Nano-Structured Silica Film for Non-Chromate Anti-corrosion Protection Layer of Metal Surfaces

摘要

Dry-gel conversion (DGC) method by hydrothermal process was developed for the growth of silica coating on metal surface. In the preparation of zeolite MFI coating, in-situ crystallization (InC) or DGC protocols were developed. Zeolitic micro-pores could be blocked by the structure directing agent, tetra-propyl-ammonium bromide, during zeolite crystallization; the inter-crystalline voidage could be minimized by reduction of crystal size and formation of structure packing. While the InC protocol generated loosely packed film made of large zeolite crystal, the DGC protocol prepared dense film comprising of nano particles. Superior protection ability of DGC-grown film is attributed to the increased film resistance associated with a dense and impermeable zeolite layer. As such, the DGC-grown film with thinner film thickness performed stronger anti-corrosion function than the InC-grown film. Furthermore, a very thin structured film exhibited satisfactory protection for AA6061 substrate during salt spray test over 1000 h. The MFI film also exhibited strong protection capability against the corrosion of H2SO4 and NaOH solution. The nano-structured silica could reduce the corrosion current of bare AA6061 substrate by five orders of magnitude as comparing to typical silica coating. The zeolite film also exhibited strong anti-corrosion capability for carbon steel and stainless steel substrates. The as-synthesized MS film containing CTAB showed strong corrosion protection property for AA6061 in NaCl solution. The strong corrosion protection property of the MS film is attributed to its blocked porous structure and hydrophobic nature. The corrosion protection capability of the CTAB-replenished MS film is stronger than the benzotriazole-replenished one. As such, the corrosion protection capability of the as-synthesized MS film is repairable due to loss of CTAB over long term service by replenishing CTAB to extend its service life.