Thermodynamic model for estimating the composition of passive films and Flade potential on Fe-Cr alloys in aqueous solutions

A thermodynamic model of a passive film is developed, in which the formation of the film on the surface of an Fe-Cr alloy in an aqueous solution is considered to be a result of the stable solid-phase chemical and adsorption equilibrium at the alloy-inner passive film layer interface. In the calculations, the Cr₂O₃ content in the passive film is determined by both the Gibbs energy change (ΔG < 0) in the chemical oxidation of the alloy components by the water oxygen and the change in the surface Gibbs energy (ΔG _S > 0) of the alloy. The ΔG _S change results in the negative adsorption of chromium atoms, which shifts the 3Fe + Cr₂O₃ ↔ 3FeO + 2Cr equilibrium toward the FeO formation in the passive film. Calculations showed that the enrichment of the passive film in chromium oxide should sharply increase in a chromium content range of 10–20% in the alloy, which agrees with the known data of X-ray photoelectron spectroscopy of the passive films. A formula is derived for estimating the Flade potential of Fe-Cr alloys, which relates the Flade potentials of individual Fe and Cr components to the FeO and Cr₂O₃ contents in the passive film.     

Esca-studies of the composition profile of low temperature oxide formed on chromium steels—I. Oxydation in dry oxygen

ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the composition of oxide films formed on chromium steels with 6–20%Cr at room temperature in pure oxygen at 0·2 atm. during different times from 2 min to 46 h. Ion etching was used to clean the surface before oxidation and to investigate the composition profile by successive stripping of the oxide layer.It is demonstrated that the total chromium content in the oxide is proportional to the concentration in the metal, but that it varies with depth inside the film. At the oxide-gas boundary the chromium concentration is less than in the metal, while chromium is enriched at the metal phase boundary. It is suggested that the oxide consists of (Fe,Cr)₂O₃ in the outermost zone and FeCr₂O₄ at the inside, with the reservation that the method is not sensitive to deviations from stoichiometry.The thickness of the layer has been determined as 25 Å from the intensity ratio of the deconvoluted ESCA peaks of the metallic and oxidized states. The thickness and the composition profile of the film do not vary noticeably with the oxidation time.     

XPS analysis of the passive film formed on austenitic stainless steel coated with conductive polymer

Improvement of the passivation behavior of Type 304 austenitic stainless steel (SS) by coating with conductive polymers (CPs), like polyaniline (PANI) and poly(o-phenylenediamine) (PoPD), followed by exposure in an acid solution has been demonstrated. The passive films formed on SSs (after peeling off the polymer layer) are compared with those formed during anodic polarization under the same exposure condition. The passive films beneath the CPs are thicker and less hydrated than those formed on uncoated stainless steel. The polymer layer enhances the enrichment of chromium and nickel in the entire passive oxide, forming a more protective film than that formed during anodic polarization. The elemental distribution within the passive film is different in the two modes of passivation. The type of the polymer influences on the composition of the passive film. The best passivation is obtained by PoPD, with the passive film resulting in significant resistance of the SS to pitting corrosion in the 3% NaCl solution. The oxide film of this steel is characterized, in its inner and outer layers, by the highest ratio of Cr(OH)₃/Cr₂O₃ and the lowest content of iron species.     

The open-circuit ennoblement of alloy C-22 and other Ni-Cr-Mo alloys

The open-circuit corrosion and anodic oxidation behavior of the C-series of Ni-Cr-Mo alloys (C-4, C-276, C-2000, and C-22) and alloy 625 have been studied at 25°C and 75°C in 1.0 mol·L⁻¹ NaCl+1.0 mol·L⁻¹ H₂SO₄. A combination of open-circuit potential, potentiostatic polarization, and electrochemical impedance spectroscopy were employed in the study. The composition of the films formed was determined by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Passive oxide film resistances increase and defect oxide film concentrations decrease as films thicken and chromium and molybdenum segregate to the alloy/oxide and oxide/solution interfaces, respectively. The high-chromium alloys exhibit higher film resistances and lower film defect concentrations consistent with the more positive potentials observed on these alloys. The results show that the observed ennoblement in corrosion potentials with time is coupled to the Cr/Mo segregation process and the suppression of defect injection at the alloy/oxide interface. By all measures, C-22 exhibited the best passive properties. For more information, contact D.W. Shoesmith, University of Western Ontario, Department of Chemistry, London, Canada, N6A 5B7; (519) 661-2111, ext. 86366; fax (519) 661-3022; e-mail dwshoesm@uwo.ca.     

Passivation of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys in 1 M H2SO4 and 1 M NaOH solutions

The corrosion mechanisms of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys have been investigated by potentiodynamic and potentiostatic polarization. Very good passivation of the Al–Cr–Fe surface is exhibited from 1 M H₂SO₄ to 1 M NaOH solutions, which was confirmed by ICP-OES analysis over a period of 273 days. Potentiostatically formed passive films analysed by XPS revealed chromium enrichment in the outermost layer of the aluminium oxy-hydroxide film. Although Al–Cu–Fe–Cr showed passivation during potentiodynamic polarization, heavy active corrosion at OCP was revealed by ICP-OES. For the Al–Cu–Fe–Cr alloy, the 10% content of Cr is insufficient to maintain a protective “chemically stable” Cr oxide/hydroxide.     

An investigation of thin oxide films thermally grown in situ on Fe24Cr and Fe24Cr11Mo by auger electron spectroscopy and X-ray photoelectron spectroscopy

AES depth profiling and XPS have been used for the characterization of thin oxide layers thermally grown in situ in the UHV-analysis chamber on pure iron, chromium and the alloys Fe24Cr and Fe24Cr11Mo at a temperature of 384°C. The apparent oxide film thickness and the film composition were monitored as a function of oxygen exposure. The oxidation rate of the Fe24Cr alloy was found to lie in between that of pure iron and chromium. The films formed have a duplex structure, the outer part being iron oxide, the inner part mostly chromium oxide. Alloying with molybdenum decreases the rate of oxidation by a mechanism involving the formation of a barrier layer rich in molybdenum at the oxide-metal interface. No molybdenum is found in the outer part of the oxide film.     

Nature of passive films on Fe26Cr alloy

Anodic oxide films formed on Fe26Cr alloy in pH 2.0 H₂SO₄ solution have been investigated using electrochemical techniques, and Auger electron spectroscopy (AES) combined with ion sputtering and subsequent quantitative thin film analysis. Films formed on electropolished (EP) and cathodically reduced (CR) surfaces have been examined in terms of their thickness, composition and open-circuit stability as a function of potential and time of anodization. Passive films on CR surfaces appear to be composed of either a (Fe,Cr)₂O₃ type of oxide, or an inner layer of FeCr₂O₄ and an outer layer of (Fe,Cr)₂O₃ depending on the anodizing potential. The Cr/Fe ratio in the films varied with both potential and time of anodization, but all films were enriched in Cr relative to the underlying alloy. Oxide thicknesses were $\text{≈17}\text{A}\text{?}\text{(±5?10\%)}$ irrespective of the conditions of formation. The somewhat thicker EP prior oxide contained Cl^? which was removed by CR. If no CR was performed, subsequent passive films on EP surfaces contained various amounts of incorporated Cl^? depending on the anodizing potential. The Cl^?-containing films were also Cr rich, and their composition and thickness were potential dependent. Open-circuit decay profiles and surface reactivity measurements demonstrated that the Cl^?-containing films break down faster than Cl^?-free anodic films with preferential removal of Fe and Cl^? occurring from the outer part of the film. The stability of both types of film increased with time of anodization.     

An XPS study of the passivity of a series of iron—chromium alloys in sulphuric acid

The composition and structure of passive films formed on a series of iron—chromium alloys in de-aerated 1M H₂SO₄ were quantitatively studied by XPS. The chromium content of passive film increased drastically at ca. 13 %Cr of the bulk alloy composition, whereas no composition change was found in the surface of the substrate alloy immediately under the passive film. The high corrosion resistance of iron—chromium alloys with high chromium content is attributed to the protective nature of hydrated chromium oxy-hydroxide, which is the main constituent of passive films on the alloys containing chromium of 12.5 at. % or more.     

Optimization of Cr/Mo molar ratio in FeCoCrMoCBY alloys for high corrosion resistance

The corrosion behavior of bulk metallic glasses (BMGs) (Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂)_100–xCr_x (x=0, 4, 8, 12, molar fraction, %) was investigated in 1 mol/L HCl aqueous solution with electrochemical tests. The electrochemical measurements demonstrate that the passive current density of Fe-based amorphous alloy is reduced by about one order of magnitude, and meanwhile, the stability of passive film can be guaranteed by the Cr/Mo molar ratio. The Mott–Schottky (M–S) curves show that the passive film is the densest when the molar ratio of Cr/Mo is between 1.37 and 1.69. X-ray photoelectron spectroscopy (XPS) analysis was performed to clarify chemical states of elements in the passive films. The results show that the corrosion resistance of the alloy is related to the molar ratio of Cr/Mo. The stability of passive film is determined by the synergistic action of Cr and Mo elements. The main component of the passive film is Cr³⁺ oxide. When the potential is greater than 0.5 V (vs SCE), Mo⁶⁺ ions play an important role in keeping the stability of the passive film. The appropriate molar ratio of Cr/Mo can reduce the dissolution rate of the passive film.     

Surface analytical and electrochemical study on the role of adsorbed chloride ions in corrosion of stainless steels

The pitting potential E_pit of 18/8 CrNi stainless steels with different sulfur content (0.003, 0.017 and 0.29%) has been determined from potentiodynamic polarization curves in deaerated neutral solutions (0.1 M NaCl and 1 M Na₂SO₄ + 0.1 M NaCl) with nominally identical chloride content. E_pit decreased with increasing sulfur content of the alloy and was about 0.2 V more negative in pure 0.1 M NaCl solution. The chemical composition of the passive film and the adsorbed chloride content have been determined by XPS surface analysis on mechanically polished samples passivated for one hour at potentials below the pitting potential. XPS results show that the surface films are composed in all cases of mixed iron‐chromium oxi‐hydroxides with a higher chromium content than the bulk composition. The average passive film composition (ca. 40% chromium oxi‐hydroxide) and the film thickness (2.3 ± 0.2 nm) were similar for all 18/8 CrNi steels regardless the different sulfur content. The amount of chlorides in the passive film is about twice as high on steels exposed to pure 0.1 N NaCl solution compared to the mixed 1 M Na₂SO₄ + 0.1 M NaCl solution. Thus the lower pitting potential measured in pure 0.1 N NaCl solution correlates with the higher amount of chloride ions on the passive film surface.     

The behavior of chromium and molybdenum in the propagation process of localized corrosion of steels

In order to clarify the effect of Cr and Mo on the propagation of localized corrosion of steels, the electrochemical behavior of pure chromium and molybdenum and of some stainless steels was studied under conditions similar to those existing inside occluded corrosion cells of steels. Oxygen-free FeCl₂ and/or CrCl₃ solutions were used for experiments. The surface films formed in these conditions were analyzed by means of AES and XPS. Cr is passive if the pH value >ca. 1.8 and Cr may retard the propagation of localized corrosion of steels in its early stage. The hydrolysis of Cr³⁺ may change the pH to even lower values. At such low pH and high Cl^? concentrations, Cr is active in the potential range of interest for localized corrosion of steels. Mo is passive under these more acidic conditions and the passivity of Mo in this potential range is attributed to the formation of a film of MoO₂ (or hydrated oxide), which is extremely thin but very protective. Mo is thus effective to retard the propagation of localized corrosion of steels in its more advanced stage.     

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.     

Influence of alloying on the passive behaviour of steels in bicarbonate medium

The electrochemical behaviour of passive films on carbon steel, chromium steel and high speed steel (HSS) was studied in 0.5 M NaHCO₃. The influence of the presence of 10 mM potassium chloride on passive film stability, and of the heat treatment of chromium steel and HSS on the respective film properties, was also investigated. Polarization curves showed that the alloy composition exerts a significant influence on the onset of passive film formation, chromium and HSSs passivating more easily as a result of the higher alloying content. Values of passive current, obtained both potentiodynamically and at fixed potential, were higher for HSS than for carbon and chromium steels, which was attributed to the contributions of Mo and W to the passive film composition. The presence of chloride ion delays the passivation of chromium and HSSs, but does not interfere with passive film stability, as shown by the chronoamperometric profile and passive current values. In the case of carbon steel, with no alloying elements, chloride ion diminishes film stability but does not influence passive film formation. Heat treatment, which alters the microstructure, does not alter the properties of the passive films except for a small effect on those formed potentiodynamically on HSS.     

Oxidation of Fe-8Cr-14Ni-Al-Si-Mn from 700 to 1000°C

This paper reports an investigation into reducing the Cr concentration in commercial-grade stainless steels while maintaining oxidation protection at elevated temperatures. Aluminum and Si were added as partial substitute alloy elements to enhance the reduced operation protection resulting from Cr concentration reduced by approximately 50 pct of that found in stainless steels. The goal of this study was to determine the oxidation mechanism of such an Fe, Al-Si alloy: Fe-8Cr-14Ni-1Al-3.5Si-1Mn. During the initial oxidation period the protection resulted from a thin film of Al₂O₃ over an Fe and Cr spinel. Long-term oxidation protection resulted from the gradual formation of a Cr sesquioxide (Cr₂O₂) inner oxide layer. Eventually an outer oxide layer formed that was a mixed composition spinel of Cr and Mn (MnO · Cr₂O₃). The Al₂O₃, which was part of the original protective layer flaked off early in the oxide testing, and the aluminum oxide that formed later appeared as an internal oxide precipitate.     

Selective dissolution and surface enrichment of alloy components of passivated Fe18Cr and Fe18Cr3Mo single crystals

The possible surface enrichment of chromium and molybdenum during dissolution of Fe18Cr (110) and Fe18Cr3Mo (110) alloys at constant potentials in the passive region is elucidated by taking into account quantitative information on partial dissolution rates of alloy components as measured by γ-spectrometry and on chemical composition of passivating films as measured by ESCA and AES. Combining results from all methods it is found that chromium under all conditions is enriched in the passivating films. An accumulation of chromium in the alloy is also indicated. For molybdenum, an enrichment in the passive film is observed at ? 0.2 V (SCE), whereas in the potential interval 0.1–0.9 V, the molybdenum enrichment as determined by ESCA and AES is hardly significant. On the other hand, γ-spectrometry gives a clear indication of molybdenum enrichment in both potential regions.     

Semiconductor properties and protective role of passive films of iron base alloys

Semiconductor properties of passive films formed on the Fe-18Cr alloy in a borate buffer solution (pH = 8.4) and 0.1 M H₂SO₄ solution were examined using a photoelectrochemical spectroscopy and an electrochemical impedance spectroscopy. Photo current reveals two photo action spectra that derived from outer hydroxide and inner oxide layers. A typical n-type semiconductor behaviour is observed by both photo current and impedance for the passive films formed in the borate buffer solution. On the other hand, a negative photo current generated, the absolute value of which decreased as applied potential increased in the sulfuric acid solution. This indicates that the passive film behaves as a p-type semiconductor. However, Mott-Schottky plot revealed the typical n-type semiconductor property. It is concluded that the passive film on the Fe-18Cr alloy formed in the borate buffer solution is composed of both n-type outer hydroxide and inner oxide layers. On the other hand, the passive film of the Fe-18Cr alloy in the sulphuric acid consists of p-type oxide and n-type hydroxide layers. The behaviour of passive film growth and corrosion was discussed in terms of the electronic structure in the passive film.     

The dissolution behaviour of iron,chromium, molybdenum and copper from pure metals and from ferritic stainless steels

Dissolution rates of iron, chromium, molybdenum and copper as pure metals and as components of three ferritic stainless steels in neutral solutions (distilled H₂O and 0.5 M NaCl) and acid solutions (1 M and 9.2 M of HNO₃ and H₂SO₄) are presented. The relative position of dissolution rates of the elements is in general the same in pure metal and alloy dissolution. Selective dissolution of iron and simultaneous surface enrichment of chromium is observed under all the investigated conditions. A slight surface enrichment of molybdenum is found upon exposure in the neutral and sulfuric acid solutions but not in nitric acid solution. Large amounts of copper are found in the surface of the copper-containing steel exposed to 1 MH₂SO₄. Evidence is presented which shows that an increase in electrolyte volume decreases the thickness and changes the composition of the passive film. It is believed that this hardly ever mentioned effect is due to an increased amount of dissolution of the film components upon increased volume of electrolyte.     

ESCA study of the passive film on an extremely corrosion-resistant amorphous iron alloy

X-ray photoelectron spectroscopy was applied to study the composition of the passive film formed on an extremely corrosion resistant amorphous Fe-10at.%Cr-13at.%P-7at.%C alloy in 1 N HCl. The passive film consists mainly of hydrated chromium oxyhydroxide which is a common major constituent of passive films on crystalline stainless steels. The extremely high corrosion resistance of the amorphous alloy can only in part be attributed to the formation of a protective hydrated chromium oxyhydroxide film.     

The air oxidation of two-phase Cu-Cr alloys at 700–900°C

The oxidation in air of three two phase Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt. % was studied at 700–900°C. The alloys corroded nearly parabolically, except at 900°C, when the corrosion rates decreased with time more rapidly than predicted by the parabolic rate law. The corrosion rate decreased for higher Cr contents in the alloy under constant temperature and generally increased with temperature for the same alloy composition. The scales were complex and consisted in most cases of an outermost copper oxide layer free from chromium and an inner layer composed of a matrix of copper oxide or of the double oxide Cu₂Cr₂O₄, often containing particles of chromium metal surrounded by chromia and then by the double oxide. Metallic copper was also frequently mixed with chromia. Cr-rich regions tended to form continuous chromia layers at the base of the scale, especially at the highest temperature. No chromium depletion was observed in the alloy.