Low energy ion implantation of nitrogen into oxide free aluminium surfaces was used to obtain stoichiometric and pore-free AlN layers of about 8-10 nm thickness. The electrochemical stability of these layers was investigated by anodic cyclic voltammetry, pulse measurements and X-ray Photoelectron Spectroscopy (XPS). For comparison AIN layers prepared by Physical Vapor Deposition (PVD, thickness 2 mu m) were examined as well. While AlN is hydrolysed at pH 14 as predicted by thermodynamics, electrochemical stability up to 1.6 V was observed in acetate buffer solution (pH 5.9). Surface analysis reveals that at potentials below 2 V, AlN is covered by an oxide layer of 2 nm. Molecular N-2 dissolved at the AlN/Al2O3 phase boundary was detected as a product of nitride decomposition. At higher anodic potentials, the oxide layer thickness increases whereas the AlN layer diminishes. Oxidation takes place in a two-step mechanism. Beyond a potential of 1.6 V, the electric field is sufficiently high to initiate the oxidation of N3- via tunnelling. Although migration of mobile ions (Al3+, N3-, O2-) starts. the layer composition remains nearly unchanged. At potentials above 4 V, high-field oxide growth starts until the nitride is completely decomposed. In contrast, the properties of PVD layers on steel substrates are determined mainly by the porosity of the coating. Due to substrate properties, iron dissolution and transpassive oxygen evolution were observed.