مجله علوم و مهندسی خوردگی
سال دهم شماره 1 (پیاپی 35، بهار 1399)

Polypyrrole modified coatings were coated by ammonium bifluoride doped in the polyethylene glycol (Doped Ppy) on AZ31 alloy using cyclic voltammetry. The result of nuclear magnetic resonance showed that fluoride doping occurred in the glycol molecule. Corrosion performance of the coatings was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization in 0.05 M NaCl solution. Corrosion current density of polypyrrole modified coatings (Doped Ppy) was calculated about 5 µA.cm-2 , which was 5 times lower than the coating with discrete PEG and fluoride ions. It was observed that the modified polypyrrole coatings shows the highest corrosion resistance due to the synergism effect of fluoride and polyethylene glycol. Charge transfer resistance (Rdl) of polypyrrole coating with fluoride and PEG and polypyrrole coating containing ammonium bifluoride doped in the PEG was 5950 and 10050 Ω.cm2 , which was higher about 2.5 and 4.5 compared polypyrrole coating with Rdl of 2250 Ω.cm2 .

Corrosion of steel bars in reinforcement concrete is the most important factor in the destruction of concrete structures. Till now, several protection methods for reducing the rate of corrosion of steel in reinforced concrete structures have been used, which corrosion inhibitors due to economical, easy to apply and efficiency, compared to other methods, faced with more attention. In this study, the effects of the simultaneous use of sodium nitrite and sodium-potassium tartrate inhibitors have been investigated. For this purpose, open circuit potential tests, electrochemical impedance spectroscopy and cyclic polarization were used in simulated concrete pore solution. This study aims to reduce the consumption of sodium nitrite due to environmental problems reported for it and also its destructive effects on the mechanical properties of concrete. Electrochemical impedance spectroscopy showed that the combination of these two inhibitors, improves the corrosion resistance by 91%. This rate is higher than the best performance of each of them, and finally, the synergistic parameter of 2/2 was calculated for this combination. According to the scanning electron microscopy images and the parameters obtained from the fitting of the Electrochemical impedance spectroscopy data, it seems that the simultaneous use of these two inhibitors at appropriate concentrations leads to the formation of a flat, uniform and chloride-resistant layer. Also, according to the results of the cyclic polarization test, the simultaneous use of these two inhibitors increases the pitting potential to 691 mV. Generally, it improves the resistance of steel specimens to pitting corrosion.

In the present study, spark plasma sintering (SPS) technique was used to fabricate Ti-Al2O3-HA composite. Powders were sintered at 40 MPa pressure for 5 min at 1200 and 13000C. Microstructural characterisation was carried out using scanning electron microscopy (SEM). Density, hardness, electrochemical impedance and polarization test were performed to evaluate the properties of the samples. The results showed that by increasing the sintering temperature from 1200 to 1300 0C the density of the samples increases and the apparent prosity decreases. Also with increasing temperature the hardness of the samples increased from 528HV to 612HV. The result of electrochemical impedance test showed the polarization resistance of the sample enhanced from 222 Ωcm2 to 996 Ωcm2 . Furthermore, according to polarization measurements, the corrosion current density was reduced from 16.35 μA/cm2 to 1.47 μA/cm2 . The improvement in the corrosion resistance was attributed to the reduction of prosity and the formation of stronger passive film on the sample surface with increasing sintering temperature.

Usually, in all applications where an organic coating is applied on the surface, physical and chemical surface preparations need to enhance adhesion and anti-corrosion resistance of the system. Control of surface roughness and chemistry of modified metal by conversion coatings are effective factors in the adhesion properties of organic coatings. In this project, the behavior of anti-corrosion resistance and adhesion of epoxy coatings in the presence and absence of anti-corrosion glass flakes pigments on the surface of steel prepared by applying the zirconium conversion coating and sandblast method were examined. Conversion coating was formed on the surface in the optimum condition of practical parameters. In the next step, the effect of the type of surface preparation on the anti-corrosion resistance and morphology behavior of the coating was investigated using electrochemical tests and atomic force microscope images. It was found that the samples prepared with conversion coating indicated the best anti-corrosion resistance and adhesion strength as a result of the change in the roughness and changing the surface chemistry. Finally, epoxy/glass flake coating with 10% of glass flake was applied on the prepared surface through conversion coating and it was found that adding glass flakes increase the anti-corrosion resistance of the coating and these results confirmed by the salt spray test.

Plasma electrolytic oxidation is a novel and promising method for coating the light alloys especially magnesium alloys. Several parameters affect the coating quality and its properties, where in this study; the effects of frequency on properties of coatings produced by PEO on AZ31 Mg alloy were investigated. The coating process was performed in a silicate-based electrolyte containing KF and K2TiF6 using a soft-sparking waveforms with 30 % cathodic pulse width at 1, 2, and 3 kHz. The surface morphology of the coatings was a net-like (scaffold) containing a micro-pores network, micro-cracks, and granules of oxide compounds, in which, no noticeable change on coating morphology occurred with increasing the frequency. Also, the coatings were composed of MgO, MgF2, Mg2SiO4, SiO2 (amorphous), and TiO2 (crystalline and amorphous) phases contributed by the elements from both substrate and electrolyte in the coating process. Based on the cross-sectional images, the thickness of coatings displayed no significant increase with increasing in frequency, but the porosity percentage was increased. Also, the very small pores were presented in the inner layer with increasing frequency. Corrosion behavior of the coatings was evaluated using electrochemical impedance spectroscopy (EIS) in 3.5 wt. % NaCl until 7 days of immersion. The results of the EIS showed that with increasing frequency and porosity, the larger surface of the coating is in contact with the solution, and the presence of porosity in the more compact layer of the coating allowed the access of solution to the substrate, which reduces the barrier performance of the coating.

Organic coatings are the single most widely applied materials to protect metallic substrates against corrosion. Therefore, coating researchers need to understand how a coating can guarantee protective performance during service life. This study it is tried to find how adhesion durability can affect the protective performance of a coating. An epoxy resin (Araldite GY 783) was cured with a mixture of two types of hardeners (a polyether polyamine and a cycloaliphatic polyamine) in four different mixing ratios (i.e. 20–80, 50–50, 70–30, 90–10) to achieve various adhesion behaviors, water permeability, and viscoelastic properties. Samples were exposed to humidity in the testing chamber for 300 days and adhesion was measured at wet and dry conditions over time. After drying, adhesion loss and recovery were evaluated by a pull-off test. The electrochemical behavior of coatings was studied by electrochemical impedance spectroscopy (EIS). A series of impedance spectra of coatings during exposure to humidity were recorded and their protective properties were compared. The results showed a correlation between protective properties and viscoelastic behavior of coatings. It was found that a sample with a cycloaliphatic hardener has a higher elastic modulus (1275 MPa) and a sample with a polyether polyamine hardener has a lower elastic modulus (465 MPa). Additionally, it was found that a sample with excellent initial protective properties (|Z| =1010 Ω cm2), higher elastic modulus, and low initial water permeability (0.308 g/m2hr) may quickly fail because of the adhesion failure after wet cycles.

A new model for estimating the water absorption of coatings has been developed using electrochemical impedance spectroscopy data and based on an effective medium theory. It is assumed that the volume fraction of the electrolyte varies along the cross section of coating thickness according to a power-law which is defined based on coating thickness and its capacitance capacity and the coating resistivity and permittivity profiles of immersed coating can be calculated through an effective medium theory (EMT) formula corresponding to a parallel combination of the two media (electrolyte and coating material). The total volume water uptake of proposed model was compared with the results calculated based on the Brusher-Kingsbury theory and gravimetric data for a commercial epoxy coating sample immersed in 3.5% sodium chloride solution for 120 days. The results showed that the estimated water absorption values according to this model, gravimetric method and Brusher-Kingsbury model is 7.5, 7.1 and 10.3%, respectively, and the results of this model are in good agreement with the gravimetric results.. In addition, this method made it possible to estimate the water absorption profile of the coating, changes in specific strength and permeability of the coating during immersion time.