مجله علوم و مهندسی خوردگی
سال هفتم شماره 1 (پیاپی 23، بهار 1396)

Plasma electrolytic oxidation (PEO) process using a pulsed current regime was performed on 7075 Al alloy in silicate based electrolytic bath with and without potassium permanganate as an additive. The results showed that with adding potassium permanganate to the electrolyte, an oxide coating containing Mn as the major element, Si and Al with a higher thickness was formed, which showed different microstructure and surface morphology than that produced in bath containing no additive. According to the observations, oxygen reduction became intense, the sparking quality on the surface was improved and the occurrence of destructive discharges were prevented and thus, a uniform and compact coating with lower defects was achieved. The corrosion behavior of the PEO coatings was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique in 3.5 % NaCl solution at pH 4. The results showed that the coating produced in the presence of potassium permanganate had the higher corrosion resistance due to its higher thickness and compactness of the inner layer.

Well-known corrosion and wear resistance of electroless nickel phosphorus (NiP) coatings led studies on upgrading of mechanical and chemical properties and feasibility of its application in different industries. In this study, the effect of coating applying time on corrosion and wear resistance of the electroless nickel phosphorus-nano graphene oxide composite coating (NiP/GO) on st37 steel substrate was studied, comparing the simple electroless nickel phosphorus coatings. According to the polarization and electrochemical impedance spectroscopy results, NiP/GO deposit possessed more corrosion resistance than NiP coatings and the appropriate coating time was 60 minutes. The results also revealed that the presence of graphene oxide in nickel phosphorus coatings and increasing of coating applying time causes increasing of the coating microhardness by 279.8 HV and the friction coefficient by 19.33%.

In this study, the ion beam sputtering method was used to deposit the optimized ZrN coating with 1 μm thickness on AZ91 magnesium alloy at 400 °C and then the HA coatings were created on them at temperatures of 150 °C, 200, 250, 300, and 350 °C for 360 minutes. The profilometry and indentation methods for thickness and film adhesion evaluation showed that, from 150 to 300 °C the thickness decreases from 8.1 μm to 3.3 μm and dp/dr ratio increases from 0.07 to 0.2 kg/μm. The analysis of the x-ray diffraction pattern by Williamson - Hall method showed that, by increasing the temperature to 300 °C the grain growth is dominated therefore the grain size reaches to 0.36 μm, but with increasing the temperature to 350 °C, grain size decreases to 0.12 μm. The potentiodynamic polarization test showed that, all coated samples have corrosion potential close to AZ91 about 1.54 Volt vs. SCE and the corrosion current density in the deposited samples at 300°C showed the minimum current density about 0. 33 μA/cm2.

Plasma electrolytic oxidation was employed for producing alumina, alumina/titania composite and alumina/ titania composite coatings containing sodium tungstate additive. These coatings were produced in a silicatebased electrolyte on 7075 aluminum alloy using unipolar and bipolar waveforms with anodic/cathodic duty aspect ratios of 1 and 0.5. The results showed that the morphology of the coatings surfaces depends on the used waveform. By increasing the cathodic duty cycle, pancake-like morphology was converted to crater-like one. The incorporation of TiO2 nano-particles into the coatings, led to the widening the micro-pores when unipolar waveforms were used. The results revealed that the incorporation of TiO2 nano-particles doesn’t depend on the used waveforms. Titania nano-particles were incorporated into the coatings via a physical entrapping mechanism, while the adsorption mechanism of tungstate ions was electrophoretic. The corrosion resistance of the coatings was increased by increasing the cathodic duty cycle. Furthermore, the use of titania nanoparticles led to a reduction of corrosion resistance and the use of tungstate additive in bipolar waveform compensated the reduced resistance. The corrosion currents recorded 3, 5 and 3 nano-amperes for simple, composite and additive contained composite coatings respectively.

Inconel 738 superalloy is widely used for hot-section components of gas turbines. These hot-section components are exposed to an extremely oxidative environment at high temperatures and consequently their life span is dramatically decreased. Considering this problem, NiCrAlY coatings can be effectively used to protect the hotsection components. In this study, laser cladding process was used to coat a NiCrAlY alloy on the surface of Inconel 738 substrate for the purpose of improving the high-temperature oxidation resistance. In order to compare the oxidation behavior, oxidation tests at temperature of 1200 ˚C were individually conducted on Inconel 738 substrate and freestanding laser-cladded NiCrAlY coating. Laser cladding of NiCrAlY alloy resulted in the formation of hypo-eutectic dendritic γ/β structure. Moreover, the results of oxidation tests showed that the oxidation resistance of the laser-cladded coating was significantly better than that of Inconel 738 substrate and this coating can be an effective choice for protecting the hot-section components made of Inconel 738 superalloy.

In this study two step method was used in order to fabricate superhydrophobic surface with high corrosion resistance and chemical stability on Ti substrate. At first shot peening treatment was applied on Ti surface and then etching operation was carried out on it. Finally, silane operation was done on Ti surface in order to reduce the surface energy. The effect of shot peening treatment on wettability, corrosion behavior and long term chemical stability of fabricated superhydrophobic surface was studied. The results indicated that, the network fiber structure is in nanometer scale which bonded with functional group of methyle tricolor silane on Ti substrate. Corrosion resistance of superhydrophobic surface was analyzed by immersion, potentiodynamic polarization and EIS test in 3.5%wt NaCl solution. Also long tern stability of fabricated coating was studied by immersion in 3.5%wt NaCl solution for different times. The results showed that the highest contact angle of 159° was obtained for pre- shot peened samples. The corrosion resistance evaluation studies also indicate the high corrosion resistance of pre shot peened sample. Also the surface with high corrosion resistance indicate highest chemical stability and contact angle.

In this study stress corrosion cracking behavior of X70 pipeline steel after shielded metal arc welding has been investigated. For this purpose, the alloy was welded by low, medium and high rates of heat input. Microstructures of specimens were studied using light and electron microscopes. Stress corrosion cracking susceptibility was evaluated using slow strain rate tests in C2 simulated soil solution in free corrosion potential. Scanning electron microscopy was used to examine the fracture surfaces. Corrosion of different areas of weld was evaluated by potentiodynamic polarization test method. Results showed that by increasing heat input, tensile strength and hardness of welded metal reduces, larger heat affected zone (HAZ) is produced and the grain size is increased. Slow Strain Rate Test (SSRT) results showed that increasing the welding heat input will reduce the susceptibility to SCC. SEM images obtained from samples confirmed brittle fracture of specimens tested in the corrosive environment. It was also observed that the produced cracks might grow in transgranular mode. Potentiodynamic polarization tests showed the lowest rate of anodic dissolution for welded samples with higher heat inputs. Overall, results showed that by reducing heat input the susceptibility to SCC increases; this is due decrease in areas which are sensitive to crack growth and initiation.