOOH. ‘Chimneys’ oriented in the water flow direction grow out of the crust. γ-FeOOH is reduced to Fe₃O₄ which becomes the principal constituent of the scale. Scales on cast iron components from central heating systems closely resemble those found in the present work.     

Corrosion behaviour of X52 pipeline steel in high H2S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract

High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H₂S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H₂S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H₂S concentration at a temperature of 90°C. When the temperature and H₂S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe²⁺ ions and the inward diffusion of H₂S and HS^? species.     

Corrosion of titanium in phosphoric acid at 250 °C

Corrosion studies of a commercially pure titanium in phosphoric acid solutions at 250 °C were carried out by immersion test in an autoclave. At lower phosphoric acid concentration (0.1 mol/L), the corrosion was mild. At higher phosphoric concentration (1.0 mol/L) corrosion, a 25 μm-thick white corrosion products layer was formed on the samples after 24 h immersion. XRD analysis shows that the white layer consists mainly of titanium oxide phosphate hydrate (π-Ti₂O(PO₄)₂·2H₂O). The corrosion product shows the morphology of fiber bundles. A thermodynamic analysis of the formation of the corrosion product is presented.     

Evolution of micro-pores in a single crystal nickel-based superalloy during 980°C creep

The evolution of micro-pores in a single crystal nickel-based superalloy during creep at 980 °C/220 MPa was investigated by X-ray computed tomography. Time-dependent ex-situ 3D information including the number, volume fraction, distribution and morphology of micro-pores was analyzed. The results reveal that the signifi cant formation and growth of micro-pores occur at the end of secondary/beginning of tertiary creep stage. The irregular large pores as well as high density pores located at strain...     

Microstructure–properties relationships in martensitic stainless steel resistance spot welds

The paper addresses the phase transformations and mechanical response of martensitic stainless steel resistance spot welds. The fusion zone microstructure consists of carbon-rich martensite together with a relatively high amount of retained delta ferrite along the grain boundaries with a transition in solidification mode from equiaxed to columnar dendritic grains across the fusion zone. The heat affected zone microstructure is featured by martensitic matrix together with carbide precipitation. The very high hardness of the fusion zone and the heat affected zone, the sharpness of the notch at sheet/sheet interface, which is located in the hard microstructural zone, and the presence of delta ferrite in the weld nugget play important roles in failure characteristics and mechanical performance of the joint.     

Microstructural change in gravity cast Mg−Ni alloys with Ni contents

The effects of Ni content on the microstructures of gravity cast Mg−Ni alloys were examined. Different shapes of primary solid particles and microstructures of the eutectic matrix were observed with varying Ni content. The eutectic matrix in the hypoeutectic alloy consisted of rod-like Mg₂Ni phase with a small amount of α-Mg phase. The eutectic matrix in the near-eutectic and hypereutectic alloys consisted of eutectic cells oriented in different directions. A transition of lamellae to fiber occurred. The formation of cellular structure and transition of lamellae to fiber resulted from two-phase instability in the coupled region induced by constitutional undercooling.     

Solubility of hemimorphite in ammonium sulfate solution at 25 °C

The solubility of natural hemimorphite in ammonium sulfate solution was measured by isothermal solution method at 25 °C and the dissolved residue of hemimorphite was investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) methods. The results show that zinc and silica in hemimorphite simultaneously dissolve in ammonium sulfate solution. The solubility of zinc in solution increases rapidly from 4.5381 mmol/kg in 0.5469 mol/kg ammonium sulfate solution to 11.5083 mmol/kg in 3.7038 mol/kg ammonium sulfate solution. The solubility of silica in solution increases slowly from 2.5509 mmol/kg in 0.5469 mol/kg ammonium sulfate solution to 7.2891 mmol/kg in 3.7038 mol/kg ammonium sulfate solution. The dissolved residue is the characteristic of hemimorphite Zn₄Si₂O₇(OH)₂·H₂O based on the results of the XRD, SEM and FTIR. Thus, no phase transition occurs in the dissolution process of hemimorphite in ammonium sulfate solution.     

Oxidation of P92 steel weldment between 600 and 800 °C in air

P92 alloy with a composition of Fe-9.1Cr-0.5Mo-1.7W (wt.%) was welded, and its oxidation behavior was studied at 600, 700 and 800 °C for up to 6 months in air. The oxidation resistance decreased in the order of the base metal, weld metal, and the heat affected zone. The morphology and the composition of the scales formed on these samples were similar. The scales were either uniform in thickness or nodular. The scales consisted mainly of Fe₂O₃. As oxidation progressed, thick, nodular oxide scales formed. The alloying elements such as Cr, W, and Mn tended to incorporate in the lower part of the oxide scale.     

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.