The present study systematically investigates the effect of Al2O3 coatings with thicknesses approximately ranging from 20 to 100 nm on the biocorrosion of AZ31-type alloy. The coating was carried out by means of the atomic layer deposition, and the thickness, composition, morphology and conformality of the coatings were studied by spectral ellipsometry, SEM, SEM-EDX, AFM, XRD, ED-XRF and XPS. Ringer solution, phosphate-buffered saline (PBS) and simulated body fluid (SBF) were utilised as media for the biocorrosion study, and the dynamics of the process were examined by three independent methods during 14 days of sample immersion. Supplementary electrochemical studies were conducted, and a substantial variation was identified in the corrosion rate and stability of the coatings across different solutions, with SBF yielding the most favourable outcomes. The coatings demonstrated high efficacy in mitigating biocorrosion during the initial three days of the study. The reduction in hydrogen release in SBF for 100 nm thick coatings reached up to sevenfold, and mass loss was almost two orders of magnitude. With extended immersion times, the effect of the coatings diminished, yet persisted even following a 14-day immersion period. The MTT test utilising preosteoblasts MG-63 cells for all samples exhibited either no or low levels of toxicity.