Air-formed Film Research Articles

Neutron Reflectivity in Corrosion Research on Metals.

Neutron reflectivity (NR) is potentially a powerful tool for characterizing chemical and morphological changes in thin films and at buried interfaces in corrosion science. While the scope of NR is limited by its inherent demands for low surface roughness and high sample planarity, these drawbacks are compensated for by the unique ability to detect light elements and distinguish between isotopes. Furthermore, the generally weak absorption of neutrons by matter allows the use of bulky sample environments and in situ experiments. In particular, the layer thickness range of 3-100 nm accessible by NR is appropriate for studying air-formed films and passive films, which are crucial for the ability of metallic materials to resist corrosion, as well as for investigating the interaction of metal surfaces with hydrogen and its compounds, e.g., water. Also, NR is suitable for studying early stages of oxide growth on metals at high temperature, including the transition from Cabrera-Mott-type films to Wagner-type growth. Here, we outline key characteristics of NR as applied to the study of corrosion of metals, exemplified by earlier work, and discuss perspectives for future work in the field. The aim of our work is to stimulate the application of the unique capabilities of NR to corrosion science.

An Experimental High-Throughput to High-Fidelity Study Towards Discovering Al–Cr Containing Corrosion-Resistant Compositionally Complex Alloys

Compositionally complex alloys hold the promise of simultaneously attaining superior combinations of properties, such as corrosion resistance, light-weighting, and strength. Achieving this goal is a challenge due in part to a large number of possible compositions and structures in the vast alloy design space. High-throughput methods offer a path forward, but a strong connection between the synthesis of an alloy of a given composition and structure with its properties has not been fully realized to date. Here, we present the rapid identification of corrosion-resistant alloys based on combinations of Al and Cr in a base Al–Co–Cr–Fe–Ni alloy. Previously unstudied alloy stoichiometries were identified using a combination of high-throughput experimental screening coupled with key metallurgical and electrochemical corrosion tests, identifying alloys with excellent passivation behavior. The alloy native oxide performance and its self-healing attributes were probed using rapid tests in deaerated 0.1-mol/L H2SO4. Importantly, a correlation was found between the electrochemical impedance modulus of the exposure-modified air-formed film and self-healing rate of the CCAs. Multi-element extended x-ray absorption fine structure analyses connected more ordered type chemical short-range order in the Ni–Al 1st nearest-neighbor shell to poorer corrosion resistance. This report underscores the utility of high-throughput exploration of compositionally complex alloys for the identification and rapid screening of a vast stoichiometric space.Graphical

Effect of air-formed film on corrosion behavior of magnesium-lithium alloys

It is recently suggested that air-formed film plays an important role in controlling corrosion resistance of Mg-Li alloys. However, the structure of the air-formed film and its effect on corrosion resistance of Mg-Li alloys has not been fully understood. Firstly, the air-formed films formed on α and β phases in a dual-phase LZ91 Mg-Li alloy after exposure to laboratory air for up to 48 h have been examined by SEM under the assistance of ultramicrotomy. Then, the effect of the air-formed film on surface potential and, consequently, corrosion/oxidation behavior of the alloy has been investigated. Finally, in order to exclude the influence from α phase, the structure of the air-formed film on β phase and its effect on corrosion/oxidation behavior of Mg-Li alloys have been studied based on a single-phase LA141 Mg-Li alloy. The results show that the air-formed film is thin and negligible on α phase but thick on β phase after prolonged exposure to laboratory air. The thick air-formed film on β phase has a multilayer structure with an inner layer consisting of MgO/Mg(OH)2 and outer layer consisting of Li2CO3, which greatly elevates the surface potential of β phase in air. Both LZ91 and LA141 Mg-Li alloys firstly undergo uniform corrosion and then filiform corrosion when immersed in NaCl solution and the pre-existed air-formed film on β-Li phase can retard the occurrence of filiform corrosion in the alloys.

Electrochemical evaluation of cr-mn austenitic stainless steel in aqueous sulphuric acid and influence of thiocyanate ions

PurposeLow nickel austenitic stainless steel (ASS) has attracted much attention worldwide because of its economical price. This study aims to investigate the effect of different corrosive environments on the corrosion behavior of chrome-manganese (Cr-Mn) ASS. The tests were carried out as a function of H2SO4 concentrations, temperature and addition of ammonium thiocyanate (NH4SCN) (0.01 M). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were used to study the corrosion behavior of Cr-Mn ASS. It was observed that with increasing H2SO4 concentration, temperature and with the addition of NH4SCN solution, icorr, icrit and ipassive values increased. EIS data show decreasing charge transfer resistance value with increasing concentration and temperature. Higher corrosion rate with increasing temperature and concentration of H2SO4 is related to the anions (SO42−), which is responsible for reducing the stability of passive films. With the presence of 0.01 M NH4SCN thiocyanate (SCN− anion), there is a higher dilution of the passive film resulting in a higher corrosion rate. Energy-dispersive spectroscopy (EDS) analysis reveals the adsorption of sulfur on the surface in NH4SCN containing a solution. The significant presence of counter ions and the adsorbed sulfur species on the steel surface play a vital role in corrosion behavior.Design/methodology/approachAll the experiments were performed on a 3 mm thick sheet of Cr-Mn ASS (202 ASS) in hot rolled condition. The samples were then annealed at 1,050°C for 1 h, followed by water quenching. For microstructural examination, they were electrochemically etched in 10 Wt.% oxalic acid solution at 1 amp for 90 s. A computer-controlled Potentiostat (Biologic VMP-300) was used. After the cell was set up, the working electrode (WE) was electrostatically cleaned at −1 V vs saturated calomel electrode (SCE) for 30 s to remove the air-formed film. Then, WE were allowed to attain stable open circuit potential (OCP) for 1 h, following by the EIS test and potentiodynamic polarization test. The polarization test was started from a cathodic potential (−1.2 V vs SCE) and continued up to an anodic potential (1.6 V vs SCE) a scan rate of 0.1667 mV/s. EIS experiment was conducted on the same instrument by using a sinusoidal AC signal of 10 mV in a frequency range of 1,000,000 to 0.01 Hz at OCP.FindingsPotentiodynamic polarization graph shows that with the increase in sulphuric acid concentration. Increasing temperature from 20°C to 80°C in 0.5 M H2SO4 solution increases the corrosion rate (icorr) of Cr-Mn ASS. On the addition of 0.01 M NH4SCN to the sulfuric acid solution (0.1, 0.5 and 1 M) the corrosion rate increases drastically almost four to five times. EDS and XRD analysis shows the presence of sulfur over the oxide film and preferential site for dissolution of Cr and Mn at the steel surface when NH4SCN is added to the sulfuric acid solution.Originality/valueA study on the corrosion behavior of Cr-Mn ASS is scanty according to the author’s knowledge. Therefore, the present study will investigate the corrosion behavior of Cr-Mn ASS on SO4−2 anions, temperature and the addition of SCN− ion in sulfuric acid.

Anodic Polarization Behavior of Zn in Aqueous MgCl2 Solutions

Zn-coated steels have been widely used in various engineering fields such as automobiles and construction since they have excellent corrosion resistance in mild atmospheric corrosion environments. In order to improve the corrosion resistance of the coated steels, Zn-alloy coatings containing Mg have been newly developed and they have shown superior corrosion resistance compared with Zn coatings. However, Mg should dissolve into aqueous solutions preferentially in the early stage of immersion because of one of the active metallic elements. Therefore, it is considered that better corrosion resistance of Zn-alloy coatings containing Mg is related to the presence of Mg2+ ions in corrosion media. However, the effect of Mg2+ on the improvement of corrosion resistance of Zn and Zn-alloy coatings is still unclear. In this study, the effect of Mg2+ on the anodic polarization behavior of Zn is investigated by polarization measurements. The sample used in this study was Zn plate (99.7%, Goodfellow) and it was ground down to JIS#2000 grit with SiC abrasive papers. Then the sample was degreased in EtOH ultrasonically and then dried in air. The test solutions was aqueous solutions of MgCl2 of various concentrations ranging from 0.05 M to 1 M. Some of the test solutions were deaerated by Ar bubbling at least for 20 h. Anodic polarization curves for Zn were measured at a sweep rate of 0.5 mV/s. Anodic polarization curves for Zn was measured both in aerated and deaerated MgCl2 solutions. For the measurements in deaerated solutions, cathodic polarization treatment for reduction of air-formed films on Zn was done in NaCl solutions without Mg2+ ions. It was found from the results of anodic polarization curves for Zn in aerated MgCl2 solutions that anodic dissolution was enhanced with increasing concentration of MgCl2 and that Tafel slopes of the curves were relatively small in the order of tens of mV/dec independent of MgCl2 concentration. On the other hand, anodic polarization behaviors of Zn in deaerated MgCl2 solution were almost the same with Tafel slopes of 40 mV/dec independent of MgCl2 concentration. These results suggests that surface film formed on Zn in aerated solutions is related to anodic polarization behavior in MgCl2 solutions and that Mg2+ ions improve anti-corrosion performance of the film.

Study of the air-formed oxide layer at the copper surface and its impact on the copper corrosion in an aggressive chloride medium

The electrochemical behaviour of pre-oxidized copper surfaces prepared by heating in air at 90 °C was studied to evaluate the impact of the air-formed oxide layer at the copper surface on the corrosion in an aggressive chloride medium. In a first part, the evolution of the chemical composition and the structure of the air-formed oxide layer with the heating time were investigated by cyclic voltammetry in an alkaline medium in which the different oxide species can be discriminated, X-ray photoelectrons spectroscopy experiments (XPS) and water contact angle measurements. It was demonstrated that short heating times produce Cu2O-rich films, while CuO-rich films were obtained after long heating-time periods. Furthermore, cyclic voltammetric data strongly support a stratification of the corrosion products in the mature oxide layer. In a second part, the protection efficiency of air-formed films, aged from 1 to 22 days, against the copper corrosion was study using a potentiodynamic technique and the electrochemical impedance spectroscopy (EIS) in an aerated 0.2 M NaCl aqueous electrolyte. Results demonstrate that the chemical composition of the air-formed oxide layer has an important impact in the overall corrosion process. Results are tentatively explained in terms of composition and structure of the oxide layer.

Synergistic effects of cavitation erosion and corrosion for nickel aluminium bronze with oxide film in 3.5% NaCl solution

Cavitation erosion-corrosion of nickel aluminium bronze with oxide films formed under two different conditions, oxide formed in air for 1 week and oxide formed after 3 months immersion in 3.5% NaCl solution, was conducted using a 19.5kHz indirect ultrasonic vibratory system at a peak-to-peak amplitude of 80 (±0.2) μm in 3.5% NaCl solution. The synergistic interactions between the cavitation erosion and corrosion were determined via experiment comprising of three different tests: pure cavitation erosion test in distilled water, pure in-situ electrochemical corrosion tests using open circuit potentials (OCP) and polarization scanning, and a combination of cavitation erosion-corrosion kept at OCP in 3.5% NaCl solution to understand the overall synergism existing between cavitation wear and corrosion. The results were then analysed and compared using mass loss analysis. Scanning electron microscopy was used to analyse microstructural characteristics of the cavitated sample surfaces, as well as the transverse-sections of the surface features. It was seen that samples with water formed film exhibited high synergy and was more susceptible to selective phase attack and phase boundary erosion in comparison to air-formed film.

Comparative study of the native oxide on 316L stainless steel by XPS and ToF-SIMS

The very thin native oxide film on stainless steel, of the order of 2 nm, is known to be readily modified by immersion in aqueous media. In this paper, X-ray photoelectron spectroscopy (XPS) and time of flight secondary ions mass spectrometry are employed to investigate the nature of the air-formed film and modification after water emersion. The film is described in terms of oxide, hydroxide, and water content. The preferential dissolution of iron is shown to occur on immersion. It is shown that a water absorbed layer and a hydroxide layer are present above the oxide-like passive film. The concentrations of water and hydroxide appear to be higher in the case of exposure to water. A secure method for the peak fitting of Fe2p and Cr2p XPS spectra of such films on their metallic substrates is described. The importance of XPS survey spectra is underlined and the feasibility of C60+ SIMS depth profiling of a thin oxide layer is shown.

ChemInform Abstract: The Corrosion Behavior of Sputter-Deposited Amorphous Chromium- Zirconium Alloys in 6 M HCl Solution.

Abstract Amorphous binary CrZr alloys containing 11–66 at% Zr were successfully prepared by a magnetron sputtering method. Amorphous CrZr alloys thus prepared were spontaneously passive in 6 M HCl solution. Their corrosion rates were four orders of magnitude lower than chromium metal and one-third of zirconium metal and decreased with alloy chromium content. XPS analysis revealed that airformed films and passive films on the amorphous CrZr alloys were composed of double oxyhydroxides of chromium and zirconium. This appears to be responsible for the high corrosion resistance of the amorphous CrZr alloys in comparison with their constituent elements; the formation of chromium-containing passive film is more protective than the film on zirconium metal, despite the fact that chromium metal dissolves actively in 6 M HCl solution.

ChemInform Abstract: An XPS Study of the Corrosion Behavior of Sputter-Deposited Amorphous Cr-Nb and Cr-Ta Alloys in 12 M HCl Solution.

Abstract Sputter-deposited amorphous Cr-Nb and Cr-Ta alloys possessing higher corrosion resistance than that of alloy constituents were spontaneously passivated in 12 M HCl solution. Their open circuit potentials were higher than those of alloy constituents in 12 M HCl solution, and the passive current densities of amorphous Cr-Nb and Cr-Ta alloys were lower than those of niobium and tantalum metals, respectively, and decreased with increasing chromium concentration. XPS analysis revealed that air-formed films and passive films on the amorphous Cr-Nb and Cr-Ta alloys were composed of double oxyhydroxides of chromium and niobium or tantalum. The higher corrosion resistance of the amorphous Cr-Nb and Cr-Ta alloys in comparison with their constituent elements is attributed not only to the uniformity and homogeneity of the amorphous alloys but also to the formation of the double oxyhydroxide film of chromium and niobium or tantalum, both of which act synergistically in improving the corrosion resistance, partly because the double oxyhydroxide films possess high activity for cathodic oxygen reduction with a consequent ennoblement of the open circuit potential.

X-ray photoelectron spectroscopy study of the passive films formed on thermally sprayed and wrought Inconel 625

There is a well known performance gap in corrosion resistance between thermally sprayed corrosion resistant coatings and the equivalent bulk materials. Interconnected porosity has an important and well known effect, however there are additional relevant microstructural effects. Previous work has shown that a compositional difference exists between the regions of resolidified and non-melted material that exist in the as-sprayed coatings. The resolidified regions are depleted in oxide forming elements due to formation of oxides during coating deposition. Formation of galvanic cells between these different regions is believed to decrease the corrosion resistance of the coating. In order to increase understanding of the details of this effect, this work uses X-ray photoelectron spectroscopy (XPS) to study the passive films formed on thermally sprayed coatings (HVOF) and bulk Inconel 625, a commercially available corrosion resistant Ni–Cr–Mo–Nb alloy. Passive films produced by potentiodynamic scanning to 400 mV in 0.5 M sulphuric acid were compared with air-formed films. The poorer corrosion performance of the thermally sprayed coatings was attributed to Ni(OH) 2, which forms a loose, non-adherent and therefore non-protective film. The good corrosion resistance of wrought Inconel 625 is due to formation of Cr, Mo and Nb oxides.

Passivation behavior of Type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF 6 salt

Passivation behavior of type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF 6 salt was studied using electrochemical polarization, X-ray photoelectron spectroscopy (XPS) and time of flight-secondary ion mass spectroscopy (ToF-SIMS). Cathodic polarization to 0 V vs. Li/Li + resulted in most but not complete reduction of the air-formed film from oxides to metal on the stainless steel, as confirmed by XPS. For complete elimination of the air-formed film, the surface of the stainless steel was scratched under anodic polarization conditions. At 3 V vs. Li/Li + where an anodic current peak appeared, only an indistinct layer was recognized on the newly scratched surface, according to ToF-SIMS analysis. Above 4 V vs. Li/Li +, substantial passive films were formed, which were composed of oxides and fluorides of iron and chromium. The origin of oxide was due to water contained in the non-aqueous alkyl carbonate solution, and that of fluorides were the result of the decomposition of electrolytic salt, LiPF 6, especially at higher potential. The resultant passive films were stable in the non-aqueous alkyl carbonate solution containing LiPF 6 salt.

Effects of Corrosion-Resistant Elements on the Corrosion Resistance of Amorphous Bulk Ni-Cr-Mo-Ta-Nb-4P Alloys in Concentrated Hydrochloric Acids

Rod-shaped amorphous bulk Ni-Cr-Mo-Ta-Nb-P alloys resistant to concentrated hydrochloric acids were prepared by copper-mold casting. Most of them were spontaneously passive and immune to corrosion in 6 and 12 M HCl. Corrosion resistance was improved remarkably when both chromium and molybdenum were added to the alloy with sufficient tantalum contents. Spontaneous passivation was due to presence of stable air-formed films, in which chromium was particularly concentrated in addition to enrichment of tantalum. The angle resolved XPS revealed that chromium and molybdenum were rich in the inner part of the film. The high corrosion resistance was interpreted in terms of the high stability of the air-formed oxide film consisting of the outer double oxide of chromium and tantalum with a high stability in concentrated hydrochloric acids and the inner molybdenum oxide acting as the diffusion barrier.

Passivity of polycrystalline NiMnGa alloys for magnetic shape memory applications

The corrosion and passivation behaviour of bulk polycrystalline martensite Ni 50Mn 30Ga 20 and austenite Ni 48Mn 30Ga 22 alloys was compared in electrolytes with different pH values. Linear anodic and cyclic potentiodynamic polarisation methods and anodic current transient measurements have been conducted for the alloys and their constituents to analyze free corrosion, anodic dissolution and passive layer formation processes. Electrochemically treated alloy surfaces were characterized with scanning electron microscopy (SEM) and angle-resolved x-ray photoelectron spectroscopy (XPS). The electrochemical response of both alloys is in principal similar and is dominated by the Ni oxidation. In acidic solutions (pH 0.5 and 5) a slightly higher reactivity is detectable for the martensitic alloy which is mainly attributed to enhanced dissolution processes at the multiple twin boundaries. In weakly acidic to strongly alkaline solutions (pH 5–11) both alloys exhibit a low corrosion rate and a stable anodic passivity. While air-formed films comprise NiOOH, Ga 2O 3 and MnO 2, passive films formed in near neutral media (pH 5–8.4) are composed of Ni(OH) 2, NiOOH and Ga 2O 3 in the outer region and of NiO, MnO 2 and MnO in the metal-near region.

Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys

X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to 98% relative humidity at 50 °C. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH) 2 and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH) 2. A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formed.

SnO2:F Coated Duplex Stainless Steel for PEM Fuel Cell Bipolar Plates

Duplex 2205 stainless steel was deposited with ˜0.6 µm thick SnO2:F coating; coated steel was characterized for PEMFC bipolar plate application. Compared with bare alloy, interfacial contact resistance (ICR) values of the coated 2205 steel are higher. SnO2:F coating adds its own resistance to the air-formed film on the steel. In a PEMFC anode environment, a current peak of ca. 25 µA/cm2 registered at ca. 30 min for coated 2205 steel. It stabilized at ca. 2.0~-1.0 µA/cm2. This peak is related to the complicated process of coating dissolution and oxide-layer formation. Anodic-cathodic current transfer occurred at ca. 200 min polarization. In a PEMFC cathode environment, current was stable immediately after polari¬zation. The stable current was ca. 0.5~2.0 µA/cm2 during the entire polarization period. AES depth profiles with tested samples and ICP analysis with the tested solutions confirmed the excellent corrosion resistance of the SnO2:F coated 2205 alloy in simulated PEMFC environments.

Spontaneous passivity of amorphous bulk Ni–Cr–Ta–Mo–Nb–P alloys in concentrated hydrochloric acids

Rod-shaped amorphous bulk Ni–Cr–Mo-22 at.%Ta-14 at.%Nb–P alloys resistant to concentrated hydrochloric acids were prepared by copper-mold casting. Alloys of amorphous single phase and mixture of nanocrystalline phases in the amorphous matrix were all spontaneously passive in 6 and 12 M HCl and were immune to corrosion in 6 M HCl, although the corrosion weight loss was detected for heterogeneous alloys in 12 M HCl. Spontaneous passivation is due to presence of stable air-formed films in which chromium was particularly concentrated in addition to enrichment of tantalum and niobium. The angle resolved X-ray photoelectron spectroscopy revealed that chromium and molybdenum are rich in the inner part of the film. The major molybdenum species is in the tetravalent state, although penta- and hexavalent state molybdenum is also included. The high corrosion resistance was interpreted in terms of the high stability of the outer triple oxyhydroxide, Cr1−x−yTaxNbyOz(OH)3+2x+2y−2z, and the effective diffusion barrier of the inner Mo4+ and Cr3+ oxide layer.

Anodic behavior of high nitrogen-bearing steels in PEMFC environments

High nitrogen-bearing stainless steels, AISI Type 201 and AL219, were investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) environments to assess the use of these materials in fuel cell bipolar plate applications. Both steels exhibit better corrosion behavior than 316L steel in the same environments. Type 201 steel shows similar but lower interfacial contact resistance (ICR) than 316L, while AL219 steel shows higher ICR than 316L. X-ray photoelectron spectroscopy (XPS) analysis shows that the air-formed films on Type 201 and AL219 are composed of iron oxides, chromium oxide, and manganese oxide. Iron oxides dominate the composition of the air-formed film, specially the outer layer. Chromium oxide dominates passive films. Surface film thicknesses were estimated. The results suggest that high nitrogen-bearing stainless steels are promising materials for PEMFC bipolar plates.

Corrosion prevention of iron with novel organic inhibitor of hydroxamic acid and UV irradiation

Corrosion prevention by self-assembled monolayers (SAM) of monomer and polymer inhibitor on iron covered with air-formed oxide films was investigated by cyclic voltammetry in borate buffer solution. Anti-corrosion efficiency of the SAM-coated Fe electrodes depends on UV irradiation duration on Fe electrodes prior to coating and inhibitor concentration to form SAM. The 1-h UV-irradiated Fe electrodes coated with SAM exhibits the most effective corrosion resistance despite the anti-corrosion efficiency of air-formed films on Fe was linearly increased with UV irradiation. The addition of monomer in polymer solution improves the stability and corrosion resistance of SAM.

SnO 2:F coated ferritic stainless steels for PEM fuel cell bipolar plates

Ferrite stainless steels (AISI441, AISI444, and AISI446) were successfully coated with 0.6 μm thick SnO 2:F by low-pressure chemical vapor deposition and investigated in simulated PEMFC environments. The results showed that a SnO 2:F coating enhanced the corrosion resistance of the alloys in PEMFC environments, though the substrate steel has a significant influence on the behavior of the coating. ICP results from the testing solutions indicated that fresh AISI441 had the highest dissolution rates in both environments, and coating with SnO 2:F significantly reduced the dissolution. Coating AISI444 also improved the corrosion resistance. Coating AISI446 steel further improved the already excellent corrosion resistance of this alloy. For coated steels, both potentiostatic polarizations and ICP results showed that the PEMFC cathode environment is much more corrosive than the anode one. More dissolved metallic ions were detected in solutions for PEMFC cathode environment than those in PEMFC anode environment. Sn 2+ was detected for the coated AISI441 and AISI444 steels but not for coated AISI446, indicating that the corrosion resistance of the substrate has a significant influence on the dissolution of the coating. After coating, the ICR values of the coated steels increased compared to those of the fresh steels. The SnO 2:F coating seems add an additional resistance to the native air-formed film on these stainless steels.