مجله علوم و مهندسی خوردگی
پیاپی 1 (پاییز 1390)

In this study, the electrochemical behavior of passive films anodically formed on 316L stainless steel in 0.05 M sulfuric acid has been examined using electrochemical impedance spectroscopy (EIS). For this purpose, passive films were formed potentiostatically at selected potentials for 1 h and then EIS measurements were done. For EIS measurements, an excitation voltage of 10 mV and an applied frequency ranging from 100 kHz to 10 mHz have been used. Results showed that the interfacial impedance and the polarization resistance (Rpol) initially increase with applied potential, within the low potential passive. However, at a sufficiently high potential passive (E > 0.5 V), the interfacial impedance and the polarization resistance decrease with increasing potential. Also, an electrical equivalent circuit based on the point defect model (PDM) was suitable to the actual passivation process. In this electrical equivalent circuit, the total impedance was taken to be the sum of impedance of steel/passive film interface, passive film and passive film/solution interface.

Oxidation & hot corrosion behavior of Ti-Al/Al2O3 composites produced by combustion synthesis method with different conditions of production (exposed to salt coating Na2S 4),was studied in air at 800°C. Electron microscopy and X-ray diffraction (XRD) tests were used to study the morphology and compositions of the corrosion products. The hot corrosion kinetics approximately follows a parabolic rate law. In the samples exposed to salt, holes and severe corrosion attack were observed. The oxide scales formed on the composites were predominantly rutile and Al2O3. Addition of more aluminum in the produced composites leads to a reduction in oxide scale thickness, i.e. the oxidation resistance of the oxide scale has generally improved.

This paper presents the works done to synthesize undoped and HCL doped polyaniline/clay nanocomposite (PAniCN) by chemical oxidative polymerization of aniline monomers in the presence of Cloisite 30B nano-clay powders. Scanning electron microscopy (SEM) was used to characterize the morphology and particle size of as-synthesized nanocomposites. FTIR analysis was used for characterizing and comparing the chemical structure of nanocomposites. The nanocomposites were added to a commercial zinc rich ethyl silicate primer to modify its barrier properties. Unmodified and modified primers were applied with the same dry film thickness on the carbon steel panels, separately. The corrosion protection performances of different primers were evaluated via determining open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) during immersing in 3.5% sodium chloride solution for a period of 90 days. Results revealed that although undoped modified primer showed the highest barrier properties, non-conducting nature of this nanocomposite affect onto the cathodic protection properties of modified primer. On the other hand, HCL doped nanocomposite modified primer showed good barrier properties and this conductive nanocomposite did not have negative effect onto the cathodic protection mechanism of the primer.

TiCx coatings are well known for a very high melting point, high hardness, good corrosion and wear resistance, so, these properties have made them of particular interest for a broadrange of application industries. In this research, nanostructured TiC was coated onto H11 hot-working steel at three different operating temperatures, such as 470, 490 and 510 (as a thermodynamic- kinetic parameter), by the pulsed DC plasma-assisted chemical vapor deposition (PACVD) method. The phase and structure properties and corrosion behavior of thin film were studied by X-Ray direction, X-ray photoelectron spectroscopy (XPS), raman spectroscopy, SEM, Polarization test and EIS. The results indicate that coating which was applied at 490 °C shows the formation of a homogeneous and uniform layer with particle size of about 7 nm and high corrosion resistance, and its corrosion behaviors possessed the positive open circuit potential (OCP) of about 10 mV vs. Standard calomel electrode (SCE) and lowest current density of about 0.43 (μA/Cm-2) in 3.5% NaCl solution.

In the present study nanocrystalline Co-W coating was deposited on copper substrate at different current densities. All coatings were single phase solid solution with constant grain size. These coatings showed spherical nodular structure. By increasing the current density, the number of nodules and microcracks increased. The coating produced at higher current density showed more corrosion resistance. This could be due to the lower exchange current density of water reduction in the present of oxygen on this coating. According to cyclic polarization in 3.5% NaCl solution, these coatings were resistant against pitting corrosion in the present of ionic chloride. It was seen that by increasing the current density, the tungsten content of coating decreased. This is probably due to the low diffusion of tungsten ion complexes. It was assumed that lower microhardness and wear resistance was related to lower tungsten content at higher current densities.

Nanocomposite coatings containing polyaniline (PANI) and different concentration of zinc oxide (ZnO) as nano-additives dispersions were synthesized and studied by FTIR and SEM. Epoxy resin was used as a binder and nanocomposite coatings were deposited on aluminum 5000 series by dip-coating method. To improve the adhesion of epoxy-polyaniline-ZnO coatings, organosilane film and electrochemical anodizing pretreatment methods were used. In addition, the anticorrosive performance of the PANI-epoxy coatings were investigated in 3.5 wt.% NaCl solution at 65˚C. The coating containing epoxy ZnO-PANI (0.5%) showed better corrosion resistance as compared to the other concentration of ZnO-PANI composites.

Nanocomposite coatings were formed by addition of nano-Fe2O3 to specially developed polyurethane coatings by in situ polymerization method were applied on mild steel substratein various concentrations of nanoparticles. In order to improve dispersion of iron oxide nanoparticles in polymer matrix and better chemical interactions between nanoparticles and polymer coating, the nanoparticles were surface modified with the Tetraethyloorthosilicate (TEOS) silane. Ensuring from Interaction of nano-particles with silane and suitability of surface modification was demonstrated using FTIR test. The effects of iron oxide nanoparticles addition on the electrochemical behavior and corrosion performance of coatings were studied using electrochemical impedance spectroscopy (EIS) and polarization tests in 3.5% NaCl solution. The uniformity of coatings and dispersion of Fe2O3 nanoparticles in the polymer coatings were investigated FESEM. The results demonstrated that addition of modified iron oxide nanoparticles surprisingly reduced the corrosion rate of the nanocomposite coatings with respect to neat coating. Besides, coatings with high concentrations of nano-Fe2O3 particles (0.6 wt. % Fe2O3) showed better corrosion performance. This was as a result of properties of iron oxide nanoparticles interdiction against corrosive agents, as well as better adhesion of the coating to the substrate.