مجله علوم و مهندسی خوردگی
سال چهارم شماره 1 (پیاپی 11، بهار 1393)

Offshore structures’ complex and unique designs limit the performance of either sacrificial anode or impressed current cathodic protection. These obstacles lead to development of “Hybrid Cathodic Protection” which is devised to harness both methods benefits. In this study, the potential distribution between the anodes and the structures was determined by finding the solution to relevant systems of mathematical equations based on known boundary conditions. Firstly, BE (boundary element) method by using iso parametric linear quadrilateral elements on structure and Anodes, was used to define the systems of equations. Then, these equations were solved by a program written in FORTRAN language. Geometrical modeling, mesh generation, pre processing and post processing all were carried out by GID software. To facilitate data input, a user interface was developed in Delphi environment. A comparison between results of this program and a commercial FEM software and verification has been done. Finally, cathodic protection systems for some basic examples were designed by the proposed model and the results were cross checked with reality to show the reliability of the model.

In this research, compatibility of cold and hot applied tape with rusty coal tar gas pipelines was investigated by cathodic disbonding and adhesion tests in room temperature which reveals that using of cold applied tapes at weld joints and repairing region have not inherent problems, but they required to carful and correct surface preparations with using of suitable primer. Cathodic disbonding results presented better performance of cold applied tapes on weld joints by average cathodic disbonding radius of 23 millimeter, and better performance of hot applied tapes on damaged regions by average cathodic disbonding radius of 1.5 millimeter. Adhesion results like cathodic disbonding results were explanatory of better performance of cold applied tapes on weld joints by peeling speed of 2.33 mm/min and better adhesion of cold applied tapes on damaged regions by peeling speed of 1 mm/min.

Surface alloying of aluminum into AISI 316 austenitic stainless steel was prepared by electroplating nickel layer followed by a single-step high-activity aluminizing process. After aluminization, the surface microstructure of stainless steel consist of β-(Fe, Ni)Al as matrix with β-FeAl+ α-(Fe, Cr) precipitates. Aluminized coating on specimens with 10 and 20 μm primary thickness of nickel layer made up of two layers; a thin layer of β-NiAl and a β-(Fe, Ni)Al layer. In the case of the nickel layer with thickness of 50 μm, the coating showed three layers: β-NiAl and Ni3Al phase and layer of remained Ni was also determined. Oxidation test at 950ºC was carried out for 157 hours to evaluate the coatings. By increasing primary electroplated nickel to 100μm, the weight gain of aluminized specimens decreased from 2.95 to 1.44g/cm2. Surface morphology of oxide scale on Th 100 after oxidation for 160 h at 950°C showed the stable α-Al2O3 and it can be confirmed from the XRD results.

In this work, a vertical screw was used for applying shear stress to melted Al-Zn-Mn-Fe alloy under various times; and, the effete of this process (melt shearing) on microstructure and corrosion properties were investigated. Optical microscope and Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) were used for evaluating the microstructure changes. The effect of melt shearing process on the phase changes was investigated by XRD method. The experimental investigations showed that, melt shearing with a vertical screw has a significant effect on the microstructure. When the shearing time increases, the dendritic grains change to equiaxed grains and also the large grains alter to fine grains. EDS results depicted that, the samples which are solidified without shearing process, include some grain boundary concentrated by iron, while the iron concentration would be uniform in the sample with melt shearing process. With increasing the shearing time, the coarse and continuous intermetallics in the grain boundaries change in to discontinuous and fine intermetallics. In order to investigate the corrosion resistance of samples, the Electrochemical Impedance Spectroscopy (EIS) and cyclic polarization tests were used. The results depicted that, when the shearing time increases, due to uniform distribution of cathodic intermetallics and reduction of their size, the corrosion resistance in sea water increases considerably.

In this study the oxidation behavior of electroless Ni-B coatings on steel CK45 was evaluated using TGA at 600 °C. Samples after the plating process heat treated at 400 °C for an hour. The rate of temperature increase 5 °C/min and the duration of sample isotherm temperature at about 3 h and Oxygen was selected as the gas inputTGA. According to data sets and also SEM images of cross-sections of the coatings showed that coatings at about 9,000 and 12,000 seconds influenced by temperature and oxidation have been crack and the amount and slope of weight more drastically changed in diagram TGA. The amount of change in weight after testing was 14 mg and the percent oxidation of the coating after the test was about 4%. Due to the high hardness of the coating HV1001085, high internal stresses causing failure part of the interface between coating and substrate.

In this research, mechanism of HF effect and fluoride ions on corrosion behavior of aluminum alloy 6061-T6 in 30% nitric acid by used Electrochemical tests have been investigated by electrochemical test. Cyclic polarization and impedance electrochemical tests results showed that with increasing concentration of HF, pitting potential decreased and sensivity to pitting increased. SEM image showed that alloy surface been intensely corroded by formation of pits.The mechanism of localized corrosion have been determined by EDS-SEM. The results showed that Fluoride ions with creating fluoride layers on aluminum alloy 5000 series fortified corrosion resistance but formation of H2SiF6 acid that has intenstified the pitting corrosion of the 6061-T6 alloy surface.

In this research the corrosion resistance of a nanocomposite which was by optimum dispersion of nanoclay particles in the Epoxy-Polyamide matrix, was studied. Several important parameters such as mixing method, mixing time and temperature were examined by X-ray diffraction (XRD), FTIR and transmission electron microscopy (TEM). The optimum dispersion condition has obtained at 50-60 °C using ultrasonic mixing in a short time (1 hour). Finally, the corrosion resistance of nanocomposite was evaluated by using results of electrochemical impedance (EIS) method, salt spray and 100% relative humidity chambers. The EIS results showed that, the epoxy polyamide nanocomposite has good corrosion resistance in 270 days in comparison to epoxy polyamide varnish. The results obtained from this study provide good understanding of the relationship between the optimum dispersion conditions and corrosion control performance.