Electrochemical behaviour of thermally sprayed Cr3C2-NiCr coatings in 0.5 M H2SO4 media

The electrochemical behaviour of coated Cr3C2-NiCr steel in aerated 0.5 M H2SO4 solution was studied by means of electrochemical a.c. and d.c. measurements. A complete structural characterization of the coated steel before and after electrochemical tests was also carried out to access the corrosion mechanism of coated steel, electrolyte penetration through the coating, and to confirm the results obtained using electrochemical techniques. Two types of Cr3C2-NiCr coatings produced by a high velocity oxy-fuel spraying system (HVOF) were studied. Differences between coated steels are related to the spraying parameters reflecting their behaviour against corrosion phenomena. The electrochemical behaviour of the coated steel was strongly influenced by porosity and the presence of microcracks in the coating. Once the electrolyte reaches the steel substrate, it corrodes in a galvanic manner resulting in coating detachment from the steel.

Comparative study of Cr3C2-NiCr coatings obtained by HVOF and hard chromium coatings

In the present work the corrosion resistance of micro-cracked hard chromium and Cr3C2-NiCr (HVOF) coatings applied on a steel substrate have been compared using open-circuit potential (E-OC) measurements, electrochemical impedance spectroscopy (EIS) and polarization curves. The coatings surfaces and cross-section were characterized before and after corrosion tests using optical microscopy (OM) and scanning electron microscopy (SEM). After 18 h of immersion, the open-circuit potential values were around -0.50 and -0.25V/(Ag vertical bar AgCl vertical bar KClsat) for hard chromium and Cr3C2- NiCr, respectively. The surface analysis done after 12 h of immersion showed iron on the hard chromium surface inside/near surface cracks, while iron was not detected on the Cr3C2-NiCr surface even after 18 h. For longer immersion time hard chromium was more degraded than thermal sprayed coating. For hard chromium coating a total resistance values between 50 and 80 k Omega cm(2) were measured and two well-defined time constants were observed, without significant change with the immersion time. For Cr3C2-NiCr coating the total impedance diminished from around 750 to 25 k Omega cm(2) as the immersion time increased from 17 up to 132 h and two overlapped time constants were also observed. Polarization curves recorded after 18 h of immersion showed a lower current and higher corrosion potential for Cr3C2-NiCr coating than other samples studied. (c) 2005 Elsevier Ltd. All rights reserved.

Electrochemical and structural characterization of heat-treated Cr3C2-NiCr coatings

The influence of heat-treatments on the electrochemical behavior of thermal spray Cr3C2-NiCr coatings prepared by high velocity oxygen fuel (HVOF) was studied in NaCl solution, at 25 degrees C, using open-circuit potential (E-OC) and electrochemical impedance spectroscopy (EIS) measurements. Coating characterization were performed before and after the heat-treatments and electrochemical tests by scanning electron microscopy, X-ray diffraction, and Auger electron spectroscopy. In addition to the changes in the original powder composition occurring during HVOF process, heat-treatment performed at 450 degrees C caused no significant changes in electrochemical response compared with untreated sample, and at 760 degrees C the main difference was the formation of a thin and defective layer of Cr2O3 at the coating surface, which increased the total impedance at the first day of immersion. Higher influence on the electrochemical was noted for samples treated at 880 degrees C, which also showed higher E-OC and total impedance, and lower corrosion current. This behavior was interpreted considering the formation of a chromium oxide layer on the coating surface, dissolution and decomposition of smaller carbide particles and their surface enrichment with Cr due to C diffusion and dissolution into the matrix, and possible Ni, Cr, and Fe diffusion to coating/substrate interface. (c) 2006 the Electrochemical Society.

WC-CoCr coatings sprayed by high velocity oxygen-fuel (HVOF) flame on AA7050 aluminum alloy: electrochemical behavior in 3.5% NaCl solution

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Comparison of crystallinity between natural hydroxyapatite and synthetic cp-Ti /HA coatings

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Influence of HVOF parameters on the corrosion and wear resistance of WC-Co coatings sprayed on AA7050 T7

Thermal spray WC-based coatings are widely used in the aircraft industry mainly for their resistance to wear, reworking and rebuilding operations and repair of worn components on landing gear, hydraulic cylinders, actuators, propeller hub assemblies, gas turbine engines, and so on. The aircraft industry is also trying to use thermal spray technology to replace electroplating coatings such as hard chromium. In the present work, WC-Co coatings were built up on an AA 7050 aluminum alloy using high velocity oxygen fuel (HVOF) technology and a liquid nitrogen cooling prototype system. The influence of the spray parameters (standard conditions, W19S, increasing the oxygen flux, W19H, and also increasing the carrier gas flux, W19F) on corrosion, friction, and abrasive wear resistance were also studied. The coatings were characterized using optical (OM) and scanning electron (SEM) microscopy, and X-ray diffraction (XRD). The friction and abrasive wear resistance of the coatings were studied using Rubber Wheel and Ball on Disk tests. The electrochemical studies were conducted using open-circuit potential (E(oc)) measurements and electrochemical impedance spectroscopy (EIS). Differences among coated samples were mainly related to the variation of the thermal spray parameters used during the spray process. No significant differences were observed in the wear resistance for the coatings studied, and all of them showed a wear rate around 10 times lower than that of the aluminum alloy. The results of mass loss and wear rate were interpreted considering different mechanisms. Comparing the different spray parameters, the oxygen flux (higher flame temperature) produced the sample which showed the highest corrosion resistance in aerated and unstirred 3.5% NaCl solution. Aluminum ions were detected on the surface almost immediately after the immersion of samples W19S and W19F in chloride solution, showing that the electrolyte reached the substrate and galvanic corrosion probably occurred. For sample W19H, aluminum ions were not detected even after 120 min of immersion in NaCl solution. (C) 2008 Elsevier B.V. All rights reserved.

Cr3C2-NiCr and WC-Ni thermal spray coatings as alternatives to hard chromium for erosion-corrosion resistance

Cr3C2-NiCr and WC-Ni coatings are widely used for wear applications at high and room temperature, respectively. Due to the high corrosion resistance of NiCr binder, Cr3C2-NiCr coatings are also used in corrosive environments. The application of WC-Ni coatings in corrosive media is 14 not recommended due to the poor corrosion resistance of the (pure Ni) metallic matrix. It is well known that the addition of Cr to the metallic binder improves the corrosion properties. Erosion-corrosion performance of thermal spray coatings is widely influenced by ceramic phase composition, the size of ceramic particles and also the composition of the metallic binder. In the present work, two types of HVOF thermal spray coatings (Cr3C2-NiCr and WC-Ni) obtained with different spray conditions were studied and compared with conventional micro-cracked hard chromium coatings. Both as-sprayed and polished samples were tested under two erosion-corrosion conditions with different erosivity. Tungsten carbide coatings showed better performance under the most erosive condition, while chromium carbide coatings were superior under less erosive conditions. Some of the tungsten carbide coatings and hard chromium showed similar erosion-corrosion behaviour under more and less erosive conditions. The erosion-corrosion and electrochemical results showed that surface polishing improved the erosion-corrosion properties of the thermally sprayed coatings. The corrosion behaviour of the different coatings has been compared using Electrochemical Impedance Spectroscopy (EIS) and polarization curves. Total material loss due to erosion-corrosion was determined by weight loss measurements. An estimation of the corrosion contribution to the total weight loss was also given. (c) 2007 Elsevier B.V. All rights reserved.

Highly corrosion resistant siloxane-polymethyl methacrylate hybrid coatings

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Corrosion and Wear Studies of Cr3C2NiCr-HVOF Coatings Sprayed on AA7050 T7 Under Cooling

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effects of synthesis and processing on supersaturated rare earth-doped nanometric SnO2 powders

This paper presents and discusses some of the results of the effects of processing on rare earth-doped nanosize SnO2. Several relevant factors that may influence the characteristics of the final product are studied. The influence of two preparation routes and two heat-treatment conditions on the incorporation of dopants is investigated. The route whereby a soluble salt is used as the dopant source is found to provide the highest degree of dopant incorporation, even under the least favorable heat-treatment conditions.

Electron scattering and effects of sources of light on photoconductivity of SnO2 coatings prepared by sol-gel

Electro-optical properties of sol-gel derived 2 mol% antimony or niobium doped tin dioxide films have been measured. The electron density has been calculated considering all the relevant scattering mechanisms and experimental conductivity data measured in the range -197 to 25 degrees C. The results support the hypothesis that both ionised impurity scattering and grain boundary scattering have comparable effects in the resistivity of coatings, for free electron density congruent to 5 x 10(18) cm(-3). We have measured variation of photoconductivity excitation with wavelength using xenon and deuterium lamp as light sources. Results show that the main band in the photoconductivity spectrum is dependent on the spectral light source emission, the excitation peak reaching 5 eV (deuterium lamp). This band is due to the recombination process involving oxygen species and photogenerated electron-hole pairs. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Synthesis and photoluminescence study of La1.8Eu0.2O3 coating on nanometric alpha-Al2O3

In this work the La1.8Eu0.2O3 coating on nanometric alpha-alumina, alpha-Al2O3@La1.8Eu0.2O3, was prepared for the first time by a soft chemical method. The powder was heat-treated at 100, 400, 800 and 1200 degrees C for 2 h. X-ray powder diffraction patterns (XRD), transmission electronic microscopy (TEM), emission and excitation spectra, as well as Eu3+, lifetime were used to characterize the material and to follow the changes in structure as the heating temperature increases. The Eu3+ luminescence data revealed the characteristic transitions D-5(0) --> F-7(J) (J = 0, 1 and 3) of Eu3+ at around 580, 591 and 613 nm, respectively, when the powders were excited by 393 nm. The red color of the samples changed to yellow when the powder was annealed at 1200 degrees C. The decrease in the (D-5(0) --> F-7(2))/(D-5(0) --> F-7(1)) ratio from around 5.0 for samples heated at lower temperatures to 3.1 for samples annealed at 1200 degrees C is consistent with a higher symmetry of the Eu3+ at higher temperature. The excitation spectra of the samples also confirms this change by the presence of a more intense and broad band at around 317 nm, instead of the presence of the characteristic peak at 393 mn, which corresponds to the F-7(0) --> L-5(6) transition of the Eu3+. The lifetimes of the D-5(0) --> F-7(2) transition of Eu3+ for the samples heat-treated at 100, 400, 800 and 1200 degrees C was evaluated as 0.57, 0.72, 0.43 and 0.31 ms, respectively. (C) 2006 Elsevier Ltd. All fights reserved.

X-ray reflectivity of zirconia based sol-gel coatings on borosilicate glasses

In this work the technique of X-ray reflectometry was applied to study zirconiumsulfate films deposited by sol-gel dip-coating process on a borosilicate glass surface. The influence of withdrawal speed and temperature of thermal treatment on the film structure are analyzed. The thermal evolution of the density and thickness of the film was compared with these properties measured for a monolithic xerogel by helium picnometry and thermomechanical analysis. The fitting of experimental curves by classical reflectivity model showed the presence of an additional layer at the top surface of the coating. Layer thickness increases with increase of withdrawal speed in agreement with the Landau-Levich model. The apparent and real densities are similar for coatings fired below 400 degrees C, which shows that the films are free of pores. The shrinkage during firing is anisotropic, occurring essentially perpendicular to the coating surface. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Characterization of tin oxide based sol-gel coatings on borosilicate glasses by X-ray reflectivity

The X-ray reflectivity technique was applied in the study of tin oxide films deposited by sol-gel dip-coating on borosilicate glasses. The influence of the withdrawal speed and temperature of thermal treatment on the film structure was analyzed. We have compared the thermal evolution of the density and the shrinkage of the films with these properties measured for the monolithic xerogel by helium picnometry and thermomechanical analysis. In agreement with the Landau-Levich model, the layer thickness increases by increasing the withdrawal speed. Nevertheless, it decreases with the increase of the thermal treatment temperature, due to the densification process. The values of apparent density are smaller than the skeletal density, which shows that the films are porous. The comparison between the film and the monolith indicates that shrinkage during firing is anisotropic, occurring essentially perpendicular to the coating surface.

Improvement of the corrosion resistance of polysiloxane hybrid coatings by cerium doping

Polysiloxane hybrid films were deposited on stainless steel by dip-coating using a sol prepared by hydrolytic co-polycondensation of tetraethoxysilane (TEOS) and 3-methacryloxy propyltrimethoxysilane (MPTS), followed by radical polymerization of methacrylic moieties. The TEOS/MPTS ratio was chosen equal to 2 and the Ce/Si ratio varied between 0.01 and 0.1. The effects of cerium concentration and valence (Ce(III) and Ce (IV)) on the structural features of polysiloxane films were studied by X-ray photoelectron spectroscopy (XPS) and (29)Si nuclear magnetic resonance (NMR). The corrosion protection of stainless steel by the hybrid coatings was investigated by XPS, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves, after immersion in saline and acid solutions. The NMR results have shown for Ce(IV) doped films a high degree of polycondensation of up to 89%. Electrochemical analysis has evidenced that hybrid films with the lowest Ce concentration act as an efficient diffusion barrier by increasing the corrosion resistance and reducing the current densities up to 3 orders of magnitude compared to bare stainless steel. The analysis of structural effects induced by Ce(III) and Ce(IV) species, performed by XPS, indicates that the improved corrosion protection of Ce(IV) doped films might be mainly related to the enhanced polymerization of siloxane groups. (C) 2010 Elsevier B.V. All rights reserved.