فهرست مطالب s.r. allahkaram

The purpose of this research is to investigate the effect of carbon black nanoparticles and graphene oxide on increasing the protective properties of epoxy coating on plain carbon steel. The results of the Kelvin scanning probe (SKP) test showed that in the sample containing carbon black nanoparticles and graphene oxide, exposure during three weeks In 100% humidity, the average surface voltage potential increased from -655 mV to -117 mV, while in the sample without the presence of nanoparticles, the surface voltage potential decreased from -329 mV to -506 mV in the same period of time. The presence of carbon black nanoparticles and graphene oxide in the coating prolongs the movement path of the electrolyte, including water, oxygen and corrosive ions, and delays the degradation of the epoxy coating.

In order to improve the corrosion resistance of polymer (ABS) components, electroplated nickel-chromium coatings and solutions were introduced to increase resistance to corrosion and remove surface micro-defects in chrome cathodic arc coatings during the coating process. Six solutions were examined: micro-etching with argon plasma, ion bombardment, increasing coating thickness, creating chromium nitride and titanium nitride multilayers, creating coatings with gradient bias voltage, and removing chromium layers during the plating process. Combination coatings of Cr-CrN-Cr and Ti-TiN-Cr with multilayer structures were used on electroplated polymer substrates for coating car handles. Surface treatments were applied in the common sublayer-coating interface. The corrosion behavior of the coatings was investigated using the Copper Accelerated Salt Spray (CASS) test method. The results of CASS showed that surface treatment improves the corrosion resistance of coatings by 50-60%. By eliminating droplets and inclusions on the surface using surface treatments and changing the columnar structure of the coating with bias voltage changes prevents the formation and growth of diffusion paths throughout the defects and prevents the formation of corrosion cells. Furthermore, by removing the electroplated chromium layer and applying coatings onto nickel electroplating, corrosion resistance was improved by up to 80%, providing a hopeful solution for improving the corrosion resistance of coated parts.

Corrosion of steel in concrete is the most important reason for the premature destruction of concrete structures. The use of corrosion inhibitors is a suitable way to control the corrosion of reinforcement in concrete. These chemicals are added in small amounts to the corrosive medium to reduce the rate of corrosion by affecting the rate of anodic, cathodic, or both reactions. In this paper, we investigate the corrosion inhibitory effects of disodium hydrogen phosphate, sodium molybdate, and sodium nitrite in simulated concrete pore solution in two concentrations of 1.8% and 3.5% NaCl. Hence according to the tests performed in simulator solution containing 1.8 wt.% NaCl salts, the nitrite, phosphate, and molybdate inhibitors added in 0.2, 0.0002, and 0.15 M, resulted in the efficiencies of 69, 59 and 81 percent, respectively. Also, in scanning electron microscopy micrographs, it was found that the use of these three inhibitors dramatically reduces the number and distance of surface pits compared to the control sample.

Electroless deposition of Ni-P coating on AZ31 magnesium alloy was carried out after forming conversion coating on the samples by immersing in saturated NaHCO3 solution. Effect of changing immersion time in NaHCO3 solution on surface morphology, chemical composition and phase structure of coatings were studied by SEM/EDS and XRD analyses. Hardness and corrosion resistance of the coatings were studied by microhardness test and polarization test in 3.5 wt.% NaCl solution. The results indicated significant effect of pre-treatment process on morphology and therefore corrosion resistance of Ni-P coated samples. By increasing conversion film formation time to optimum time, smooth and compact Ni-P morphology was obtained. Corrosion potential was nobler for Ni-P coatings in comparison with Magnesium alloy substrates and electroless Ni-P coating improved corrosion resistance of the substrate. In addition the microhardness of the Ni-P alloy coatings was examined and Ni-P coatings enhanced hardness, significantly. Increasing the conversion film formation time to a certain amount resulted in a better morphology of the Ni-P films with less porosity and thus better corrosion resistance. Any increase in conversion film formation time more than the optimum amount led to a lower corrosion resistance.

Pure cobalt coatings were electrodeposited on copper substrate by means of direct electric current in a chloride solution at different current densities in the range of 10-70 mA cm -2. The surface morphology and microstructure were investigated via X-ray diffraction analysis and scanning electron microscopy. Corrosion behavior of cobalt coatings was also studied in a 3.5 wt% NaCl solution using potentiodynamic polarization and impedance spectroscopy techniques. The results showed that corrosion resistance of deposits was strongly influenced by the coating’s morphology. Co deposit obtained in lower current densities exhibited the highest corrosion resistance, due to their lower grain boundaries and so the least density of active sites for preferential corrosion attacks.

Electroless Ni-P coatings have found a variety of applications due to their unique properties. For improvement of the electroless nickel-phosphorous binary coatings qualities, addition of a third element is being developed. Meanwhile, Cobalt as an effective element in improving the coatings characteristics such as corrosion and wear resistance, hardness, electromagnetic properties, etc., has attracted attentions of many researchers. In this study, applying the electroless Ni-Co-P alloy deposit on St37 substrate was considered and the achieved coating was heat treated at 400oC for 1 hour. Influence of bath parameters such as pH, concentration of cobalt sulphate and reducing agent on plating rate was investigated. Morphology and composition of the coatings were characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). Corrosion behavior of the binary and ternary alloy deposits were characterized using potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) confirming the effect of cobalt addition on improvement of the coatings corrosion resistance.

The effect of cobalt ion concentration of electroplating solution on morphology, phase structure and corrosion resistance of zinc-cobalt alloy electrodeposits that were obtained from Sulfate- acid solutions by direct current on St37 steel has been studied. SEM, EDS and XRD were used to study the surface morphology, chemical composition and phase structure of coatings. Respectively corrosion behavior of Zn-Co alloy coatings were studied by Tafel polarization experiment in 3.5 wt% NaCl solution. In addition the microhardness of the Zn-Co alloy coatings was examined. The results showed that increasing of cobalt ion content of electroplating solution, increases cobalt content as well as microhardnees of the coating. The cobalt ion in the plating bath strongly affects the chemical content and phase structure, as well as corrosion stability, of Zn-Co alloys. It was also noticed that corrosion resistance of the deposits were highly influenced by the composition and morphology of the coatings. Zn-Co alloy coatings containing 1.2 wt% Co showed the highest corrosion resistance due to its single phase.