The role of chromium and molybdenum in passivation of amorphous Fe-Cr-Mo-P-C alloys in deaerated 1 M HCl

The effect of open circuit immersion on the passivation behavior of amorphous Fe-8Cr-Mo-13P-7C alloys with 0, 2 and 6 at.% molybdenum by potentiostatic polarization in de-aerated 1 M HCl was investigated. The polarization was made at 400 mV (SCE) which is in the passive region of chromium but in the transpassive region of molybdenum, whereas the open circuit potential of these alloys is in the passive region of molybdenum but in the active region of chromium. Open circuit immersion of molybdenum-bearing alloys results in the formation of molybdenum-enriched passive film, but passivation at 400 mV is based on the formation of chromium-enriched passive film and the molybdenum-enriched film formed by open circuit immersion is converted to the chromium-enriched film at 400 mV. Polarization at 400 mV immediately after immersion leads to rapid enrichment of chromium to the level of 50% cations within 100 s, but polarization after open circuit immersion of molybdenum-bearing alloys for 10 min results in a gradual increase in chromium content to the level of 50% of cations in 5 h. The current density of molybdenum-bearing alloys after open circuit immersion is always higher than that of the alloys polarized immediately after immersion. Open circuit immersion of molybdenum-free alloy for 10 min gives rise to serious damage of the alloy surface and hence prevents passivation even if prolonged polarization is carried out.     

Ion beam analyses of surface films formed on amorphous Fe12Mo18C alloy in 1 N HCl

The Rutherford backscattering with 2 MeV He⁺ and the ¹⁶O(d,p)¹⁷O¹ nuclear reaction with 1.8 MeV D₂⁺ were utilized to determine the concentration profiles of iron, molybdenum and oxygen in the surfaces of an amorphous Fe12Mo18C alloy polarized in 1 N HCl at potentials from ? 0.17 to 1.6 V(SCE). The thickness of the surface film was estimated as 20–200 nm. In the primary active region, selective dissolution of iron and carbon causes enrichment of molybdenum ions in the corrosion product film and of metallic molybdenum in the topmost part of the underlying alloy. A further potential increase led to a decrease in the molybdenum content in the film by transpassive dissolution. Molybdenum ions were not concentrated in the film formed in the stable passive region of 0.5–1.5 V(SCE). The high passivating ability of the amorphous alloy even in the aggressive HCl was ascribed to the homogeneity of the alloy structure as well as to the effect of molybdenum.     

Untersuchungen zum Aktiv/Passiv-Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Medien. Teil 3. Resultate der Impedanzmessungen und Passivierungsmodell einer Legierung

Investigations into the active/passive transition of 304 stainless steel in organic media containing water and hydrogen chloride Part 3. Results of the impedance measurements and passivation model of an alloy Steady state polarization curves and electrode impedances were measured during the active/passive transition of type 304 stainless steel in dearated ethanolic solution containing hydrogen chloride and different amounts of water. The passivation potential and the critical current density for passivation strongly depend on the water content of the solution. The impedance measurements in the active/passive transition show the same sequence of diagrams independent of the water content of the solution. They indicate the onset of passivation before the maximum current density and show two time constants related to two different passivating species on the alloy surface. The experimental results were interpreted on the basis of a reaction model with parallel dissolution and passivation mechanism of the iron and the chromium compound of the alloy. The resulting total surface composition (related to the steady-state polarization curves) can be described with a reaction model of iron–the alloy behaviour is that of pure metal. The fundamental passivation reaction is described as a potential dependent equilibrium between adsorbed Me(II)- and passivating Me(III)-hydroxide, water molecules being directly involved in the formation of this primary passivating film. In the case of stainless steel this primary passivating film mainly consists of chromium (III) adsorbates. Finally, a general model for the passivation is proposed: The passivation of a pure metal or of an alloy can be understood as the coupling of the stepwise deprotonation of the water molecules at the interface metal/solution and the formation of a high cation charge density in this adsorbed hydroxide/oxide film to build up the passive layer. The effect of water content, pH, adding of passivating species to the solution or the alloying with chromium on the passivation potential and the critical current density thus can be explained.     

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.     

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.     

The effect of water on the anodic behaviour of FeMo alloys in organic solvents

The presence of molybdenum in FeMo alloys impedes the anodic dissolution process of these alloys in organic solutions of sulphuric and hydrochloric acids. The inhibiting action of molybdenum increases with a lower water concentration in the solution. As the solutions are dehydrated, there is a noticeable increase in the stability of molybdenum compounds occurring on the alloy surface as a result of anodic dissolution.     

Three-dimensional modelling of selective dissolution and passivation of iron-chromium alloys

A three-dimensional model has been developed for modelling the selective dissolution and passivation of alloys. The model has been used to simulate the passivation of iron-chromium alloys. The real structure of the alloy is taken into account (bcc in the present case), as well as the structure of the initial surface. The passivation is modelled in considering the formation of “oxide” nuclei, resulting from the presence of local chromium-rich clusters. During the dynamic evolution of the model, based on the Monte Carlo method, surface diffusion and dissolution of atoms occur according to probabilities dependent on the nature of the atom (Cr or Fe) and on its chemical environment. The conditions of simulation can be changed through a set of parameters defining the rules for surface diffusion, selective dissolution and number of Cr atoms in the Cr clusters required to initiate locally the passivation. The effects of these parameters on the simulation have been tested for an alloy containing 22 at.% Cr and compared with experimental data. The results show that the diffusion of Fe has little influence on the course of passivation while the diffusion of Cr has a marked effect. When the number of surface chromium atoms required to form a nucleus of passive film increases, the passivation becomes less rapid, with a marked effect on the composition of the passivated layer. The extent of the chromium enrichment in the passivated surface obtained in the model for the initial stages of passivation is not as high as the one measured experimentally in the stationary state of passivity.Other simulations have then been performed with various chromium contents in the alloy. The results show the existence of a transition, which is not sharp but progressive, between alloys that cannot be passivated to alloys that are passivated.     

An X-ray photo-electron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steels in HC1

X-ray photo-electron spectroscopy has been used to investigate the correlation between composition of surface films and the beneficial effects of molybdenum addition to high purity, 30Cr ferritic stainless steels in improving the corrosion resistance properties in HCI. It has been found that the passive films formed consist mainly of hydrated chromium oxy-hydroxide and the composition of the films on 30Cr and 30Cr-2Mo stainless steels is essentially the same, except for the existence of a small amount of hexavalent molybdenum on the latter steel. The surface film formed in the active region contains a large amount of hexavalent molybdenum. The beneficial effects of molybdenum have been interpreted as follows: molybdenum eliminates the active surface sites through the formation of molybdenum oxy-hydroxide or molybdate on these site, on which it is difficult to form the stable passive film. This leads to the appearance of a homogeneous steel surface and to the formation of a homogeneous passive film.     

The role of alloying elements in improving the corrosion resistance of amorphous iron base alloys

X-ray photo-electron spectroscopy has been used to investigate the correlation between the compositions of a surface film and the underlying alloy and the beneficial effects of alloying elements. The addition of alloying elements less noble than iron increases the corrosion resistance in the active region by improving the protective quality of the corrosion product film in which the cations of alloying elements are significantly concentrated. Even if the passivity of alloying elements is not stable in the passive region of alloys, the alloying asists the formation of passive hydrated iron oxy-hydroxide film by decreasing the dissolution rate of alloys prior to the passive film formation. The improvement of corrosion resistance by alloying with the elements more noble than iron has been interpreted in terms of the decrease in the anodic activity of alloys.     

The role of alloying elements in improving the corrosion resistance of amorphous iron base alloys

X-ray photo-electron spectroscopy has been used to investigate the correlation between the compositions of a surface film and the underlying alloy and the beneficial effects of alloying elements. The addition of alloying elements less noble than iron increases the corrosion resistance in the active region by improving the protective quality of the corrosion product film in which the cations of alloying elements are significantly concentrated. Even if the passivity of alloying elements is not stable in the passive region of alloys, the alloying asists the formation of passive hydrated iron oxy-hydroxide film by decreasing the dissolution rate of alloys prior to the passive film formation. The improvement of corrosion resistance by alloying with the elements more noble than iron has been interpreted in terms of the decrease in the anodic activity of alloys.     

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.     

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.     

The dissolution of CuNi alloys in hydrochloric acid—I. Rotating disc electrode measurements

The dissolution of pure copper, 90 Cu10 Ni and 60 Cu40 Ni in hydrochloric acid in the concentration range 1/3-3 M was studied using potentiodynamic sweeps and steady-state potential measurements on a rotating disc electrode. Some results are also reported for rotating ring-disc experiments. In the active region copper dissolves as a cuprous species from all three alloys, and in the passive region a film of cuprous chloride formed by a solid state mechanism is probably responsible for passivity. At higher anodic potential cupric species are produced. The alloy behaviour cannot be represented by a simple linear combination of the two partial currents.In the case of the $\text{60}\text{40}$ alloys, behaviour consistent with a partially blocked anode has been observed at higher current densities.     

A study of the initial stages of iron passivation in neutral solutions using the quartz crystal resonator technique

The initial period of growth of a passive film of iron in borate solutions (pH 7.4 and 6.7) is studied using the quartz crystal resonator technique (EQSN) and pulsed chronoamperometry. Dependences of the surface layer thickness on time are obtained at the metal passivation and prepassivation potentials. Regions corresponding to different stages of passive layer formation are found in anodic current transients, which allowed the ambiguous effect of atomic hydrogen on kinetics of hydrogenated iron dissolution to be explained. It is shown that the iron hydrogenation promoter prevents formation of a primary passive film by accelerating iron dissolution at prepassivation potentials.     

Influence of alloying elements on the corrosion performance of alloy 33 and alloy 24 in seawater

The nitrogen-bearing alloys (alloy 33 and alloy 24) and stainless steel type 316L were used in this investigation in order to study the effect of alloying elements on electrochemical behavior and on the nature of passive film in seawater. Scanning electron microscopic studies were carried out to identify the pit morphology of the alloys. Surface analysis of the alloys by x-ray photoelectron spectroscopy after passivation showed that nitrogen and chromium are enriched at the surface of the passive film.     

Effects of an applied magnetic field on the dissolution and passivation of iron in sulphuric acid

The effects of an applied magnetic field (MF) on the electrochemical state, anodic dissolution and passivation of iron in sulphuric acid solution were studied by potentiodynamic scanning polarisation measurements, potentiostatic polarisation measurements and scanning electron microscopy observation. The magnetic field reduced the fractional surface film coverage on the electrode by enhancing the film dissolution process. This made the electrode prone to active dissolution. With increasing applied potentials the magnetic field accelerated the anodic dissolution at relatively low potentials, changed the oscillation or passivation to permanent active dissolution at intermediate potentials, and maintained the passive state at high potentials. Potentials for the onset of passivation moved in the noble direction when the magnetic field was imposed. An electrode kinetics formulation for the effects of the magnetic field on the dissolution and passivation is proposed. In the presence of a magnetic field and at specific anodic potentials, scalloping occurred due to accelerated localized dissolution. The scalloping areas were on both sides of the electrode and oriented parallel to the direction of the earth’s gravitation field. The ratios of the scalloping area caused by a 0.4 T magnetic field on the whole electrode surface were 0.69 (at 200 mV), 0.66 (at 350 mV) and 0.75 (at 400 mV), respectively. In contrast, uniform electrode surfaces were observed at these anodic potentials in the absence of the magnetic field. Uneven dissolution of iron in the presence of a magnetic field was related to the relative configuration between the magnetic field direction and the electrode surface and also to the special concentration gradient of reactive species at the electrode circumferential area.     

Relation between individual properties of alloy components and anodic behavior of multicomponent alloys in acidic environments: I. Active dissolution and passivation of Fe-Cr alloys in sulfuric acid solutions

The active dissolution and passivation of Fe-Cr alloys are analyzed with the use of concepts of the appearance, operation, and blockade of active dissolution sites on the actual metal surface. General regularities of the effect of chromium on the rate of the anodic process at the change in the alloy composition and electrode potential are considered. The less noble, but easily passivable chromium component provides a dual effect on the anodic behavior of the binary alloys; it stimulates the anodic reaction at the low content in the alloy and suppresses the process at the high content. The principally different electrochemical properties of the alloy components result in the step-by-step passivation of the alloy with an increase in the potential, which is reflected by regular breaks in perfect anodic curves.     

Anodic passivity of some titanium base alloys in aggressive environments

The passivity of titanium, binary Ti‐15Mo and ternary Ti‐15Mo‐5Al alloys in hydrochloric acid solutions was studied by potentiostatic, potentiodynamic, linear polarization and electrochemical impedance spectroscopy (EIS) techniques. The anodic passivity of binary Ti‐15Mo and ternary Ti‐15Mo‐5Al titanium alloys differs from that of the base metal in hydrochloric acid solutions. The corrosion potentials of both alloys are nobler than of the titanium because the beneficial effect of molybdenum. The critical passivation current density for binary Ti‐15Mo alloy is higher than of titanium; this fact can be explained by the instability of the constituent phases in hydrochloric acid solutions. Ternary Ti‐15Mo‐5Al alloy exhibits two critical passivation current densities (i_cr1 and i_cr2) with higher values than of the base metal and two critical passivation potentials (E_cr1 and E_cr2); at the first critical passivation potential (E_cr1) the porous titanium trioxide (Ti₃O₅) is formed and at the second critical passivation potential (E_cr2) this oxide is converted to a still higher valence oxide, the compact and protective titanium dioxide (TiO₂). The dissolution current densities in the passive range of alloys are higher than of the base metal due the dissolution of the alloying elements in this potential range. The alloys are more resistant than titanium presenting lower corrosion rates. A three time constants equivalent circuit was fitted: one time constant is for the double layer capacity (C_dl) and for the passive film (R_p); another time constant is for the charge transfer reactions visualised by a constant phase element (CPE) and a resistance (R₁); the third time constant is for diffusion processes through the passive film represented by a resistance (R₂) and a Warburg element (W).     

Stability of the passive state of Zr–Nb crystalline alloys

Stability of the passive state was assessed for pure Zr and four Zr–Nb alloys (2.5, 5.0, 10.0, 25.0 at.% Nb) after 24 h exposure to naturally aerated 3.5% NaCl solution (pH = 6). Open circuit potential values indicated that all the Zr–Nb alloys studied and the pure Zr undergo spontaneous passivation due to spontaneously formed oxide film passivating the metallic surface. It also indicated that the addition of increasing Nb contents to pure Zr seems to improve the protection characteristics of its spontaneous oxides. Potentiodynamic polarization curves showed an increase of the breakdown potential and the extent of the passive range following an increased niobium amount in the alloy. This leads to the conclusion that a progressive increase of niobium content positively affects the localized corrosion behaviour of the Zr–Nb alloys by enhancing the electrochemical stability of the passive film. Electrochemical impedance spectroscopy (EIS) studies showed high impedance values for all the samples, and its increase on increased niobium content in the alloy indicated an improvement in the corrosion resistance of the spontaneous oxide film. The fit obtained suggests a single passive film present on the metal surface, with resistance improving with the increase of niobium amount in the alloy. All these electrochemical results show the beneficial effect of increasing niobium contents on the passive state stability of the Zr–Nb alloys.     

An XPS study of anodic behavior of amorphous nickel-phosphorus alloys containing chromium,molybdenum or tungsten in 1 m CHl

The surfaces of amorphous Ni-18P, Ni-IOCr-20P, Ni-9Mo-19P and Ni-5W-18P alloys immersed or anodically polarized in 1 M HCl solution were analyzed in connection with their corrosion and anodic behavior. All alloys were more corrosion-resistant than crystalline nickel metal because of formation of phosphate-containing surface films on the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys and because of spontaneous passivation due to formation of passive hydrated chromium oxyhydroxide film on the Ni-10Cr-20P alloy. The latter alloy was stable up to the transpassive region of chromium although intrusion of phosphate in the film was responsible for the higher passive current density in comparison to the amorphous Fe-Cr-13P-7C alloy of the same chromium content. The formation of thick porous phosphate films containing nickel, and molybdenum or tungsten by anodic polarization was not effective in passivating the Ni-18P, Ni-9Mo-19P and Ni-5W-18P alloys, and they suffered pitting corrosion by anodic polarization.