Abstract
A novel method to functionalize the surface of inorganic coating by growing organic coating has been investigated based on microstructural interpretation, electrochemical assessment, and quantum chemical analysis. For this purpose, inorganic coating with magnesium aluminate, magnesium oxide, and titanium dioxide was prepared on magnesium alloy via plasma electrolytic oxidation (PEO), and, then, subsequent dip-coating method was used to tailor organic coating using diethyl-5-hydroxyisophthalate (DEIP) as organic molecules. The incorporation of TiO2 particles worked as a sealing agent to block the micro-defects which resulted mainly from the intense plasma sparks during PEO. In addition, such incorporation played an important role in enhancing the adhesion between inorganic and organic coatings. The use of DEIP as organic corrosion inhibitor resulted in a significant decrease in porosity of inorganic coating. Quantum chemical calculation was used to clarify the corrosion inhibition mechanism which was activated by introduction of DEIP. Thus, the electrochemical analysis based on potentiodynamic polarization and impedance spectroscopy tests in 3.5 wt% NaCl solution suggested that corrosion resistance of magnesium alloy sample was enhanced significantly due to a synergistic effect arising from the hybrid inorganic and organic coatings. This phenomenon was explained in relation to electron transfer behaviour between inorganic and organic coatings.
Highlights
Magnesium and its alloys have been applied in a wide range of lightweight engineering applications, such as the automotive, mobile, and biomedical fields, due to their good mechanical reliability based on low density[1, 2]
The surface morphologies of hybrid inorganic-organic coatings was illustrated in Fig. 1a–m; this process consisted of a two-step strategy involving plasma electrolytic oxidation for formation of the inorganic coatings and chemical treatment to load the surface with DEIP
A novel comprehensive method was proposed in this study to develop the heterogeneous multi-layer which resulted from the growth of DEIP organic coating on top of inorganic coating by plasma electrolytic oxidation in the electrolyte containing TiO2 particles
Summary
Magnesium and its alloys have been applied in a wide range of lightweight engineering applications, such as the automotive, mobile, and biomedical fields, due to their good mechanical reliability based on low density[1, 2]. Surface modification techniques, such as conversion coating, electroplating, anodisation, plasma electrolytic oxidation (PEO), organic coating, and vapour-phase process, have been developed to overcome the surface matter[7,8,9] Among these methods, PEO coating was reported to be an eco-friendly technique suitable for valve metallic materials in which protective inorganic coating formed on the metal substrate with an aid of plasma-enhanced electrochemical reactions[7]. It has been believed that organic compounds were adsorbed onto the metal surface with an aid of their heteroatoms which possessed high basicity and electron density We considered diethyl-5-hydroxyisophthalate (DEIP) as kind of phenolic organic compounds would exhibit good efficiency of corrosion inhibition due to its hydroxyl (OH) and methoxy (OCH3) groups for succinct electron transfer
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