مجله علوم و مهندسی خوردگی
سال سوم شماره 2 (پیاپی 8، تابستان 1392)

In this study, pulse electrodeposition of Ni-Fe coating in sulphate bath was performed and effects of pulse parameters on corrosion properties were investigated. Field Emission Scanning Electron Microscopy (FESEM) was used to study the morphology of coatings. The chemical composition of deposits was examined by Energy Dispersive X-ray Spectroscopy (EDS) attached with Scanning Electron Microscopy and Atomic Adsorption Spectroscopy (AAS). Corrosion testing of coatings by potentiodynamic polarization in solution 3.5% NaCl was investigated. With pulse current technique, corrosion resistance of Ni-Fe coating increased. In Ip=20 (mA/cm2) and f=10 (Hz) corrosion current of coating received to 0.3 (μA/cm2). With varying the duty cycle at duty cycle of 25%, corrosion current of 0.19 (μA/cm2) obtained. Incorporation of 2.59 (At%) TiO2 in Ni-Fe matrix increased the corrosion current of deposit.

Corrosion and other destructive factors in corrosive areas have caused irreparable damages to Iranian economy as well as to the concrete structures in such areas. Therefore, it is necessary to investigate and identify the behavior of concrete in these areas in order to prevent early destruction. Obviously, the most important factor in destruction and reduction of useful life of concrete structures is the penetration of Cl ion and the related corrosions. One of the methods to make the concrete impenetrable is to use different pozzolans for improving its structure. In this investigation, the performance of Nano-silica pozzolan (2, 3, 5, 7 & 10 %) is studied for the purpose of increasing resistance against Cl ion penetration in simulated conditions of Oman sea environment (tidal and immersion) in ordinary and Self-consolidating concrete. The results show positive performance of the instances containing Nano-silica in contrast with the concrete without it. Meanwhile, comparing ordinary and Self-consolidating concrete, we found out higher persistence of Self-consolidating concrete against Cl ion and destructive conditions of tidal environment than those of ordinary concrete.

In this study, two Cr-Mn-(Ni-free) austenitic steels were fabricated by vacuum induction furnace. Then plates with 10 mm thickness were fabricated by hot-rolling. Electrochemical behaviour of two Cr-Mn steels in 0.1 M H2SO4 solution was investigated by open-circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis. The results indicated that the open circuit potentials of two Cr-Mn austenitic steels were found to shift towards positive direction. Also, the potentiodynamic polarization curves suggested that two Cr-Mn austenitic steels showed comparable passive behaviour. Mott–Schottky analysis revealed that the passive films formed on two Cr-Mn austenitic steels behave as n-type and p-type semiconductors. Also, Mott–Schottky analysis indicated that the donor densities are in the range of 1021 cm-3.

In this research, corrosion inhibition of 6061-T6 aluminum alloy in nitric acid by inhibitor of thiourea in two type of solution using tests electrochemical and quantum mechanical calculations have been investigated. Cyclic polarization and electrochemical impedance results showed that with increasing concentrations of inhibitors, pitting potential rosed and sensitivity to pitting diminishes. Anodic and cathodic polarization curves showed that the inhibition was mixed type inhibitors and the inhibition efficiency increases with increasing inhibitor concentration. Inhibitor adsorption on the surface from the Langmuir isotherm can follow. Adsorption free energy calculations revealed that corrosion inhibition is type of physical adsorption. Quantum chemical calculations showed that the inhibitor adsorption on the alloy surface through nitrogen and sulfur atoms as well as π electrons done and reduces the energy difference between the HOMO and LUMO significant role has increasing inhibition efficiency.

The corrosion protection, morphological and adhesion properties of a room temperature phosphate coating containing manganese ions (Mn) were studied on a steel substrate. For this purpose, phosphate coating solutions containing different concentrations of Mn were prepared. The steel samples were treated by the phosphate coatings at room temperature, pH of 3.1 and immersion time of 30 min. Then, an epoxy / polyamide coating was applied on the phosphated samples. The corrosion protection properties of the samples (in 3.5% NaCl solution) were studied by a potentiostatic polarization technique. The microstructure and morphology of the surface treated sample were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Adhesion strength values of the epoxy coatings were measured by a pull-off test before and after exposure to 3.5 wt% NaCl solution. Results showed that addition of Mn (especially at 7 gr/lit) to the phosphate coating caused an improvement of its corrosion protection properties compared to the sample without additive. A less porous and more compact phosphate coating formed in the presence of Mn than the sample without additive. The lowest adhesion loss of the epoxy coating was observed on the sample treated by a phosphate coating containing Mn. Generally, results showed that Mn as an additive reduced adhesion loss and improved corrosion protection properties of a room temperature phosphate coating on steel.

In this paper, St-12 steel surfaces were coated using hexafluorozirconic acid conversion coatings. Influence of different pH (2, 4, 6) on corrosion resistance of coated samples was initially studied. Electrochemical and anti corrosion properties in solutions with different pH were studied by use of Polarization and EIS measurements. The results showed that pH=4 hinders corrosion of steel much better compared to pH= due to less acidity. On the other hand, pH=6 also couldn’t offer good corrosion protection for steel, since it is too alkaline to result in proper oxidation of substrate. Next, questions of interest associated with how the concentration of MnSO4 additive (0.01, 0.1 and 1 g/l) affects corrosion resistance, morphology and composition of the coating. The properties of the coated surface was examined by a range of characterization techniques including DC polarization and EIS tests for comparing the corrosion resistance of differently coated samples; AFM to study morphology and surface roughness; SEM to evaluate surface morphology; in addition to EDS and XPS to analyze chemical composition. Results showed that the sample coated in solution with pH=4 and Mn2+ concentration of 0.01 g/l possess improved corrosion resistance (current density decreased about fifty percent in comparison to steel coated just with Zr) and more uniform morphology. The result of XPS for the sample coated in optimized Mn2+ containing solution showed that the coating is composed of various zirconium oxide states and manganese compounds.

Ni-W alloy coating is a novel coating that can be used as an environmentally safe substitute to hard chromium coating. To improve corrosion properties of this coating, nanocrystalline and amorphous Ni-W coatings containing Al2O3 nanoparticles were electrodeposited from three different baths by direct current method. Scanning electron microscopy, EDS and X-ray diffraction analysis were used to characterize the coatings, and electrochemical impedance spectroscopy and electrochemical polarization measurements were carried out to evaluate the corrosion behavior of nanocomposite coatings. It was found that nanoparticles can codeposit with the the Ni-W matrix; however the amount of embedded nanoparticles depends on bath chemical composition. In addition, nanoparticles can result in a more compact coating with fewer defects. Electrochemical measurements reveal that nanoparticles have the capability to improve corrosion resistance of Ni-W alloy coatings, though the levels of improvement depend on coatings structure which is affected by bath chemical composition.