In-situ High-Resolution Microscopy on Duplex Stainless Steels

The aim of the present work is the characterization of materials in-situ by using a modified Near-Field Scanning Optical Microscope (NSOM) with a home-built tuning fork head that is used to obtain optical images concurrently with topography of some systems. The first example is related to finding precursor sites for pitting corrosion on Duplex Stainless Steels (DSS). Precursor sites for pitting on DSS were found to be inclusions that are complex in structure. Metastable pits develop in these inclusions at temperatures below the Critical Pitting Temperature (CPT), but stable pits with large corrosion current) occur at the CPT. The inclusions were analyzed and found to be inhomogeneous in nature and consisting of a mixture of various elements (Si, Al, Mg, Ca, Ti, Mn and S). After analysis of the particles, in-situ observation of the particles in 2.5%NaCl and 1 M HCl solutions showed that they developed metastable pits. The pits and corrosion products developed in particles present in the austenite grains and in particles contained within the ferrite matrix.