| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 101 === There are a lot of methods to enhance the anti-corrosion capability of Magnesium alloy. Electrochemical plating, anodizing process, chemical conversion coating etc, are those of them. One method is to use chemical conversion coating to form a CaCO3 layer onto the Mg alloy to isolate it from the surroundings. This could prevent it from the possibilities of corrosion to the base material.
This experiment used AZ91D Mg-Al alloy , the widely used in electronic industry, and dip into CaCO3 solutions to form LDH and CaCO3 layer for corrosion resistance. First is to dip samples into carbonic acid to form precursor layer. SEM/EDS show that the surface exists many crack lines and already change to high amount of oxygen and carbon. FTIR and GAXRD detect the LDH grain structure after dipping into alkaline carbonic acid solution. And finally dipping into CaCO3 solutions, SEM show it is mostly calcite grain type and polarization curves indicate the corrosion resistance capability is enhanced.
We may observe some pores on the surface by SEM inspection. This kind of structure should decrease the capability of corrosion resistance. Using Focus Ion Beam for digging a hole across the poor and well coated areas, compare the under structure by FE-SEM inspection.
Under 4~5kx magnification, a precursor layer with cracks are found between CaCO3 and base substrate. After observation, the poor CaCO3 coating area has thinner precursor layer; furthermore there is an area without CaCO3, the precursor layer could not be found. This indicates the precursor layer plays an important role in the formation of CaCO3 coatings. The preliminary corrosion should be necessary and how to keep corrosion a certain depth and uniformity is essential in the process of CaCO3 coating formation.
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