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.     

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 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 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.     

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 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.     

Phase-Field simulation of phase decomposition in Fe−Cr−Co alloy under an external magnetic field

Phase decomposition during isothermal aging of a Fe−Cr−Co ternary alloy under an external magnetic field is simulated based on the phase-field method. In this simulation, since the Gibbs energy available from the thermodynamic CALPHAD database of the equilibrium phase diagram is employed as a chemical free energy, the present calculation provides the quantitative microstructure changes directly linked to the phase diagram. The simulated microstructure evolution demonstrates that the lamella like microstructure elongated along the external magnetic field is evolved with the progress of aging. The morphological and temporal developments of the simulated microstructures are in good agreement with experimental results that have been obtained for this alloy system.     

The effect of phosphorus addition on the corrosion behavior of ARC-MELTED Ni10TaP alloys in 12 M HCl

The corrosion resistance of arc-melted Ni10TaP alloys containing 0, 10 and 20 at% phosphorus in 12 M HCl solution at 30 °C was investigated. The alloys containing 0 and 10 at% phosphorus suffer severe corrosion. The addition of 20 at% phosphorus to crystalline Ni10Ta alloy results in a three-orders-of-magnitude decrease in the corrosion rate. The open circuit potentials of the Ni10Ta alloys containing 0 and 10 at% phosphorus stay almost constant in the active region of nickel, while the open circuit potential of the Ni10Ta20P alloy increases almost linearly in the initial 2 h. The Ni10Ta alloy consists of intermetallic Ni₈Ta and immersion in 12 M HCl results in faceting dissolution. Ni10Ta10P alloy is composed of major Ni₈Ta and Ni₃P phases and minor Ni₂Ta and Ni₂P phases. Immersion of Ni10Ta10P alloy leads to preferential dissolution of the Ni₈Ta phase and to continuous thickening of the corrosion product film consisting mostly of tantalum as cations. Ni 10Ta20P alloy consists of Ni₂Ta, Ni₃P, Ni₂P and NiP phases. Immersion of Ni10Ta20P alloy gives rise to initial increase in elemental phosphorus on the surface as a result of selective dissolution of nickel and selective oxidation of tantalum. The formation of elemental phosphorus with a high cathodic activity is responsible for the initial ennoblement of the open circuit potential and for the formation of the passive film in which tantalum is highly concentrated. The higher corrosion resistance of Ni10Ta20P alloy than Ni10Ta10P alloy is attributable to the formation of the Ni₂Ta phase with a higher tantalum content than the Ni₈Ta phase which is the readily corroded major intermetallic phase in the Ni10Ta10P alloy.     

Effect of Cr on oxidation of Ni-xAl alloy

The oxidation of the Ni-10Cr-xAl（x=7%, 10%, molar fraction） alloys was studied at 1 100℃ under 0.1 MPa 02 and compared with that of the binary Ni-7Al and Ni-10Al alloys. The results show that the binary alloys form NiO external scales associated with an internal oxidation of Al. On the contrary, the ternary Ni-10Cr-7Al and Ni-10Cr-10Al form external alumina scale in contact with the alloy substrate. An addition of 10%Cr（molar fraction） into Ni-7Al and Ni-10A1 inhibits the internal oxidation of aluminum and considerably reduces the critical content of A1 needed to form exclusive alumina scales with respect to binary Ni-Al alloy, providing an example of third-element effect.     

Oxidation mechanism of FeNi2025Cr5Al alloys-influence of small amounts of yttrium on oxidation kinetics and oxide adherence

S.I.M.S. and optical spectrometry were used to determine the composition of oxide scales developed on several FeNi2025Cr5AlY alloys, oxidized at high temperature (1273–1573K) under a pure oxygen atmosphere. The oxide scales were made up of an alumina layer containing a certain amount of spinel, independently of the iron and nickel content; yttrium enters in solid solution in the alumina layer. The growth of the alumina layer depends mainly on anionic diffusion, but does not obey a parabolic rate. The amount of yttrium in the alloys must be limited to 300 ppm (approximately the solubility of this element in the alloys), in order to have an optimum effect on oxidation kinetics and oxide adherence. This improvement in adherence is due to yttrium solute atoms in the alumina, rather than to mechanical keying by yttria particles.     

The effect of grain boundary character distribution on the high temperature deformation behavior of Ni–16Cr–9Fe alloys

The objective of this work was to test the Thaveeprungsriporn model for the dependence of creep rate on the coincident site lattice (CSL) fraction. The model attributed the large reduction in creep rate in alloys with a high population of CSL boundaries to the greater difficulty of extrinsic grain boundary dislocation (EGBD) absorption at coincident site lattice boundaries (CSLBs) vs. high angle boundaries (HABs). Ease of EGBD absorption was assessed by measuring the annihilation rates of EGBDs in both CSL-related and HABs following an anneal at 360 °C. Results showed that EGBDs are annihilated at HABs at a rate that is on average three times that at CSLBs, implying a grain boundary diffusion coefficient in CSLBs that is 12 times lower than that in HABs. The expectation that a reduction in EGBD absorption would lead to greater matrix hardening was investigated using nano-hardness measurements. Results showed that the hardness in the vicinity of CSLBs is greater than that near HABs, and the grain-averaged hardness increases with the fraction of contiguous CSLBs. Further, strain hardening is greater in CSL-enhanced samples than in reference, solution annealed samples. These results taken together substantiate the hypothesis that CSLBs impede dislocation absorption into the grain boundary, thereby increasing lattice hardening and internal stress in the sample, resulting in a reduced creep rate.