R. Di Maggio

The use of electrochemical techniques to study the corrosion behaviour of organic coatings on steel pretreated with sol–gel zirconia films

The two main features of a protective organic coating are its adhesion and corrosion protection. In order to improve both, chemical pretreatments have been used. The use of chromates was very popular, but recently they have been highly restricted because of their toxicity, so that chromate-free pretreatments have been developed and tested. An interesting alternative seems to be the deposition on the metallic surface of thin layers of zirconia by the sol–gel process. In this study thin films of amorphous zirconia on low carbon steel sheets have been obtained by the dip-coating technique, using two different complexing reagents. Control of the hydrolysis allowed the formation of ZrO2 films suitable as pretreatments, promoting the adhesion of organic coatings. The behaviour of these samples was compared with steel samples pretreated in conventional phosphatation baths. The adhesion of a polyester organic coating was evaluated by the pull-off technique, by measuring the detachment of cross-scratched samples after salt fog chamber testing, or by swelling the organic coating in methyl pyrrolidone. According to the results, the samples pretreated with zirconia layers showed promising performance, in comparison with commercial chemical treatments (tricationic phosphate and iron phosphate). The organic coating adhesion on zirconia films was found to depend strongly on the process parameters, e.g. concentration of the precursors solutions and chelating agents, which determine the thickness and the amount of organic residuals of the amorphous zirconia. Electrochemical impedance spectroscopy (EIS) was also used in sodium sulphate solutions to evaluate adhesion and the corrosion behaviour of these materials. No barrier properties of the zirconia films were observed. Resistance to delamination was studied by using samples where an artificial defect was made by mechanical tools of different diameter. The information obtained by EIS was in good agreement with the data obtained by salt fog chamber tests.

Dry and wet corrosion behaviour of AISI 304 stainless steel coated by sol-gel ZrO2CeO2 films

Abstract ZrO 2 CeO 2 coatings were deposited on 304 stainless steel substrates by dip coating in alkoxide solution. The coatings were transparent and continuous at thickness below about 700 nm. The films were used as barriers against oxidation for 304 stainless steel at high temperature. The best performance (no oxidation up to 16 h at 750 °C) was obtained with a coating approximately 1 μm thick. The corrosion resistance of the coated samples in an aggressive electrolyte (15 wt. % HCl solution) was studied by electrochemical impedance spectroscopy, a technique frequently used to test the protective properties of organic and metallic coatings on metals. The protective action of the deposited coatings was discussed as a function of the film thickness and quality.

Effect of the chemical modification of the precursor of ZrO2 films on the adhesion of organic coatings

ZrO2 films were deposited on low carbon steel by a sol-gel process as a chemical pretreatment before the application of a polyester paint. The films were obtained by a dip-coating technique using solutions of Zr(OBun)4 containing complexing agents (acetylacetone or acetic acid). These additives modified the alkoxide at the molecular level, so a new precursor was formed in solution. This new compound shows slower rates of hydrolysis and condensation than Zr(OBun)4, which allows the stability of solutions and the morphology of the gel to be controlled. Moreover, the length and the temperature of the thermal treatment influence the structure of the gel by reducing the amount of organic residues. The behavior of the films in promoting the adhesion of organic coatings was evaluated by measuring the detachment of the paint from a cross-scratch of samples which had been exposed for different times in a salt fog chamber. Tests using electrochemical impedance spectroscopy were also carried out. According to the results, most of the samples pretreated with zirconia layers showed a good performance, even better than commercial chemical treatments, when acetic acid was used as a modifier of the alkoxide precursors.

ZrO2-CeO2 films as protective coatings against dry and wet corrosion of metallic alloys

Abstract ZrO 2 -CeO 2 films were deposited on metals by sol-gel processing. The coatings were effective barriers against both corrosion in aggressive electrolytes and oxidation at high temperature. The protective action of the deposited coatings, was mainly dependent on their thickness and above all on the absence of any defects. Coatings withstood up to 80 h in acid environment and up to 18 h at 750°C in air. A strict control of the film thickness and quality could be performed by modifying the solutions of alkoxide precursors of ZrO 2 -CeO 2 used for the deposition. The corrosion behaviour in 0.1 M H 3 SO 4 and in 15 wt.% HCl was studied by electrochemical impedance spectroscopy. The electrochemical test on coated samples in different electrolytes, provided information about the nature and size of the defects with respect to the different preparation procedures. In order to evaluate the protective capacity of films independently of the substrate, superalloy sheets were coated by the same solutions and tested in the same acid environment, and the results were compared with those of 304 stainless steel-coated samples.

Characterization of modified thick thermal barrier coatings

In gas turbines and diesel engines, there is a demand for thick thermal barrier coatings (TTBCs) due to the increased process combustion temperatures. Unfortunately, the increased thickness of plasma-sprayed thermal barrier coatings (TBCs) normally leads to a reduced coating lifetime. For that reason, the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally on elastic modulus reduction of the thick coating to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot-corrosion resistance or better mechanical properties are needed. In this article, several modified zirconia-based TTBC structures with specific microstructural properties are discussed. Coating surface sealing procedures such as phosphate sealing, laser glazing, and sol-gel impregnation were studied as potential methods for increasing the hot-corrosion and erosion resistance of TTBCs. Some microstructural modifications also were made by introducing segmentation cracks into the coating structures by laser glazing and by using special spraying parameters. These last two methods were studied to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, a scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The effect of sealing procedures on the basic thermal and mechanical properties of the coatings was studied. In addition, some correlations between the coating properties and microstructures are also presented, and the advantages and drawbacks of each modification procedure are discussed.

Raman and X-ray diffraction study of boehmite gels and their transformation to α- or β-alumina

Abstract The structural evolution of gel-derived boehmite and alkali-doped boehmite samples, undergoing different heat treatments up to 1200°C, was followed by both X-ray diffraction and Raman scattering measurements, collected at room temperature. Our observations reveal the presence of different transition alumina phases at intermediate temperatures, before formation at the highest temperatures of stable α-alumina or β + β″-alumina, for alkali-loaded Al2O3. In particular, Raman data from samples treated at 400°C are consistent with a phase change from pseudo-boehmite to the γ-alumina structure, while the dehydroxylation process is not yet completed. The process appears independent of the presence of alkali: identical evolution was presented up to 800°C by all our samples. After final annealing at 1200°C the presence of alkali induces the formation of stable β + β″-alumina phases whereas, in the absence of alkali, a phase identified as Θ-Al2O3 is observed at 1000°C and, finally, stable α-Al2O3 is obtained at 1200°C.

XRD Microstructural Characterization of Tetragonal Pure Zirconia Powders Obtained by Controlled Hydrolysis of Zirconium Alkoxides

A new preparation procedure to obtain tetragonal pure zirconia powders is reported together with a detailed analysis of the profile of X-ray Diffraction (XRD) peaks. The crystallization kinetic up to 800°C is described through r.m.s. microstrain and crystallite size distributions. The results of two methods of profile analysis are compared. After thermal treatments up to 100°C the samples of amorphous gel prepared crystallize in the tetragonal structure. The monoclinic phase occurs only above this temperature. Moreover the tetragonal to monoclinic transformation has a strong effect in changing the shape of the distributions. Studying the crystallite size distributions we can infer a critical size of about 300 A for the tetragonal crystallites to transform. The shape of the mean crystallite of a fully tetragonal sample is also described.

Kinetics of phase separation and thermal behaviour of gel-derived Al2O3 doped by Cr2O3: an X-ray diffraction and fluorescence spectroscopy study

Alpha-alumina crystallization from boehmite obtained from a gel was studied as a function of heating temperature and residence time. X-ray analysis allowed the determination of phases and their crystallite size; fluorescence spectroscopy was also studied as a complementary identification technique. The results reveal the general behaviour of gel-derived Al203 phases and their transformation. The occurrence of an amorphous intermediate state was ruled out.

Characterization and photocatalytic activity of TiO2 by sol–gel in acid and basic environments

The sol–gel synthesis of water-based sols of nanocrystalline TiO2 in a large pH range (1.3–10.6) was carried out by acid hydrolysis of titanium(IV) isopropoxide with acetic acid or malonic acid acting also as complexing agents. Subsequent peptization was achieved in acid medium, in the case of acetic acid, and in basic medium (through triethylamine), in the case of malonic acid. The TiO2 particles were characterized using X-ray diffraction, Raman, high-resolution transmission electron microscopy and dynamic light scattering–electrophoretic light scattering. Methyl orange (MeO) and methylene blue (MB) were used as model contaminants to investigate the degradation activity of the different sols under UV irradiation. MB molecules adsorbed on the TiO2 surface of the basic sol were found as trimeric species. The basic TiO2 sol was effective for the degradation of both dyes, whereas the acid sols gave satisfactory results only for MeO.

Crystallization in J-1 polymer/carbon-fibre composites: bulk and interface processes

The process of crystallization, during isothermal treatments from the melt, of a composite of a J-1 polymer (a polyamide homopolymer produced by Du Pont) containing a single carbon fibre was studied. Two main crystalline morphologies develop in the polymer, depending on the temperature. This was directly observed during the treatment of the sample in a hot-stage chamber placed under a light microscope, and confirmed by both X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses. Moreover, a transcrystalline layer grows at the interface with the carbon fibre. The kinetics of bulk and interface crystallization were evaluated and compared by measuring, at different temperatures, the radii of crystals and the thickness of the transcrystalline layer with time. Moreover, bulk crystallization kinetics, measured by the depolarization of the light passing through the polymer sample during the isothermal treatment, indicated the apparent presence of two regimes of bulk crystallization.