Corrosion processes are found in all areas of industry. Understanding them is therefore important for further technical progress. With the information we obtain by investigated corrosion processes, we can develop more corrosion-resistant alloys and improve corrosion protection. In the master's thesis we studied the influence of surface refinement and how the modified layer affects the corrosion properties of selected steels.
Selected steels 16Mo3, 10CrMo9-10 and 13CrMo4-5 were used to increase the concentration of aluminum in the surface layer by powder-pack aluminization. The aluminization process took place in a filling, which is a mixture of different powders. The filling consisted of a diffusion element (6 wt. % Al), an activator (3 wt. % NH4Cl) and an inert filler (91 wt. % Al2O3). The process took place at two different temperatures (900 and 730 °C) and two different annealing times (2 and 4 hours).
In order to characterize the modified surface, a microstructural analysis was performed after aluminization using a light and scanning electron microscope (SEM). A thicker layer was obtained at a higher diffusion aluminization temperature (900 °C) than at 730 °C diffusion aluminization. From the results, we can conclude that in the modified layer in the sampels aluminized at 900 °C were obtained intermediate phases FeAl, Fe3Al and solid solution α. In the samples aluminized at 730 ° C, the intermediate phases FeAl2, Fe2Al5, FeAl, Fe3Al and solid solution α were obtained in modified layer. The corrosion properties of the modified surface were determined by electrochemical measurements. The properties were determined by corrosion potential measurements, linear polarization measurements, potentiodynamic measurements, and electrochemical impedance spectroscopy. Based on the results, it was determined that aluminide coating improved the corrosion resistance of selected steels. Aluminization brought the corrosion potential of the steels to more positive values. The average corrosion potential aluminized samples were in general more positive than potential for steels, showing at formation of more protective surface. We also reduced the corrosion current density of aluminized samples by two orders of magnitude compared to base steel, indicating improved corrosion resistance. To estimate the corrosion rate, we calculated the corrosion rate for individual steels and aluminized samples. The aluminizing process on all steels reduces corrosion rates by 55 to 241 times.
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