Synthesis Of MFI Zeolite Coatings And Their Corrosion Properties On Aluminum Alloy

Zeolites are hydrated highly crystalline aluminosilicates. They have been widely used in catalysis, adsorption and separation due to their high temperature resistance, chemical stability and strong mechanical strength. Zeolites are environmentally friendly. Moreover, high silica zeolites are hydrophobic due to the lack of ionic sites. Therefore, it has been reported that zeolite MFI coatings synthesized by in situ hydrothermal synthesis is corrosion resistant. During the in situ hydrothermal synthesis process, the support is directly brought in contact with the strong alkali synthesis solution at high temperature and high pressure. The conditions of strong alkali synthesis solution with high temperature and high pressure can be hardly used in anti-corrosion of metallic industry. Besides, the metal could be easily corroded during the in situ synthesis process. Therefore, the goal of this work is to fabricate corrosion resistant zeolite MFI coatings on aluminum alloy plates by a mild and effective method. Seeded growth method and steam assisted crystallization have been investigated. The main experimental results and conclusions are summarized as follows:(1) Seeded Growth MethodThe key question is how to form a dense and continuous seed layer on an aluminum alloy plate. Influences of different seed solutions with water or ethanol as dispersants are investigated. In order to improve the adhesion of the seed layer on a metal surface and modify the intergranular voids between seeds, in this work, a seed paste containing clear silica sol and seed is used to form a compact and continuous seed layer. The effects of seed/sol ratio (SSR) in the seed paste on the seed layer and as-synthesized zeolite coatings are examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and direct current polarization measurement. The SSR value is found to be closely related to the coverage, continuity and orientation of the seed layer and zeolite coating. This is due to the binding forces between seed layer and the substrate and among the seed crystals, which increase with the silica sol amount. Cracks increased and orientation of seed layer became worse with decreasing the SSR value. If the SSR value is too large, the coverage of the seed layer is not satisfied. Therefore, there is a balance in quantity between the sol and the seed. The optimum SSR value in the seed paste is6.0.Three seed crystal sizes (100,500and1000nm) are used to investigate the effects of the seed size upon the formation of seed layers, zeolite coatings and the corrosion protection role of the zeolite coating with SSR=6.0. It is found that the zeolite coating synthesized from the seed layer with the seed of500nm is more protective than those with smaller or larger seeds.Three different seeding methods (dip-coating, spin-coating and brush-coating+leveling) are examined. Compared with dip-coating and spin-coating method, a compact and continuous seed layer on rough and nonporous aluminum alloy surface can be more easily formed by brush-coating+leveling method.The zeolite MFI coatings synthesized by seeded growth are compared with those synthesized by in situ crystallization. The zeolite coatings synthesized by in situ crystallization is random-oriented and dense, and its thickness (-20μm) is several times thicker than the thickness of the zeolite coating synthesized by seeded growth (-3μm). Although the thickness of the zeolite coating by seeded growth (150℃3h) is several times thinner than that by in situ crystallization (175℃16h), they have similar corrosion-resistant performance, indicating that a thin b-oriented zeolite coating of only several micrometers thick is sufficient to significantly reduce corrosion of aluminum alloy plates.(2) Steam Assisted CrystallizationWell crystallized zeolite MFI coatings have been synthesized by a steam assisted crystallization method on aluminum alloy plates. It is found that the crystallinity and coating properties are affected by the synthesis conditions such as alkalinity, crystallization time, and silica sources. The optimum conditions for this method are:the alkalinity of the gel solution is0.10, crystallization time is12h, and silica source is silica sol.In this paper, dense and highly b-oriented zeolite MFI coatings have been synthesized by a milder seeded growth method, which have the same corrosion resistant ability as the coatings synthesized by in situ crystallization. These results will not only have a great impact on anti-corrosion of metals, but also promote the applications of zeolite coatings in other fields.