Thermodynamic properties and phase equilibria in the Cr-P system

A Knudsen effusion method with mass-spectrometric analysis of gaseous phase has been applied to investigate the thermodynamic properties of the chromium phosphides (1341 to 1704 K) and Cr-P liquid alloys (1664 to 1819 K). Simultaneously, DSC has been used to measure heat capacities of chromium phosphides Cr₃P and Cr₁₂P₇ in the temperature range of113 to 873 K. The entropies of formation of chromium phosphides calculated according to the second and third laws of thermodynamics agree within the limits of experimental error. The Gibbs energies of formation of the phosphides from solid Cr and P₂ gas have been approximated with the following equations (in J/mol): A_fG⁰(Cr₃P) = −(244 112 ±2800) + (70.95 ±1.80)T ΔG⁰(Cr₁₂2P₇) = −(1563 678 ±15 350) + (440.6 ±9.90)T Thermodynamic properties of liquid solutions have been described with the ideal associated-solution model assuming that CrP, Cr₂P, Cr₃P, and Cr₃P₂ complexes exist in the melt. The phase diagram computed with the help of the thermodynamic data agrees with the published information.     

ESCA studies of the composition profile of low temperature oxide formed on chromium steels—II. Corrosion in oxygenated water

ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the chemical composition of passive films formed on chromium steels with 4–30% Cr at 25 and 70°C in oxygenated water for different times. The composition-depth profile of the films has been investigated by successive stripping of the film by ion etching.The composition of the film varies with exposure time: short time yields an Fe---Cr ratio corresponding to that of the alloy; prolonged exposure produces increasing enrichment of chromium through the whole layer, with the highest concentration at the outer surface. The passive film generally seems to consist of two layers: outermost one of (Cr(OH)₃) and underneath one of iron-chromium oxide (Fe_1+xCr_2−xO₄. The low alloy steels (3·9 and 7·8%Cr) were passive for a limited time during which the Cr enrichment was high (8 times); after breakdown of the chromium-rich passive layer, it was replaced by FeOOH.When a dry-formed oxide film, which consists mainly of iron oxide, is exposed to water, its composition slowly changes until that of a water-formed film is reached.     

Silicon as an alloying element in ferrite stainless steels containing 8–13% Cr

A jumpwise improving of the passivation ability and pitting-resistance of the Fe-(8–13)% Cr-(0.32–2.7)% Si alloys upon surpassing the limit of ∼14–15 at % by the chromium and silicon summary concentration is found. It is suggested that the nature of the critical summary concentration C _Cr + C _Si is identical to that in Fe-Cr binary alloys; it is caused by the silicon building-in to the alloy’s crystal lattice and its substitution for chromium in the statistical proportion to its atomic part in alloys. When the summary concentration C _Cr + C _Si approaches ∼14–15 at %, each elementary cell in the Fe bcc lattice must necessarily contain either a Cr atom or the proportional number of Si atoms. The improved passivation ability and pitting-resistance of the Fe-Cr-Si alloy, compared with the Fe-Cr alloys, is explained by the fayalite formation at the alloy surface. Original Russian Text ? I.I. Reformatskaya, I.G. Rodionova, A.N. Podobaev, I.I. Ashcheulova, E.V. Trofimova, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 6, pp. 591–597.     

Distribution and characterization of high temperature air corrosion products on iron-chromium alloys by Raman microscopy

Raman microscopy has been used to study the nature and distribution of corrosion products formed on iron and iron-chromium alloys in air at high temperatures. Fe and Fe-Cr alloys containing 2, 5, 14, and 18% Cr were oxidized at 400, 600, and 850°C for 2 hr, in addition samples of each alloy were oxidized for 24 hr at 400°C to obtain thicker scales at this temperature. The corroded samples showed varying distributions of the oxides Fe₂O₃, Fe₃O₄, Cr₂O₃, and FeCr₂O₄. Fe₂O₃ and Fe₃O₄ were formed exclusively on the pure iron and the 2 and 5% chromium alloys at all temperatures and on the 14% chromium alloy at 400°C. The 14 and 18% Cr alloys formed scales containing Cr₂O₃ and FeCr₂O₄ at the higher temperatures (600 and 850°C). Examples of small regions of Fe₂O₃ being formed within Cr₂O₃-FeCr₂O₄ scales are suggested as possible indications of breakaway corrosion initiation sites.     

Transition from oxidation to sulfidation of Fe-Cr alloys in gases with low oxygen and high sulfur pressures

Fe-Cr alloys with 17–30% Cr show in H₂-H₂O-H₂S mixtures at 1273 and 1073 K a transition from protective oxide scale formation to rapid sulfidation. The critical oxygen pressure to stabilize the oxide formation increases with increasing sulfur pressure of the gas and decreasing Cr content of the alloy. Cr₂O₃ with traces of Fe₂O₃ is formed under these conditions. Below the critical oxygen pressure, a primarily formed Cr₂O₃ film becomes overgrown by (Fe, Cr)S. The kinetic boundary of oxidation-sulfidation, which lies in the stability field (Fe, Cr)S + spinel Fe_1+xCr_2–xO₄ of the Fe-Cr-O-S phase diagram, is explained with the help of the Fe-Cr-O-S phase diagram and the assumption that Fe diffuses faster through the (Cr, Fe)₂O₃ solid solution than does Cr.     

Chemical and phase composition of nanosized oxide and passive films on Ni-Cr alloys. II. XPS analysis of films produced by anodic passivation of alloys in 1 N H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

XPS data of thin (1 to 2 nm) oxide films formed by the anodic passivation of Ni-2 at % Cr and Ni-6 at % Cr alloys in 1 N H₂SO₄ are discussed. Thermodynamic calculations of the solid-phase chemical reaction 3NiO + 2Cr = Cr₂O₃ + 3Ni are carried out taking into account the changes in the surface energy at the alloy-oxide film interface along with the Gibbs energy change in the alloy oxidation reaction.     

Effects of nitrogen on the passivity of Fe-20Cr alloy

Influences of nitrogen on the passivity of Fe-20Cr-(0, 1.1)N alloys were examined by in situ electrochemical techniques. Nitrogen was incorporated in the form of (Fe, Cr)-nitrides in the passive film, and Cr was enriched in the film of the alloy with nitrogen. Photocurrent analysis demonstrated that the structure of passive film formed on Fe-20Cr-1.1N alloy is Cr-substituted γ-Fe₂O₃ with (Fe, Cr)-nitrides. Mott-Schottky analysis revealed that the film formed on Fe-20Cr-1.1N contained higher Cr⁶⁺ and lower Cr³⁺ vacancy concentrations compared with that on Fe-20Cr alloy. All of these results were associated with the enhanced protectiveness of the film on Fe-20Cr-1.1N.     

Electrochemical behavior of surface films formed on Fe in chromate solutions

The stability of passive films formed on Fe in K₂Cr₂O₇ solutions during exposure at open-circuit potential or by potential cycling is studied in a chromate-free solution. The electrochemical behavior of chromate-passivated Fe is investigated with cyclic voltammetry combined with LASER light reflectance measurements which allow an in situ determination of the thickness of the iron oxide film. The electrochemical behavior of chromate-passivated Fe in chromate-free solutions strongly depends on passivation treatment. Passivation of iron by immersion at open-circuit in chromate solution leads to a passive film, in which both Fe and Cr species dissolve almost independently of the presence of the other one: Fe oxide by reductive dissolution and Cr oxide by oxidative dissolution in the corresponding potential regions. Passivation of iron by potential cycling in chromate solutions leads to much less loss of the otherwise soluble oxidized chromate and reduced ferrous species in subsequent electrochemical experiments (trapping in a protective film). Concerning the dissolution behavior, the film formed on iron by cycling in chromate solution behaves similarly as the passive film on Fe-17Cr alloy. However, the remnant passive film after reductive or oxidative dissolution on the Fe-Cr alloy is of truely protective nature as compared to the films formed on iron in chromate solutions, which show only a small contribution to the potential drop.     

A new theoretical approach to the thermodynamic calculation of high-temperature oxidation of Ni-Cr alloys

In calculations, not only the Gibbs formation energy of nickel and chromium oxides and thermodynamic activities of the alloy components, but also the Gibbs surface energy of a Ni-Cr alloy is taken into account. The method is based on the adsorption model of the alloy oxidation, according to which the adsorption of an alloy component with the smaller surface energy, namely, nickel, at the alloy-oxide film interface boundary shifts the dynamic equilibrium between the alloy and oxide-film components toward the formation of NiO. For the oxidation of Ni-Cr alloys, which are solid solutions with an fcc lattice, at 1123 to 1473 K, concentration boundaries of the chromium content in the alloy are calculated for the case of the prevailing formation of Cr₂O₃ oxide, which determines the high heat resistance of Ni-Cr alloys. Original Russian Text ? Yu.Ya. Andreev, A.A. Shumkin, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 3, pp. 239–244.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C,Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr₂O₃ (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr₂O₃ by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.     

The oxidation of Fe-19.6Cr-15.1Mn stainless steel

The oxidation behavior in air of Fe-19.6Cr-15.1Mn was studied from 700 to 1000°C. Pseudoparabolic kinetics were followed, giving an activation energy of 80 kcal/mole. The scale structure varied with temperature, although spinel formation occurred at all temperatures. At both 700 and 800°C, a thin outer layer of -Mn₂O₃ formed. The inner layer at 700°C was (Fe,Cr,Mn)₃O₄, but at 800°C there was an intermediate layer of Fe₂O₃ and an inner layer of Cr₂O₃ + (Fe, Cr,Mn)₃O₄. Oxidation at 900°C produced an outer layer of Fe₃O₄ and an inner layer of Cr₂O₃+(Fe,Cr,Mn)₃O₄. Oxidation at 1000°C caused some internal oxidation of chromium. In addition, a thin layer of Cr₂O₃ formed in some regions with an intermediate layer of Fe₃O₄ and an outer layer of (Fe,Mn)₃O₄. A comparison of rates for Fe₃O₄ formation during oxidation of FeO as well as for the oxidation of various stainless steels, which form spinels, gave good agreement and strongly suggests that spinel growth was rate controlling. The oxidation rate of this alloy (high-Cr) was compared with that of an alloy previously studied, Fe-9.5Cr-17.8Mn (low-Cr) and was less by about a factor of 12 at 1000°C and by about a factor of 100 at 800°C. The marked differences can be ascribed to the destabilization of wustite by the higher chromium alloy. No wustite formation occurred in the high-Cr alloy, whereas, extensive wustite formed in the low-Cr alloy. Scale structures are explained by the use of calculated stability diagrams. The mechanism of oxidation is discussed and compared with that of the low-Cr alloy.     

High-Temperature Scaling of Cu–Al and Cu–Cr–Al Alloys: An Example of a Non-Classical Third-Element Effect

The oxidation of three Cu–xCr–2Al and three Cu–xCr–4Al alloys (x ≅ 0,4,8 at.%) has been investigated at 800°C in 1 atm O₂. Oxidation of a binary Cu–Al alloy containing 2.2 at.% Al produced external scales composed mainly of copper oxides with small amounts of Al-rich oxide in the inner region, while the internal oxidation of Al was almost absent. The addition of 3.9 at.% Cr to this alloy was able to decrease the oxidation rate but was insufficient to prevent the oxidation of copper. Conversely, addition of 8.1 at.% Cr to the same binary alloy promoted the rather fast formation of a protective Al₂O₃ layer in contact with the metal substrate, with a simultaneous large decrease in the oxidation rate, producing a form of third-element effect. On the contrary, all the Cu–xCr–4Al alloys formed an internal Al₂O₃ layer after an initial stage during which all the alloy components were oxidized, so that the only effect of the presence of chromium was to decrease the duration of the fast initial stage. The third-element effect due to chromium additions to Cu–2Al is related to a transition from the formation of external scales composed of mixtures of Cu and Al oxides to the external growth of Al₂O₃–rich scales as a consequence of a thermodynamic destabilization of copper oxides associated with the formation of solid solutions between Al₂O₃ and Cr₂O₃.     

Thermodynamisch-kinetische Deutung der Passivierungserscheinungen bei Fe-Cr-Legierungen

Thermodynamic-kinetic interpretation of the passivation phenomena with Fe? Cr alloys By superimposing two thermodynamic kinetic current output/potential curves corresponding to iron and chromium, respectively, it is possible to plot the selfpassivation curve for Fe-Cr alloys, which is similar to the potentiostatic polarisation curve. For the ideal Fe-Cr alloy, it is possible to draw conclusions concerning the Fe and Cr passivity zones and the composition of the oxide layers on the passivated Fe-Cr alloy at the Flade reference potential.     

Photoelectrochemical study of the growth of the passive film formed on Fe-20Cr-15Ni in a pH 8.5 buffer solution

The structural change of the passive film on Fe-20Cr-15Ni at its growth stage was examined in-situ using a photoelectrochemical technique. The photocurrent spectra showed that the passive film on Fe-20Cr-15Ni for 0.5 h ∼ 25 h in a pH 8.5 buffer solution was composed of (Cr, Ni)-substituted γ-Fe₂O₃ mixed with NiO. Photocurrent spectral analysis suggested that a crystalline film of (Cr, Ni)-substituted γ-Fe₂O₃ formed before 30 min, with its thickness exceeded that of the space charge layer after 1 h of passivation. NiO particles appeared to have gradually precipitated from (Cr, Ni)-substituted γ-Fe₂O₃ film in the early stage of passivation of 1 h.     

High-temperature corrosion of Cr2O3-forming alloys in CO-CO2-N2 atmospheres

The corrosion of Fe–28Cr, Ni–28Cr, Co–28Cr, and pure chromium in a number of gas atmospheres made up of CO–CO₂(–N₂) was studied at 900°C. In addition, chromium was reacted with H₂–H₂O–N₂, and Fe–28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H₂–H₂O–N₂ produced a single-phase of Cr₂O₃. In a CO–CO₂ mixture, a sublayer consisting of Cr₂O₃ and Cr₇C₃ was formed underneath an external Cr₂O₃ layer. Adding nitrogen to the CO–CO₂ mixture resulted in the formation of an additional single-phase layer of Cr₂N next to the metal substrate. Oxidizing the binary alloys in CO–CO₂–N₂ resulted in a single Cr₂O₃ scale on Fe–28Cr and Ni–28Cr, while oxide precipitation occurred below the outer-oxide scale on Co–28Cr, which is ascribed to the slow alloy interdiffusion and possibily high oxygen solubility of Co–Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe–28Cr, but varied between the different materials, decreasing in the order chromium >Fe–28Cr>Ni(Co)–28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr₂O₃ is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affects the permeability of the oxide.     

Photoelectrochemical analysis of the effects of pH and sulfate ions on the structure and the composition of the passive film formed on Fe-20Cr-15Ni alloy

The effects of pH and sulfate ions on the structure and compositions of the passive film formed on Fe-20Cr- 15Ni were examined using a photoelectrochemical technique and Mott-Schottky analysis. The photocurrent spectra for the passive film formed on Fe-20Cr-15Ni in the buffer solutions are composed of two spectral components, one of which is generated from Cr-substituted γ-Fe₂O₃ and the other of which is generated from NiO. However, the passive film formed in sulfate solutions showed only the photocurrent spectrum of Cr-substituted γ-Fe₂O₃, suggesting that the formation of NiO in the passive film is suppressed because of a severe selective dissolution of Ni in the presence of sulfate ions. Mott-Schottky plots confirmed that the base structure of the passive film on Fe-20Cr-15Ni is n-type (Cr, Ni)-substituted γ-Fe₂O₃ regardless of solution pH and sulfate ions. The photocurrent intensity, flat band potential, and donor density for the passive film varied depending on the solution pH or the presence of sulfate ions in the solution, due primarily to the Cr enrichment in the film caused by the preferential dissolution of Fe and Ni that is more appreciable in highly acidic solutions containing sulfate ions.     

Carbon transport through oxide scales on Fe-Cr alloys

The penetration of carbon through Cr₂O₃ layers was studied for a series of different Fe-Cr alloys using a radioactive tracer method. Preoxidized samples were exposed at 900°C for 700 hr in a H₂ -H₂O -CO-CO₂ atmosphere tagged with¹⁴C; carbon penetration profiles were then determined, and the lateral distribution of carbon was observed by autoradiography. Even minute amounts of carbon (0.05 ppm) within the scale and in the alloy could be detected. The carbon uptake into different Fe-Cr alloys decreased with increasing Cr content to a minimum for the alloys with 12.5–20% Cr, indicating low porosity and good adherence of the Cr₂O₃ layers. Poor scale adherence was observed for Fe-10% Cr but could be improved by Ce additions. Porosity increased with contents >20% Cr of the alloys. Pore formation could be induced by impurities, e.g., SiC particles distributed on the surface.     

Chemical and phase composition of nanosize oxide and passive films on Ni-Cr alloys. I. XPS studies of films obtained by alloy oxidation in air

XPS data for thin (less than 100 nm) oxide films obtained by oxidation of Ni-4 Cr and Ni-12.5 Cr alloys at 500°C (0.5 h) are discussed. Thermodynamic analysis of 3 NiO + 2 Cr = Cr₂O₃ + 3 Ni solid-phase reaction is given, in which both the Gibbs energy change in the thermochemical process and the change in the interface energy at the alloy-oxide film boundary are taken into account.     

Tunnel scanning microscopy and spectroscopy in studying Fe-Cr stainless steels

Scanning tunnel microscopy (STM) and scanning tunnel spectroscopy (STS) are used in studying highly pure iron and chromium and Fe-x%Cr (2.4 ≤ x ≤ 60 wt %) alloys at the air boundary, as well as some of the alloys in 0.01 N H₂SO₄. The obtained probability coefficients (α) of the electron tunnel transfer from a specimen to the needle and the slopes (β) of the logarithmic U _t /log (I _t ) dependences as functions of the chromium content in Fe-Cr alloys confirm the critical compositions of the alloys containing ∼6.5 and 10–13% chromium, which is in accord with the results of the steady-state and transient electrochemical measurements. A correlation between these critical compositions of the alloys and sharp changes in the histograms of α and β values is observed. Pronounced extreme properties of the Fe-25.2% Cr alloy surface, which corresponds to the inclusion of Cr atoms in tetrahedral voids of the alloy crystal lattice, are noticed. Original Russian Text ? E.V. Trofimova, E.V. Kasatkin, I.I. Reformatskaya, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 3, pp. 245–255. The work was financially supported by the Russian Foundation for Basic Research, project no. 04-03-32337.