مجله علوم و مهندسی خوردگی
سال نهم شماره 4 (پیاپی 34، زمستان 1398)

The high-temperature erosion-oxidation behavior of a Ni-20Cr-3Mo-4Fe alloy containing 0, 2, and 4 wt.% of Si was studied. The specimens were exposed to 700 °C silica sand mixed with 0.5 wt.% of NaCl-KCl salt for 250 h. The difference in the thickness of the samples before and after the erosion-oxidation test was the measure for the material loss. The sample comprising 4 wt.% of Si showed a thickness reduction of about 45 percent higher than the Si-free specimen. Since the oxide scale is mostly removed by the eroding particles, the specimens were oxidized in the atmospheres of moving air and air+chlorine in a tube furnace at 520 °C for 100h to better understand the behavior of oxide scales. The Si bearing alloys were interestingly of higher corrosion resistance. The results of microscopic evaluations were indicative of the effects of Si on the reduction of oxidation rate, deteriorating spalling resistance, and increasing the Cr interdiffusion coefficient in the alloy and a subsequent reduction in the Cr-depleted zone.

In this study, the inhibition effect of a Schiff base with three C=N groups and three aromatic rings on corrosion of 1018 carbon steel in 0.1 M hydrochloric acid is investigated by two electrochemical methods, impedance spectroscopy and polarization, at 25 °C. The inhibition efficiency increases by increasing the concentration of Schiff base from 100 to 500 ppm and the highest inhibition efficiency was about 96% at 25 °C for the concentration of 500 ppm of Schiff base. The results of the polarization method showed that the Schiff base in acidic solution acts as a mixed inhibitor. The charge transfer resistance (Rct) for 1.0 M HCl solution increases from 39.5 Ω.cm2 to 949.7 Ω.cm2 by adding 500 ppm of Schiff base. The effect of temperature on the inhibitor performance is investigated by electrochemical polarization method in the range of 25-65 °C and it shows decreasing of inhibition efficiency with increasing temperature. The adsorption of Schiff base molecules on the surface of 1018 carbon steel at all temperatures follows the Langmuir adsorption isotherm. The values of ΔGads obtain -32.122 kJ and -35.124 kJ mol-1 at 25 °C and 65 °C, respectively, which shows increment in its absolute value. These results show that the adsorption of Schiff base molecules on the surface of 1018 carbon steel is a physico-chemical adsorption.

In this study, the effect of hydride formation on the corrosion and mechanical behavior of Zr-1% Nb alloy in 0.5 M LiOH solution was investigated. Hydride phase was created by electrochemical method in 1 M H2SO4 solution at room temperature. In this method, the hydrogen atoms adsorb on the surface and then penetrated into the bulk, which causes formation of the hydride phase. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirm the formation of the hydride phase in the sample. More hydride is formed in the sample with increasing current density, which led to the creation of more cracks. In addition, these cracks reducing the mechanical properties (about 12%) and significantly increased the corrosion rate of the sample due to the ionic and electronic changes in the oxide layer (about 100 time).

Some aniline Derivatives as organic corrosion inhibitors were selected and the inhibition effects of these compounds was studied with density functional theory (DFT) calculations. Some aniline derivatives containing substituted halides (except I), alkyls (methyl, isoporopyl and ter-butyl), OH, SH, NH2, OCH3 and NO2 were selected. For this purpose some electronic parameters such as EHOMO, ELUMO, the energy gap (ΔE), dipole moment (μ), electronegativity (χ), global hardness (η), the fraction of electrons transferred (ΔN) as well as the thermodynamic parameters such as standard Gibbs free energy (ΔGᵒ ) and standard reduced potential (Eᵒ ) were calculated. In order to find optimized structures and determination of quantum chemical parameters related to corrosion, the calculations were performed by using Gaussian software with DFT/B3LYP method and 6- 311++G(d,p) basis set in the gaseous and aqueous phases. After performing the calculations, it was determined that the most appropriate position on the aniline ring is para position. The NH2 > OCH3 > OH > SH > aniline and halides < alkyls > NO2 order was obtained by comparing their inhibitory properties based on the ΔN and E° parameters. The results of the calculations of the derivatives containing methyl or methoxy groups and last experimental studies are in good agreement. Both studies show the order of inhibition for compounds as para> ortho> meta, confirming the validity of the calculations.

In this research, pure zinc, Zn-SiC and Zn-TiO2 nanocomposite coatings were fabricated through pulse electroplating process from a sulphate bath. While pulse electroplating, the effect of several parameters like maximum current density (ip), frequency and duty cycle were investigated. In order to characterize the obtained coatings, surface morphology of the coatings was investigated by scanning electron microscopy (SEM & FESEM), composition of the coatings by elemental energy dispersive spectroscopy (EDS) analysis, microstructure by means of X-ray (XRD) method, corrosion measurement by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), surface topography by the use of atomic force microscopy (AFM) and also microhardness of the coatings were investigated. The highest amount of incorporated SiC nanoparticles was 8.73 Vol. % and for TiO2 nanoparticles was 7.12 Vol. %. Also, minimum corrosion current density for pure zinc coating was 7.1, for Zn-SiC nanocomposite coating was 4.49 and for Zn-TiO2 nanocomposite coating was 4.53 μA/cm2. It was observed that utilization of simultaneous of composite coating system and also pulse electroplating process leads to significant increase in corrosion resistance. XRD analysis also showed that preferred orientation for pure zinc coating is through (002) plane. By incorporation of SiC and TiO2 nanoparticles into the zinc matrix, the intensity for (002) plane decreases and (101) plane intensity increases. Average roughness for three samples of Zn, Zn-SiC and Zn-TiO2 coatings was 528.11, 627.32 and 815.41 nm respectively. The microhardness of the coatings was also improved after SiC and TiO2 incorporation.

In the present study, the effect of completion of isothermal solidification (IS) and NaCl concentration on potentiodynamic polarization behavior (PDP) of transient liquid phase bonded (TLP) 2205 duplex stainless steel (DSS) was investigated. The PDP results indicated that the pitting potential was decreased from 1.171VAg/AgCl in the as-received base metal to 0.373VAg/AgCl in the TLP bonded specimen before completing the IS in 0.1M NaCl test solution. By completion of the IS, the pitting potential of TLP bonded specimen increased to 1.180VAg/AgCl which was similar to that of the as-received base metal (1.171VAg/AgCl). This improve of pitting potential was strongly related to the uniform distribution of Cr and Mo across the bonding zone due to removing (Mo, Cr)-rich secondary phases from the bonding zone after completion of the IS. Before completion of the IS, the pitting potential of TLP bonded specimen decreased significantly with the increasing chloride concentration, while after it, the pitting potential was almost equal to that of the as-received base metal until 1.5M NaCl, but with increasing to 2.5M NaCl, the stable pitting attack occurred in the bonding zone and therefore, the pitting potential was decreased in the TLP bonded specimen. Furthermore, with increasing chloride concentration from 0.1 to 2.5M, the pitting corrosion was extended in the bonding zone before completing the isothermal solidification, which could be due to decreasing the pitting potential of TLP bonded specimen with increasing the NaCl concentration.

Corrosion of steel in concrete is the most important reason for the premature destruction of concrete structures. The use of corrosion inhibitors is a suitable way to control the corrosion of reinforcement in concrete. These chemicals are added in small amounts to the corrosive medium to reduce the rate of corrosion by affecting the rate of anodic, cathodic, or both reactions. In this paper, we investigate the corrosion inhibitory effects of disodium hydrogen phosphate, sodium molybdate, and sodium nitrite in simulated concrete pore solution in two concentrations of 1.8% and 3.5% NaCl. Hence according to the tests performed in simulator solution containing 1.8 wt.% NaCl salts, the nitrite, phosphate, and molybdate inhibitors added in 0.2, 0.0002, and 0.15 M, resulted in the efficiencies of 69, 59 and 81 percent, respectively. Also, in scanning electron microscopy micrographs, it was found that the use of these three inhibitors dramatically reduces the number and distance of surface pits compared to the control sample.