مجله علوم و مهندسی خوردگی
سال دوازدهم شماره 4 (پیاپی 46، زمستان 1401)

The aim of this research is to investigate the effect of 7 variables of the plasma electrolytic oxidation (PEO) coating process, including the concentration of calcium acetate and potassium hydroxide salts of the electrolyte, frequency, current density, duty cycle, treatment time, and the distance between the anode and cathode on the corrosion current density of the coatings and studying the correlation between the corrosion resistance with the surface roughness and wettability of the coatings on pure titanium using the Taguchi design of experiments method. The surface roughness of the coatings was evaluated with the help of a surface profilometer and their wettability was assessed by measuring the contact angle of their surface with a drop of water. The corrosion resistance of the coatings was investigated using the potentiodynamic polarization test after 72 hours exposure of the samples to Ringer's physiological solution. The surface structure and chemical composition of the optimal coating were studied by a scanning electron microscope and X-ray diffractometer, respectively. The results showed that duty cycle and frequency were the most effective and the least important variables on the corrosion current density of coatings, respectively. Also, the optimal conditions for forming a coating with the lowest corrosion current density (0.0018 μA/cm2), included the concentration of 7.5 and 3 g/l of calcium acetate and potassium hydroxide salts in the electrolyte, respectively, frequency of 2000 Hz, current density of 5 A/dm2, duty cycle of 50%, treatment time of 15 minutes, and the distance between anode and cathode of 3 cm. In general, the corrosion resistance of the coatings decreased with the increase in their surface roughness and wettability.

"Risk" is a mathematical expression obtained by multiplying the probability of an event occurring by its consequences. For many years, a variety of qualitative, weighting, and scoring methods have been used in the pipeline risk assessment in the industry, but the output of these methods has been qualitative, while in areas Scoring and indexing methods are not used to make reasonable conclusions. That is why the development of a methodology whose results are more accurate, applicable and more cited is very interesting. In this study, a new methodology was developed to evaluate the corrosion risk of gas pipelines where the probability of failure due to soil side corrosion and internal corrosion threats is presented as a function of the time to failure. The consequences of a disability are also determined by the nature of the fluid and the environment affected by the leakage, and ultimately the risk of failure is presented in the form of "Outcome per unit of time". To evaluate the applicability of this methodology, a 10-km segment of a 30-inch pipeline was evaluated by the present methodology. The results showed that the highest risk of this pipeline was for the parts that were encased inside the casing and the risk was 15096.44 $/ year.

Al2O3-C refractories have been widely used in the continuous steel casting process due to their remarkable properties at high temperatures, such as low thermal expansion, thermal conductivity, and high thermal shock resistance. In the present study, the mechanisms and factors of the destruction of Al2O3-C tundish stoppers in the continuous casting process of Khuzestan Steel Company have been investigated. Samples were taken from different stopper sections. In order to study the causes of corrosion and failure, the microstructure and chemical composition of the samples have been examined by scanning electron microscope (SEM), energy dispersive Xray spectrometer (EDS), and X-ray diffractometry (XRD), respectively. The results showed that the destruction of the tundish stopper started with the oxidation of refractory carbon during the pre-heating process. It affects the reduction of corrosion resistance of black refractory due to the two aspects of creating porosity and increasing the wettability of refractory. In addition, refractory decarburization leads to a decrease in the thermal shock resistance of the stopper and will cause a decrease in its useful life of that. After the refractories are exposed to melt and slag, the main stage of corrosion and destruction of the refractories begins. At this stage, the molten steel and slag penetrate the refractory to such an extent that the particles separated from the refractory network merge with the molten steel/slag, and the refractory base is destroyed. Finally, the slag completely covers the refractory, and the alumina particles are dispersed as scattered islands in the ground. Refractory destruction is especially clear in the slag line; As long as the diameter of the stopper in the slag line is reduced (necking), it will not be possible to continue the casting process.

In the present study, the corrosion behavior of projection friction stir spot welding of 2024 aluminum alloy has been investigated and compared with the corrosion behavior of the base metal. Welding of sample was performed with the rotation speed of 2500 rpm and the dwell time of 12 seconds. The penetration depth of the tool on the surface of the sample was 0.1 mm, the diameter of the tool was 16 mm, and a circular protrusion with a diameter of 10 mm and a height of 0.4 mm was placed below the working table. By using Tafel polarization test method and standard Scanning Kelvin Probe through potential chang of weld surfaces and base metal as well as chang in corrosion current density, the corrosion behavior of projection friction stir spot welding was investigated. The results showed that the corrosion potential of the weld area is reduced compared to the base metal, although this reduction has not led to a significant increase in corrosion current density. Also, the welding zone has higher corrosion current density than the base metal region.

A smart nanostructured multilayered coating based on sol-gel/ polyelectrolytes/inhibitor/graphene oxide is developed via layer-by-layer (LbL) technique to provide effective corrosion protection for Al2024 substrates. The multilayered coating is consisting of a strong polycation (i.e., poly diallyldimethylammonium chloride (PDADMAC)), a weak polyanion (i.e., poly acrylic acid (PAA)), a corrosion inhibitor (i.e., Benzotriazole), and graphene oxide (GO). The local pH change in case of corrosion cause the loss of electrostatic attraction between polyelectrolytes, leading to increased permeability of the polyelectrolyte layer. Consequently, the corrosion inhibitor is released to prevent the corrosion process, providing a self-healing effect. Besides, the graphene oxide acts as a barrier layer, preventing the permeation of water and corrosive agents. The morphology, structure and chemical compositions are characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analysis (EDS). To ensure the deposition of graphene oxide layer, Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy are performed. UV-visible spectrophotometry (UV-vis) is used to study the pH responsive release of corrosion inhibitor. To study the corrosion resistance and self-healing properties, electrochemical impedance spectroscopy (EIS) is performed in 3 % NaCl. The samples with polyelectrolyte layer show improved corrosion protection compared to the sol-gel sample (control). However, the samples with graphene oxide show the highest resistance, which is ~10X higher than the control sample after 2 weeks and its value is equal to 105.42 ohm.cm2.

Due to the efficiency of 17-4PH steel in gas turbines and contact with external particles such as dust and foreign objects, physical deposits such as oil vapors will undergo cracks, wear, and chemical corrosion during operation and will be removed from the cycle of use. Part replacement is not cost-effective due to the high cost of raw materials. In this research, the corrosion resistance of 17-4PH steel cladding and stellite6 on 17-4PH steel substrate was investigated by the Tafel polarization test; In the cyclic polarization curve for 17-4PH steel, the direct scan current density is lower than the reverse scan, and positive hysteresis is seen in the cyclic polarization curve. Also the partially formed passive layer is unstable and fragile. In the 17-4PH cladding, the direct scan current density is lower than the reverse scan, and positive hysteresis is observed in the polarization curve. Also the formed passive layer is resistant to the formation of new holes. In the cover of stellite6, according to the shape of this cover, it has high pitting corrosion resistance in this solution; It forms a negative hysteresis curve and its inverse curve is located in a lower current density than the straight curve; Therefore, the corroded areas can repair themselves by forming a passive layer. By examining the results, it was found that the corrosion rate of stellite6 and stainless steel claddings has decreased by 91% and 50% compared to the substrate.