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Formation of Cerium Conversion Coatings on AZ31 Magnesium Alloy
Basit Raza Fazal,Sungmo Moon 한국표면공학회 2016 한국표면공학회지 Vol.49 No.1
This review deals with one of the surface modification techniques, chemical conversion coating and particularly cerium-based conversion coatings (CeCC) as a promising substitute for chromium and phosphate conversion coating on magnesium and its alloys. The CeCCs are commonly considered environmentally friendly. The effects of surface preparation, coating thickness, bath composition, and e-paint on the corrosion behavior of CeCCs have been studied on the AZ31 magnesium alloy. This review also correlates the coating microstructural, morphological, and chemical characteristics with the processing parameters and corrosion protection. Results showed that the as-deposited coating system consists of a three layer structure (1) a nanocrystalline MgO transition layer in contact with the Mg substrate, (2) a nanocrystalline CeCC layer, and (3) an outer amorphous CeCC layer. The nanocrystalline CeCC layer thickness is a function of immersion time and cerium salt used. The overall corrosion protection was crucially dependent on the presence of coating defects. The corrosion resistance of AZ31 magnesium alloy was better for thinner CeCCs, which can be explained by the presence of fewer and smaller cracks. On the other hand, maximum corrosion protection was achieved when AZ31 magnesium samples with thin CeCCs are e-painted. The e-paint layer further restricts and hinders the movement of chloride and other aggressive ions present in the environment from reaching the magnesium surface.
Acid Pickling/polishing of AZ31 Magnesium Alloy
Basit Raza Fazal,Sungmo Moon 한국표면공학회 2016 한국표면공학회지 Vol.49 No.3
This article reports a new chemical bath for preparing a mirror-like surface of AZ31 Mg alloy. In order to find an appropriate chemical polishing solution, four different acidic solutions of sulphuric acid, nitric acid, acetic acid and a specially designed mixture of nitric acid and acetic acid were investigated in view of the changes in surface appearance, roughness and dissolution rate of AZ31 Mg alloy. The surface scales on AZ31 Mg alloy were readily removed by all the acidic solutions, but a reflective surface was produced only by etching in the specially designed solution, and only after a specific etching time. The surface roughness increased with etching time in sulphuric acid, nitric acid, and acetic acid, but it lowered after a specific etching time in the specially designed mixture of nitric acid and acetic acid. Dissolution rate of the alloy in the specially designed mixture of nitric acid and acetic acid appeared to be more than twice than that in separate nitric acid or acetic acid. In this work, we recommend the mirror-like surface of AZ31 Mg alloy obtained by polishing for an optimum time in a mixture of nitric acid and acetic acid for following surface finishings, chemical conversion coating, electroplating, electrophoretic painting and anodizing treatment.
Basit Raza Fazal,Sungmo Moon 한국표면공학회 2017 한국표면공학회지 Vol.50 No.3
This research was conducted to investigate the electrochemical properties of the thin air-formed oxide filmcovered AZ31 Mg alloy. Native air-formed oxide films on AZ31 Mg alloy samples were prepared by knifeabrading method and the changes in the electrochemical properties of the air-formed oxide film were investigated in seven different electrolytes containing the following anions Cl<SUP>-</SUP>, F<SUP>-</SUP>, SO4<SUP>2-</SUP>, NO3<SUP>-</SUP>, CH3COO<SUP>-</SUP>, CO3<SUP>2-</SUP>, and PO4<SUP>3-</SUP>. It was observed from open circuit potential (OCP) transients that the potential initially decreased before gradually increasing again in the solutions containing only CO3<SUP>2-</SUP> or PO4<SUP>3-</SUP> ions, indicating the dissolution or transformation of the native air-formed oxide film into new more protective surface films. The Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) showed that there was growth of new surface films with immersion time on the air-formed oxide film-covered specimens in all the electrolyte. The least resistive surface films were formed in fluoride and sulphate baths whereas the most protective film was formed in phosphate bath. The potentiodynamic polarization curves illustrated that passive behaviour of AZ31 Mg alloy under anodic polarization appears only in CO3<SUP>2-</SUP>, or PO4<SUP>3-</SUP> ions containing solutions and at more than -0.4 VAg/AgCl in F<SUP>-</SUP> ion containing solution.