مجله علوم و مهندسی خوردگی
سال پنجم شماره 2 (پیاپی 16، تابستان 1394)

Active Screen plasma nitriding is a new process that has recently developed surface engineering. Also while the elimination of the problems the conventional plasma nitriding، many developments in this field has provided. In this research، the effects of plasma nitriding parameters investigated on samples with different geometry. Samples were prepared were nitrided under the atmosphere content of 20%H2-80%N2، at the temperature of 520 Co، the duty cycles of 30%، 50% and 80% and with the frequencies of 10 kHz for 6 hours. Then the property of grooves surfaces and the behavior of corrosion investigate by experiments of the SEM، XRD، Roughness Micro Hardness measurement and test corrosion. The results of the experiments showed that the surface of the plasma nitriding samples are covered by Hexagonal form of the particles that the formation of this particles in plasma nitriding samples are due to sputtering of active screen and deposition on the surface during the process. With increasing thickness of the groove، frequency and duty cycle، roughness of surfaces raise. Also micro hardness rise with increasing the thickness of the groove and the duty cycle resulting from the increase in percent of deposition particle nitride. Hollow cathode phenomena occurred in sample with 2mm groove and 80% duty cycle in CPN. This will result in over heating of the sample which leads to a decrease hardness of the surface and an increasing in the Roughness of the surface. Also the results of the experiments XRD showed that، in the surface all of the samples form of γ : Fe4N and ε: Fe2-3N phases.

In this study, the influence of black pepper on the corrosion behavior of copper samples in sulfuric acid has been investigated. The corrosion test and morphology study were carried out by potentiodynamic polarization and Scanning Electron Microscopy (SEM) respectively. Fourier Transform Infrared (FTIR) technique was used for determination function groups in black pepper. The black pepper, as inhibitor, decreased corrosion rate of Cu in acidic media by affected cathodic and anodic polarization curves. The present of black pepper has shown more effect on the cathodic polarization curve. The results showed that Inhibition Efficiency (IE) increased when the black pepper concentration increased and become highest IE=86% at 548 mg/L. the SEM image confirmed that the black pepper as natural inhibitor prevented localized corrosion of copper.

Recent research findings have indicated that ionic liquids can be used as effective alternatives for harmful organic inhibitors. In this paper, the effect of an imidazolium based ionic liquid (1,4 -di [1' –methylene- 3'- methyl imidazulim bromide]- benzene) on the corrosion of mild steel in 1M H2SO4 is studied through electrochemical impedance spectroscopy and polarization measurements as well as scanning electron microscopy. The slope of anodic and cathodic branches revealed that the compound is a mixed-type inhibitor. Moreover, the trend and magnitude of the parameters extracted from polarization curves and impedance spectra showed the significant impact of concentration on the performance of inhibitor adsorbing on the metal surface based on Langmuir isotherm.

The aim of this paper is identification of conditions of burial environment, corrosion events occurred in copper/bronze objects and characterization of relationship between soil environment and corrosion events in archaeological metals of Haft Tappeh site. For this purpose, 20 metal samples were selected from Haft Tappeh collection. The samples were observed and analyzed by optical microscopy, SEM-EDS and XRD methods. The different soil conditions were examined such as pH, physical nature and soluble salts. The results showed that the soil environment of Haft Tappeh is heavily corrosive and amount of soluble salts –especially chloride- are significant in soil. On the other hand, the copper/bronze samples are corroded heavily and in some cases, whole artefact has converted to corrosion products. The corrosion morphology and chemical nature of corrosion products in the copper/bronze samples showed that the main mechanism occurred in Haft Tappeh bronzes is active corrosion or “Bronze Disease”. Also, some events such as selective dissolution of copper, are obviously visible in these metal objects. Finally, the corrosion phenomena in Haft Tappeh samples are strongly related to corrosive nature of soil in this site.

The present work deals with the process of electrochemical deposition of cobalt–tungsten /polychlorotrifluoroethylene (Co-W-PCTFE) nanocomposite coatings in the tartrate bath and electrochemical corrosion behavior in 3.5 wt.% NaCl solution. Surface morphology and composition of the composite coatings are characterized by energy dispersive X-ray (EDX), scanning electron microscopy (SEM). Corrosion behavior of coatings is carried out applying opencircuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS) and polarization techniques in 3.5 wt.% NaCl solution. The study reveals reduce in corrosion current density, important substitute in corrosion potential towards the positive direction with the incorporation of PCTFE particles in the Co-W matrix. Therefore, the significant improvement in corrosion resistance observed for Co-W-PCTFE nanocomposite coatings (7.587kΩ cm2) compared to Co-W (2.325kΩcm2). The results of SEM images indicated to changing morphology of alloys coatings in the presence of PCTFE particle sizes from micrometric to nanometeric sizes. The EDX analyzes confirmed the forming of Co-W-PCTFE nanocomposite coatings.

A study has been carried out to investigate the incorporation of sodium citrate into coatings formed on 1010 aluminum using DC plasma electrolytic oxidation (PEO). The effects of sodium citrate as electrolyte’s additive into silicate-base electrolyte, on the voltage response, microstructure, compositions and corrosion resistance of PEO coatings were investigated by scanning electron microscopy (SEM), energy-dispersive spectrum (EDS), X-ray diffraction (XRD) and cyclic polarization tests. To improve the corrosion resistance, Si3N4 nanopowder was added to electrolyte as suspension to fabricate nanocomposite coating. Results indicated that adding sodium citrate into electrolyte and its incorporation into the coatings will lead to more noble corrosion potential than samples without additive. Corrosion current density increased by adding this additive but current of anodic branch in maximum potential for all samples except one, was decreased by increasing of additive’s concentration.

In the present study، the microstructure and corrosion resistance of AISI 321 austenitic stainless steel، which is widely employed in the oil and gas industries due to high corrosion resistance، with GTAW process by welding with various constant current and pulsed current were investigated. Polarization tests were performed in NACL 3. 5% in order to analyze the corrosion behavior of base metal and welded test coupons and compare their corrosion resistance. Micro structural studies by metallographic of samples with optical microscope (OM) and scanning electron microscope (SEM) and polarization test results revealed that using PCGTAW (Pulsed current gas tungsten arc welding) reduce heat input up to 50%، dendrite branches break down and reduced heat inputs and HAZ width and increase corrosion resistance of weld metal that`s similar to base metal corrosion resistance.