Iron-chromium Alloys Research Articles

Analysis of the passive film on iron chromium alloys by potential modulated ellipsometry and impedance spectroscopy

Impedance measurements have been combined with ellipsometric measurements. The optical properties change with varying potential due to the formation of a thin layer or to changes in the composition of the passive film. This combination of techniques can, therefore, give more insight into the passivation mechanism of iron chromium alloys. The results obtained emphasize the important contribution of hydrogen adsorption to the chromium and the 17% Cr alloy corrosion mechanism. The measurements are also in agreement with recently obtained XPS data and further support the model for chromium passivation published before.

Investigation of passive layers on iron and iron-chromium alloys by electrochemical impedance spectroscopy

The semiconductor properties of passive layers anodicaly generated on Fe and FeCr alloys in near neutral aqueous solutions, varying, at our convenience, the oxide formation potential E f, were investigated using electrochemical impedance spectroscopy (EIS). The impedance of the system is characterized by two capacitive contributions in the 1 mHz-65 kHz frequency range. From the Mott-Schottky plots the flat band potential E FB and the donor concentration N D can be determined as a function of E f and Cr content. A comparative study of the kinetics of the ferro-ferricyanide electron transfer reaction on passive Fe and FeCr substrates were performed to analyse the relation between the electronic properties of the anodic oxide layers and the mechanism of the redox couple electrode reaction.

The effect of an oxygen atmosphere on the creep-fatigue failure of iron-chromium alloys at 600°C: Aghion, E., Ferreira, J. and Eliezer, D. Proc. INFACON 6, Cape Town, South Africa, 1992. Vol. 2, Chromium Steel and Alloys (The South African Institute of Mining and Metallurgy, Johannesburg, Republic of South Africa, 1992) pp 241–244

The effect of an oxygen atmosphere on the creep-fatigue failure of iron-chromium alloys at 600°C: Aghion, E., Ferreira, J. and Eliezer, D. Proc. INFACON 6, Cape Town, South Africa, 1992. Vol. 2, Chromium Steel and Alloys (The South African Institute of Mining and Metallurgy, Johannesburg, Republic of South Africa, 1992) pp 241–244

Application of electrochemical impedance spectroscopy and rotating ring—disc measurements on FeCr alloys

Three different ironchromium alloys have been studied in pure 0.5 M H 2SO 4 and with an addition of chloride. The investigations have been performed in the active and activepassive range with a combination of Electrochemical Impedance Spectroscopy and a rotating ringdisc electrode to give quantitative information about the charge and information about the adsorption of reacting intermediates. In sulphuric acid the emission efficiency ralative to ferrous ions is totally located in the positive range, which means that iron species have a passivating role. The emission efficiency in sulphuric acid with chloride is located in the negative range. The iron containing surface species never have, therefore, a passivating role but behave as an intermediate in a dissolution path of the alloy. It has been found that it is possible to discriminate between the individual contributions of different species to the surface process by selecting the proper detection on the ring.

Porous sintered iron materials for elements of magnetic circuits: J. Ciekot and B. Weglinski, (Polytechnic of Wroclawski, Poland), Metall Proszkow, Vol 25, No 2, 1992. (In Polish)

The electropolishing of iron, the ferritic iron-chromium alloys, Fe-13Cr and Fe-24Cr, and austenitic type 304 stainless steel has been investigated in H3PO4-H2SO4 electrolytes using rotating disk electrodes. The electropolishing of stainless-steel-type alloys takes place in the transpassive region and is mass transport controlled. It is favoured by high temperatures. The anodic polarization of the Fe-Cr alloys in 65% H3PO4-20% H2SO4-15% H2O is insensitive to the elemental composition of the alloy and is the same as for the austenitic type 304 stainless steel. At 70°C electropolishing is obtained for a wide concentration range of H3PO4-H2SO4, but in the absence of H3PO4, iron passivity prevents the establishment of electropolishing conditions.

Study of Diffusion of Iron in Iron Telluride by Mössbauer Spectroscopy and Reaction between Iron-Chromium Alloy and Tellurium

Study of Diffusion of Iron in Iron Telluride by Mössbauer Spectroscopy and Reaction between Iron-Chromium Alloy and Tellurium

Structure effects on corrosion failure in iron-chromium alloys

Structure effects on corrosion failure in iron-chromium alloys

Passivation thresholds in iron-chromium alloys prepared by ion-beam sputtering

The threshold contents of chromium required for chromium-like passivity of Fe-Cr alloys have been determined using sputtered thin films with lateral concentration gradients. Clear thresholds are found for Fe-Cr alloys at 10 and 17 at.% for passivation, and 17 and 13 at.% for reactivation, in sulphuric acid solution. The former two values coincide with the critical compositions for 1st-2nd-3rd nearest-neighbour (p c{1,2,3}) and 1st-2nd nearest neighbour (p c{1,2}) site percolation for the b.c.c. lattice, respectively. The reactivation threshold was found to change from p c {1,2} to 13 at.% when the passivation potential was altered from −250 mV(SCE) to a high value, such as 500 mV(SCE).

Atomistic modelling of diffusional phase transformations

A simple Monte Carlo model has been used to simulate diffusional phase transformations occurring in binary alloys during thermal ageing. The results of the simulation are compared directly with atomic-scale chemical information obtained from position-sensitive atom probe microanalysis. A simple pair potential model is found to give a good match with the ageing behaviour of spindodally decomposing iron-chromium alloys. Initial results on the modelling of nucleation and growth are presented, and compared with phase separation of copper-cobalt alloys.

Hot pressed wear resistant iron-chromium alloy with hard phase mechanically alloyed surface

Hot pressed wear resistant iron-chromium alloy with hard phase mechanically alloyed surface

Electrodeposition and characterization of amorphous Fe-Cr-P-C alloys

Electrodeposition of iron-chromium alloys was investigated in a divided and undivided cell. The influence of the current density on the composition of the alloy was determined. Energy dispersive spectroscopy and Auger electron spectroscopy were used to evaluate alloy composition. These alloys were determined to be amorphous by X-ray diffraction and transmission electron microscopy. The corrosion behaviour of the alloys was studied in 0.5m HCl, and they were found to be more corrosion resistant than conventional chromium electrodeposits, with corrosion currents ranging between 40–60 μA cm−2, compared to 1850 μA cm−2 for conventional chromium. The alloy electrodeposition mechanism was studied by cyclic voltammetry.

The corrosion behaviour of iron-chromium alloys in 0.5 M sulphuric acid

The corrosion of five different iron-chromium alloys was studied in 0.5 M H2SO4, using polarization curves and impedance measurements. The data obtained were compared with the data obtained for pure iron and chromium. It was concluded that the passive current density for the alloys with a Cr content ≤9% is diffusion controlled. For alloys with a higher Cr content, 13 ≤Cr% ≤ 25, and the pure metals this is not the case. The passive film on the lower alloyed iron-chromium alloys is a porous film just after the active-passive transition. This porosity decreases with increasing chromium content and/or potential improving the protective quality of the film.

Three-dimensional characterization and modelling of spinodally decomposed iron-chromium alloys

The iron-chromium system has a spinodal region within which a random solid solution is thermodynamically unstable with respect to small composition fluctuations. The resulting decomposition yields Cr-enriched α′ and Fe-rich α phases forming complex and often interconnected morphologies. The resultant physical properties of the alloy are dependent on the scale and morphology of the microstructure and the amplitude of composition fluctuations. In this paper, the results of a series of thermally aged Fe-Cr specimens that have been analyzed with the position sensitive atom probe (POSAP) are compared with the results from a Monte Carlo simulation of the decomposition process. The resulting microstructures have been analyzed in terms of scale, composition amplitude and morphology. The scale has been measured by an autocorrelation analysis and the composition amplitude calculated from an analysis of the frequency distribution of block compositions. New morphological analysis techniques based on the measurement of a fractal dimension, and counting the number of handles within the microstructure, have been applied to characterize the detailed morphology on a subnanometre scale.

Two-element track autoradiography of boron and nitrogen in various materials

Abstract A non-destructive method of quantitative autoradiographic analysis of boron and nitrogen in a single object is described. Plastic track detectors (PTDs) were used to record tracks of particles from 10B(n, α)7Li and 14N(n, p)14C reactions. The potentialities of the method in determining the nitrogen content as a function of boron content in an object using a CR-39 detector are estimated. The application of the method to microanalysis of individual precipitates in the structure of iron-chromium alloys is demonstrated.

Oxidation at defects in scales on iron-chromium alloys

A prior investigation on the lateral spreading of oxide into defects in Wustite scales on iron was extended to study the same phenomena in Fe-Cr alloys. Included were two Fe-Cr-Mo alloys and an Fe-25Cr-6Al alloy. Three types of experiments were conducted to study flaws introduced to simulate damage to protective oxide layers caused by particle erosion. It was found that outer scales of Wustite on the Cr-Mo alloys spread into flaws in much the same way as Wustite on unalloyed iron. However, inner scales of (Fe,Cr)3O4 on the Cr-Mo alloys and the scale formed on the Fe-25Cr-6Al alloy had only a slight tendency to spread into flaws. These results are consistent with the known higher diffusion coefficients and higher defect concentrations in Wustite than in other oxide phases.

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.

The oxidation of Iron-Chromium-Manganese alloys at 900�C

The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900°C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt. %, and chromium at 5, 12, and 20 wt. %. The scales formed on the low-chromium alloys consisted of (Mn,Fe)2O3, α-Fe2O3, and Fe3O4. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr2O4 with Cr2O3 underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.

High Temperature Corrosion of Iron Chromium Alloys by Tellurium

High Temperature Corrosion of Iron Chromium Alloys by Tellurium

The effects of boron additions on the oxidation of Fe-Cr alloys in high-temperature steam: Analytical results and mechanisms

The oxide films formed on iron-chromium alloys in superheated steam have been investigated using a variety of microanalytical techniques, most notably electron energy loss spectroscopy. The addition of boron dopants leads to the rapid formation of a microcrystalline film of composition (Cr)xB1}-x2O3,which resists further oxidation. Analysis of the near-edge structures associated with each core-loss edge after different oxidation times allows us to postulate various mechanisms for the observed behavior.

Thermal oxidation study of thin magnetite-based coatings from iron-chromium alloys

The thermal stability of ferritic stainless steel conversion coatings for selective solar absorbers was studied in air and under low oxygen pressure (partial vacuum 2 × 10 -2 Pa) using optical techniques, X-ray diffraction, reflection high energy electron diffraction and secondary ion mass spectrometry analyses. Stability of optical selectivity was demonstrated up to 450°C in air and 500°C under vacuum. However, changes in the coating composition were observed in this temperature domain: the oxygen diffuses into the coating and the oxidation of the substituted magnetite phase (which is the main coating component) leads to α-Fe 2O 3 and (Fe 1− x Cr x ) 2 O 3 in air and to FeCr 2O 4 under low oxygen pressures. At higher temperatures substantial degradation of the optical properties was observed: the solar absorptance decreased and the emittance increased, causing high radiance losses. This phenomenon is related to substrate oxidation (which produces an increase in the coating thickness) and to large chemical modifications induced by chromium diffusion into the coating from the substrate.