مجله علوم و مهندسی خوردگی
سال هشتم شماره 4 (پیاپی 30، زمستان 1397)

This paper presents a review on corrosion mechanisms in archaeological bronzes buried in soil. The archaeological bronze objects show different corrosion mechanisms due to placing in different burial environments with various conditions and characteristics, by which various corrosion morphologies are observed in these objects. The main corrosion events in archaeological bronzes are occurrence of uniform corrosion with formation of noble patina and corrosion products such as oxides and basic carbonates of copper on the surface, and also active corrosion or bronze disease due to influence of chloride ions on copper and formation of basic copper chlorides. Furthermore, different environments lead to formation of different morphologies in archaeological bronzes that are called as type I and type II corrosion morphologies, which form under effecting of chloride ions amount in the soil environment. On the other hand, microbially influenced corrosion occurs in archaeological bronzes buried in anaerobic soils leading to formation of copper sulphide products. In order to explain these corrosion mechanisms in archaeological bronzes, some examples of studies on archaeological bronze objects from Iran are presented here.

In this research, the corrosive effect of sweet crude oil was studied by Electrochemical Impedance Spectroscopy (EIS), Tafel polarization and Electrochemical Noise (ECN), on Carbon steel samples immersed in the extracted water of desalter vessel. According to the EIS results, after 4 hours, the impedance modulus at 0.01 Hz frequency were 1376 at 25 °C and 625 Ohm.cm2 at 85 °C. The relatively low variations of impedance modules at 25 and 85 °C can be caused by the formation of Iron carbonate film on carbon steel at temperatures above 60 °C, in the presence of CO2 of the sweet crude oil. Also, the results of Tafel polarization confirmed that the corrosion rate due to the significant increase of temperature, was equal to 5.9 MPYand 10.7 MPY, at 25 and 85 °C respectively. The ECN results at 65 and 85 °C demonstrated that the large contribution of low-frequency events on these immersed carbon steel samples is an indicative of general corrosion due to the discontinuous formation of carbonate film on the carbon steel surface. The discontinuous formation of carbonate film was also confirmed by scanning electron microscopy(SEM) micrographs. Then, for inside corrosion control of the desalter vessel, the best protective commercial coating has been selected considering the corrosion rate of carbon steel samples immersed in the extracted water of desalter vessel. To do so, the EIS tests in the special immersion cell and pull off tests were done on the solvent free Epoxy coatings with different hardners. Although all three solvent free coating samples can exhibit a suitable corrosion resistance at 80 °C, but on the comparison between them, the performance of epoxy coating with phenolic hardener is better than epoxy coating with poly amino- amide hardener.

Temperature effect on the corrosion behavior of API 5L-X65 in the presence of 0.05, 0.1, and 0.5M concentration of Carbonate, Bicarbonate and Carbonate/Bicarbonate was investigated. Potentiodynamicpolarization and Mott-Schottky tests at varying temperatures were used to investigate the corrosion behavior and “the semiconductor” properties of X65 steel’s passive layer. Following the addition of 3% NaCl these tests were repeated and data compared. The polarization curves showed that the passive layer in the carbonate solution was more stable than in the bicarbonate solution: as the passive current density was lowered. It was also revealed that in the bicarbonate solution, passivation happened (for which lowering the temperature was more effective). Mott-Schottky test was more temperature dependent than the polarization test (in carbonate and carbonate/bicarbonate solutions), as the temperature increase did not show any significant effect on the passivation current of steel. On the other hand, the Mott-Schottky test showed that in a solution without chloride ion the increase in temperature resulted in increase of the electron donor density (ND). This change was related to an increase in the number of spot defects of the passive film. The semiconductor behavior of passive layer was n-type, and the electron donor densities in the order of 1021-1022 cm-3. At concentrations of 0.05 and 0.1 M of carbonate and carbonate/bicarbonate solutions (in the presence of chloride ion) the increase in temperature increased the ND. But as the solution concentration increased to 0.5 M, the temperature increase had less effect on the increase of ND. Two sets of results in carbonate and carbonate/bicarbonate solution concentration, (in the presence of chloride ion) by Mott-Schottky test were in good agreement with that of polarization test (where the temperature increase did not alter the passivation current density).

The presence of organic molecules containing active groups in the aqueous extract of Kiwi fruit shell converted it as a low cost and rich source of green inhibitors in corrosion control of carbon steel. In the present study, the synergistic inhibition effect of Kiwi fruit shell extract and zinc nitrate on the corrosion behavior of carbon steel in 3.5 wt.% NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. By means of scanning electron microscopy (SEM) the morphology and chemistry of surface were investigated. In addition, the Fourier transform infrared (FT-IR) spectroscopy was applied to recognize the functional groups of Kiwi fruit shell extract and its mixture with zinc nitrate. The EIS results indicated that after 24 h immersion of steel sample in NaCl solution containing 200 ppm Kiwi fruit shell extract and 600 ppm zinc nitrate the maximum inhibition efficiency of 85% was achieved. The potentiodynamic polarization analysis demonstrated that the corrosion inhibition dominantly occurred through anodic half-reactions. The interactions and complex formation of organic molecules of Kiwi fruit shell extract with iron cations and also the zinc cations adsorption on the cathodic sites resulted in the deposition of effective corrosion resistant film on the mild steel surface. Moreover, the images obtained from SEM analysis proved that the increase in the concentration of zinc nitrate resulted in the steel surface corrosion resistance increment so that with addition of 600 ppm zinc nitrate to the solution a uniform layer with less porosity was formed over the surface. These observations confirm the successful adsorption of inhibitor molecules on mild steel surface.

Conversion Coatings are one of the effective ways for improving adhesion of organic coatings to metallic substrates and consequently increasing anti-corrosion performance of a fully coated system. In the current research, multi-cationic conversion coating formed from sources such as hexafluorotitanic acid, sodium molybdate, phytic acid and nickel sulphate was studied and investigated. Results obtained from electrochemical impedance spectroscopy and polarization revealed that conversion coating which formed in the presence of sodium molybdate has not appropriate polarization resistance and the reason for this weak performance investigated by field emission scanning electron microscope. Obtained results showed that in the presence of sodium molybdate, there were abundant micro-cracks on the surface of the coating. After optimizing the multicationic conversion coating, an epoxy coating applied on the surface of bare steel and conversion coated one. The results obtained from adhesion and salt-spray tests indicated that the multi-cationic conversion coating causes improvement in adhesion strength of epoxy coating to steel substrate and finally increased the anti-corrosion performance of the system.

The main objective of the present study was to apply a polymer coating made of Polycaprolactone/chitosan (PCL/Ch) /1%wt. Baghdadite to an anodized AZ91 alloy through the immersion method in an attempt to improve these alloy's resistance to corrosion, bioactivity, and biocompatibility. The results of the corrosion test were indicative of an improvement in the corrosion resistance and a 103-fold decrease in the corrosion current density of the anodized AZ91 alloy as a result of applying the polymer-ceramic coating. Moreover, applied the polymer-ceramic coating increased the roughness of AZ91 from 0.329 ± 0.02 to 7.792 ± 0.34 μm. Simulated body fluid (SBF) was used to evaluate the apatite formation ability of the samples. The results showed the formation of an apatite layer on the sample surface which can be considered as a bioactivity criterion.