مجله علوم و مهندسی خوردگی
سال هفتم شماره 2 (پیاپی 24، تابستان 1396)

7075 aluminum alloys are widely used in industry, due to their properties such as specific strength and good fatigue resistance. The nitride multilayer coating, which applied by physical vapor deposition (PVD) process have shown good improvement in abrasive resistance, but their columnar structure led to decreasing corrosion resistance behaviors. In this paper, multilayer coatings of Ti/TiN with 6 layers applied on Al 7075 as a substrate by high vacuum magnetron sputtering. Then GIXRD, FESEM and AFM were used to assessed phase behavior, structure and morphology of the coating, respectively. As well as the corrosion behavior of coating was investigated by using electrochemical impedance in simulated body fluid such as Ringer’s solution at 37 ºC. The results show production of homogeneity and uniformity multilayer coating, with cutting the column structure of titanium nitride, which increasing the charge transfer resistance by 56 times compared with Al 7075 and also no pitting was seen at the coated samples after 72 hours of immersion, with enhancing of corrosion resistance by 284 % compare to 48 h of immersion.

In the current study corrosion fatigue properties of 6063-T5 alloy were studied via standard test methods and the results were compared with the fatigue properties of the specimens at the same conditions. Results revealed that shot peening process caused a considerable increase on fatigue life of alloy. But on the other hand that presence of an aggressive environment around the alloy concurrent with applying a cyclic stress, Significantly reduced the fatigue life of the alloy. Further studies showed that the fatigue cracks initiate and propagate both from specimen external surface and internal bulk, while in shot peened specimens, these cracks initiate from underneath external surface. Furthermore, in propagation step, these cracks experience an evidence deviation from their expected direction or branch formation in case of facing to an impurity, phase or precipitation i.e. Mg 2Si. microstructural studies on Corrosion Fatigue fracture surfaces showed the occurrence of hydrogen embrittlement phenomenon. Furthermore, there was a significant different between the fatigue fracture surfaces compare to corrosion fatigue fracture surface. Dimensions, and crack propagation of fatigue specimens were completely different from corrosion fatigue cracks.

In this study, the effect of processing time on microstructure and corrosion behavior of coatings fabricated by plasma electrolytic oxidation (PEO) on pure titanium substrate was investigated. All PEO processes were performed in aluminate based electrolyte and under constant applied voltage (420 V) and at three times (180, 240 and 300 s). By studying the surface morphology of coatings, it was found that the coating formed at 180 s had the most compact and uniform structure with smallest micropores and the least porosity. The XRD pattern of this coating showed that the coating is composed of rutile, anatase and TiAl2O5 phases. Also, studying the corrosion behavior of coatings with potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in 3.5 wt.% NaCl solution proved that the coating formed at 180 s exhibited the most noble corrosion potential, the lowest corrosion current density and the most polarization resistance and so the best corrosion resistance among all samples.

In this research, the anti-corrosion resistance of titanium/zirconium based conversion coating on Al-2024 substrate was investigated. Ti/Zr solution practical parameters such as acids concentrations(1, 5, 10 g/lit) solution pH (3, 4, 5), and immersion time (1, 3, 5 min) were optimized Obtained result from DC (Direct polarization) and EIS (Electrochemical Impedance Spectroscopy) revealed that Ti/Zr conversion coating with the acid concentration of 1 g/lit for both H2ZrF6 and H2TiF6 solution pH equal to 4 and immersion time of 3min had the highest anti-corrosion performance. Corrosion current density and low frequency impedance values for untreated substrate are 48.1 μA/cm2 and 1439 ohm.cm2 respectively. These values for optimized Ti/ Zr conversion coating are 6.1 μA/cm2 and 4206 ohm.cm2. Also, field emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM) images indicated the presence of fine and compact particles on the surface of Ti/Zr treated substrate

Artificial neural network model is a high precision predictive tool for unknown values without performing the related experiments. It can be developed and utilized for prediction of nonlinear corrosion processes. In this way, owning a numer of input values, the output can predicted, exactely. In present work, first, the effect of holding time, hydrodynamic conditions and temperature were investigated on the inhibiting efficiency of 7-Hydroxyphenoxazone on steel corrosion in 1.0M HCl solution by electrochemical impedance spectroscopy (EIS). Then, the experimental variables such as concentration, immersion time, and hydrodynamic condition were taken as the input values and the corrosion inhibiting efficiency as the output value of artificial neural network model. Results showed that by increasing immersion time up to 8 hours, with 100ppm of 7-Hydroxyphenoxazone, the polarization resitance increases from 1660 to 2260 Ωcm2 and inhibition efficiency reaches to 91%. Also, by increasing rotational speed up to 500 rpm and temperature up to 550C, the inhibition efficiency decreases from 86.6% to 24% and 60%, respectively. The prediction results of artificial neural network indicated the good agreement with experimental data and the trained values of artificial neural network predicted the inhibiting efficiency values with average error less than 1%.

Polyaniline and polyaniline-clay nanocomposite were synthesized in supercritical carbon dioxide medium. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were used to determine the morphology and chemical structure of the synthesized samples, respectively. Synthesized polyaniline and polyaniline-clay nanocomposites were used as biocompatible corrosion inhibitor to reduce the corrosion rate of carbon steel in hydrochloric acid solution. For this purpose, polyaniline and polyaniline-clay nanocomposite was individually added to a 0.5 M hydrochloric acid solution and corrosion of carbon steel immersed in the solution, was evaluated using electrochemical impedance spectroscopy technique (EIS) and Tafel polarization. Tafel polarization results confirmed the corrosion rate reduction from 15 mm/ year to 6 mm/year in the presence of polyaniline-clay nanocomposite. Based on EIS Results the polarization resistance increased from 11 ohm.cm2 to 36 ohm.cm2 when polyaniline-clay nanocomposite was added to the acidic solution, and these results confirmed the reduction of corrosion rate at a rate higher than 60 percent.

In this study, alumina was coated on Al2024 and Al6061 using plasma electrolytic oxidation method and the effects of Al substrate and applied voltage on the properties of coated samples were investigated. Scanning electron microscopy and X-ray diffraction were used to study the microstructure of coated samples and microhardness and corrosion tests were employed to evaluate the final properties of the coats. The results showed that with increase of applied voltage, α-alumina increased in the coats of both alloys however it enhanced more moderately for Al2024. Generally, the amount of α-alumina in Al2024 specimens was less than that of Al6061. The presence of more alloying elements in Al2024 led to the more metallic ions in its coat preventing the gammato- alpha alumina transition. Moreover, the highest corrosion resistance belongs to the Al2024 coated by 400 V and the Al6061 coated by 425 V and the corrosion potential values for them were measured -0.571 V and -0.549 V, respectively. The corrosion test also indicated that 67-68% α-alumina in the coatings can lead to the highest corrosion resistance. The microhardness test indicated that the maximum hardness value belongs to the samples coated by 450 V so that the values for the Al2024 and Al6061 coats were 1360 and 1475 Vickers, respectively.