Effect of Surface Modification of Zinc Powders with Organosilanes on the Corrosion Resistance of Thin Zinc Pigmented Organic Coated Steel Sheet

Abstract

MasterConventionally, zinc (Zn) pigmented coating has been used in heavy industry, such as bridges, offshore construction and ships, to protect steel from corrosion. Zn particles can provide both galvanic protection and a barrier protection to coatings on steel panels. More recently, Zn pigmented coating has been used in the automotive industry to impart weldability to pre-coated steel sheetsin this context, Zn pigmented coating is called “Pre-sealed coating”, or “Weldable primer”. Pre-coated metal sheets have many advantages, such as good corrosion resistance, reduced production cost and simple manufacturing procedures. However, in the automotive industry, the main function of Zn particles in the coating is to provide an electrical pathway between the welding electrode and the steel substrate. For that reason, the pre-sealed coating should be thin enough (< 10 m) to guarantee weldability. Due to the limited coating thickness, the demands for higher increasingly corrosion resistance are difficult to meet. Efforts have been made to increase the corrosion resistance of Zn pigmented coating, reducing the coating thickness to maintain or improve the electrical resistance spot-weldabilitythese efforts include optimizing the size and shape of Zn particles, optimizing the pigment/binder ratio, and incorporating corrosion-inhibitive pigments or conductive pigments to improve corrosion resistance and weldability. However, incorporating these pigments can degrade the mechanical properties of the organic coating, particularly if the coating is thin. Hence, improvement of the current systems without addition of other functional pigments would be a promising strategy.Therefore, in this work, surface modification of Zn powders using aqueous organosilane (OS) solutions was used to improve the corrosion resistance of thin ZRCs. To investigate the effects of OS properties on the surface modification, six different OSs were tested as surface modifying materials: N-propyltriethoxy silane (PES), which has a short propyl chainbis-1,2-triethoxysilyl ethane (BTSE), which has six hydroxyl groups and ethyl side chain between siloxane groups3-aminopropyletriethoxysilane (APS), which has an amine side chain at the end of the middle of moleculebis-trimethoxysilylpropyl amine (BTSPA), which has an amine side chain in the middle of molecule3-glycidyloxypropyl trimethoxysilane (GPTMS), which includes an alkyl epoxy chainand bis-triethoxysilylpropyl tetrasulfide (BTSPS), which contains tetrasulfide groups in the middle of molecule.Scanning electron microscopy (SEM), the scanning vibrating electrode technique (SVET), cross sectional analysis, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) were conducted to investigate the surface characteristics of modified Zn particles. Corrosion resistance of Zn pigmented coatings containing OS modified Zn particles was evaluated using a salt sprayed test (SST). Electrochemical impedance spectroscopy (EIS) was used to describe the corrosion protection mechanism of the coating and to determine the key factors that cause corrosion. In the general corrosion test, Zn particles with OS-modified surfaces showed higher resistance to corrosion than did non-treated Zn particles. The electrochemical activity of Zn particles was diminished by surface modification with OSs. The passivation layer formed by OSs on the surface of Zn particles was indirectly characterized using XPS depth profile analysis of surface-modified Zn electrodesthe thickness of modified surface layer depended mainly on the chemical structure of the OS. FT-IR analysis showed that the intensities of specific analysis peaks were proportional to the thickness of the passivation layerthis suggests that covalent bonding is formed between not only OS molecules but also the OS and the Zn.In addition, the activity of the passivation layer on the Zn particle was indirectly evaluated using SVET. The current-density of Zn electrodes treated with OSs having unreacted functional groups was higher than that of non-treated bare Zn electrodes. Current-density signified electrochemical activity in the samples. Unreacted functional groups of OSs are polar and electronegative in solution, so when they were exposed to the sodium chloride solution they attracted ions dissolved in it. A higher potential deviation of OS treated Zn electrodes , measured by SVET, was induced by formation of ion rich layer on the electrodes. This large potential drop was converted to high current-density and indicated that the OSs are reactive or electrochemically active. The activity results revealed the reactive characteristics of OSs.Coatings that included Zn particles treated using OSs that have functional groups resisted corrosion better than those treated using OSs that lack them. In contrast, corrosion resistance of Zn-pigmented coating was not significantly improved. For example, using alkylsilane (PES and BTSE) as modification materials resulted in no significant improvement of corrosion resistance. Electrochemical impedance spectroscopy was used to characterize the corrosion protection mechanism. Coatings containing Zn particles treated with APS, BTSPA, GPMS and BTSPS showed superior anti-corrosion properties. For instance, water uptake by the coating during the early corrosion stages was hampered owing to enhancement of the coating’s barrier property. This barrier property improvement was related to the presence of unreacted OS functional groups, which could react with the resin matrix and form an interfacial bond that increased the barrier property of the coating. Moreover, corrosion activity of the interface between substrate and coating was promoted by surface modification of Zn particles. Nonetheless, surface modification of Zn particles did not affect the disbonded area between coating and substrate in later stages.The coating that contained Zn particles treated with BTSPA had the greatest resistance to corrosion