The Novel Surface Pretreating Approaches For Magnesium Alloy And Preparation Of Nano-particle Modified Automotive Metallic Basecoat

This dissertation includes two sections: the novel surface pretreating approaches for magnesium alloy and preparation of nano-particle modified automotive metallic Basecoat.Magnesium and its alloys have the advantages of low density and high strength/weight ratio and can be used as ideal structural materials for a number of light-weight engineering applications. However, the poor corrosion resistance limit their uses. The conventional conversion coatings based on chromium compounds have been shown highly toxic carcinogens.In this dissertation , we investigated the formation mechanism of SAMs on Mg alloy using Long-chain carboxylate ions and alkylphosphonic acids as organic molecules, and a novel conversion coating based on molybdate/phosphate composite. Some main conclusions are summarized as follows:1. Alkylcarboxylate (C_nH_2n-1O₂Na, n=12,14,18) SAMs were firstly successfully formed on the Mg alloy substrate in the ethanol solutions and characterized by the contact angle measurement, ATR-FTIR, ellipsometry and XPS.The ellipsometry result showed that the tilting angles of the C12, C16 and C18 alkyl chain on Mg alloy substrate were 40.8°, 33.3° and 27.7°, respectively, similar to those SAMs on aluminum substrate. ATR-FIIR indicated that alkylcarboxylate SAMs attached to Mg alloy substrate through monodentate bonding, namely, one carboxylate oxygen to the surface. EIS tests confirmed the corrosion protection of SAMs for Mg alloy substrate with protective efficiency (PE) even up to 98.5%, and the longer the alkyl chain of alkylcarboxylate and the assembled time were, the better the corrosion protection of alkylcarboxylate SAMs were. Furthermore, we studied the adsorption of alkylphosphonic acids(C_nH_2n+1P(O)(OH)₂ , n=8,10,18) on Mg alloy in aqueous or ethanol solutions, respectively. The contact angles showed that highly hydrophobic SAMs from on Mg alloy substrate with 113°, 125°and 135°in aqueous and 105°, 102.8°and 112.1°in ethanol solutions corresponding to C₈H₁₇PO₃H₂,C₁₀H₂₂PO₃H₂ and C₁₈H₃₇PO₃H₂,respetively.XPS and ATR-FTIR results displayed that alkylphosphonic acids headgroup attached to Mg alloy substrate through bidentate bonding in aqueous solution, while in ethanol solution through monodentate bonding. The film growth mechanism in aqueous solution was a corrosion/precipitation process.2. The conversion coatings based on molybdate(Mo) and molybdate/phosphatecomposite (Mo/P) on magnesium alloy surface were investigated. XPS ,XRD and SEM showed that the Mo/P coating was composed of complicated phases with a uniform "alveolate-crystallized" structure on the surface. However, molybdating coating presented an amorphous structure with "network-like" formed on Mg alloy substrate. According to the SEM and electrochemical results, molybdate-phosphating coating had a better corrosion protection than that of molybdating coating and reached its best performance at 1/2 of MoO₄^2-/H₂PO₄^- molar ratio. The growth mechanism of Mo/P coating was a corrosion/precipitation process which was very similar to the phosphating process currently used in steel surface. The difference was that the Mo/P coatings contained a lot of netlike oxides, such as MoO₃, MgO, Al₂O₃, to provide additional corrosion protection for Mg substrate. The better corrosion resistance of molybdate-phosphating coating was also confirmed in salt water immersion tests via comparing with the traditional chromate coatings.3. For the preparation of Nano-particle modified automotive metallic basecoat. We employed an agitator bead mill accompanying with an ultrasonicator to de-agglomerate the nanofine TiO₂ powder in butyl acetate to investigate the effects of ultrasonic pretreatment, dispersant dosage, agitation speed, solid content of TiO₂ suspension and bead diameter on the grinding effiency. It was found that the grinding process combined with ultrasonic pretreatment could shorten grinding times from 2.5 h to 1～1.5 h to reach a limiting mean size of about 110nm, saving around 40% of the specific energy inputt. Increasing the dispersant content did not obviously change the limiting mean size of the nanofine TiO₂ suspensions but reduced the size distribution. Higher agitation speed produced smaller limiting mean sizes at shorter grinding times, but too high agitation speed caused wear in the mill, contaminating the suspension.4. Furthermore, we studied the nano-particles modified automotive metallic basecoat of polyester/amine resin system with flip-flop effect, and found that adding 5% nano-BaSO₄ to substitute nano-TiO₂ in basecoat could decreased haze and provide higher abrasion resistance. Adding some IPDI blocked polyisocyanate could also improve the abrasion resistance. The scale-up tests of nano-particles modified automotive metallic basecoat were carried out in plant to finally obtain automotive metallic basecoat with very good flip-flop effect.