<p>This study investigates the corrosion resistance of an aluminum phosphate sol–gel derived non-stoichiometric coating in the standard 3.5 wt.% NaCl solution, and also in an industrial 18 wt.% H₂SO₄ solution. Phase analysis of the coating was performed by regular X-ray diffraction (XRD), and grazing incidence X-ray diffraction (GI-XRD) techniques, and also surface observations were performed by scanning electron microscope (SEM). The corrosion resistance of the obtained coating was then investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) test methods. Results showed by applying the AlPO₄ amorphous coating on the AISI 304 stainless steel substrate, corrosion current density (<i>i</i>_corr) in the 3.5% NaCl environment decreased significantly from 3.5 × 10^–7&#xa0;to 9.12 × 10^–9 A/cm², and, mitigated from 1.67 × 10⁻⁶&#xa0;to 1.58 × 10⁻⁷ A/cm² in the 18 wt.% H₂SO₄ aqueous solution. This was explained by the different corrosion mechanisms in saline and acidic environments, as well as the ceramic nature and lack of grain boundaries due to the amorphous structure of the coating.</p>