مجله علوم و مهندسی خوردگی
سال دوم شماره 3 (پیاپی 5، پاییز 1391)

In this study, polyurethane/zinc/clay nanocomposite containing nanomaterials percentenges 10wt% nanozinc and 0.5wt%, 1wt%, 1.5wt%, 2wt% nanoclay were analyzed. Electrochemical impedance spectroscopy, conductance and salt fog tests with structural measurements including Transmission Electron Microscopy, X-ray spectrometry and Atomic Force Microscopy were performed on the samples. The results of the microstructure measurements showed the appropriate dispersion of nanomaterials in the coatings. Also the size and type of nanomaterials and surface topology was determined using microscopic analysis. Electrochemical tests results showed that the addition of nanoclay improves corrosion resistance of the coatings and the best corrosion performance obtained for nonocomposite containing 2wt% nanoclay that this improvement may be due to increasing the barrier properties with nanoclay. The other hand, conductance of the coating decrease with nanoclay and decline of this decreasing path occurred after 2wt% nanoclay. Therefore the attainment of optimal corrosion resistant Nanocomposite coatings may be due to interface effect of two parameters: barrier effect and conductance effect.

In this paper, Corrosion behavior of stainless steel 304L was evaluated in used catalyst solution in Sulfiran plant in temperature range 35-45°C. The electrochemical techniques consisted of polarization curves and electrochemical impedance spectroscopy (EIS) were used in laboratory tests. It was found that the corrosion rate of S.S 304L alloy was predicted to be less than 5mpy. In the meantime, there is possibility of pitting corrosion on stainless steel 304L. The linear polarization resistance (LPR) and weight loss techniques were used for determination of corrosion rate in pilot test. In addition of surface analysis techniques such as XRD, XRF and SEM were applied in this research. The evaluations of metallurgical are carried too. Results of pilot tests confirmed founding’s of laboratory tests. Investigation of metallurgical aspect of stainless steel 304L corrosion showed some corrosion in the from of wide shallow pits. Wet H2S agent was main factor in corrosion induced damages because of sensitization of the investigated SS304L alloy. It seems the alloy has been sensitized during cooling down in solution annealing heat treatment. Corrosion was initiated from sensitized grain boundaries. Presences of O2 in Sulfiran solution reduces corrosion of the alloy through repairmen of iron oxide passive layer. For prevention and mitigation of resulted damage, unsensitized SS304L alloy should be used.

In the present work the effect of SiO2 on corrosion resistance of hot dip galvanized coating on low carbon steel was studied.At first, conditions of forming a good galvanized coating with suitable thickness obtained and with use of SiO2 particles of under 50 microns particle size, the composite galvanized coating formed.The SiO2 particles added in fluxing step to flux bath and then galvanizing process was performed. To study the behavior of coatings, different electrochemical techniques such as open circuit potential measurements, anodic polarization and salt spray test used. The effect of SiO2 on surface morphology and thickness of galvanized coating was studied using scanning electron microscopy and optical microscopy.According to results of salt spray test, corrosion rate of coating obtained from flux solution containing 0.1 percent SiO2 particles, in compared with the pure zinc coating, decrease about 58 percent. According to result of this research, decrease in coating thickness about 34 percent and modification of phase structure of galvanized coating are the important results of presence of SiO2 in coating that increase the corrosion resistance of coating.

Susceptibility of the brasses containing 15, 30, 35 and 37 wt.% zinc to crevice corrosion was studied using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy techniques and immersion tests in which a glass lens was designed for providing a crevice. NaCl aqueous solutions containing 2, 3.5 and 5% NaCl supplied media of different concentrations of chloride ions. The results showed Increasing of zinc content in the brasses decreased their susceptibility to crevice corrosion. Increasing of chloride ion concentration in the media from 2 to %3.5 caused decreasing of the susceptibility, however more increasing of chloride ion concentration from 3.5 to %5 caused severe attach in the brasses. Also, it was found that there is a boundary layer at a distance (d) far off the gap opening on each specimen in which the height of gap from the lens surface could be calculated and reported as a critical gap (y) for the alloy in the related chloride. Calculations showed that with increasing zinc content in the brasses alloys, y as the susceptibility criterion increased from 13.39-17.4 μm, considerably.

The effect of particle concentration on erosion-corrosion behavior of AA5052 aluminum alloy has been investigated in 3.5 wt. % NaCl solution containing 250-500 μm SiO2 erosive particles. The tests were conducted using a jet impingement rig at a jet velocity of 6.5 m/s, an impingement angle of 30° and three sand concentrations of 30, 60 and 90 g/l. Synergistic effect between erosion and corrosion was calculated in accordance with ASTM G119-09. Results showed that the pure erosion and erosion-corrosion rates increased as the sand concentration increased to 60 g/l. However, the rate of increase in both the erosion and the erosion-corrosion rates were decreased at concentration above 60 g/l that could be due to higher work hardening of the surface at higher sand concentration. Results also showed that at the sand concentrations tested synergy was negative. This could be due to formation of protective layer on erosion-corrosion samples that resulted in lower rates during erosioncorrosion tests.

In this study, the corrosion resistance of AISI 316L stainless steel in a simulated concrete solution (0.1 M NaOH + 0.1 M KOH) has been examined using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). For this purpose, passive films were formed at open circuit potential for 1 until 12 hours and then potentiodynamic polarization and EIS measurements were done. For EIS measurements, an excitation voltage of 10 mV and an applied frequency ranging from 100 kHz to 10 mHz have been used. The polarization curves suggested that AISI 316L stainless steel showed comparable passive behaviour in a simulated concrete solution. EIS results showed that the best electrical equivalent circuit presents two time constants: The high medium-frequencies time constant can be correlated with the charge transfer process and the low frequencies time constant has been associated with the redox processes taking place in the surface film. Also, EIS results showed that passive film and charge transfer resistance and therefore polarization resistance increase with formation time from 1 until 12 hours, due to the thickening of passive film.

In this research, electrochemical behavior of conventional and nanostructure composite tungsten carbide coating with WC-20Cr-7Ni metal substrate prepared by HVOF thermal spray on carbon steel in 5wt% sulfuric acid corrosive media was studied. Coatings structures were determined by X-ray diffraction analysis (XRD) and their corrosion resistance was studied by electrochemical potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Corrosion products on the test specimens and in the sulfuric acid corrosive media were analyzed by Scanning electron microscopy with energy dispersive X-ray analyzer (SEMEDX) and Atomic absorption spectrometer respectively. Results showed that heat treated nanostructured WC-20Cr-7Ni has better corrosion resistance and lower corrosion current density compared with non heat treated coatings. Nickel of The non heat treated coat was solved in the corrosive media but after heat treatment there is not any Nickel in the solution. Chromium oxides as a corrosion products were stick to the surface of coatings.