Study On The Colored Hydrophobic Coatings On The Surface Of Magnesium And Its Alloys

Magnesium is one of lightest metals and has outstanding physical and mechanicalproperties such as high stiffness/weight, good castability, good vibration and shockadsorption, and high damping capacity. Magnesium and its alloys are expected to beapplied to many industrial applications such as components for automobiles, aircraft,communications and portable telephones and personal computers, etc. However, thepoor corrosion resistance in acid and marine environments seriously impedes the use ofmagnesium and its alloys. With the development of society, people also demand for thedecoration performance of magnesium alloy and require varies colors in many areas.Thus, it is crucial to design a surface that integrates decorative and anticorrosionproperties and this will be of great significance to expand the application of magnesiumalloys.In the present work, a colored surface was prepared on pure magnesium, AZ31magnesium alloy and AZ61magnesium alloy through a low-cost and simplehydrothermal reaction in ultrapure water. The effects of hydrothermal time onmorphology and color were researched. After modification with dodecafluoroheptyl-propyl-trimethoxysilane （G502）, a hydrophobic/super-hydrophobic coating weresuccessfully fabricated on the sample surface. The scanning electron microscopy（FE-SEM）, X-ray photoelectron spectroscopy （XPS）, digital camera and contact angleinstrument were used to characterize the surface of the sample. The corrosion behaviorof the hydrophobic surface in3.5%NaCl solution was investigated with theelectrochemical measurements, and the main results are as follow:SEM results showed that the surface morphology of the samples prepared in thesame conditions was relatively similar. It was rather rough and covered by a petal-likemicro-and nano-sheets structure and there were a large number of voids between thepetals. XPS results showed that the surface compositions of pure magnesium afterhydrothermal process were mainly Mg（OH）₂and MgO, and those of AZ31and AZ61were mainly Mg（OH）₂and MgO, plus a small amount of Al（OH）₃and Mg₁₇Al₁₂. Digitalcamera results showed that the samples could present different colors whenhydrothermal time changed. The wettability results showed that6h was the besthydrothermal time of pure magnesium and AZ31, under this condition the contact angleof pure magnesium and AZ31magnesium alloy can reach144.3°and143.3°, respectively. The best hydrothermal time of AZ61magnesium alloy was12h，thecontact angle could get to150.1°. Polarization measurements showed that the corrosionpotential of the hydrophobic sample was slightly more positive than that of theuntreated sample, and the corrosion current density decreased largely. The hydrophobiccoating on magnesium alloy mainly prevented the anodic dissolution process. Theresults of electrochemical impedance spectroscopy show that the inhibition efficiency ofhydrophobic coating was up to99%, indicating the hydrophobic surface can effectivelyprevent the erosion of NaCl. In addition, the anti-corrosion model of hydrophobicsurface was established and the formation mechanism of the hydrophobic surface wasinvestigated. The results showed that the G502and the magnesium substrate wereconnected via a chemical bond and they formed a mesh structure. The anti-corrosionability of these metals from strong to weak is pure magnesium＞AZ61magnesium alloy＞AZ31magnesium alloy.