ESCA studies of Cr-Co alloys

ESCA examination of films formed on Cr-Co alloys after immersion in 0.1M NaCl for 24 h has shown that the thickness of passive films decreased with an increase in chromium content. Surface films consisted of chromium and cobalt oxides as Cr₂O₃ and CoO. The amount of CoO in the surface film of the alloy was decreased with an increase in chromium but Cr₂O₃ was found at a greater depth in the passive film at any composition. Cr₂O₃ was a major component of the surface film when the chromium content in the alloy was 10% or higher. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate. Both Co-10 wt.% Cr and Co-30 wt.% Cr alloys investigated showed a lower corrosion rate than the Co-5 wt.% Cr alloy. Corrosion rate measured could be correlated to the surface film composition and structure as determined by ESCA.     

ESCA studies of Si-Fe alloys

ESCA examination of films formed on Si-Fe alloys after immersion in 0.1 M NaCl for 24 h has hown that the thickness of passive films decreased with an increase in silicon content. A thick passive film containing oxidized silicon and oxidized iron was formed on Fe-20 wt% Si and the oxidized iron was about three times higher than the oxidized silicon in the passive film. However, an obvious reduction in the oxidized iron in the film on Fe-30 wt % Si was observed. Oxidized iron was detected up to a depth of 1.0 nm and at a depth greater than 1.0 nm from the surface, the film was exclusively in oxidized silicon. The film was exclusively silicon oxide when the silicon content was increased to 50 wt %. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate. Fe-50 wt % Si was found to have a corrosion rate smaller than those lower silicon alloys. This relates to the surface film composition and structure as determined by ESCA.     

ESCA studies of Fe-Ti alloys

Fe-Ti alloys containing 5 to 47% Ti have been studied by ESCA. The alloys were exposed to 0.1 M NaCl for 24 h under open-circuit potential (OCP) during which passive films were formed. The passive film consisted of FeO and TiO₂ in the inner layer while Fe₂O₃, water, and hydroxyl groups were present in the outermost monolayers, irrespective of composition. The thickness of the passive layer was reduced from 4.4 nm to 1.0 nm with increasing Ti content. The amount of iron oxide in the passive layer also decreased with increasing Ti. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate of the alloys. Alloys with 5–28% Ti showed a relatively high corrosion rate but that with 47 wt.% Ti had a much lower corrosion rates.     

ESCA-studies of the surface film formed on stainless steels by exposure to 0.1 M NaCl solution at various controlled potentials

After immersion in a 0.1 M NaCl solution for 1 h at various controlled potentials, the surface films formed on two commercial stainless steels, 18–12 and 29-4-2, have been studied using a surface analysis technique ESCA (Electron Spectroscopy for Chemical Analysis) combined with ion etching. The influence of controlled potential in the lower region, between the OCP and the critical pitting potential, on the structure and chemical composition of the passive film is in limited agreement with the prior work. At higher controlled potentials above the pitting potential, the surface films consist of two components: the passive film and corrosion products. Changes to both oxidized chromium and metallic iron form major differences in the depth profiles in comparison with the previous results with the passive film. At higher controlled potentials oxidized alloying elements tend to the higher oxidation states on the outermost part of surface together with a possibility of enhanced adsorption of chloride ions. There is a great amount of oxidized molybdenum and chloride ions mainly from the corrosion products on the surface at higher applied potential. Their contents increase with the controlled potential.     

Effect of different alloyed layers on the high temperature oxidation behavior of newly developed Ti2AlNb-based alloys

The application of titanium aluminide orthorhombic alloys (O-phase alloys) as potential materials in aircraft and jet engines was limited by their poor oxidation resistance at high temperature. The Ti₂AlNb-based alloys were chromised (Cr), chromium-tungstened (Cr-W) and nickel-chromised (Ni-Cr) by the double glow plasma surface alloying process to improve their high temperature oxidation resistance. The discontinuous oxidative behavior of Cr, Cr-W and Ni-Cr alloyed layers on Ti₂AlNb-based alloy at 1093 K was explored in this study. After exposing at 1093 K, the TiO₂ layer was formed on the bare alloy and accompanied by the occurrence of crack, which promoted oxidation rate. The oxidation behavior of Ti₂AlNb-based alloys was improved by surface alloying due to the formation of protective Al₂O₃ scale or continuous and dense NiCr₂O₄ film. The Ni-Cr alloyed layer presented the best high-temperature oxidation resistance among three alloyed layers.     

Effect of Al on the electrochemical corrosion behaviour of Pb free Sn-8.5 Zn-0.5 Ag-XAl-0.5 Ga solder in 3.5% NaCl solution

The effect of Al on the electrochemical corrosion behaviour of Pb-free Sn-8.5 Zn-0.5 Ag-XAl-0.5 Ga solder in 3.5% NaCl solution was investigated by using potentiodynamic polarization techniques. The X content in the solder varied from 0.1 to 3 wt.%. Polarization studies revealed that an increase in Al content upto 1.5 wt.% decreased the corrosion current density (I_corr), corrosion rate of the solder and shifted the corrosion potential (E_corr) towards more noble values. However, higher content of Al, i.e. 3 (wt.%) in the five-element solder enhanced the corrosion rate and resulted in a significant increase in the E_corr towards more negative values. Passivation behaviour was noticed in all the solders having varying Al content, but the passive film formed at 1.5 wt.% Al was most stable due to its low passivation current density (i_p) and low critical current density (i_cc) value in comparison to the other solders. XPS and Auger depth profile results revealed that the passive film consisted of oxides/hydroxides of Al and Zn formed on the surface of the solder with Sn being formed in the subsequent layer. Considerable aluminium segregation occurred towards the surface principally as Al₂O₃/Al(OH)₃ with increase in Al content to 1.5 wt.% in the five element solder. The formation of Al₂O₃ seemed to prevent the oxidation of zinc on the surface of the solder.     

Effect of the chemistry and structure of the native oxide surface film on the corrosion properties of commercial AZ31 and AZ61 alloys

The purpose of this study has been to advance in knowledge of the chemical composition, structure and thickness of the thin native oxide film formed spontaneously in contact with the laboratory atmosphere on the surface of freshly polished commercial AZ31 and AZ61 alloys with a view to furthering the understanding of protection mechanisms. For comparative purposes, and to more fully describe the behaviour of the native oxide film, the external oxide films formed as a result of the manufacturing process (as-received condition) have been characterised. The technique applied in this research to study the thin oxide films (thickness of just a few nanometres) present on the surface of the alloys has basically been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Corrosion properties of the alloys were studied in 0.6 M NaCl by measuring charge transfer resistance values, which are deduced from EIS (electrochemical impedance spectroscopy) measurements after 1 h of exposure. Alloy AZ61 generally showed better corrosion resistance than AZ31, and the freshly polished alloys showed better corrosion resistance than the alloys in as-received condition. This is attributed to a combination of (1) higher thickness of the native oxide film on the AZ61 alloy and (2) greater uniformity of the oxide film in the polished condition. The formation of an additional oxide layer composed by a mixture of spinel (MgAl₂O₄) and MgO seems to diminish the protective properties of the passive layer on the surface of the alloys in as-received condition.     

Electrochemical and surface behavior of hydyroxyapatite/Ti film on nanotubular Ti-35Nb-xZr alloys

In this paper, we investigated the electrochemical and surface behavior of hydroxyapatite (HA)/Ti films on the nanotubular Ti-35Nb-xZr alloy. The Ti-35Nb-xZr ternary alloys with 3-10 wt.% Zr content were made by an arc melting method. The nanotubular oxide layers were developed on the Ti-35Nb-xZr alloys by an anodic oxidation method in 1 M H₃PO₄ electrolyte containing 0.8 wt% NaF at room temperature. The HA/Ti composite films on the nanotubular oxide surfaces were deposited by a magnetron sputtering method. Their surface characteristics were analyzed by field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS) and an X-ray diffractometer (XRD). The corrosion behavior of the specimens was examined through potentiodynamic and AC impedance tests in 0.9% NaCl solution. From the results, the Ti-35Nb-xZr alloys showed a solely β phase microstructure that resulted from the addition of Zr. The nanotubular structure formed with a diameter of about 200 nm, and the HA/Ti thin film was deposited on the nanotubular structure. The HA/Ti thin film-coated nanotubular Ti-35Nb-xZr alloys showed good corrosion resistance in 0.9% NaCl solution.     

ESCA studies of nitrogen-containing stainless steels

ESCA examination of films formed on nitrogen-containing stainless steels after immersion in 0.1 M NaCl revealed that nitrogen was enriched in the outer 0.5 nm of the film at a concentration a few times higher than in the substrate. The N(1s) binding energy varied from 399.6–399.7 eV at the outermost surface of the film to 398.3–398.8 eV in the film at a depth of about 0.5 nm. This indicates a change in the chemical state of nitrogen upon exposure of the nitrogen-containing stainless steels to the solution, and furthermore, that the surface nitrogen enrichment could be of significance in the improved pitting resistance.     

Corrosion of stainless steels in chloride solution: An XPS investigation of passive films

Five commercial steels ranging from the martensitic stainless steel containing 12% chromium to the superferrite containing 29% chromium, 4% molybdenum, and 2% nickel have been studied by XPS. In addition, a pure iron-chromium alloy containing 7% chromium has been investigated. Armco iron and pure chromium (99.99%) were included as references. The formation of the passive films (or corrosion) occurred in deoxygenated 0.1 M NaCl solution (pH=5.6), from which the samples were transferred directly to the XPS chamber under controlled atmosphere (Ar). Concentration profiles (at.-%) of the alloy constituents in their oxidized and metallic states have been determined separately from the measured XPS depth profiles. Forc= 12% chromium the passive films have the following structure: there is a depletion of Cr in the inner region, followed by an enrichment (concentration maximum) in the central region of the films. The height of this maximum increases, and its position shifts towards the surface with increasing chromium content in the alloy. The outermost monolayers are rich in water and hydroxyl groups. Various significant properties of the films change drastically at the critical chromium concentration of about 12%. This behaviour is rather independent of the other components (Mo, Ni, Cu) present in the alloys and is discussed in terms of a phase transition in the films which is controlled by the chromium concentration.     

ESCA-studies of the structure and composition of the passive film formed on stainless steels by various immersion times in 0.1 M NaCl solution

This report deals with the structure of the passive film formed on stainless steels during immersion in a 0.1 M NaCl solution for various immersion times. The film was examined using ion etching and Electron Spectroscopy for Chemical Analysis (ESCA).Through the chemical-composition profiles a three-factor model was developed to describe the structure of the passive films: a hydrated layer in contact with the solution, an oxide layer consisting of iron and chromium oxides having maxima at depths of 3 and 10 Å, respectively, and a metallic layer enriched in nickel. There is a smooth transition between the layers, with the thickness of the outer two layers being about 15 A. The maximum concentration of iron in the oxidized state decreases with increasing immersion time concomitant with an increase in both the maximum concentration of chromium in the oxidized state and the maximum concentration of nickel in the metallic state.It is found that air-formed films have similar structures to films formed by exposure to the solution. Longer air exposure thickens the air-formed film, with the maximum of both oxidized iron and chromium nearly equal in magnitude in comparison with the solution formed films. Films formed by short-time exposure only to the solution are somewhat thinner indicating that the total history is important in determining the structure of these passive films, whereas passive films formed by exposures of 15 h to the solution are no longer strongly dependent on prior air exposure.     

Study of the corrosion products formed on a multiphase CuAlBe alloy in a sodium chloride solution by micro-Raman and in situ AFM measurements

The corrosion products formed on a multiphase Cu-11.40Al-0.55Be (wt.%) alloy in 3.5% NaCl at open circuit potential, and their evolution with immersion time were studied mainly by micro-Raman and in situ AFM measurements. The aluminium content of each phase affects the formation of the corrosion products on them. After 1 day of immersion, γ₂ precipitates were more susceptible to dealuminization, while α′ phase exhibited a high corrosion stability. The corrosion products evolved with immersion time, and CuCl₂ and a Cu₂O/CuO double layer film were the stable products formed on all the phases after long times.     

Influence of Mo on the Microstructure and Corrosion Behavior of Laser Cladding FeCoCrNi High-Entropy Alloy Coatings

FeCoCrNi and FeCoNiCrMo_0.2 high-entropy alloy powders were prepared by gas atomization. Two kinds of coatings were prepared on the surface of 304 stainless steel by laser cladding technology. The effect of Mo element on the microstructure of laser cladding FeCoCrNi coating and its corrosion behavior in 3.5 wt.% NaCl solution was investigated. Both FeCoCrNi and FeCoCrNiMo_0.2 powders exhibit a single-phase FCC structure. Due to the remelting and multiple heat treatments during the preparation of the laser cladding coating, a small amount of σ and μ phases appeared in the FeCoCrNiMo_0.2 coating. The microstructures of the two coatings from the bonding area to the top layer are planar, columnar and equiaxed grains, respectively. The addition of the Mo element causes the dendrite size in the middle region of the FeCoCrNiMo_0.2 coating increases significantly and exhibits obvious orientation characteristics. FeCoCrNiMo_0.2 coating has high corrosion potential (−0.01 V_SHE) and low current density (0.94 × 10⁻⁷ A/cm²) in 3.5 wt.% NaCl solution, showing excellent corrosion resistance. The passivation film formed on corroded the FeCoCrNiMo_0.2 coating contains high content of oxides of Cr and Mo. The addition of the Mo element enhances the compactness and pitting resistance of the passivation film.     

Effect of a small increase in the Ni content on the properties of a laser surface clad Fe-based alloy

Two Fe-based alloys with a small variation in the Ni content, Fe-15.2Cr-5.1Ni and Fe-15.7Cr-7.1Ni (wt.%), were fabricated on a martensitic stainless steel 1Cr13 substrate by laser surface cladding (LSC) using a CO₂ laser and Ar shielding gas that was blown into a molten pool. Both LSC alloys exhibited typical rapid directional solidification structures. However, 2 wt.% Ni increase led to ∼9% increase in the weight fraction of austenite, and ∼5% increase in the area proportion of interdendritic regions, which contained the higher Cr contents. These microstructural changes caused a great reduction in the microhardness and great improvements in the resistance to electrochemical corrosion in 0.5 M H₂SO₄ solution and high temperature oxidation in air at 900 °C. The reasons for these differences are discussed in detail.     

Studies on corrosion protection of Al2024 T6 alloy by electropolymerized polyaniline coating

Aqueous electropolymerized polymer coatings on aluminum alloys can replace the carcinogenic chromate coating. In this work, the corrosion protection ability of electropolymerized polyaniline coating on Al2024 T6 alloy has been found out. The polyaniline coatings on aluminum were formed by galvanostatic and potentiostatic methods from a bath of 0.5 M oxalic acid containing 0.1 M aniline. The corrosion protection ability of the coating was found out by Tafel polarization and EIS in 1% NaCl solution. It has been found that polyaniline coated aluminum alloy has exhibited higher corrosion activity due to the presence of pores in the coating. However, the corrosion resistant property of polyaniline coated aluminum has been found to be improved to 90% by post-treatment in cerium containing solution at 60 °C.     

Preparation of anti-corrosion films by microarc oxidation on an Al-Si alloy

Thick ceramic films over 140 μm were prepared on Al-7% Si alloy by ac microarc oxidation in a silicate electrolyte. The film growth kinetics was determined by an eddy current technique and film growth features in different stages were discussed. The microstructure and composition profiles for different thick films were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. Their phase components were determined by X-ray diffraction. The electrochemical corrosion behaviors of bare and coated alloys were evaluated using potentiodynamic polarization curves, and their corrosion morphologies were observed. In the initial stage of oxidation, the growth rate is slow with 0.48 μm/min due to the effect of Si element though the current density is rather high up to 33 A/dm². After the current density has decreased to a stable value of 11 A/dm², the film mainly grows towards the interior of alloy. The film with a three-layer structure consists of mullite, γ-Al₂O₃, α-Al₂O₃ and amorphous phases. By microarc discharge treatment, the corrosion current of the Al-Si alloy in NaCl solution was significantly reduced. However, a thicker film has to be fabricated in order to obtain high corrosion-resistant film of the Al-Si alloy. Microarc oxidation is an effective method to form an anti-corrosion protective film on Si-containing aluminum alloys.     

Electrochemical corrosion behavior,Vickers microhardness,and microstructure of Co–Cr and Ni–Cr dental alloys

Electrochemical corrosion behaviors, Vickers microhardness, microstructure, and electrical properties of Magnum H50 (Co=64.5%, Cr=29%, Mo=6.5% ) and Nikkeli–Kromi–Polttosekoitus (Ni=65.2%, Cr=22.5%, Mo=9.5%, X=2.8% Nb, Si, Fe, and Mn) dental alloys have been investigated. The corrosion potential for the Co_64.5Cr₂₉Mo_6.5 alloy in HCl was higher than that of the Ni_65.2Cr_22.5Mo_9.5X_2.8 alloy. The corrosion rate with 0.5 M HCl for the Ni_65.2Cr_22.5Mo_9.5X_2.8 alloy was measured as being high and the corrosion resistance as being small as compared with the values for the Co_64.5Cr₂₉Mo_6.5 alloy. Vickers hardness of the Co_64.5Cr₂₉Mo_6.5 alloy was higher than that of the Ni_65.2Cr_22.5Mo_9.5X_2.8 alloy. Also Vickers hardness values of the used alloys were decreased by increasing indentation load. The thermal conductivity and minimum shear stress values of the used alloys are calculated.     

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.     

Broad-beam laser cladding of Al-Cu alloy coating on AZ91HP magnesium alloy

The resistance to wear and corrosion of AZ91HP Mg alloy was improved by laser cladding Al-Cu alloy. It was found that the clad layer was characterized by AlCu₄ and Mg₁₇Al₁₂ grains embedded in a AlMg matrix. The bonding zone exhibited a white-light planar crystal band with thickness of 10-13 μm. The heat-affected zone formed a eutectic structure due to the Mg diffusion. The microhardness and wear resistance of the coating were improved due to the formation of the hard phases AlCu₄ and Mg₁₇Al₁₂. Owing to the formation of dense Al₂O₃ oxide film, the coating exhibited better corrosion resistance in 3.5 wt.% NaCl solution.     

Corrosion behavior and composition analysis of chromate passive film on electroless Ni-P coating

A passive film was formed on electroless Ni-P coating (ENPC) in a bath of K₂Cr₂O₇ 30 g/l. XPS and electrochemical methods were employed to analyze its chemical compositions and corrosion behaviors. The potentiodynamic polarization tests indicated the corrosion current of the passivated sample was 1/30 that of as-plated ENPC. The XPS analysis illustrated the film comprised Cr, Ni and O. The film thickness was evaluated to be a few nanometers according to the sputtering rate of Ar⁺ ion. High-resolution XPS spectra suggested that the detected Cr in film was in the form of trivalent compounds, Cr₂O₃ and Cr(OH)₃.