Secondary electron yield (SEY) of air-exposed metals intends to be increased because of air-formed oxide, hydrocarbon and other contaminants. It enhances the possibility of secondary electron multipacting in high-power microwave systems, resulting in undesirable occurrence of discharge damages. Al2O3 coatings have been utilized as passive and protective layers on device packages to provide good environmental stability. We employed atomic layer deposition (ALD) to produce a series of uniform Al2O3 coatings with appropriate thickness on Ag-plated aluminum alloy. The secondary electron characteristics and their variations during air exposure were observed. According to the secondary electron emission of composite layer, the escape depth of electron needs to exceed coating thickness to some extent in order to demonstrate SEY of metallic substrates. Based on experimental and calculated results, the maximum SEY of Ag-plated aluminum alloy has been maintained at 2.45 over 90-days of exposure without obvious degradation by applying 1 nm Al2O3 coatings. In comparison, the peak SEY of untreated Ag-plated aluminum alloy grows from an initial 2.33 to 2.53, exceeding that of 1 nm Al2O3 sample. The ultra-thin ALD-Al2O3 coating substantially enhanced the SEY stability of metal materials, with good implications for the environmental dependability of spacecraft microwave components.