مجله علوم و مهندسی خوردگی
سال دوم شماره 4 (پیاپی 6، زمستان 1391)

Zn-SiC nanocomposite coatings were prepared from a sulfate bath solution by direct current electrochemical deposition. In this method, applied current density and concentration of nanoparticles in the bath were investigated as key parameters. Morphological and corrosion properties of these coatings were investigated by FESEM and potentiodynamic polarization in 1M NaCl solution respectively. In case of current density, an optimum percent of codeposited SiC nanoparticles was obtained at 0.1 A/cm2 by 0.96 vol.%. This was the highest amount of codeposited SiC nanoparticles between all samples produced by direct current electrodeposition.

Development of inert anodes in the Hall–Heroult for aluminum production has been one of the major challenges in recent years. In the present study, the use of Nickel base alloy of Ni- 10Cu-11Fe-6Al-1.86Sn has been studied for using as an inert metal anode in aluminum production industry. Hence, in order to create a corrosion resistant surface oxide layer, the alloy got surface heat treatment. After surface heat treatment at 1000ºC for periods of 30, 70 and 100 hours on the alloy, the behavior of oxidized samples examined in molten cryolite at 930ºC for 70 hours. EDS and XRD studies showed that with increasing time of surface heat treatment, has increased thickness of the oxide layer formed on the samples respectively about 0.95 μm, 35.48 μm and 41.93 μm and after surface oxidation on the samples, in the oxide layers formed the corrosion resistant oxides of NiO, Al2O3, SnO2 and Fe2O3. So that the oxidized sample for 100 hours has the highest corrosion resistance in corrosive cryolite– alumina environment. Finally, the mechanism of corrosion occurred in the corroded samples were studied in the mentioned environment.

In this research, sheets of aluminum alloys 6082 and 5050 were joined to stainless steel 304 by friction stir welding. The corrosion behavior of parent metal, heat affected zone and boundary line of joints in each side of the sheets was studied in 3.5% NaCl media by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that maximum current density belonged to the galvanic cell of the boundary lines of the joints. For the aluminum sheets, the corrosion resistance of HAZ was lower than parent metals. On the other hand the corrosion resistance of stainless steel 304 was the same in all regions.

Stray current from DC Traction systems (metro) can affect buried metallic structures from corrosion point of view. In order to investigate these effects on cathodically protected gas distribution pipelines, the metro lines and the gas pipeline maps were superimposed on the urban map of city of Tehran in order to prepare a comprehensive locational map. 213 intersections and almost 100150 meters of parallel sections of pipelines were found with respect to the railway right of way, which were specified for field measurements. The potential measurements were carried out in specific time periods of the day and the data was collected for one hour at each test point. In order to thoroughly investigate the effects of dynamic stray current on the pipelines, 24-hour measurements pipe-to-soil potentials were carried out. The results showed that dynamic stray current from DC traction appears as potential fluctuations on pipeline applied cathodic potentials. The fluctuations faded away during the period of metro shut down systems between 11:30 pm to 4:30 am. These fluctuations can cause the pipeline potential to exit the safe potential zone and can result in corrosion of the zones, where the DC stray current leaves the pipeline and enters the soil. In this study, a maximum potential shift of 100 mv in the pipe-to-soil potential was considered as a commencement criterion for effective dynamic stray current that requires further investigations. Results showed that a potential fluctuation range between 30 mV to 3 V exists in measured points on pipelines, which can exceed the proposed criterion and can cause damage. Corrosion rates influenced by stray currents were also calculated using coupons at a specific region. Results showed that coupons connected to the pipeline had a corrosion rate of 4.4 mpy (with cathodic protection and under influence of stray currents) which was about 1.5 times more than that of free corrosion rate of 2.6 mpy (for unconnected coupons to the pipelines, i.e. without cathodic protection).

In this study the characteristics of ZnNi/SiC nanocomposite coatings electrodeposited by pulse and pulse reverse current were evaluated. Morphology, structure and composition of the coatings were studied by using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction Analysis. Electrochemical Polarization and Electrochemical Impedance Spectroscopy (EIS) were used to evaluate corrosion properties of different coatings. Final results showed that among nanocomposite coatings, the direct pulse current deposited sample with the duty cycle of 42.85% have better corrosion behavior, due to its more dense structure, less porosity and more Ni content, than other samples.

Ni-SiC composite coatings are widely used in industry (equipment high wear resistance coatings, corrosion resistant coatings, ship and boiler coatings,. .. ). Its unique properties in high wear resistance, corrosion resistance and tribological properties. Researchers tend to investigate this coating. This study covers Ni-SiC composite coating under pulsed current using a rotating disk electrode. Coating properties were investigated by changing the duty cycle and frequency of pulsed current in waat's bath. Percent of the particles in the coating changes by the frequency and duty cycle. Amount of nanoSiC particles inside the deposit reached its maximum value in frequency of 10 Hz and 80% as duty cycle. In this condition, amount of nanoSiC particles inside the deposit was calculated at 16.2% as volume percent. The size of SiC powders is50 nm in this state. Least roughness amount is 0.38 micrometer. It obtained at a frequency of 100 Hz and 50% as duty cycle. In nano composite coating with increasing of particle volume percent in coat the corrosion current density decreased from7.09μA/cm2to 0.015 μA/cm2. Also EIS resistant is increased from 1029 Ω to 7232 Ω. With increasing rotation speed of the electrode the morphology changed from composition of the pyramid-shaped and flower cabbage to flower cabbage mode completely.

In this research, the Ni-Mo-PCTFE nanocomposite coatings in the presence of various concentrations of PCTFE were prepared on copper sheets by electrodeposition from citrate bath. Effect of PCTFE concentration on the corrosion resistance was determined. Surface morphology and composition of the composite coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) measurements. Also, corrosion resistance of coatings was evaluated using open-circuit potential (EOCP) measurements, electrochemical impedance spectroscopy (EIS) and Tafel polarization techniques in 3.5 wt.% NaCl solution. The study reveals significant shift in corrosion potential towards the noble direction, decrease in corrosion current density and increase in charge transfer resistance with the incorporation of PCTFE particles in the Ni-Mo matrix. The highest value of charge transfer resistance was obtained for Ni-Mo-PCTFE coating deposited from solution with concentration of 8 g /L PCTFE.