ESCA studies of Ni-Cr alloys

ESCA examination on Ni-Cr alloys has shown that a thin passive film was formed after 24 h immersion in 0.1 M NaCl. The film contained only chromium oxide in the form of Cr₂O₃. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate of the alloys. Both Ni-10 wt. % Cr and Ni-20 wt. % Cr alloys showed a slightly higher corrosion rate than the Ni-40 wt. % Cr alloy.The present ESCA study of the Ni-Cr system is part of our programme which involves an examination of the four binary alloy systems Fe-Si, Cr-Co, Ni-Cr, and Mo-Ni [1]. The aim is to correlate the structure and composition of the passive films formed in 0.1 M NaCl to the corrosion behaviour in the same solution.     

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.     

Ageing of plasma-mediated coatings with embedded silver nanoparticles on stainless steel: An XPS and ToF-SIMS investigation

Nanocomposite thin films (∼170 nm), composed of silver nanoparticles enclosed in an organosilicon matrix, were deposited onto stainless steel, with the aim of preventing biofilm formation. The film deposition was carried out under cold plasma conditions, combining radiofrequency (RF) glow discharge fed with argon and hexamethyldisiloxane and simultaneous silver sputtering. XPS and ToF-SIMS were used to characterize Ag-organosilicon films in native form and after ageing in saline solution (NaCl 0.15 M), in order to further correlate their lifetime with their anti-fouling properties. Two coatings with significantly different silver contents (7.5% and 20.3%) were tested. Surface analysis confirmed the presence of metallic silver in the pristine coating and revealed significant modifications after immersion in the saline solution. Two different ageing mechanisms were observed, depending on the initial silver concentration in the film. For the sample exhibiting the low silver content (7.5%), the metal amount decreased at the surface in contact with the solution, due to the release of silver from the coating. As a result, after a 2-day exposure, silver nanoparticles located at the extreme surface were entirely released, whereas silver is still present in the inner part of the film. The coating thickness was not modified during ageing. In contrast, for the high silver content film (20.3%), the thickness decreased with immersion time, due to significant silver release and matrix erosion, assigned to a percolation-like effect. However, after 18 days of immersion, the delamination process stopped and a thin strongly bounded layer remained on the stainless steel surface.     

Improvement of corrosion resistance of NiTi sputtered thin films by anodization

Anodization of sputtered NiTi thin films has been studied in 1 M acetic acid at 23 °C for different voltages from 2 to 10 V. The morphology and cross-sectional structures of the untreated and anodized surfaces were investigated by field emission scanning electron microscopy (FE-SEM). The results show that increasing anodization voltage leads to film surface roughening and unevenness. It can be seen that the thickness of the anodized layer formed on the NiTi surface is in the nanometer range. The corrosion resistance of anodized thin films was studied by potentiodynamic scan (PDS) and impedance spectroscopy (EIS) techniques in Hank's solution at 310 K (37 °C). It was shown that the corrosion resistance of the anodized film surface improved with increasing voltage to 6 V. Anodization of austenitic sputtered NiTi thin films has also been studied, in the same anodizing conditions, at 4 V. Comparison of anodized sputtered NiTi thin films with anodized austenitic shape memory films illustrate that the former are more corrosion resistant than the latter after 1 h immersion in Hank's solution, which is attributed to the higher grain boundary density to quickly form a stable and protective passive ?lm.     

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 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.     

Amorphous and crystalline IrO2 thin films as potential stimulation electrode coatings

Amorphous and crystalline iridium oxide thin films with potential use as coating materials for stimulation electrodes were studied. Characterization of these films by cyclic voltammetry and impedance spectroscopy has revealed a considerable decrease in impedance and an increase in charge capacity of iridium oxide thin films after an electrochemical activation process in 0.9% NaCl solution. The surface morphology of these films was studied by scanning electron microscopy. The two types of IrO₂ films were also compared under conditions relevant to applications as stimulation electrodes. The results indicate that amorphous IrO₂ films have significantly higher charge storage capacity and lower impedance than crystalline IrO₂ films. This makes the amorphous films a preferable coating material for stimulation applications.     

Preparation of superhydrophobic films on titanium as effective corrosion barriers

Stable superhydrophobic films were prepared on the electrochemical oxidized titania/titanium substrate by a simple immersion technique into a methanol solution of hydrolyzed 1H,1H,2H,2H-perfluorooctyltriethoxysilane [CF₃(CF₂)₅(CH₂)₂Si(OCH₂CH₃)₃, PTES] for 1 h at room temperature followed by a short annealing at 140 °C in air for 1 h. The surface morphologies and chemical composition of the film were characterized by means of water contact angle (CA), field emission scanning electron microscopy (FESEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The water contact angle on the surface of this film was measured to be as high as 160°. SEM images showed that the resulting surfaces exhibited special hierarchical structure. The special hierarchical structure along with the low surface energy leads to the high surface superhydrophobicity. The corrosion resistance ability and durance property of the superhydrophobic film in 3.5 wt.% NaCl solution was evaluated by the electrochemical impedance spectroscopy (EIS). The anticorrosion properties of the superhydrophobic film are compared to those of unmodified pure titanium and titania/titanium substrates. The results showed that the superhydrophobic film provides an effective corrosion resistant coating for the titanium metal even with immersion periods up to 90 d in the 3.5 wt.% NaCl solution, pointing to promising future applications.     

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.     

Effects of metal doping on the physical properties of ZnTe thin films

Zinc telluride (ZnTe) thin films were sublimated on a glass substrate using closed space sublimation (CSS) technique. ZnTe thin films of same thickness were tailored with copper (Cu) & silver (Ag) doping, considered for comparative study. X-ray diffraction (XRD) patterns of as-deposited ZnTe thin film and doped ZnTe samples exhibited polycrystalline behavior. The preferred orientation of (111) having cubic phase was observed. XRD patterns indicated that the crystallite size had increased after silver and copper immersion in as-deposited ZnTe thin films. Scanning electron microscopy (SEM) was used to observe the change of as-deposited and doped sample's grains sizes. EDX confirmed the presence of Cu and Ag in the ZnTe thin films after doping respectively. The optical studies showed the decreasing trend in energy band gap after Cu and Ag-doping. Transmission also decreased after doping. Resistivity of as-deposited ZnTe thin film was about 10⁶ Ω cm. The resistivity was reduced to 68.97 Ω cm after Cu immersion, and 10⁴ Ω cm after Ag immersion. Raman spectra were used to check the crystallinity of as-deposited, Cu and Ag-doped ZnTe thin film samples.     

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.     

Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques

Potentiostatically anodized oxide films on the surface of commercial pure titanium (cp-Ti) formed in sulfuric (0.5 M H₂SO₄) and in phosphoric (1.4 M H₃PO₄) acid solutions under variables anodizing voltages were investigated and compared with the native oxide film. Potentiodynamic polarization and electrochemical impedance spectroscopy, EIS, were used to predicate the different in corrosion behavior of the oxide film samples. Scanning electron microscope, SEM, and electron diffraction X-ray analysis, EDX, were used to investigate the difference in the morphology between different types of oxide films. The electrochemical characteristics were examined in phosphate saline buffer solution, PSB (pH 7.4) at 25 °C. Results have been shown that the nature of the native oxide film is thin and amorphous, while the process of anodization of Ti in both acid solutions plays an important role in changing the properties of passive oxide films. Significant increase in the corrosion resistance of the anodized surface film was recorded after 3 h of electrode immersion in PSB. On the other side, the coverage (θ) of film formed on cp-Ti was differed by changing the anodized acid solution. Impedance results showed that both the native film and anodized film formed on cp-Ti consist of two layers. The resistance of the anodized film has reached to the highest value by anodization of cp-Ti in H₃PO₄ and the inner layer in the anodized film formed in both acid solutions is also porous.     

Poly(o-anisidine) coatings on brass: Synthesis,characterization and corrosion protection

Poly(o-anisidine) (POA) coatings were synthesized on brass by electrochemical polymerization of o-anisidine in aqueous salicylate solution by using cyclic voltammetry. These coatings were characterized by cyclic voltammetry, UV–visible absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The corrosion protection aspects of POA coatings on brass in aqueous 3% NaCl solution were investigated by potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). The potentiodynamic polarization measurements show that the POA coating has ability to protect the brass against corrosion. The corrosion potential was about 0.204 V versus SCE more positive for the POA coated brass than that of uncoated brass and reduces the corrosion rate of brass almost by a factor of 800. The corrosion behavior of the POA coatings was also investigated by EIS through immersion tests performed in aqueous 3% NaCl solution. The evolution of the impedance parameters with the immersion time was studied and the results show that the POA acts as a protective coating on brass against corrosion in 3% NaCl solution. The water uptake and delamination area were also determined to further support the corrosion protection performance of the POA coating.     

Characterization of ceramic PVD thin films on AZ31 magnesium alloys

Ceramic thin films have been widely used to protect the metal substrate as coatings in the past years. In order to improve the poor corrosion resistance of AZ31 magnesium alloy, the study in this paper used the electron beam evaporation method to prepare ceramic PVD films on its surface with TiO₂ and Al₂O₃ as donors, respectively. Atomic force microscopy (AFM), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) were used to investigate the surface morphology, composition and microstructure of the thin films. Both films deposited on AZ31 took on compact top surface morphologies and grew as amorphous structures on substrate. AES test not only showed that films compositions deviated the standard stoichiometric ratios, but also found that element Mg diffused into films and existed as magnesium oxide in the TiO_x film as well as the AlO_x film. In the electrochemical corrosion test, the AlO_x coating on AZ31 exhibited the largest electrochemical impedance in a 3.5% NaCl solution. But it did not show better corrosion resistance than others for the poorer adhesion. Even if its thickness was small, the TiO_x coating on AZ31 exhibited the best corrosion resistance in this study. According to the observation and analysis, the damage of these films on AZ31 in aggressive solutions was mainly due to the existence of pores, microcracks, vacancies and poor adhesion between coating and substrate.     

Growth of sputter-deposited Ni-Ti thin films: Effect of a SiO2 buffer layer

In-situ X-ray diffraction (XRD) during the growth of Ni-Ti thin films was chosen in order to investigate their texture development using a deposition chamber installed at a synchrotron radiation beamline. Near-equiatomic films were co-sputtered from Ni-Ti and Ti targets. The texture evolution during deposition is clearly affected by the substrate type and the ion bombardment of the growing film. On naturally oxidized Si(100) substrates, the Ni-Ti B2 phase starts by stacking onto (h00) planes, and as the thickness increases evolves into a (110) fibre texture. For the deposition on thermally oxidized Si(100) substrates, this pronounced cross-over is only observed when a substrate bias voltage (-45 V) is applied. The oxide layer plays an important role on the development of the (100) orientation of the B2 phase during deposition on heated substrates (≈470 °C). If this layer is not thick enough (naturally oxidized Si substrate) or if a bias voltage is applied, a cross-over and further development of the (110) fibre texture is observed, which is considered as an orientation that minimizes surface energies. Electrical resistivity measurements showed different behaviour during phase transformation for the Ni-Ti film deposited on thermally oxidized Si without bias and those on thermally oxidized Si(100) with bias and on naturally oxidized Si(100) without bias. This is related to stresses resultant from the fact that the Ni-Ti films are attached to the substrates as well as with the existence of distinct textures. PACS 81.15.Cd; 61.10.Nz; 68.55.Jk     

Surface characteristics and electrochemical behaviour of sputter-deposited NiTi thin film

Thin nanocrystalline amorphous NiTi film was deposited on Si substrate using DC magnetron sputtering. The as-deposited NiTi thin film was crystallized by heat treatment at 500 °C for 1 h. The crystal structure, surface morphology, microstructure and surface chemistry of the deposited films were studied using X-ray diffraction, atomic force microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS), respectively. Corrosion behaviour was assessed in Ringer’s solution at 37 °C by open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy as a function of exposure time. OCP values indicate that the tendency for the formation of a spontaneous oxide film is greater for the NiTi thin films than the bulk NiTi. Long time exposure to Ringer’s solution was found to have a great effect on the corrosion behaviour of the samples. Significantly low corrosion current density was obtained for the annealed NiTi film from the potentiodynamic polarization curves indicating a typical passive behaviour, but as-deposited film and bulk NiTi alloy exhibited breakdown of passivity at potentials approximately +1.4 V (vs. SCE). XPS showed that the oxide film formed on the annealed NiTi thin film mainly composed of Ti oxides, and no evidence of Ni was found up to 8.2 nm beneath the top surface, suggesting the excellent corrosion resistance of this sample in Ringer’s 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.     

Investigation of heat-treatment and pre-treatment on microstructure and electrochemical properties of cerium nano-oxide films on AA7020-T6 by sol-gel methods

In this paper cerium nano-oxide films were applied on AA7020-T6 alloy by sol-gel method. Potentiodynamic polarization and EIS studies have been used to study the corrosion behavior of cerium oxide nano films in 3.5% NaCl. Microstructural and phase properties of cerium oxide were investigated by SEM and XRD. The results showed that heat-treatment temperature and pre-treatment have an important effect on microstructure and electrochemical properties of cerium nano-oxide films. It can be seen from the results that with increasing heat-treatment temperature from 150 to 300 °C, the corrosion resistance of the films increased. It is related to increase the condensation of the films with adding temperature. Also, it can be seen that with adding temperature from 300 to 400 °C, the corrosion resistance of the films decrease. This is an important case related to crystallization of the cerium oxide films between 300 and 400 °C which showed that crystallized ceria films illustrate less corrosion resistance with respect to an amorphous film. Although with applying cerium oxide films the corrosion resistance of the films increased but still the passive region of the ceria films was tiny. So that in this research especially pre-treatment (etching in NaOH solution for 1 min, washing with deionized water for 5 min, etching with acid solution which contained several acids (H₂SO₄, HF, HCl, H₃PO₄), washing with deionized water for 5 min and after that following the samples in boiling deionized water for 1 h) was applied on samples before ceria treatment. The results showed that after applying this pre-treatment the passive region of the films increased extremely. It is related to formation of the thick and porous alumina films after applying pre-treatment which are similar to millepore.     

The improvement of corrosion resistance of Ce conversion films on aluminum alloy by phosphate post-treatment

A phosphate post-treatment process for Ce conversion film on aluminum was studied. SEM (scanning electronic microscope), XPS (X-ray photoelectron spectroscopy) and electrochemical measurements were used to characterize the properties of the films. After the post-treatment the micro-cracks on the film surface obviously diminished, and corrosion resistance of the conversion film in NaCl solution increased. The conversion film, without post-treatment, was mainly composed of hydrated cerium oxides, and the dehydration of the film may cause cracking of the films. After phosphate treatment, stable cerium phosphate CePO₄ was formed on the surface, and the content of crystal water decreased greatly, leading to improvement of the film performance with less micro-cracks.