مجله علوم و مهندسی خوردگی
سال ششم شماره 1 (پیاپی 19، بهار 1395)

Corrosion of reinforced concrete structures is one of the major factors in creating intensive damages in concrete structures installed in south region of Iran. This phenomenon has been in the focus of many studies recently. Since during corrosion assessment stage there is a difference between results of accelerated and field tests, it is necessary to conduct field evaluation survey in this regard. In the current article corrosion behaviour of reinforced concrete cylinders, containing micro silica, corrosion inhibitors and without any additive were studied using Electrochemical Impedance Spectroscopy (EIS). The samples were exposed to corrosive environment in Northern margin of Persian Gulf in a corrosion research center for 8 years. Analysis of observed spectra in terms of modified Randles circuit appeared to be a reasonable approximation. In addition micro silica added samples have 1.8 and 13.4 times better charge transfer resistance comparing to the corrosion inhibitor added and without any additive samples, respectively.

Ternary coatings of RuO2-TiO2-IrO2 and RuO2-TiO2-Ta2O5 were synthesized in two steps; first by sol-gel procedure and then thermal decomposition at 450oC on Ti substrate. Electrocatalytic activity of the coatings toward oxygen evolution reaction was investigated by electrochemical methodes including cyclic voltammertry, electrochemical impedance spectroscopy in a 1mol.L-1 Na2SO4 solution in order to evaluation of their application as anode in cathodic protection systems. Electrochemically determined active surface area values revealed 1.97cm2 active surface for the coating containing IrO2 and 0.71cm2 for the coating containing Ta2O5. In the case of the coating containing Ta2O5, addition of Ta2O5 has led to decreasing the active sites of RuO2 as the main electrocatalyst for oxygen evolution. Morphological analysis of the coatings was conducted by providing the scanning electron microscope images and energy dispersive x-ray spectrums of coatings. The images clearly confirmed that IrO2 aggregates at the cracks and spaces between the island like structures of ternary coating of RuO2-TiO2-IrO2 and creates extra active sites. Enhancement of uniformity as a result of occupation of cracks improves the stability of the coating containing IrO2.

Electroless deposition of Ni-P coating on AZ31 magnesium alloy was carried out after forming conversion coating on the samples by immersing in saturated NaHCO3 solution. Effect of changing immersion time in NaHCO3 solution on surface morphology, chemical composition and phase structure of coatings were studied by SEM/EDS and XRD analyses. Hardness and corrosion resistance of the coatings were studied by microhardness test and polarization test in 3.5 wt.% NaCl solution. The results indicated significant effect of pre-treatment process on morphology and therefore corrosion resistance of Ni-P coated samples. By increasing conversion film formation time to optimum time, smooth and compact Ni-P morphology was obtained. Corrosion potential was nobler for Ni-P coatings in comparison with Magnesium alloy substrates and electroless Ni-P coating improved corrosion resistance of the substrate. In addition the microhardness of the Ni-P alloy coatings was examined and Ni-P coatings enhanced hardness, significantly. Increasing the conversion film formation time to a certain amount resulted in a better morphology of the Ni-P films with less porosity and thus better corrosion resistance. Any increase in conversion film formation time more than the optimum amount led to a lower corrosion resistance.

In this research, corrosion behavior of explosively bonded 304 stainless steel-Ck45 carbon steel plates was investigated. Explosive welding carried out in constant explosive ratio with 4 and 5 mm standoff distances. The results showed that by standoff distance increasing, the shape of interface transited to the wavy vortex type, and locally melted zones formed. Due to shock hardening, the hardness increased in near of the interfaces, and maximum hardness was related to the sample with 5 mm standoff distance. Polarization and impedance tests results showed that maximum corrosion rate was related to the sample with maximum standoff distance. Due to the formation of locally melted zones and increasing in impact kinetic energy in the sample with maximum standoff distance, corrosion rate was increased.

Sodium perchlorate is produced electrochemically using Pt/Nb anode due to their high service life and high oxygen evolution overpotential. Sodium chlorate, sodium chloride and sodium dichromate are the main components of the electrochemical cell for perchlorate production. In this research the effect of sodium dichromate concentration has been investigated on corrosion behavior of Pt/Nb anode. Platinum was electroplated on niobium substrate. The obtained samples were then exposed to accelerated life test in perchlorate production cell with 0, 1 and 3 gr/lit sodium dichromate for 3, 10, 40 and 70 hour at constant current density of 5.9 A/cm2. Cyclic voltammetry and electrochemical impedance spectroscopy tests was done on the samples at each time extension. The samples finally were characterized using SEM. The result showed that passivation of niobium substrate which tends Nb2O5 to form on it, is the main mechanism by which the sample is deactivated. In addition presence of sodium dichromate has a significant effect on corrosion of the anode, although it is known as a cathodic inhibitor. Finally it was observed that 3 gr/lit is the optimum concentration for sodium dichromate in which the anode has the highest corrosion resistance in sodium perchlorate production cell.

This paper evaluates the function of talc as a reservoir and its ability to absorb and release corrosion inhibitors. In this sense, the performance of released corrosion inhibitors is studied by using electrochemical analysis such as electrochemical impedance spectroscopy as well as surface analysis such as scanning electron microscopy and Energy-dispersive X-ray spectroscopy. Results indicated that talc can adsorb benzothiazole )BTH( as an organic inhibitor and release it in a sodium chloride solution. The trend and magnitude of some parameters including low frequency impedance modulus, polarization resistance and current density revealed the effective role of inhibitor-doped talc in controlling the corrosion of mild steel in sodium chloride solution. The scanning electron microscopy results showed that in the presence of benzothiazole, formation of a compact film on the substrate surface, increasing the corrosion resistance.

In current study effect of severe shot peening pretreatment on plasma electrolytic oxidation coating of MgAZ31B is investigated. It was revealed that shot peening increases surface energy and discharge sites enhances finally energy of discharges reduces. In short term, absorption of nanoparticles is improved by more than 20% while it reduces to 50% of non-pretreated sample. Changes in surface roughness and porosity volume lead to reduction of wetting angle from 48 to 29 degrees for non-pre-treated and pre-treated sample, respectively. Although shot peening reduces porosity level, application of titania nanoparticles reduces thickness of coating and therefore their simultaneous application reduces corrosion resistance. Most passive corrosion potential, -1.45V, is ascribed to pretreated sample that were coated in nanoparticle-free electrolyte.