Protection Of Fe-Mn-Si Alloy With Tannic Chemical Conversion Film And Tannic/TiO₂ Composite Film

A blue chemical conversion film was obtained on Fe30Mn2.7Si alloy by treating the alloy with tannic acid solution, then, tannic/TiO₂ composite film treated at 400 ℃ for 30 minutes was composited on the surface of tannic chemical conversion film prepared on Fe30Mn2.7Si alloy substrate by sol-gel method and the dip-coating process. Surface morphology, structure and compositions, and electrochemical corrosion resistance of tannic chemical conversion film and Tannic/TiO₂ composite film formed on the Fe30Mn2.7Si alloy have been studied by X- ray diffraction, fourier transform infrared spectroscopy, scanning electron microscopy, energy diffraction spectroscopy, anodic polarization and electrochemical impedance spectroscopy. The results show that the tannic chemical conversion film is probably constituted of amorphous and pored alloying tannate chelate, and the surface of two-layer T1O2 composite film is homogeneous and compact, and has no visible pore, and the thickness of this film is about 8 μm.In 1 mol l^-1 Na₂SO₄ solution, tannic/TiO₂ composite film can be passivated, but tannic chemical conversion film and Fe30Mn2.7Si alloy are active, and the corrosion potential E_corr of two-layer TiO₂ composite film increases more 325 mV than that of Fe30Mn2.7Si alloy, more 63 mV than that of tannic chemical conversion film, while the corrosion current density I_corr decreases by one order of mangnitude both.The passive film of tannic/TiO₂ composite film is composed of a mixture of Fe₂O₃, SiO₂ and TiO₂ in 1% NaOH solution, which improves the stability, uniformity and compactness of tannic/TiO₂ composite film. The corrosion potential E_corr of two-layer TiO₂ composite film increases more 535 mV than one of Fe30Mn2.7Si alloy, more 61 mV than one of tannic chemical conversion film. The critical passivation current density I_c and passive current density I_p decrease by one order of mangnitude than that of chemical conversion fllm.and two orders of mangnitude than that of Fe30Mn2.7Si alloy. The pitting corrosion potential E_b of two-layer TiO₂ composite film is 1050 mV in 3.5% NaCl solution. It is by far higher than 1Cr18Ni9Ti stainless steel. But tannic chemical conversion film is active, as the same of Fe30Mn2.7Si alloy. The electrochemical impedance spectroscopy...