| The silica-based coatings and anodic oxide layer used as the corrosion protection of aluminum exhibits excellent passivating properties. However, undesirable structure of the coating, especially with limited thickness and porous structure, limits the application of aluminum and its alloys in variety of corrosive surroundings. The combination of complex silicone based layer and ordered nanotexture films proposed as overall layer with good physical compatibility and chemical homogeneity is beneficial for corrosion protection, and is potential route for aluminum metal protection. Consequently, the thesis is aimed at the formation mechanism of microstructures of composite films, and analyzing its protective properties against corrosions as cathodic protective films exposed to 3.5 wt. %Na Cl solution. The main contents are as follows:1. The combination of Si-O-Si and O-Si-R were induced by self assembly effect, with tetraethylorthosilicate(TEOS), anilino methyl triethoxysilane(KH42) and 3-amino propyltriethoxy silane(KH550) as silicone precursor. The couple agent(KH42 or KH550) modified film displays complete coverage and surface density after heating process. The use of KH42 with big mole for sol will result in low viscosity and heating temperature, particularly in low thickness of film. The presence of KH550 with long group naturally presents clear surface and non-defect in the structure, and responsible for the electrochemical corrosion and self-protection of 0.89 wt. % samples. Compared with substrate, a 100~300 m V positive shift of organic-inorganic hybrid film for corrosion potential with a decrease as low as 0.02074 μA/cm2 for corrosion current density.2. The solid/colloid oxides are introduced into the intermingle sol with silicane gelatinoids for ordered organic-inorganic film to strengthen the density and improve corrosion resistance. The results show that silica composite colloid with solid oxides tends to accumulate, while colloidal oxide composite films present increasing thickness of film and well adherence to micro structure on matrix, the Icorr of composite oxide film with Nano-particles(Si O2) could be only 156.4 n A/cm2.Compared to the encapsulation-polycondensation mechanisms for Si O2 modified salicane film, the liquid oxide modified film performs better adhesion and maintenance to original microstructure of substrate. The increased content of oxide-phase deposition in modified silicon film can perform better coverage, and thereby improve corrosion protection on substrate. Electrochemical measurement shows that the corrosion potential of Ti O2 modified silica composite film is-0.657 V and corrosion current density 5.15 n A/cm2, which is much lower than that of the original silicane-based oxide film.3. Homogeneous sols are prepared with tri-assembly, TEOS, KH550 and MWCNTs-R as precursor and reactant. The effect of nanoparticles to viscosity of colloids accords with the order of ηMWCNTs> ηMWCNTs-OH>ηMWCNTs-COOH. MWCNTs and MWCNTs-OH can improve residual tube in the composite film, while MWCNTs-COOH can reach a rather clear surface due to the possible polycondensation reaction between the carboxyl and Si-O-Si. The enchased carbon nanotubes could reduce the resistance(<1 M?·cm2) of film by seepage of their long structure. The non-regular particles modified composite films perform excellent anti-corrosion properties with the order of Icorr-KTM> Icorr-KTM-OH> Icorr-KTM-COOH. And the KTM samples take biggest contact angle(85°), while the KTM-COOH samples perform the best corrosion protection(0.01267 μA/cm2) to substrate.4. Well ordered and barrier-type anodic oxide films are prepared in 15~20 wt. % H2SO4 acid and mixed H3BO3-H2SO4-based acid electrolyte, separately. A detailed research on transverse and longitudinal direction growth of pore cell and its anti-corrosion properties has been done. Results show that the average diameter of the pores for those well ordered structure is at the range of 7~9 nm. The hardness of SAA structure(20~30 μm) in 2~3 A/dm2 could be up to 94.3 k Pa, and performs excellent corrosion protection(0.5~2 n A/cm2) to substrate; the hardness of BSAA structure(8~9 μm) in 1.5 A/dm2 condition increases to 144 k Pa, and while the corrosion current density is over 19.2 n A/cm2. Compared with the traditional pore cell structure, the composite structure of hexagon hollow cell bracing structure is improved, and the anti-corrosion properties and stability are increased. The structure of porous film displays a hierarchical layer with tiny flaks after boiling water sealing process, both of which exhibit excellent corrosion protection. Barrier-type anodic oxide films prepared in mixed electrolyte have caused the pores to enlarge 10 nm diameter and density below 1×1015 /m2. The electric double layer between dissolving Al/Al2O3 and electrolyte facilitates the growth of ordered porous layer both in transverse and longitudinal directions. The galvanostatic anodizing process produces a thicker film with systolic-parallel tendency. Due to the weak dissolution effect of organic acid, the anodic oxide films grow with relatively lower pore density(<1×1015 /m2) and thickness(6.5 μm), but with an improved corrosion protection(0.4376 n A/cm2). The effect of increasing temperature to anti-corrosion properties of barrier-type structure may be due to composition and distribution of Al2O3. From the results of potentiodynamic polarization measurement, it is found that the most efficient protection can be obtained in 20 ℃, while porous and hierarchical films show a satisfied stability in 30 ℃. A series of tri-texured oxide layer after via in situ hydration and healing process shows a significant effect on the microstructure and corrosion protection.5 In order to enforce passivation property and seal of nano-texured oxide film, silica-based organic-inorganic composite coatings are meant to plug ordered porous structure in situ with colloid of special composition. In such layered structure, the sols in weak acid system can not only promotes the cross linking extent between micelles, but also provides an overall coating to cover the rough substrate surface. Well-defined narrow particle size distribution could be achieved under homogeneous conditions of micellar agglomeration, accompanying a remarkable anti-corrosion properties improvement. Unique composite oxide coating with Ti O2 homogeneously distributed in oxide pores is successfully prepared by flow-assembly of the mixture dispersion and absorption of micelle to Al2O3. The Ti O2/Si O2 modified oxide film shows an effective pore filling after heating treatment and excellent stability due to the composite oxide film during long-term immersion experiment in 3.5wt. % Na Cl solution. In general, the open current potential of the sample shifts to positive(about 0.1 V) and the Ti O2/Si O2 modified film shows a current density(<0.486 n A/cm2) lower than that of non-modified film, and a higher impedance of 358 M?·cm2. |
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