Composite Anodization/snap Technology Of Magnesium Alloy And Corrosion Protection Of Film

Magnesium and its alloys have been widely applied in fields of aerospace, automobile, mobile, electron, etc. Unfortunately, magnesium has poor corrosion resistance due to their high chemical activity. Usually, the surface treatments are frequently used for corrosion protection of magnesium alloys. In this paper, an anticorrosion bilayer was use to protect AZ31B magnesium alloy, which was made by first applying anodization to modify the magnesium alloy surface and subsequently depositing a SNAP (self-assembled nanophase particle) film. This bilayer has high thickness, good compactness, excellent corrosion protection and strong adhesion, which has made up for porous anodized film. Therefore, the bilayer would take an important position in the corrosion protection for magnesium alloy.A new anodized electrolyte without Cr, F and P was developed, and the anodized film was prepared on the surface of AZ31B magnesium alloy under the direct current mode. Electrolyte composition and technological conditions of anodization were optimized by systemic experiment: KOH 60g/L, Na2SiO3 70g/L, Na2B4O7 60g/L, Na2CO3 30g/L, benzotriazole (BTA) 5g/L, current density 1.5A/dm2 and anodized time 10min.Structure of the anodized film and effect of BTA on the structure of the anodized film were studied by SEM, EDS, XRD and XPS. The results indicated that the anodized film was consisted of the inner dense layer and outer loose layer, the main elements of the anodized film were Mg, O and Si, and its phase structure composition were Mg2SiO4 and MgO. On the other hand, there was the decrease of porosity and the improvement of the compactness for the anodized film coated on magnesium alloy by the addition of BTA into electrolyte. According to effects of the electrolyte composition, technological conditions and BTA on the anodized process, the properties and the structure of anodized film, the growth model of anodized film and the adsorption model of BTA on the surface of magnesium alloy were proposed, respectively. The growth of anodized film was consisted of four stages: anodic dissolution and surface passivation, initial growth, quick growth and balanced growth.Under the hydrolysis reaction and crosslink reaction, the SNAP film coated on magnesium alloy was prepared by the precursors of 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetraethoxysilane (TEOS), crosslink agent of triethylene tetramine (TETA). Based on the effects of precursors, acetic acid solution, ethanol and TETA, sintering temperature and sintering time on the corrosion protection of the SNAP film, the optimal preparation conditions of SNAP film was obtained, the molar composition of SNAP sol n(GPTMS):n(TEOS):n(C2H5OH):n(H2O):n(TETA)=3:1:0.25:15:0.5, and the speed of the withdrawal, the deposition time, the sintering temperature and sintering time were 1mm/s, twice, 100℃and 30min, respectively.The results of TEM and the simulation of molecular kinetics demonstrated that the formation of the nanoparticles by hydrolysis reaction could stably suspend in SNAP sol for 3-4 months. The transition state energy of precursor was lower during the hydrolysis reaction, which was in favor of the reaction. For the crosslink reaction, the nanoparticles could be connected with each other by TETA to form a large silica network structure, and the mechanism of the crosslink reaction was considered that epoxy group of GPTMS was most likely to react with No.2 N of TETA most firstly, and then react with No.1 N of TETA. Thermal decomposition kinetics of SNAP sol indicated that the addition of TETA could increase obviously the thermal stability of SNAP sol. Formation mechanism of SNAP film coated on magnesium alloy showed that the improved properties (thermal stability of SNAP sol, compactness and corrosion protection of the SNAP film) were due to high degree of cross linking of organic-inorganic Si-O-Si network structure, high content of inorganic component and high density of Mg-O-Si bonds.The bilayer of the anodization/SNAP film coated on magnesium alloy was prepared by the anodization and SNAP sol. The bilayer was consisted of the inner SL layer and the outer dense layer, and the SL layer with good compactness and high thickness was made of the SNAP film and the loose layer of the anodized film. The dense layer near to the magnesium substrate was thin, but had better compactness than the SL layer. Before 165h, the SNAP/loose layer played a main role in the anticorrosion performance due to its good compactness, strong adhesion and high thickness. After 165h, the inner dense layer occupied the dominant place in the corrosion protection of the magnesium substrate. Because the inner dense layer not only was dependent on its limited recovery caused by the formation of corrosion products plugging the pores and cracks existing in the inner dense layer, but also relied on its compact structure to inhibit the corrosive particles.In a word, the bilayer has good corrosion protection for AZ31B magnesium alloy.