مجله علوم و مهندسی خوردگی
سال پنجم شماره 1 (پیاپی 15، بهار 1394)

In this study, API 5L X65, which is widely used in gas pipelines was selected. Electrochemical potential and current noise tests have been carried out on this alloy, in a solution containing 0.5 % Na S and in the presence of CO2 gas. Moreover, the effect of Methyl Diethanol Amine (MDEA) addition into the test solution was studied. MDEA is used in the petroleum industry as a H2S and CO2 removal, as well as for controlling pH. The mentioned solution is a simulator solution for the sour gas fluid in the gas transmission pipelines in South Pars Gas Company. The XRD results revealed the formation of same FeS layer in the simulator solution and in the field study. Prior to electrochemical noise data analysis, DC trend removal had been carried out. The results showed X65 samples were reached to optimum condition of general and localized corrosion after addition of an optimum amount of MDEA into the simulator solution. The optimum amount of MDEA was 300 ppm, which results in optimized corrosion resistance.

In this study, effects of aluminum and chromium alloying elements on the electrochemical behavior of Hadfield austenitic manganese steel was investigated. To do this, first casting 3 blocks of Hadfield steel that first block was without any addition, the second block was containing 1.73% Cr and the third block was containing 1.68% Al. Then annealing heat treatment was applied to prepared blocks at temperature 110 ºC for 2 hours. The electrochemical behaviors of samples were evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy methods in the 3.5 wt.% NaCl solution. The results of electrochemical tests indicated that the sample containing aluminum; possess lower corrosion current, more noble corrosion potential and higher Corrosion resistance than the other samples. Because of aluminum addition, carbide particle volume and number of grain boundaries was reduced and this lead to galvanic cells formation inhibition.

Anodizing was carried out in sulphuric acid bath at various current densities for 45 minutes. Abrasion and Vickers microhardness tests were done. The abrasion weight loss and the Vickers microhardness were measured. Polarization tests were conducted in a 3.5% NaCl solution to assess the corrosion resistance of the coatings. Influence of anodic current density on microhardness, abrasion resistance, corrosion resistance and thickness of the oxide layer, have been investigated. The morphology of anodic oxide layer was examined by scanning electron microscopy (SEM). Results indicated that mechanical and corrosion properties of coatings are affected by using different anodic current densities. Increasing the anodic current density, results in more densed pores and improves the properties of the alloy.

In this study, synthesis of self-healing CeO2-chitosan (CeO2-CHIT) nanocomposite coatings on AA5083-H321 by dip immersion method was investigated. Corrosion and self-healing behavior of the coatings were studied by using electrochemical impedance spectroscopy (EIS) and salt spray. SEM, TEM, TGA and FTIR were used to characterize the resulting coatings. The result showed that upon increasing the cerium ion concentration in chitosan (CHIT) films from 1 to 5 mM, corrosion current density values significantly decreased. Also, the results revealed that the presence of CeO2 nanoparticles in CHIT coatings doped with Ce (III) ions (CHITCE) significantly enhanced the corrosion resistant of the coating. Thus, CeO2-chitosan nanocomposite coating doped with 5mM Ce(III) ions was selected as the optimum coating to study the self healing properties. This coating showed high impedance after 12hr of immersion in 3.5% NaCl. With further increase in immersion time from 12 to 24, 48 and 72 h, this trend continued to increase in corrosion resistance. This result showed that this coating exhibits ‘self-healing” type behavior.

In this study, the corrosive behavior of carbon steel C1010 was investigated in the solution of distilled water containing 30% by weight of mono ethylene glycol (MEG) and four different mixed inorganic inhibitors in their optimum concentration. Indeed, to increase the synergetic effect of inorganic inhibitors, four mixedinhibiting systems including nitrite-silicate, molybdate-phosphate, molybdate-nitrite and molybdate silicate were utilized. The electrochemical measurements were conducted by the polarization and electrochemical impedance spectroscopy (EIS) techniques at constant temperature of 70 °C after 1 and 60 days of immersion time. The obtained results from both the polarization and EIS techniques had a good agreement in the prediction of corrosion behavior. For all of the inhibiting systems, anticorrosive resistance of metal has been increased rather to the non-inhibiting system during the immersion time. In addition, the inhibitor of molybdate-nitrite showed the best anticorrosive function.

One of the most current heat resistant nickel alloys is 35Cr-45Ni alloy, which is used in Ethylene furnace reformer tubes at 1050°C. This alloy is operated simultaneously in high temperature, oxidation, and carburization and creep conditions. After long-term of service, carbides making alloying elements, not only causes the formation of hard carbides; but also causes growthing them. This microstructure has a low-toughness along with creep cavities and cracks which is formed during service; therefore the tube repair-welding will be impossible. In this study the role of solution annealing on mechanical properties of long-term used tubes was investigated.The samples of aged used tubes were annealed under various conditions. The temperatures was 1100°C and 1200°C during 2, 4, 6 and 8 hours. Then mechanical tests and structural studies were applied on annealed and aged tube. The structural examination revealed that after annealing, the continuous carbides lose their cohesion in grain boundaries and potential cracked parts disappeard. The result of mechanical test also showed that solution annealing at 1200°C during 6 or 8 hours increases the strength by 70% and the flexibility more than twice. Consequently, makes the alloy properties similar to the cast alloy properties.

Monel 400 is one of the most important nickel-based alloys which possesses high corrosion resistance, weldability and strength and is employed extensively in marine industries particularly in heat exchangers and boilers. It is generally used as tubes in water cooling systems especially where sea water is used as cooling water. In the current work, the causes of premature failure of a Monel heat exchanger tube of a water cooling system of an oil refinery in the south of Iran which is fed directly from Persian Gulf sea water were studied by means of different microscopical and analytical techniques. The results indicated the presence of sulfate and thiosulfate reducing bacteria as well as acid producing bacteria in the water entering the heat exchanger, in addition to formation of sulfide compounds on the internal surface of the tubes. Consequently, the main cause of failure of the tube was determined to be pitting corrosion which was dominantly initiated due to microorganisms in the sea water.