dc polarization

The increasing application of magnesium alloys in various industries on the one hand and its low corrosion resistance on the other hand, have been a new challenge for researches around the world. In this study, vanadium conversion coating was investigated as a method for protection of AZ31 alloy. In the first section, the importance of surface pretreatment for applying conversion coating was studied. In the second part, the duration of immersion in the solution was investigated and the results showed that the coating reaches to its maximum resistance (8100 Ohm.cm2) in 60 minutes immersion. Then, in the next section, it will be seen how the addition of 1 g/l copper sulphate not only reduced the optimum immersion time to 30 minutes, but also increased the polarization resistance of the coating from 250 to 14300 Ohm.cm2. In addition it is clarified that this significant change is in compliance with the polarization test in which the corrosion current density decreased from 33.4×10-6 to 5.3×10-7 A/cm2. In the final section, the images of the FE-SEM showed that the surface of vanadium conversion coating was inherently cracked. Also, reducing the number of pits and increasing the thickness of compact and uniform layer next to the surface are the reasons of higher corrosion resistance for vanadium conversion coating in the presence of copper sulphate.

In this study, a commercial zirconic acid based pretreatment was performed on hot dip galvanized steel sheets via immersion process. The effect of pH on morphology, composition and electrochemical behavior of the conversion layer was investigated. Field emission scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray spectroscopy (EDS) were carried out to study the morphology and composition of the surface after treatment. Also, DC polarization and EIS were used to evaluate the corrosion behavior of the treated surface in 3.5 wt.% NaCl. Electrochemical results revealed that the best anti-corrosion performance with the lowest current density and highest Rp value, belongs to the conversion coating obtained in acid concentration of 3% and pH= 5 for galvanized steel samples. EDS results disclosed that conversion coating consisted of: Fe, Zn, O, Zr and C confirming a dense ZrO2 layer is formed on galvanized steel samples. Furthermore, FE-SEM images showed that a uniform and compact layer of conversion coating is obtained on galvanized steel surface in pH=5 which corroborates EDS and electrochemical consequences.

effect of practical parameters such as solution pH، temperature and dipping time on coating morphology and anticorrosion resistance of coating was evaluated. Anti-corrosion resistance of so-called nanoceramic conversion coating on cold rolled steel (CRS) substrates in different solution pH، temperatures and immersion’s time، utilizing DC Polarization was studied. The conversion coating which was formed at dipping temperature of 20 °C، pH=5 and immersion’s time of 60 seconds had the best anti-corrosion resistance. The morphology of the conversion coating surface formed at different pH was investigated using Atomic Force Microscopy (AFM). Images obtained from the microscope indicate that the conversion coating which formed in pH=5 was more consistent and had finer aggregates in comparison with pH=4. 25 coating. Field Emission Scanning Electron Microscopy (FE-SEM) of conversion coating formed in 20 and 40 °C indicated that surface of conversion coating with higher temperature had micro cracks and was not uniform، also nodules on the scale of tens of nm (40–60 nm) in size، was observed on the surface. J. Color Sci. Tech. 6 (2012)، 9-18© Institute for Color Science and Technology.

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.