Evaluating oxidation behavior of amorphous aluminum phosphate coating

The aim of this study was to investigate the oxidation properties of amorphous aluminum phosphate coating. Aluminum phosphate precursor solution was prepared by a sol-gel process and then applied on AISI 304 stainless steel using dip coating technique. To evaluate the oxidation behavior, samples were placed in an electrical furnace upto 1100 °C for 100 h in air with weight measurements performed at regular ten-hour intervals. Phase composition analysis of the coatings before and after cyclic oxidation process were performed by X-ray diffractometer (XRD). The surface and cross-sectional morphology of the coatings were observed using scanning electron microscopy (SEM) analysis equipped with energy dispersive spectroscopy of characteristic X-rays (EDS). The amorphous-nanocrystalline structure and distribution of nanocrystals in the amorphous matrix were studied by transition electron microscopy (TEM). According to SEM images a uniform, continuous and crack-free coating was achieved. XRD analysis as well as TEM observations showed that the amorphous structure of coating remained unchanged after annealing at 500 °C for 15 min however; an amorphous-nanocrystalline structure was obtained after annealing at 1100 °C for 1 h. Weight change measurements after 100 h oxidation test revealed that the trace of weight gain against oxidation time for both coated and un-coated substrates were parabola in nature and the range of the weight change of the bare substrate was about 30 times greater than that of observed for the aluminum phosphate coating. In general, the results showed that the synthesized amorphous aluminum phosphate is capable of surface protecting of metals/alloys against degradation at harsh environments.