Preparation and Corrosion Resistance of Superhydrophobic Phosphate Chemical Conversion Coatings for Magnesium Alloys
Magnesium (Mg) alloys are promising materials for a number of applications in aviation,spaceflight,high-speed rail and automotive industry due to their combination of light weight,excellent shock absorption and recyclability.However,the relatively high corrosion susceptibility of Mg alloys seriously restricts their widespread use.Use of reliable surface protection coatings is an effective method to prolong the service life of Mg alloys and break through the bottleneck of Mg alloy application.Phosphate conversion coatings (PCC) stand out among lots of surface protection technologies for Mg alloys due to its advantages of simplicity,high efficiency and green environmental protection.However,the defects and micro-cracks on the surface of PCC provide a permeable path for water and aggressive ions in a corrosive environment,thus seriously weakening the corrosion protection ability of PCC.Earlier studies have focused on improving the density of PCC by optimizing the process,but in practice the effect on enhancing the durability of PCC is not satisfactory.At present,the construction of high corrosion resistance coatings by integrating PCC with other surface treatment technologies is a research hotspot in the field of Mg alloy corrosion protection.Water is an important factor that causes electrochemical corrosion of magnesium alloys,so hydrophobic treatment is an effective measure to strengthen its corrosion resistance.In recent years,the superhydrophobic surface inspired by lotus leaf has shown great competitiveness and application potential in the field of metal corrosion protection due to its excellent hydrophobic properties.As an environmental-friendly organic phosphonic acid,phytic acid has strong chelating ability with metal ions to generate insoluble chelating precipitate.Based on this characteristic,the work aims to develop a simple,efficient,cheap and eco-friendly cyclic self-assembly process and prepare a superhydrophobic composite coating on Mg alloy matrix by combining PCC.By combining biomimetic superhydrophobic technology and chemical conversion technology,the traditional phosphate chemical conversion film was modified by post-treatment,and the superhydrophobic phosphate chemical conversion film was prepared on Mg alloy matrix.The SEM results showed that the defects of PCC were effectively sealed and film density was significantly improved after the modification.The contact angle test results showed that the modified film presented excellent superhydrophobicity with the static water contact angle as high as 156.5° and the rolling angle as low as 5°.Electrochemical test results showed that the corrosion resistance of modified superhydrophobic film was significantly increased.Its corrosion current density decreased by one order of magnitude and its low-frequency impedance modulus (|Z|0.01 Hz) increased by one order of magnitude,and the|Z|0.01 Hz was still higher than 104 Ω·cm2 even after 12 d of immersion.The salt spray test indicated that the modified superhydrophobic coating withstood salt spray for more than 100 h.In addition,due to the barrier effect of the "air cushion",the coating exhibited excellent hydrophobic and self-cleaning properties.Therefore,it has potential application value in industrial production due to its simple preparation process and readily available raw materials.
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