مجله علوم و مهندسی خوردگی
سال یازدهم شماره 1 (پیاپی 39، بهار 1400)

In the present study, the effect of different concentrations of calcium acetate (10 and 20 g/l) and potassium hydroxide (1 and 3 g/l) salts in electrolyte of the plasma electrolytic oxidation process on corrosion resistance of coatings formed on pure titanium has been investigated. After observation the surface structure of the coatings, it was found that the lowest porosity (about 4.86%) was related to the coating formed in the electrolyte containing 20 g/l calcium acetate and 1 g/l potassium hydroxide. This coating also showed the highest contact angle with water droplets (67.89 degrees) or in other words, the lowest wettability. The results of evaluation of corrosion behavior of coatings using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests after 48 hours of immersion in Ringer's solution indicated that the mentioned coating had the lowest corrosion current density (3.06 × 10-8 Ω/cm2 ) and the highest corrosion resistance (448272 Ω cm2 ) among all coatings. The high resistance of this coating against corrosion was due to low porosity percentage and thus reducing the penetration of corrosive ions into the substrate and also low wettability.

In order to strengthen the epoxy phenolic coating, glass flakes and cerium nitrate have been used. For this purpose, 6 samples with different combinations of cerium nitrate and glass flake were prepared and applied on ST37 steel. The self-healing properties of the samples at the scratch site were investigated by field emission scanning electron microscope (FESEM), elemental map analysis (MAP), and energy dispersive spectroscopy (EDS). According to the results, 58.52% of the complexes formed in the scratch area are composed of cerium nitrate (in sample EP2CN1.5G-04). In order to measure the anti-corrosion performance, electrochemical impedance spectroscopy (EIS), polarization and salt spray tests were performed. The obtained results showed a significant improvement in the protective performance of coatings containing glass flake and cerium nitrate compared to the pure epoxy. The results indicated that the presence of glass flakes and the combination of its protective mechanism with the self-healing properties of cerium nitrate increased Rct (charge transfer resistance) in sample 04 compared to sample 01 by 11.2 and 4.8 times, respectively, after 18 days of immersion. This sample also had the highest RP and the lowest icorr compared to other under-studed samples. These results prove the release of cerium nitrate in addition to the barrier properties of glass flakes. According to the results, it was observed that the EP2CN1.5G04 sample had better anti-corrosion performance than other samples.

In this study, synthesize of the non-isocyanate-based polyhydroxy urethane from linseed oil and the effect of increasing the carbonation conversion percentage on the resulted polyurethane coating properties were studied. For this purpose, double bonds of the linseed oil were converted to the epoxy rings at the first step, and then these epoxy rings were turned into the carbonate rings with carbonation reaction with different percentages (45%, 75% and 100%), which will break these rings and form urethane bond by reacting with amine hardeners. The Fourier-transform infrared spectroscopy (FTIR) was used to characterize the samples )In the epoxidation step, the 1689 cm-1 peak, which corresponds to the carbonate double bonds in linseed oil, became smaller, and the 823 cm-1 peak, which corresponds to the epoxy rings, formed, and then in the second stage, 823 cm-1 become smaller and the formation of the peak of 1800 cm-1 represent the carbonate ring formed instead of epoxy rings). Differential scanning calorimetry (DSC) was used to assesses the curing conditions. After that, the optimal curing temperature and time were determined (suitable time for curing is 2 hours for preheating at 100°C and then 2 hours to complete the cure process at 180°C), and finally, the physical, mechanical and anti-corrosion properties of samples were compared through Pull-off, Crosscut, hardness, impact, bending and Electrochemical Impedance Spectroscopy (EIS) tests. The results of the characterization tests confirmed the epoxidation of linseed oil and carbonation of the epoxidized linseed oil. Also, coating properties showed improvement of adhesion and hardness and reduction of flexibility by increasing the carbonation conversion and increasing of the anti-corrosion properties first and then decreasing it by increasing the percentage of carbonation. After 2 weeks immersion in corrosive electrolyte, sample with 75% of carbonation revealed anti-corrosion resistance equal to 5.38×106 ohm.cm2 while solvent-base polyurethane indicated 1.03×107 ohm.cm2 . Finally, polyurethane resine synthesized from 75% carbonated linseed oil represents the optimum properties for coating application.

This research studied the effect of two-stage over aging treatment on the 17-4 PH stainless steel corrosion behavior. First, the steel was solution annealed at 1035 °C for 1h, then it was subjected to a two-stage over aging treatment known as H1150-M treatment. Hydrogen induced cracking and stress corrosion cracking standard tests were carried out under annealed and over aged conditions. Microhardness tests were conducted to evaluate the mechanical properties under different testing conditions. Microstructural examinations were performed by the optical microscope and field emission scanning electron microscope. The results showed that under annealed condition, the microstructure consisted of lath and coarse martensite and interdendritic δ-ferrite that transferred to severely tempered and layered martensite after over aging treatment with the formation of reversed austenite. In the latter case, the reversed austenite was about 26% compared to 7% before over aging treatment. Under the hydrogen induced cracking test, the over aged 17-4 PH steel exhibited no cracking, unlike the annealed condition. The results also showed that 17-4 PH steel under both annealing and over aging conditions, is suspectable to stress corrosion cracking; meanwhile, time to failure was substantially higher in the latter condition.

Platinum anodes, including platinum coating on the substrate of niobium, are used for electrolysis of sodium chlorate. Due to the high price of these anodes, their corrosion resistance in this environment is important. In this research the effect of current density has been investigated on corrosion the behavior of Pt/Nb anode. Platinum coating was deposited on the niobium substrate by electrodeposition method. The samples were then exposed to accelerated life test in chlorate electrolysis cell at current densities of 0.5, 1.2 and 2 A/cm2 for 3, 10, 40 and 70 hours at constant temperature of 35°C. Cyclic voltammetry and electrochemical impedance spectroscopy was done on the samples at each time extension. The samples finally were characterized using SEM. The results showed that at any current density, the solution in the vicinity of the anode is acidic and with increasing current density, the corrosion of the coating through the formation of chloroplatin complexes, from local to uniform, is converted at a high rate.As well as, it was revealed that the Pt/Nb anodes are deactivated in to two manners; Spontaneous dissolution of platinum coating and growth of passive layer or dissolution of coating together with passive layer. The electrolysis with current density of 1.2 A/cm2 is the best operating condition in which the platinum anode has its highest corrosion resistance, lowest destruction and optimum catalytic effect.

In this study, the effects of Tri-sodium citrate, Sodium potassium tartrate and Ethylene di-amine tetra-acetic acid as organic corrosion inhibitors on the corrosion behavior of carbon steel in a simulated concrete pore solution contaminated with 3.5 wt% NaCl, were investigated through open circuit potential, Electrochemical impedance spectroscopy and Cyclic polarization analysis. Morphology and surface roughness of the specimens was evaluated by scanning electron microscopy. EIS analysis showed that after 48 hours of exposure to NaCl solution, the citrate and tartrate were able to achieve 91.2% and 87.3% corrosion inhibition efficiency at their best performance respectively. Inhibition Mechanism of each inhibitor was analyzed Based on the parameters obtained from the EIS tests. According to Cyclic polarization results, the dominant mechanism of inhibition is through aggregating on anodic sites and consequently limiting the anodic reactions. SEM images showed that the addition of 0.1 M of citrate or tartrate to the corrosive medium, causes a relatively persistent layer on the surface of the metal with low porosity level which immunes the carbon steel from pitting corrosion to a large extent.