Inducing aluminum oxide growth at room temperature and atmospheric pressure through low dose gamma-ray irradiation

Abstract

The corrosion resistance of the aluminum is well-known and it is attributed to passivation, a process that occurs when the metal surface reacts to the environment to form a spontaneously thin oxide film. Basically, the natural aluminum oxide film is composed of two layers, an internal layer composed entirely by Al2O3, and a second one made by Al(OH)3. In normal conditions of temperature and pressure, the Al(OH)3 film takes a long time to grow, something around several weeks or months. However, in the present work, through the use of low doses gamma-ray irradiation in an environment composed by air and water vapor, it was possible, in a few days, to increase the layer of Al(OH)3 on a sample of pure aluminum. Using a Cs-137 disk as a gamma-ray source, three aluminum samples were irradiated during 14, 8 and 5 days, receiving absorbed doses of 2.85 mGy, 1.49 mGy and 0.94 mGy, respectively. Comparing the Raman spectrum of the two irradiated samples with another non-irradiated sample, it was possible to verify that the sample irradiated during 14 days had the greatest values of peak intensity and peak area, indicating a higher concentration of aluminum hydroxide when compared to other samples. Moreover, the polarization curves of samples were obtained to analyze the nature of the film formed on the surface. From the results, it was possible to observe that the gamma-ray irradiation led to an improvement in passive film quality (higher resistance), once the irradiation resulted in a lower passivation current density (ipass). To confirm the consistence of previous analysis, more two samples were examined through X-ray photoelectron spectroscopy (XPS), where a sample irradiated during five days showed an increase in the percentage of aluminum hydroxide (related to XPS O 1s peak) in comparison to a non-irradiated sample. Then, it can be pointed that the irradiation, in a low dose, induced the growth of the aluminum hydroxide layer of passive film and improved the film's resistance in a few days. Moreover, the results show the utility of applying low doses of ionizing irradiation to change metal oxide surfaces.