Fabrication Of Hierarchical Structure Superhydrophobic Surface On The Magnesium Alloy And Its Corrosion Resistance

Magnesium alloys, because of the excellent physical properties and processingperformance, especially the advantage of high specific strength, there are prospects for thedevelopment in the field of aviation, automotive industry, transport and chemical industry.However, magnesium metal with active chemical properties can not generate passive filmindependently, while easily corroded, which makes magnesium alloy restricted in industryapplications that need to be exposed to the harsh environment. It’s a necessary approach toimprove the corrosion resistance of magnesium alloy for its rapid development of application.In this paper, we build different conversion film on the surface of the magnesium alloysubstrate to cut off the connect route with external environment, thereby improving thecorrosion resistance of the magnesium alloy. The specific research contents are as following:1. Conversion films which is built with ammonia as the alkali source and nitrate ionsinvolved in the hydrothermal reaction, though the X-ray diffraction （XRD） indicates that120℃and180℃conversion films are both Mg（OH）₂, there are different surface structuresunder different reaction temperatures. After the low surface energy processing, both surfacesshows hydrophobic property with the static water contact angle of132.3+1°and145.1+1°,respectively. By immersion in the3.5%sodium chloride aqueous solution of72h, theimpedance of two samples are reduced significantly. Though providing protection for themagnesium alloy, it’s a short effect. There are obvious corrosion pits after immersion.2. Using different anions Cl^-and SO₄^2-participate in the hydrothermal reaction, theresulting conversion films shows different morphology with the NO₃^-conversion film.However, these three samples are all Mg（OH）₂. SO₄^2-conversion film, after low surfaceenergy processing, transforms a super-hydrophobic surface while the static water contactangle is161.2+1°. Soaked in3.5%sodium chloride aqueous solution, the corrosion currentdecreased with time gradually. The corrosion rate becomes slow, which is a common decisionof the hydrophobic property and corrosion product depositing.3. Using urea as indirectly alkali source growing in situ on the magnesium alloysubstrate surface, we get three samples with different morphologies while the differenttemperatures. XRD analysis shows that the resulting conversion coatings of120℃and150℃are hydromagnesite. But the hydromagnesite decompounds when the temperature increased to 180℃, then the180℃conversion film is Mg（OH）₂. After low surface energy processing, thestatic water contact angles are160.1+1°、152.6+1°and151.3+1°, respectively. Otherwise, the120℃and180℃conversion films show a good conrrosion resistance, while the impedanceis about106order of magnitude.4. Different from urea, the principle of hexamethylenetetramine as alkali source ishydrolysis reaction which generates HCHO and NH3. Three different morphologies can beobtained under different reaction temperatures. XRD shows that120℃conversion film isMg（OH）₂, and150℃and180℃conversion films are hydromagnesite. Three samples showthe static water contact angle of152.8+1°、102.9+1°and98.6+1°after the low surface energyprocessing. Then make120℃conversion film soaked in3.5%sodium chloride solution, after24h immersion, the impedance of sample is about120k, and quickly reduce.