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ToF-SIMS, XPS, voltammetry and EIS investigation of the anti-corrosion properties of thin (10, 50 and 100nm) alumina coatings grown by atomic layer deposition at 160°C on steel is reported. Surface analysis shows a thickness-independent Al₂O₃ stoichiometry of the coating and trace contamination by the growth precursors. The buried coating/alloy interface has iron oxide formed in ambient air and/or resulting from the growth of spurious traces in the initial stages of deposition. Electrochemical analysis yields an exponential decay of the coating porosity over four orders of magnitude with increasing thickness, achieved by sealing of the more defective first deposited 10nm.