This study investigated the anodization of tin in oxalic acid electrolyte at low applied voltages of U <= 5 V. Under such conditions, the anodization product consists of micron-sized SnC2O4 crystalline particles and the morphology of the product changes during the anodization process in three distinctive stages. In the initial stage I, Sn dissolves into the electrolyte at high current density. This process is associated with the precipitation of highly oriented SnC2O4 crystals on the Sn surface. Stage II is a passivation process associated with the formation of a SnC2O4 surface layer with unique pyramidal microstructure via in situ conversion. Stage III is the localized creation of smaller SnC2O4 polyhedron crystals. The anodic reaction mechanisms for each stage are discussed in detail and a mathematical model is established to quantify the critical condition that induces the anodic passivation. (C) 2011 Elsevier Ltd. All rights reserved.