SCIENCE AND ENGINEERING CORROSION
Year:2016 | Volume:6 | Issue:11 (21)|Papers Count:7

In this study, Zinc-Nickel electrophosphate coating was applied on galvanized steel. microstructure, corrosion resistance and porosity of coating were characterized using scanning electron microscopy, polarization test and electrochemical method respectively, Zn-Ni electrophosphate coating was applied on galvanized steel substrate using cathodic electrochemical method at different operation times, the result of this study indicated that, by using cathodic electrochemical method, coating with the desired properties can be obtained at low temperature bath. Result of microscopic study indicated that, in low operation time, the surface of the substrate not fully covered by phosphate crystals, consequently, coating with poor corrosion resistance can be obtained, also at the high operation time, excessive growth of phosphate crystals leads to high porosity in the coating. Therefore, by increasing the operation time from 20 min to 40 min the corrosion current density increase from 0.68mA/cm2 to 2.54 68mA/ cm2 consequently, the optimized time for obtaining the coating with desired properties is 20 minutes. Also the deposition mechanism of Zn-Ni electrophosphate coating was investigated in this study.

In this study, causes of premature failure due to corrosion of air cooler tubes in ISOMAX unit of an oil refinery were investigated. Corrosion products and the scales formed on the surfaces of tubes were examined using x-ray diffraction and x-ray fluorescence spectroscopy. In addition, scanning electron microscopy and energy dispersive spectroscopy were employed to study the morphology of the voids and the effect of aggressive species on their growth. The results showed that two different mechanisms were responsible for failure of the tubes. Underdeposit corrosion due to formation of the scales on the interior surfaces supported by the presence of ammonium bisulfide salts and high amounts of oxygen played a significant role in pitting corrosion on one of the tubes. On the other hand, erosion-corrosion due to high fluid velocity was the major failure mechanism in the other two tubes.

In this research, the surface and cross section of an API 5L X70 pipeline steel was investigated by SEM observation and EDS analysis in order to determine type and morphology of inclusions. Then, the electrochemical hydrogen charging technique using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate was utilized to create hydrogen induced cracks (HICs) in X70 steel. After hydrogen charging experiments, the cross section of hydrogen charged steels was polished up to 1 mm then etched with 2% nital solution to find HIC cracks. The HIC crack initiation and propagation was studied by Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) techniques. The results showed that there are various types of inclusions including oxide, sulfide and nitride inclusions at the cross section of as-received X70 steel; however, the accumulation of inclusions was higher at the center of cross section than in other regions. But, in hydrogen charged specimens, only some special inclusions, such as sulfide and nitride type inclusions, can initiate HIC cracks. Oxide inclusions and precipices has not any role in HIC crack initiation and propagation, but they reduce the fracture toughness of X70 steel. Moreover, the EBSD results showed that the crack initiates in an area with weak or random texture. Other effective parameters in crack growth, such as orientation of grains involved with HIC crack, was discussed.

In this study, the effect of Fe2O3 particle concentration on the protective properties of the Ni-Fe2O3 composite coating formed on AISI304 substrate was evaluated. The electrodeposition process was conducted in a plating bath without and with different concentration of Fe2O3 particles. The surface morphology and structure was characterized using scanning electron microscope (SEM), Electronic diffraction line scans (EDS) and X-ray diffraction (XRD). The corrosion behavior of the coating was evaluated by measuring the variation of corrosion potential during the immersion time and electrochemical impedance spectroscopy (EIS) method. The results revealed that in pH=4 and a current density of 20 mA/cm2, The precipitation of Fe2O3 particles in nickel matrix reached to its maximum value when the concentration of the particles in the electrolyte was 20 g/lit. However, the microhardness results and corrosion behavior of coated samples was revealed that the homogeneous dispersion of second phase particles in matrix plays a dominant role in the promotion of surface and protective properties of the coating. Nickel coating increased the corrosion resistance of AISI304 more than double. The addition of Fe2O3 particles into the electroplating bath caused an increase in corrosion resistance of Nickel coating and the coating fabricated in the bath containing 15g/lit Fe2O3 provided the highest corrosion resistant which was almost 5 times higher than corrosion resistance of the steel substrate.

This study investigates the effect of cathodic protection level, on corrosion of X-52 pipeline steel under simulated coating disbondment. In this regard, an experimental setup which could simulate the effect of different levels of Cathodic Protection (CP) on corrosion under disbonded coating was built. Then using C2 solution (which is a solution with near-neutral pH that CO2/N2 is purged in it) coupons made from X-52 pipeline steel were placed under the simulated coating disbondment. Results showed with increasing the level of cathodic protection under simulated coating disbondment the depth of protection increased, while corrosion decreased. The pH changes under the simulated coating disbondment was a function of level of CP and also the distance from the disbanded coating Open Mouth. Cathodic protection level was increased slightly, by past the time. Corrosion rate when applying a CP of -750 mVSCE was close to that when no CP was applied (OCP). This was explained by the more acidic condition being formed locally at these points. Microscopic figures where in accordance with the weight loss measurements and showed both uniform and localized corrosion occurring under the simulated coating disbondment.

Corrosion of archaeological bronzes may cause due to effect of manufacturing processes, s well as influence of different factors in the burial environment over the time. Physico-chemical processes occur in the archaeological objects in complex and slow processes as a result of environmental conditions. This paper presents the results of an analytical study on six archaeological metal objects from the archaeological collection found in the cemetery of Toul-e-Talesh, Gilan. The aim of study was to identify corrosion stratigraphy based on chemical composition and microscopic appearence of the layers and corrosion products. To study the stratigraphy in the cross-sections of the samples, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), optical microscopy (OM), and X-ray diffraction (XRD) methods were used. Based on the results, the chemical composition of five objects is tin bronze (Cu-Sn) alloy with variable levels of tin and small amounts of lead, zinc, sulfur, iron, arsenic, antimony, phosphorus and silver and one object is made of pure copper. Two main of corrosion stratigraphy were observed in the samples: 1) a one-part internal corrosion structure including one, two or three layered structure without the outer layer, and 2) a two-part structure including internal and external corrosion layers. Presence of the internal and external corrosion layers in objects as well as a tin-rich internal corrosion layer can be related to decuprification phenomenon and re-deposition of copper compounds over the originl surface. Basic copper carbonates were found as the main corrosion products in all samples. Uniform structure of the corrosion in the studied samples is comparable with the type I corrosion in archaeological bronzes.

In this study, the effect of sintering temperature on the corrosion resistance of Silver Oxide electrodes in the KOH environment was investigated. For this purpose, initially five Ag Oxide electrodes (positive plate) with the composition of 95 wt% Silver Oxide, 4.9 wt% carbon powder and 0.1 wt% resin was prepared. Then, five Silver Oxide electrodes were sintered for 10 minutes at temperatures of 400, 450, 500, 550 and 600oC. Potentiodynamic polarization and electrochemical impedance spectroscopy methods were used to evaluate the corrosion resistance of Ag electrodes. Scanning electron microscopy was used to examine the microstructures of electrodes. Electrochemical tests results showed that with increasing sintering temperature, the corrosion resistance of Silver electrodes reduced. SEM images showed that with increasing sintering temperature, the amount and size of the pores increased. Also, SEM point analysis results had implied on reduction of electrodes oxygen contents with increasing of sintering temperatures.