The effect of oxygen on the open-circuit passivity of Fe18Cr

Electrochemical studies were conducted to determine the critical amount of oxygen necessary to prevent corrosion by maintaining the open-circuit passivity of Fe18Cr samples initially passivated at 0.6 V(NHE) in 1N H₂SO₄ solutions. Samples passivated at 0.6 V(NHE) and then released to open circuit in O₂-saturated (29.4 ppm O₂ dissolved) solution maintained a state of passivity. Samples passivated and released to open circuit in N₂-purged solution decayed to a state of active corrosion in 800–2000 min. A passive state, however, could be maintained if O₂ were added to the N₂ flow so that a minimum of 1.7 ppm O₂ was present in solution. Furthermore, this critical amount of O₂ had to be added before open-circuit decay reached 0.45 V(NHE). Auger electron spectroscopy measurements indicated that following open-circuit stabilization of passivity by O₂ the percentage chromium in the film increased with increasing open-circuit potential. X-ray photo-electron spectroscopy measurements indicated that the film thickness decreased with increasing open-circuit potential.     

The effect of Nd and Pr on the oxidation behavior of a Fe–13Cr alloy

The influence of neodymium (Nd) and praseodymium (Pr) on the oxidation behavior of a Fe–13Cr alloy has been studied in oxygen at 800 °C for 24 h. Additions (⩽0.03 wt.%) of either Nd or Pr reduce the oxidation rate of Fe–13Cr and change the morphology of the oxides formed on Fe–13Cr.     

Pitting corrosion of single crystals of the Fe-16 Cr alloy in solutions containing Cl− ions

Pitting corrosion of a Fe-16Cr alloy in solutions containing Cl⁻ ions was studied on {100}, {110} and {111} faces of single crystals. Near the breakdown potential there occurred anisotropic dissolution of the alloy and the pits took the form of regular polyhedrons. The lowest dissolution rate was observed with the {110} face (the most closely packed crystal plane) and with the less closely packed {100} plane. These planes formed the bounding faces of regular pits. The nucleation of pits occurred at subboundaries, at spots where bubbles of O began to grow, and at the three-phase boundary between metal-solution-gas.     

The dissolution and passivation kinetics of stainless alloys containing molybdenum—1. Coulometric studies of FeCr and FeCrMo alloys

The evolution of active current density with time has been measured on new surfaces of FeCr and FeCr-1.5 at. % Mo alloys. The solutions used were 1M and 4M HCl. At several potentials, and at steady-state current densities as high as 150 mA/cm¹, the inhibiting effect of molybdenum was established after the passage of a charge density equivalent to less than three monolayers of alloy. The results suggest that molybdenum acts by enriching at surface kink or step sites in the active state.     

The effect of the Cl− ion on the passive film on anodically polarized 304 stainless steel

The differential capacity-potential behaviour of AISI 304 was determined during anodic polarization in de-aerated KCl/Na₂SO₄ solutions at pH 2.4 using the single pulse technique. The behaviour characterizing potentiostatic and galvanostatic polarization in non-pitting systems is paralleled during galvanostatic polarization in pitting solutions up to the maximum potential attained prior to breakdown. Characteristic potentiostatic potentials also agree well with galvanostatic potential arrests. The potentiostatic capacity-potential behaviour of the passive region is not influenced by pitting. No capacity peak is associated with the onset of pitting and similar film thicknesses are calculated in both pitting and non-pitting environments.     

The effects of heat treatment and implantation of aluminium on the oxidation resistance of FeCrAlY alloys

The oxidation resistance of FeCrAlY alloys is known to be due to the rapid formation of a protective film containing α-Al₂O₃. The near-surface concentration of Al has been increased by ion implantation, and for a dose of 10¹⁷ Al⁺ ions cm^?2 (equivalent to 4.5 μ cm^?3 the parabolic oxidation rate constant has been reduced by a factor of 140 in tests at 1100°C. Heat treatment prior to oxidation also proved beneficial for reasons which are discussed. These relate to the diffusivity of Cr and other spinel-forming elements within the metal. Analysis of Al⁺-implanted steels after oxidation, using ESCA and EDS techniques, confirmed the high concentration of Al₂O₃ within the oxide, and SEM examination showed that the scale is much more compact than in the unimplanted case. Suggestions are made for enhancing still further the efficacy of Al⁺ implantation as a means of improving oxidation resistance and for optimizing the synergistic effect of yttrium and aluminium.     

Thermal stability of nanocrystalline Al−10Fe−5Cr bulk alloy

Thermal stability of nanocrystalline Al?10wt.%Fe?5wt.%Cr bulk alloy was investigated. The initial micro-grained mixture of powders was processed for 100 h using mechanical alloying (MA) to produce nano-grained alloy. The processed powders were sintered using high frequency induction heat sintering (HFIHS). The microstructures of the processed alloy in the form of powders and bulk samples were investigated using XRD, FESEM and HRTEM. Microhardness and compression tests were conducted on the bulk samples for evaluating their mechanical properties. To evaluate the thermal stability of the bulk samples, they were experimented at 573, 623, 673 and 723 K under compression load at strain rates of 1×10^?1 and 1×10^?2 s^?1. The annealed samples exhibited a significant increase in their microhardness value of 2.65 GPa when being annealed at 723 K, as compared to 2.25 GPa of the as-sintered alloy. The bulk alloy revealed compressive strengths of 520 MPa and 450 MPa at 300 K and 723 K, respectively, when applying a strain rate of 1×10^?1 s^?1. The microstructural stability of the bulk alloy was ascribed to the formation of iron and chromium containing phases with Al such as Al₆Fe, Al₁₃Fe₄ and Al₁₃Cr₂, in addition to the supersaturated solid solution (SSSS) of Cr and Fe in Al matrix.     

The ductility and strength of Fe−Cr−Al alloys

Metal Science and Heat Treatment -     

The effect of microstructure on the shape recovery of a Fe–Mn–Si–Cr–Ni stainless steel shape memory alloy

In this work the effect of varying the microstructure on the shape memory properties of a Fe-15Mn-7Si-9Cr-5Ni (wt.%) stainless steel shape memory alloy was evaluated using a simple bend test. The best shape recovery was obtained for a single-phase austenite microstructure and for a two-phase microstructure composed of an austenite matrix and Fe₅Ni₃Si₂ type intermetallic grain boundary phase. The maximum shape recovery was achieved at the reversion temperature of 600 °C and when the pre-stain was less than 2%.     

The oxidation behaviour of austenitic FeCrNi alloys containing dispersed phases

The high temperature oxidation behaviour of FeCrNi austenitic alloys containing 1% Ti which, in some cases, had been converted into an oxide dispersion has been examined. The oxide dispersions were produced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ powder mixture in a sealed quartz capsule at 1100°C: the samples were not in direct contact with the powders. Generally, the effect of the dispersed oxide was much less pronounced than in corresponding nickel-free, ferritic alloys. Nevertheless, the time-to-breakaway of the protective Cr₃O₃ scale which developed on Fe18CrNi alloys was substantially increased, although the differences between the untreated and the internally oxidized alloys reduced with increasing nickel content. An Fe14Cr20Ni alloy did not show any improvement after internal oxidation. Unlike the ferritic alloys, no coarsening of the dispersoid phase was observed during exposure.     

Effect of titanium on the kinetics of the σ-phase formation in a coarse-grained Fe–Cr alloy

The influence of titanium on the kinetics of the σ-phase formation promoted by an isothermal annealing at T=973 K was studied in coarse-grained quasi-equiatomic Fe–Cr alloys by using the ⁵⁷Fe Mössbauer Spectroscopy. It was found that the kinetics could be well-described in terms of the Johnson–Avrami–Mehl equation. The addition of titanium (up to 3 at%) was revealed to effect the transformation kinetics in the following way: for x_Ti⩽1.5 at%, its presence accelerates the process with the highest transformation rate for x_Ti=0.3 at%, for x_Ti⩾1.5 at%, titanium retards the formation of the σ-phase. Quantitatively, the effect of titanium on the kinetics was described in terms of a change of the effective activation energy.     

The corrosion of Fe12Cr iron-chromium alloys in o-Phosphoric acid

Corrosion rates of Fe12Cr in o-phosphoric acid have been studied as a function of temperature and acid concentration. The chemical composition and thickness of corrosion product layers on the surface of the metal have also been investigated by the technique of X-ray photoelectron spectroscopy. It is found that, within the limits of accuracy, the amount of corrosion is linearly dependent on time and independent of the thickness of the corrosion layer despite the observation that the thickness is significantly affected by temperature and acid concentration.Under all conditions studied the corrosion layer consists of a mixture of CrPO₄ and Fe₂O₃ and, deduced from empirical formulae for the mixture of corrosion products, that FePO₄ is most likely present as well. It is suggested that the variation of corrosion rate with acid concentration is associated with the amount of FePO₄ in the corrosion products. No strong correlation is found between the thermodynamic stabilities and solubilities of the constituents of the corrosion products and it is suggested that the structure of the corrosion layer which is determined by its composition is important in determining the corrosion resistance of the alloy.     

Effect of α phase morphology on fatigue crack growth behavior of Ti−5Al−5Mo−5V−1Cr−1Fe alloy

Taking a Ti−5Al−5Mo−5V−1Cr−1Fe alloy as exemplary case, the fatigue crack growth sensitivity and fracture features with various tailored α phase morphologies were thoroughly investigated using fatigue crack growth rate (FCGR) test, optical microscopy (OM) and scanning electron microscopy (SEM). The tailored microstructures by heat treatments include the fine and coarse secondary α phase, as well as the widmanstatten and basket weave features. The sample with coarse secondary α phase exhibits better comprehensive properties of good crack propagation resistance (with long Paris regime ranging from 15 to 60 MPa·m^1/2), high yield strength (1113 MPa) and ultimate strength (1150 MPa), and good elongation (11.6%). The good crack propagation resistance can be attributed to crack deflection, long secondary crack, and tortuous crack path induced by coarse secondary α phase.     

The influence of F− ions on the electrochemical reactions on oxide-covered A1

In a neutral acetic buffered solution the influence of F^? ions on a high purity A1 electrode has been studied by a potentiostatic step method. The experiments show that both the anodic and cathodic reaction rates are increased by the introduction of the F^? ions into the electrolyte. The results are explained by assuming that the F^? ions enter the oxide lattice and replace some of the O^2? ions. The change in charge distribution caused by the replacement affects the reaction rates in such a way that both the anodic and cathodic reaction rates are increased.     

The effect of annealing on the microstructure and tensile properties of a β/γ′ Ni–Al–Fe alloy

The evolution of the microstructure of a (β/γ ′) Ni–32 at.% Al–5 at.% Fe alloy during annealing has been studied by electron microscopy and X-ray diffraction. Annealing at 800°C and 1100°C causes a reverse martensitic transformation, L1₀→B2 (β), and a B2→L1₂ (γ ′) phase transformation. The lower annealing temperature leads to a higher volume fraction of the γ ′-phase but a smaller size of the γ ′-particles. The kinetic laws of the coarsening and of the increase in the volume fraction of the γ ′-phase are discussed. The orientation relationships between the β and γ ′ phases appeared to be mainly of Nishiyama–Wassermann and Bain types after 800°C annealing, while Kurdjumov–Sachs and Bain orientation relationships were predominant in the alloys annealed at 1100°C. A strong correlation between the volume fraction of the γ ′-phase and the tensile characteristics of the alloy has been established.