Convenient routes to synthesize uncommon vaterite nanoparticles and the nanocomposites of alkyd resin/polyaniline/vaterite: The latter possessing superior anticorrosive performance on mild steel surfaces

摘要

Polyaniline (PANI)/Precipitated Calcium Carbonate (PCC) composite materials are prepared, for the first time, starting from naturally occurring calcite, and are well characterized. X-ray diffraction (XRD) studies provide information for the presence of unstable vaterite form of PCC in the composites, with an average crystallite size of 26 nm, thus demonstrating the ability of PANI to stabilize, otherwise unstable, vaterite phase of CaCO_3. Thermal analytical results (TGA and DSC) also provide information for the presence of only PANI and PCC, thus providing information for the purity of the composites. This method, therefore, provides a convenient route to prepare vaterite nanoparticles. Electron Microscopic (FE-SEM) images of the composites confirm that the voids of PANI chains are filled by the spherical nannoparticles of vaterite of diameter ～ 24 nm, to result in spheres of the composites with an average diameter of 3-4 μm. FTIR spectra show that the PANI exists in its emaraldine form, weakly protonated when prepared at pH 5. Analysis of the FT-IR data for the four composites of PANI/vaterite gives the molar ratios of PANI: vaterite to be 1:4,1:2,1:1,2:1, respectively. The PANI/PCC composites show electrical conductivity of ～1.00 × 10~(-5) S cm~(-1), which is an impressive value to use these materials as anticorrosive coatings. AC impedance studies also give the conductivities of the PANI/PCC composites to be corresponding to a weakly electronically-conductive emeraldine form of PANI, with equal contributions from the ionic and electronic components, irrespective of the different amounts of vaterite or calcite present in the composites. The DC polarization test confirms equal transport numbers for ions and electrons in PANI samples. The above composites of PANI/vaterite, and a composite of 1:1 molar ratio of PANI/calcite, were mixed with alkyd resin and xylene, separately, to prepare anticorrosive coatings on mild steel (Mole percentages: 98.90% Fe, 0.26% C, 0.04% P, 0.05% S and 0.75% Mn) surfaces. All five composite coatings, with thickness ～ 40 μm, show dramatic decrease in corrosion current density, and a considerable increase in corrosion resistance, to result in several orders of magnitude lowering of the corrosion rate from that of bare mild steel surfaces and those coated with only alkyd resin. There are considerable positive shifts in the corrosion potential also, when each of the five coatings are applied, separately, on mild steel samples, which provide information for a significant overpotentials induced by these coatings on iron oxidation. All four Alkyd resin/PANI/vaterite coatings show higher anticorrosive performances (by factors of 2 × 10~4, 5 × 10~4, 1 × 10~5 and 1.67 × 10~4, respectively from that of bare mild steel) than that of the Alkyd resin/PANI/calcite coating (by a factor of 1.25 × 10~3 from the same). The improved pore-sealing by relatively smaller (26 nm diameter) and spherical vaterite nanoparticles, when compared with a little larger (38 nm) and somewhat elongated nanoparticles of calcite, is suggested to be responsible for better anticorrosive performance of the Alkyd resin/PANI/vaterite nanocomposites.