مجله علوم و مهندسی خوردگی
سال پنجم شماره 4 (پیاپی 18، زمستان 1394)

In this study, plates of 304 stainless steel and st37 steel were welded together by friction stir welding which has been set at a welding speed of 50 and 100 mm/min and tool rotational speed of 400, 600 and 800 rpm in order to study the effect of parameter variations on micro structure and corrosion resistance. Samples were cut from the cross section of the joint and metallographic imaging was performed after etching. For the corrosion resistance evaluation, samples were immersed in Ferric Chloride and acetic acid 2M and then were studied by macroscopic imaging and stereograms also Tafel polarization and electrochemical impedance spectroscopy (EIS) tests have been performed in 3.5% NaCl solution in order to verify weight loss results. The results showed that the corrosion rate of the St37 base metal is more than other parts of the joint. On the other hand, all parts of stainless steel 304 was galvanically protected. The best corrosion resistance among all the weld areas were determined for the weld at the tools rotational speed of 800 rpm due to minimum grain growth by heating.

In most oil and gas industry, the corrosion rate of metals is accelerated by the presence of sulfur containing ions such as H2S species. In this study, acidic sour water corrosion of API 5L Gr.B carbon steel in oxygen-free Na2SO4㖭 solution in the presence different concentrations of bisN-(3-nitrobenzyl)benzamide as corrosion inhibitor at temperature of 25˚C were investigated by electro impedance spectroscopy and potentiodynamic polarization technique. Potentiodynamic polarization curves showed that the inhibitor decreased the corrosion rate of API 5L carbon steel by postponing hydrogen evolution. The results showed that the inhibition efficiencies of carbon steel increased significantly with the increase of concentration of inhibitor. The inhibition efficiencies, calculated from impedance results, show the same trend as those obtained from polarization measurements.

Nickel- Silica (SiO2) nanocomposite films with various SiO2 contents (0, 2, 5 and 10 g/l) were prepared using a nickel watts plating bath. After deposition of nanocomposite coatings, the surface of coatings were treated by silane base composition by self-assemble method. The nano composite films were characterized by means of XRD, atomic force microscopy (AFM) and scanning electronic microscope. Their water repellencies were also measured. Corrosion performance of the coatings was examined using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results show that the Ni-SiO2 nanocomposite coating has corrosion resistance better than the Ni coating. Also, adding silica nanoparticles ends to increase the roughness and better water repellency. The corrosion current in the nano composite coating with 10 g/l SiO2 was 0.4 μA/cm2 and for pure nickel coating was 2 μA/cm2.

Investigation of Persian Gulf seawater corrosion effect on the carbon steels and welded pieces used in construction of metallic storage tanks is important from this point of view that this investigation may result in using this seawater to do hydro test of these tanks. If the results of this evaluation be positive, it will have lots of economic and environmental advantages. In order to achieve this target in this study, at first a 15 mm thickness plate of ASTM A283 Grade C low carbon steel was welded by E7018 electrode then quality controlled by radiography test and its microstructure evaluated by macro and micro etch tests and also hardness test was done for it. Furthermore chemical composition, pH, conductivity and SRB amount within the water has been measured too. Then corrosion rates for each one of these mentioned steel and electrode in Persian Gulf seawater were measured by electrochemical method of linear polarization resistance and also immersion test. Then corrosion product and corroded surface were analyzed by EDS and SEM methods. Results of polarization and immersion tests proved that E7018 electrode is anode compared to ASTM A283 Grade C steel and this result was confirmed by SEM images.

This paper reports on the failure analysis of tubes from a conventional shell tube type heat exchanger in one of the compression stages used to cool the compressed natural gas. The heat exchanger is a one pass type with a tube bundle with straight tubes. Both the tubes and shell being manufactured from carbon steel. The service fluid at the shell side is anti-freezing solution while it is compressed natural gas at the tube side. The cause of the failure was thoroughly investigated using visual examination, chemical analysis and metallographic studies using optical microscope while the surface of corroded material were studied using scanning electron microscope. Electrochemical studies were performed by linear polarization resistance technique, while X-ray diffraction, X-ray fluorescence and EDS analysis was conducted on residual corrosion products. The investigation revealed that corrosion had occurred on outer surface of the tubes in areas exposed to coolant fluid. The evidence indicated that, penetration of oxygen inside coolant fluid resulting in glycol oxidation and has led the production of acid which was the main cause of the corrosion. The presence of copper on the outer surface of tubes had accelerated the corrosion process by galvanic corrosion effect.

The corrosion inhibition of mild steel in 1 mol/l HCl N,N′-bis (2, 4 - dihydroxypropiophenone) - 2, 2 - dimethylpropandiimine has been investigated by using potentiodynamic polarization and electrochemical impedance spectroscopy. All electrochemical measurements suggested that this compound was a suitable corrosion inhibitor for mild steel and the inhibition efficiency increased with the increase in inhibitor concentration. In addition, charge transfer resistance increased and double layer capacitance decreased with increasing inhibitor concentrations. The effect of temperature on the corrosion behavior of mild steel with the addition of the Schiff base was studied. With increasing temperature, the inhibition efficiency decreased. Adsorption of this inhibitor follows the Langmuir adsorption isotherms. The values of the thermodynamic parameters were calculated by the corrosion currents at different temperatures and using the adsorption isotherm. The negative sign of ΔG indicated spontaneous adsorption of inhibitor on steel surface. The morphology of mild steel surface in the absence and presence of inhibitor was examined by scanning electron microscopy.

The purpose of this work is formation of the Zn-Ni alloy coating on the st12 low carbon steel specimens by plasma electrolyte saturation method in order to improve surface and corrosion properties. Electrolyte bath consisted of sulphate aqueous solution and magnesium sulphate by 35-65 ratios. Working specimens was connected to the high voltage source with direct current and and pulsed direct then was immersed in corresponding electrolyte. The morphology and composition of the coated surface was investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that NiZn8 alloy coating has been formed on the specimen’s surface with unique morphology. Results show Coatings generated by pulse current are thinner than Coatings generated by direct current and have more regular morphology. This phenomenon is due to the higher solubility and the absence of coating formation in the cutout time. The results of roughness showed coating of the pulsed, lower roughness compared to coating generated by direct current, and the pulse frequency of 100 to 1000 Hz with increasing roughness increases.Corrosion behavior of the coating was investigated by linear polarization tests in %3.5 Wt NaCl solution. The results showed that corrosion behavior improved due to surface composition changes. Because of the existence of magnesium in Ni-Zn Alloy coating, the coating potential was lower than pure zinc coating potential, therefore alloy coating protect steel surface through galvanic corrosion.