Date

2022-8-12

Author

Kaykaç, Nalan Gülçin

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Cataphoresis is the process of applying high-performance epoxy resins to metal surfaces in order to increase the corrosion resistance of metal materials by using electrochemistry principles. This process is widely used in automotive industry to ensure long time corrosion durability of the vehicles. The cataphoresis process proceeds in two main steps followed by baking. These processes can be roughly summarized as zinc phosphating and electrocoating. A multiple number of degreasing, washing and rinsing steps occur during the mass production between these main steps to ensure best performance of the coating. A typical cataphoresis line has fifteen baths to carry out these two basic operations prior to baking. In order to ensure the quality of the coating, all baths should be prepared and monitored regularly in the light of certain parameters. Within the scope of this study, in the first chapter, the effect of some operational parameters on surface quality such as roughness and some physical properties such as stone chip resistance, corrosion resistance and adhesion are evaluated. SEM, FTIR, DSC and TGA are performed to the coatings in order to understand the effectiveness of the parameters and to determine optimum operation parameters. In the first chapter, it is found that; the increase in parameters such as voltage, application time, application temperature and solvent ratio, which directly affect the film thickness, adversely affect the surface quality and mechanical properties. Pigment and resin ratios directly affect the crater problem and gloss on the surface. Crater formation is seen on the surface at low pigment ratios, while a dullness occurs on the surface at high pigment ratios. It has been observed that parameters that directly affect the viscosity and mobility of the coating solution, such as solid matter, pigment ratio, solvent ratio and bath temperature, directly affect the thickness and mechanical strength of the coating. In the second chapter of this thesis study, a DOE study is conducted to find optimum physical properties of the coating according to the results based on the preliminary analysis obtained at previous chapter and a regression equation is defined. In the third chapter of the study, to find optimum operation conditions of the zinc phosphating process prior to electrocoating, another DOE study is investigated for phosphate conversion layer. Zinc phosphate bath temperature and duration of the process are changed to reach optimum conversion layer structure to promote corrosion resistance behavior. In the fourth chapter, performance tests are applied on the zinc phosphate coatings to validate DOE results. It is obtained that uniform and fine-grained crystal structure with lower coating weight gives the best physical performance after electrocoating. In this study 180 sec immersion time at a temperature of 45 oC is found as optimum condition in terms of crystal morphology, coating weight and performance results such as corrosion, adhesion, stone chip and water immersion resistance. In the last chapter, a cross comparison of optimum conditions with a reference condition is performed. It is found that optimum condition of phosphate and electrocoating provides a better surface quality together with better physical properties while providing material, energy and cost savings as well.

Subject Keywords

Cataphoresis, Electrocoating, Zinc Phosphating, Roughness, Corrosion Resistance

URI

https://hdl.handle.net/11511/98621

Collections

Graduate School of Natural and Applied Sciences, Thesis