Study On High-performance Nano-coatings For Corrosion Protection Of Stainless Steel

Stainless steel (SS) is a kind of common anticorrosion material, which plays an important role in the development of national economy. However, its service life is much shorter than expected due to the attack from various environments. This not only leads to safety problems but also to resource and energy waste resulting from the early replacement of SS. Coating protection is a simple and effective method to improve its corrosion and fouling resistance. This is also an approach for advanced applications of SS. The SS under the protection of coating not only plays a positive important role in aircraft, high speed train, and lunar research, but also is widely used in appliances, automobiles, kitchen utensils, elevators, etc.Generally, the coating is coated on the SS by electroplating. The electroplating has many disadvantages such as complex processes, high energy consumption, and high cost. More importantly, highly toxicants and carcinogens will be produced during electroplating. This makes it banned under the End of Life Vehicle (ELV) Directive (2000/53/EC) and the Registration, Evaluation and Authorization of Chemicals (REACH) Regulation (COM(03)644(01)). Thus, the development of new, high-performance, and environmental anticorrosion coatings is an important technical problems for the promotion of domestic SS products in the international market.In the present thesis, a silica-acrylic-epoxy hybrid coating has been fabricated by using the organic-modified SiO2 nanoparticles as filler and using the acrylic-epoxy composite resin as binder. Two methods of blending and in-situ polymerization have been employed to prepare this coating. The corrosion resistance and the anticorrosion mechanisms have been studied in detail. In addition, polyaniline (PANI) coatings have been electropolymerized in H2C2O4, H2SO4, and HNO3 solutions by cyclic voltammetry in order to protect the ferritic and economic 430 stainless steel (SS) from corrosion. Morphology, corrosion resistance and anticorrosion mechanism of the PANI coatings have been investigated in detail. The main conclusions are shown as follows:1. The dispersibility of silica particles is improved after surface modification by 3-methacryloxypropyltrimethoxysilane (γMPS). The 430SS coated by the coating containing modified SiO2 (CCMS) exhibits a higher protection efficiency (99.72%) than that coated by the coating containing unmodified SiO2 (CCUS,70.27%). Some CCUS delaminates from the 430SS along the scratches during 200 h salt spray testing. However, the CCMS remains intact around the scratches; it provides a better salt spray resistance for the 430SS than the CCUS does. Even when the CCMS-coated 430SS undergoes a 500 h salt spray test, only small scale delamination is observed along the scratches. The CCMS delays the delamination from 430SS and thereby improves the salt spray resistance of CCMS-coated 430SS. This is attributed to the strong interfacial adhesion resulting from the formation of Fe-O-Si covalent bonds.2. The silica-acrylic-epoxy hybrid coating synthesized by the in-situ method obtains a stronger corrosion resistance on 430SS as compared with the same coating prepared by the blending method, and its salt spray resistance time reaches more than 1000 h. This relates to the improved dispersion of SiO2 nanoparticles by the in-situ method, thereby enhances the corrosion resistance.3. The corrosion of 430SS under the blending coating originates from the coating flaws, and then is accelerated by the O2 concentration cell and H+ self-catalysis. The corrosion products decrease the adhesion between the coating and the substrate, thus lead to the coating failure.4. All PANI coatings derived from the three acids provide good corrosion protection for SS. The protection efficiencies of the coatings decrease in the following order:HNO3-> H2SO4-> H2C2O4-PANI. The thinner HNO3-PANI exhibits a smoother and more compact morphology due to its lower growth rate and better adhesion in comparison with the H2SO4-and H2C2O4-PANI.5. The H2C2O4-PANI acts predominantly as a mere physical barrier, while the oxide layers formed by the catalytic effect at the SS/PANI interface mainly contributes to the corrosion resistance of HNO3-PANI and H2SO4-PANI although they also have physical barrier effects.