Precise determination of surface micro-galvanic behavior

Stainless steel and its alloys are widely used as a corrosion-resistant mat erial in RO and desalination plants. However, these alloys have shown varia ble corrosion performance in chloride containing environments. Localized co rrosion attack of some of these alloys, particularly in a welded metal regi on and heat affected zone are not uncommon. In duplex stainless steel (DSS) the volume fraction of the austenite phase tends to decrease in weldement compared with that of base metals because of the rapid cooling from a ferri te mono-phase temperature. This change of microstructure in welded metal af fects (decreases) the mechanical and corrosion properties of DSS. A surface potential characterization of welded metal was established for a laser-wel ded DSS with 22Cr, 4.3Ni, 3Mo and 0.17N(2) using a modified surface potenti al mapping technique (SCM). It is evident from potential mapping tests that the welded metal is more anodic to the base metal in ferric chloride solut ion. The welded metal potential maps showed significant micro-galvanic inte raction between ferrite and austenite phases. The results also showed that the modified SCM technique alone or in conjunction with other tests such as ASTM G48 could offer significant assistance as a laboratory diagnostic too l for decision-makers with regard to materials selection and early pit dete ction.