Study of the Obtaining Conditions of Vinyltretoxysilane Film as Corrosion Protection of the Galvanized Steel

ABSTRACT Galvanized steel is obtained by immersing the carbon steel in a molten zinc bath widely employed in the automotive, petrochemical, chemical, marine, aeronautical and white-good appliances. It has higher corrosion resistance than carbon steel when exposed to corrosive environments. To increase its durability and corrosion resistance, it undergoes a surface treatment that is adherent to its surface. For this reason, phosphatization and chromatization are currently used as an after treatment to galvanization, both with environmental restrictions and human health. Due to the presence of toxic and carcinogenic products – in the chromatization or phosphatization - and to minimize the impact on the environment, the development of silane films as alternative pretreatments have been studied due to their good physical barrier properties against diffusion of oxygen to the metal interface and improve corrosion resistance. The aim of this work is to evaluate a corrosion resistance performance of galvanized steel coated with vinyltriethoxysilane (VS) film under hydrolysis conditions: silane concentration (2%, 3% and 4% %v) and hydrolysis time (30 min, 45 min and 60 min). The fixed variables were: the solvent solution used was water/ethanol (50% v/v) at pH 4.0, the curing time of 120 min and the curing temperature of 150°C in an oven. The corrosion resistance of pre-treated galvanized steel with VS silane film was evaluated by the following electrochemical techniques: open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), linear polarization resistance (Rp) and potentiodynamic polarization curves in 0.1 mol.L-1 NaCl solution. The chemical properties of films were analyzed by infrared spectroscopy technique (IR). The galvanized steel and the film surface morphology were observed by scanning electron microscopy (SEM) and X-ray energy dispersion spectroscopy (EDX). The electrochemical results indicated the formation of an efficient corrosion protection film when galvanized steel is treated with film obtained at 4% silane concentration and 60 min hydrolysis time. SEM images revealed less corrosion product formation when galvanized steel is treated with silane film and EDS spectra showed Si peaks indicating the adhesion of the film to the galvanized steel surface. The spectra in the IR region showed bands of Si-containing compounds that reveal the presence of silane film on the substrate surface.摘要 镀锌钢是将碳钢浸入熔融锌浴中制得，该工艺广泛应用于汽车、石化、化工、船舶、航空航天及白色家电领域。相较于碳钢，其在腐蚀环境中具备更优异的耐蚀性能。为进一步提升其耐久性与耐蚀性，需在其表面施加附着性良好的表面处理工艺。目前，磷化与铬化处理常作为镀锌后的后处理工序，但二者均存在环境限制与人体健康风险。由于铬化或磷化处理会产生有毒且致癌的副产物，为降低对环境的影响，研究者们已开展硅烷膜作为替代预处理工艺的相关研究，因其可通过良好的物理屏障性能阻隔氧气向金属界面扩散，从而提升耐蚀性。本研究旨在评估乙烯基三乙氧基硅烷（vinyltriethoxysilane, VS）膜在不同水解条件下对镀锌钢的耐蚀性能，水解条件包括硅烷浓度（2%、3%、4%，体积分数）与水解时间（30 min、45 min、60 min）。本实验的固定变量为：溶剂体系采用体积分数50%的水/乙醇混合溶液，pH值为4.0；固化时间120 min，固化温度150℃，固化方式为烘箱加热。采用电化学技术对经VS硅烷膜预处理的镀锌钢的耐蚀性能进行表征，具体包括开路电位（open circuit potential, OCP）、电化学阻抗谱（electrochemical impedance spectroscopy, EIS）、线性极化电阻（linear polarization resistance, Rp）以及在0.1 mol·L⁻¹氯化钠溶液中进行的动电位极化曲线测试。采用红外光谱（infrared spectroscopy, IR）分析膜层的化学性质。通过扫描电子显微镜（scanning electron microscopy, SEM）与X射线能谱仪（X-ray energy dispersion spectroscopy, EDX）观察镀锌钢及膜层的表面形貌。电化学测试结果表明，当采用硅烷浓度为4%、水解时间为60 min制得的膜层对镀锌钢进行处理时，可形成高效的防腐防护膜。SEM图像显示，经硅烷膜处理后的镀锌钢表面腐蚀产物更少；EDS能谱中出现硅元素峰，证实了膜层在镀锌钢表面的附着。红外光谱测试结果显示含硅化合物的特征吸收峰，证实了基底表面硅烷膜的存在。