مجله علوم و مهندسی خوردگی
پیاپی 2 (زمستان 1390)

Anticorrosive coating has been prepared through incorporation of synthesized nanopolyaniline into the epoxy ester resin. Anticorrosive performance of the manufactured coating was evaluated in the environments with different pH values based on the HCl and NaOH for acidic and basic environments, respectively. The anticorrosion measurements were performed by the electrochemical impedance spectroscopy (EIS) technique during 95 days of immersion time. Corrosion mechanism of coating was determined by the Nyquist plots analysis in different environments. The influence of pH variations on the anticorrosion performance of coating was quantitively investigated by the electrochemical modeling. The obtained results showed smart corrosion inhibition mechanism of nanopolyaniline particles and high anticorrosive ability of the coating. Where, the reaction between released anionic inhibitor and metallic cations causes to formation of a second barrier layer in the defected situations and enhancement of coating resistance against the corrosive ions. In addition, the coating showed better performance in the environments with less corrosive ions concentration.

A nanoceramic hexafluorozirconic acid based conversion coating free of phosphate salts was investigated. The effect of solution pH on coating morphology and corrosion resistance was evaluated. Corrosion resistance of nanoceramic conversion coating on cold rolled steel (CRS) substrates in different solution pH (3.5, 4.5 and 5.5) was studied utilizing DC polarization and Electrochemical Impedance Spectroscopy (EIS). The polarization tests revealed that sample was coated in pH=4.5 have the lowest corrosion current density and nobler corrosion potential in comparison with other coated samples. The results of EIS showed that this sample (pH=4.5) has the highest polarization resistance and the lowest capacitance. According to bode diagrams, this sample indicated greater phase angle (at high frequencies) and impedance modulus (at low frequencies). The morphology of the conversion coating surface formed at different pH was investigated using Field Emission-Scanning Electron Microscopy (FE-SEM). Images revealed that the morphological structure changed with increasing pH value.

In this study, the influence of gas tungsten arc welding (GTAW) on microstructure, strength and corrosion resistance of dissimilar weld joint between stainless steel SAF 2205 and stainless steel AISI 316L was investigated. Welding was accomplished by two different types of welding wires with codes ER 347 and ER 309L at the same conditions. To verify soundness of produced welded samples, nondestructive tests including visual inspection, liquid penetrant and radiography were performed. Metallographic samples were prepared from cross-section areas of weld joints to study microstructure of different regions of weld joints by optical microscopy and scanning electron microscopy. The strength of weld joints was determined by tensile tests. Corrosion resistance of weld joints was evaluated by electrochemical polarization and electrochemical impedance tests. In the weld metal which was produced by welding wire with code ER 347, the brittle sigma phase was created resulted in decreasing corrosion resistance of weld joint. According to the results of metallurgical investigations and corrosion tests, it was founded that for produced weld joints, welding wire with code ER 309L causing more strength and more corrosion resistance than welding wire with code ER 347.

This paper describe a study on atmospheric corrosion of special Pd-Ag alloy (Ag- 5%Pd- 2.5%Cu) in four gaseous environments contain air and CO2,SO2,H2S and NO2 in two temperature 25˚Cand 50˚C and four relative humidity (40%,60%,80% and 100%). Design of experiment was performed with Minitab15 software using taguchi method L16. The product composition was investigated with XRD low angle (grazing) and factsage software also used for simulating the reaction condition. Gloss meter tests were performed to samples to find the tarnish effect of the gasses. It was found that SO2 had the most and CO2 had the least corrosive effect on samples. With increasing in time and temperature corrosion was increased. And finally the most tarnishing gas was H2S and the least effect was from CO2.

Electrochemical and corrosion behavior of Ti-Al stable intermetallics (TiAl, Ti3Al and TiAl3) in H2O2 solution at different concentrations with constant pH of 12.5 was studied in this paper. As-casted alloys were fabricated under vacuum arc remelting (VAR) process. Then the samples were examined in H2O2 solution using different electrochemical analysis containing potential-time, potentiodynamic polarization and electrochemical impedance. The results showed that the concentration of the electrolyte had a considerable effect on the corrosion of the specimens. At higher concentrations an inductive element was also observed in the EIS curves. Anodic polarization behavior of titanium aluminide specimens in H2O2 was relatively similar, all showing passivity. However, the passive current density increased by increasing Al content in the intermetallic.

Magnesium is a metal with high biocompatibility and a necessary material for human body metabolism that because its desirable mechanical properties compared with Biodegradable polymers, has been studied at recent years for production of biodegradable materials at human body. In this study, calcium containing magnesium alloy and commercial pure magnesium was casting in the same terms and their chemical composition and phases was reconnoitered by Quantometer device, XRD analysis and optical microscopy. Then corrosion behavior of these materials was investigated by tafel polarization technique in two simulated body fluid solutions (SBF and Hank solutions) and the surface morphology of materials was studied with scanning electron microscopy. This research showed that adding calcium to magnesium as an alloying element increase the formation of calcium-magnesium phosphate layer but due to formation of microscopic galvanic cells between magnesium atoms (cathode) and calcium atoms and Mg2Ca intermetallic phase (anode), the corrosion rate of alloy will be increased. Also observed that corrosion behavior of calcium containing magnesium alloys largely depended on dispersion mode of Mg2Ca intermetallic phase.

The presence of hydrogen sulfide in sour gas and oil is the major factor in the occurrence of corrosion and damage to equipment in gas and oil industries. Hence, the corrosion of steels used in these industries, such as austenitic stainless steel 304L, 316L and, carbon steel 516 grade 70 were evaluated from the corrosion products formed in the alloys studied in the acidic environment containing H2S. XRD test and Pourbaix diagrams related to each element of the alloys in terms of test solution conditions and the presence of H2S were used to investigate chemical composition of the corrosion products formed on the surface of each alloy. Also the SEM images were used to investigate the morphology of formed corrosion products. Obtained results indicate difference in the surface layer of formed corrosion products on the surface of each alloy and significance of the surface layer in different resistances to corrosion of investigated alloys in this environment. The investigations show that FeS2+MoS2 layer on the surface of stainless steel 316L is more uniform and compact than the porous layer of FeS2 on the surface of 304L alloy and carbon steel.