Precise determination of surface micro-galvanic behavior

Stainless steel and its alloys are widely used as a corrosion-resistant material in RO and desalination plants. However, these alloys have shown variable corrosion performance in chloride containing environments. Localized corrosion attack of some of these alloys, particularly in a welded metal region 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 ferrite mono-phase temperature. This change of microstructure in welded metal affects (decreases) the mechanical and corrosion properties of DSS. A surface potential characterization of welded metal was established for a laser-welded DSS with 22Cr, 4.3Ni, 3Mo and 0.17N2 using a modified surface potential mapping technique (SCM). It is evident from potential mapping tests that the welded metal is more anodic to the base metal in ferric chloride solution. The welded metal potential maps showed significant micro-galvanic interaction 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 tool for decision-makers with regard to materials selection and early pit detection.

1. The SCM detection method was able to detect the most corrosion-susceptible region of the specimen as well as to follow the progress with time of weldment deterioration with respect to the adjacent parent metal. 2. The SCM technique was able to follow with time localized pitting corrosion activity in the welded metal region during pitting incubation, initiation, and propagation stages. 3. Metallographic observations were consistent with the SCM map recorded under chloride pitting conditions. 4. Improper welding operation caused alloy segregation and interdendritic weakness in the ferrite/austenite islet that reduced corrosion resistance. 5. This high-resolution implementation of the SCM may prove useful for studies involving the grading of welded duplex steels in terms of optimum welding parameters.