Thermal cycles during welding lead to inhomogeneous conditions in the chemical composition and microstructure of both weld metal and parent metal. These properties are directly related to pitting corrosion susceptibility of low-C 13CrNiMo martensitic stainless steels via the stability of their passive films. This work describes how passive film properties vary according to the substrate microstructure and allows interpreting the way in which these changes impact on pitting resistance. Thickness and structure of the anodic films on low-C 13CrNiMo martensitic stainless steels are markedly influenced by heat treatments previously applied to the alloys. Higher retained austenite amounts preserve higher Cr contents and lead to thicker anodic films. This beneficial microstructure is also related to higher rates of anodic film formation. (C) 2010 Elsevier B.V. All rights reserved.