مجله علوم و مهندسی خوردگی
سال دهم شماره 3 (پیاپی 37، پاییز 1399)

Dual-phase steels have many applications in various industries. By studying the various properties of these steels such as mechanical and electrochemical properties, new applications can be imagined in the use of this type of steels. Although the mechanical properties of these steels have been studied in numerous studies, the corrosion properties of Dual-phase steels need to be extensively studied. In this research, using intercritical heat treatment, the effect of heat treatment temperature on the microstructure and corrosion properties of Dual-phase steels has been investigated. For this purpose, using two methods of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), the corrosion properties of heat treated samples at 740, 770 and 800 ° C were investigated. The results showed that the martensite phase has a dual effect on the corrosion behavior of steel. On the one hand, due to the higher corrosion resistance of the martensite phase than the ferrite phase, the presence of martensite increases the corrosion resistance of steel. On the other hand, due to the potential difference between the two phases of martensite and ferrite and galvanic coupling, presence of martensite reduces corrosion resistance. The highest corrosion resistance was related to the sample prepared at 770 ° C. With increasing temperature to 800 ° C, due to the increase in the percentage of martensite phase and the intensification of the galvanic coupling between the martensite phase (cathode) and the ferrite phase (anode), the corrosion resistance of the sample decreases.

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.

The effect of pulse cathodic protection (CP) on CP penetration depth under tape coating disbondment of pipeline steel has been investigated in this study and results have been compared with conventional CP. In this regard, a laboratory cell with the length of 2000 mm that can simulate the underlying conditions by applying CP was designed and made. For investigation of crevice width on CP penetration depth coupons with dimensions of 2000×10 mm and 2000×50 mm made from API X-52 pipeline steel were placed under coating disbondment with the gap size of 3 mm and different widths. By placing the cell in a simulated soil solution C2, the potential of - 1000mVSCE through two methods of conventional & pulse CP with the frequency of 10000 Hz was applied to the open mouth (OM) of the simulated coating disbondment. Result showed by applying conventional CP for both the crevice widths, potential failed sharply to Open Circuit Potential (OCP) near to the OM while for pulse CP, in contrast to conventional CP, which CP penetration depth was increased. At the crevice with the width of 50 mm CP level was measured more than the width of 10 mm, such that the entire disbondment length was protected by CP. The lower CP level for the crevice with the width of 10 mm could be due to concentration polarization resistance which restricted the penetration of the positive current within the crevice. Also, pH value under the coating disbondment with applying pulse CP at the crevice with the width of 50 mm was measured more than the pH value for the disbondment with 30 mm width other due to higher CP level under this disbondment. In summary, results showed despite the conventional CP, applying pulse CP with high frequency could be an effective method in increase of the CP penetration depth under tape coating disbondments.

In this paper, the effect of anode arrangement is investigated on the applied current density in the cathodic protection system of reinforced concrete. For this purpose, two different arrangements of anodes with different number of anodes have been studied. The results indicate that the location of anodes relative to the rods in concrete has a significant effect on the current density. And the amount of applied current density for corrosion protection is a function of the anodes location. In the case where the anodes are parallel to the rods, a lower applied current density is required, and if the anodes are vertical to the rods, a higher current density is required for protection.

To develop and improve some of the electroless nickel coating properties, nickel composite electroless coatings were introduced. In this research, Ni-P-PTFE composite coatings containing different PTFE percentages were deposited on steel substrate by electroless method. The influence of amount of PTFE on the mechanical, anti- corrosive and tribological properties of coatings was investigated. The Micro hardness and pin on disk analysis were carried out to evaluate the hardness, erosion resistance and friction coefficient of coatings. Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDX) were performed to study the morphology and elements of coatings. The results showed in the absence of PTFE the amount of P element was about 10.42 wt.% whilst it decreased to 5.02 wt.% in the presence of 20 g/l PTFE. In addition, the presence of PTFE lubricant particles reduced the friction coefficient from 0.8 for Ni-P coating to 0.1 for Ni-P-PTFE (20g/l) composite coatings. In bath containing concentrations of 20g/l PTFE, respectively, the lowest coefficient of friction was observed. The anti-corrosive behavior of coatings was studied in 3.5 wt. % NaCl by tafel polarization and electrochemical impedance spectroscopy (EIS) methods. Evaluating the effect of PTFE concentration on the corrosion behavior of the composite coatings demonstrated that the obtained coatings in presence of 20 g/L PTFE had the maximum corrosion resistance. For example, the OCP number increased from -475 mV to -350 mV. Moreover, the logarithm of the corrosion current density decreased from 1.78 μA.cm-2 to 1.15 μA.cm-2. It was due to the proper distribution of PTFE particles in coating and their chemical stability.

Conventional and nanostructured NiCoCrAlY coatings were deposited developed using the high-velocity oxy- fuel (HVOF) thermal spraying technique. The nanostructured NiCoCrAlY powder feedstock for the coatings was produced by the ball milling method. In this regard, the ball-milling method was used to produce the nanostructured NiCoCrAlY powder feedstock. The microstructures examinations of the as-received and nanostructured powders as well as their developed coatings were analyzed investigated by using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) equipped with energy-dispersive electron dispersed X-ray spectroscopy (EDS), and a transmission electron microscope (TEM). Due to the evaluation of the oxidation kinetics, the free-standing coating specimens were subjected to the isothermal and cyclic oxidation testing at 1000 and 1100 ºC respectively, under a laboratory air atmosphere. The results indicated that the as- received NiCoCrAlY coating had a parabolic oxidation behavior in short- and long-term oxidation exposure tests. For the nanostructured NiCoCrAlY coating, in contrast, the long-term oxidation kinetics deviated from parabolic behavior and showed instead sub-parabolic rate behavior. The obtained results also exemplified revealed that the nanostructured NiCoCrAlY coating had a greater oxidation resistance for to both cases of isothermal and cyclic conditions on account of the formation of a dense and slow-growing Al2O3 layer on the coating surface.