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.     

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.     

An XPS study of passive films on Nickel and alloy 600 in acids

Compositions of surface films formed on nickel and Alloy 600 in I M HCI, 0.5 M H₂SO₄ and I/3 M H₃PO₁ solutions were investigated as a function of polarization potential. The main constituent of surface films formed on Ni in 0.5 M H₂SO₄ or 1/3 M H₃PO₄ solution was hydrated nickel oxyhydroxide, in which the ratio of O₂ to OH⁻ increased when passivation occurred. The surface films formed on Ni and Alloy 600 at lower potentials in 0.5 M H₂SO₄ solution contain S² ions other than SOP_4²⁻ ions, whereas S²⁻ ions were not incorporated in the passive film. Passivation of Alloy 600 took place by the formation of hydrated chromium oxyhydroxide. Pitting led to no substantial change in the average composition of the film.     

The influence of temperature on the passivation behaviour of a super duplex stainless steel in a boric-borate buffer solution

The passivation behaviour of a super duplex stainless steel has been investigated in a boric-borate buffer solution with and without chloride additions, at different temperatures, by means of potentiodynamic and potentiostatic techniques. X-ray photoelectron spectroscopy was used to detect surface compositional changes of anodic passive films formed as a function of electrode potential and temperature. The resulting passive films were all chromium enriched with respect to the bulk composition, and their thickness and ratio were found to vary significantly. In particular, the films formed in the passive domain were more enriched with chromium compared with those grown in the secondary passive region. At a constant potential the temperature increase produced a reduction in the chromium content of the film. These findings can be explained by the different ionic mobilities and dissolution rates at the film/solution and film/alloy interfaces. Pitting corrosion was observed only in the transpassive zone. A temperature increase from 23 to 60 °C did not produce a decrease in the pitting potential. This behaviour can be explained by the parallel action of two different phenomena, the change in the composition and structure of the passive film and the velocity increase in the process, which yield localized rupture of the passivity by the nucleation and growth of pits in connection with chromium depletion at high potentials.     

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.     

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.     

An X-ray photo-electron spectroscopic study of the passivity of ferritic 19Cr stainless steels in 1 NHCl

The composition of surface films formed on 19Cr and 19Cr-2Mo ferritic stainless steels in 1N HCl has been analyzed by X-ray photo-electron spectroscopy. The composition of the passive films on both steels is essentially the same. Passivation takes place by the formation of a hydrated chromium oxy-hydroxide It has been suggested that, in general, the passive film is not uniform but contains micro-pores which are repeatedly formed and repassivated. The roles of chromium and molybdenum in increasing the corrosion resistance have been illustrated in terms of the activity and repassivation of micro-pores The origin of the micro-pores has also been suggested.     

The polarization behaviour of chromium in acidic sulphate solutions

The polarization behaviour of chromium was studied in solutions of various pH, with particular regard to the hydrogen evolution reaction and chromium dissolution. Hydrogen evolution occurred on active and passive chromium and the exchange current density was ca. 10^?6 A/cm² at pH 1.0 on both electrodes. The rate of active dissolution of chromium did not depend on pH, an observation that indicated the absence of a hydroxyl-containing intermediate in the rate determining step. The transpassive dissolution of chromium was strongly dependent on pH, and it was suggested that the rate was controlled by a base-accelerated anodic dissolution of an oxide film.     

Electrochemische Untersuchungen zur Hochtemperaturkorrosion von Chromstählen in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of chromium steels in alkali sulfate melts Electrochemical and corrosion-chemical investigations have been carried out with scaling resistant chromium steels, iron, chromium and platinum in a eutectic (Li, Na, K)₂-SO₄. It has turned out that sufficiently exact data concerning corrosion reactions can be obtained only from mass losses, not, however, from current density. The corrosion behaviour depends from potential. Comparable to the conditions in aqueous solutions potential ranges exist with passive and transpassive corrosion and with a rupture potential which depends from the chromium content of a steel. Protective oxide layers exist in the passive range where the mass loss becomes almost constant after a certain in cubation period. In the transpassive range corrosion follows an almost parabolic law with formation of an inner sulfide layer and a thicker external oxide layer where chromium is enriched. These layers are largely formed by oxidizing media carried to the metal surface via sulfur oxides; during this reaction inert marks in the steel remain unchanged. Sulfur oxides may be formed as secondary consecutive products by reactions between metal ions and sulfates. The solubility of metal ions in the sulfate melt is an important parameter for corrosion rates. Oxide ions (as reduction products of O₂) act as inhibitor on the anodic partial reaction, while SO₃ and ferric ions have a large lating effect, so that the anodic dissolution is autocatalyzed. Chlorides, too, act as stimulators in the transpassive range. Corrosion at the free corrosion potential is largely controlled by ferric ions which act as anodic and cathodic stimulators in acid melts. In neutral melts under oxygen an 18% chromium steel is passive.     

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.     

硫酸溶液中铁在过钝化区电极行为

本文应用电位衰减和交流阻抗等电化学方法对H_2SO_4溶液中处于过钝化区的铁的钝化膜的性质进行了研究。发现在钝化-过钝化过渡区域,钝化膜的厚度随着电位的升高而下降。当电位升到1800mV(VS·SCE)以后,钝化膜的厚度不再随电位的变化而变化。对不同电位下的交流阻抗频谱图进行了拟合分析,得到了一些与钝化膜的性质有关的参数。     

Effect of alloying elements on the contact resistance and the passivation behaviour of stainless steels

The effects of alloying elements (Cr, Mo) on the passivation and the transpassive transition behaviour of various commercial stainless steels, which are candidate materials for the bi-polar plates in polymer electrolyte membrane fuel cell, were investigated via a contact electric resistance (CER) technique with graphite counter part. The contact resistance, too high for a semi-conductor film with high donor density, is not the inherent ohmic resistance of the passive film. The CER potentials decrease linearly with the pitting resistance equivalent number (PREN). The passive films of stainless steels with high PREN are more readily removed compared to those with low PREN in terms of the charge necessary to remove the passive films. The change in transpassive transition behaviour of the stainless steels from that of iron to that of chromium went on steeply with an increase in the PREN.     

Influence of temperature and applied potential on the electrochemical behaviour of nickel in LiBr solutions by means of electrochemical impedance spectroscopy

The effects of temperature, applied potential and hydrogen generation on the passive behaviour of nickel were investigated in lithium bromide aqueous solution using different electrochemical techniques: open circuit potential (OCP), potentiodynamic and potentiostatic measurements, and electrochemical impedance spectroscopy (EIS). From the polarization curves, it is observed that localised corrosion resistance decreases with temperature, the repassivation of nickel is more difficult at 75 °C and the hydrogen evolution reaction is favoured with an increase in temperature. Impedance results showed that the most suitable corrosion mechanism of nickel in LiBr solutions includes the double layer and the passive film formed on the nickel surface. The passive film of nickel partially disappears when a low cathodic potential is applied.     

Influence of pH on electrochemical properties of passive films formed on Alloy 690 in high temperature aqueous environments

Passive films formed on Alloy 690 in different pH solutions at high temperatures were studied by potentiodynamic polarization, Auger electron spectroscopy, thermodynamic diagrams and the Mott–Schottky relation. The chemical compositions and electronic structures of the passive films were found to be strongly pH-dependent. In alkaline solutions, a secondary passivation was clearly observed on potentiodynamic polarization curves. The passive films were a mixture of Cr₂O₃ and FeCr₂O₄ below the flat band potential of nickel oxide and were NiFe₂O₄ above this potential. Electronic structure models, describing the electrochemical properties of the passive films, are proposed and discussed.     

Kinetics of passive layer formation on Fe 20 Cr in comparison with Fe

The passivation of Fe 20 Cr alloys was investigated in borate buffer, pH 8.4, and compared with corresponding data for Fe. After the reduction of electropolished electrodes at ?0,7 V potentiostatic pulse measurements were carried out in dependence on the polarization potential ε_P and polarization time τ_P. During the polarization, the current density i(t), the charge q(t) and the capacity C(t) were measured. Subsequent potentiodynamic reduction curves yielded the cathodic reduction charge. The kinetic data of passivation show the exponential influence of ε_P and the film thickness d_Ox on the oxide growth. Hence, the characteristics of the high field mechanism of passivation are observed which are similar to oxide films on pure metals. The agreement with the direct or inverse logarithmic law of the oxide growth, however, is poor. Various diagrams which test both laws show unexpected deviations for thick layers. Taking into account these results and those of the surface analysis, which show initial layers IL enriched with chromium at low ε_P, mixed passive layers PL at intermediate ε_P, and chromium depletion of the transpassive layer TPL at high ε_P, the results can be explained by the processes I and II shown in Fig. 2. During anodisation at low potentials I yields a slow transformation of IL into a mixed oxide PL of several nm thickness. This first oxide growth I is much slower than on passive iron. At high potentials (ε_P > 1.3 V/HESS, process II) the Cr dissolution yields the Cr-depleted film TPL, which behaves similarly to passive iron in the transpassive region. In the cathodic process the preferential reduction of Fe-oxides dominates. Hence Cr-oxides are enriched again in the reduced layer RL which differs from IL. The electronic properties of the passive layer PL are similar to those of passive iron. At low energies, however, additional surface states, which are created by chromium, change the behaviour at potentials ε_P > 1.3 V/HESS.     

Impedance of NiSi electrode in sulfuric-acid solution in the region of passive and transpassive states

The behavior of an NiSi electrode in 0.5 M H₂SO₄ in the region of passive and transpassive state (from 0.50 to 2.1 V (versus SHE)) is studied. The conclusion is made about formation of oxide film close to SiO₂ by composition on the electrode surface in the passivation process. Equivalent electric circuit modeling passive and transpassive state of nickel silicide is proposed. Thickness of oxide film and its specific resistance depending on electrode potential are calculated basing on the impedance data.     

Laser Raman microscopic studies of passive films formed on type 316LN stainless steels during pitting in chloride solution

The surface films formed on type 316LN stainless steels (SS) with different nitrogen contents, during potentiodynamic polarization in acidified 1 M NaCl solution, were characterized by Laser Raman Spectroscopy (LRS). LRS confirmed the presence of oxides and oxychlorides of iron and chromium, hydrated chlorides and nitrates in the film. Raman mapping showed increasing nitrate content in the film with increasing nitrogen content. The film on the uncorroded material showed the presence of chromium and molybdenum oxides. The improvement in pitting corrosion resistance of type 316LN SS with increasing nitrogen content was attributed to increased amount of nitrates in the passive film.     

Direct modification of a passivated iron electrode with alkyltriethoxysilanes for preventing passive film breakdown in a borate buffer containing 0.1 M of chloride ion

A passive film on an iron electrode was modified with alkyltriethoxysilanes directly. In order to examine the protective ability of the modified passive film against breakdown, the pitting potential, E_pit was measured by anodic polarization of the modified electrode in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻. The value of E_pit for the modified electrode shifted in the positive direction from that of the unmodified electrode, indicating prevention of passive film breakdown. The modified passive film was not broken down in the passive and transpassive regions of polarization curve in some cases. However, many current spikes appeared in the all curves of the modified electrodes. The modified surface of passivated electrode was characterized by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement. There were defects and clusters of associated water within the modified film and hence, Cl⁻ could permeate through the defects, leading to appearance of current spikes and occurrence of breakdown.     

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.     

Stress corrosion cracking of alloy 600 in an aqueous solution containing lead oxide

The stress corrosion cracking (SCC) behavior of Alloy 600 was studied in aqueous solutions containing lead. Electrochemical polarization and current transient experiments were performed at 315 °C in a 40% sodium hydroxide solution to assess the effect of lead on a passive film formed on Alloy 600. The influences of the alloy microstructure and the addition of an inhibitor to the environments on lead-induced SCC were investigated using C-ring and slow strain-rate tensile (SSRT) tests in demineralized high-purity water and caustic solutions containing PbO at 315 °C. The surface films on Alloy 600 were examined using Auger electron spectroscopy (AES) and energy dispersive x-ray spectroscopy (EDXS). The PbO markedly accelerated SCC of Alloy 600 in the caustic solution and the high-purity water at 315 °C. The addition of nickel boride (NiB) or cerium boride (CeB₆) to the test solutions decreased the susceptibility of Alloy 600 to SCC. Thermally treated Alloy 600 (Alloy 600 TT) tended to crack in a transgranular mode, while the solution-annealed Alloy 600 (Alloy 600 SA) tended to crack in an intergranular mode in water containing PbO.