Corrosion protection by ultra-thin (≤ 50 nm) alumina films deposited by atomic layer deposition (ALD) on copper at 250 • C was studied in 0.5 M NaCl aqueous solution by combining electrochemical and surface analytical methods. Time-of-Flight Secondary Ion Mass Spectrometry revealed a homogenous in-depth stoichiometry of the alumina coatings and their residual bulk contamination by organic and water precursor fragments. Trace contamination by a spurious Cu oxide was evidenced at the interface with the Cu substrate despite a reducing pre-treatment. Atomic force microscopy images revealed conformal substrate coverage and a granularlike coating morphology at the nanometer scale. Linear Sweep Voltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS) showed no electrochemical activity of the coating and a corrosion decrease reaching 4 orders of magnitude for coated samples in comparison with bare copper. Uncoated surface fraction was quantified by application of three different methods using the LSV and EIS data which led to results in excellent agreement. The uncoated surface fraction for 50 nm Al 2 O 3 layers was ∼0.01%, confirming ALD alumina as an excellent coating for corrosion protection with spectacularly low residual corrosion activity restricted to the bottom of channel defects connecting the substrate to environment through the coating.Copper is an important engineering metal and finds widespread application in heat exchangers, electrical power lines, water utilities, oil refineries, marine environments and microelectronics owing to its high electrical and thermal conductivity and mechanical workability. 1-3 Many studies have been dedicated to the copper corrosion mechanisms, especially in presence of aggressive ions like chloride, 4-8 and various corrosion protection methods have been explored including coating techniques such as Chemical Vapor Deposition (CVD), 9,10 plasma deposition 11 or electrochemical polymerization, 12 and the use of inhibitors 13-16 or self-assembled monolayers. 17,18 In the present study, Atomic Layer Deposition (ALD) of ultra-thin (≤ 50 nm) alumina coatings has been investigated for the corrosion protection of copper. ALD, as a CVD derived method, is an ideal technique for growing high quality nanometer-thick films. It takes advantage of sequential precursor pulsing on the substrate with selflimiting saturative steps to produce conformal films with accurate thickness control. As a result of excellent step-coverage on even difficult substrate surfaces with high aspect ratios, the grown films tend to be conformal and pinhole-free. [19][20][21][22] Alumina (Al 2 O 3 ) was chosen as a well-behaved and thoroughly-studied ALD material by virtue of its high dielectric properties, good adhesion to many surfaces, amorphous structure, thermal stability and mechanical properties. 21,23-25 ALD alumina for corrosion protection has already been studied alone and/or in combination with titania on stainless steel, 23,26-28 Mg alloy, 29 Al alloy, 30 silver 31,32 and copper. 33-36 Nanolamina...