Corrosion Behavior of Duplex Stainless Steels in Acidic-Chloride Solutions Studied with Micrometer Resolution

Sammanfattning: The local corrosion behavior of duplex stainless steel (DSS)is affected by a wide variety of factors. Localized corrosionof DSS frequently starts at micrometer scale inclusions orprecipitates, which are often segregated in theaustenite-ferrite boundary regions. Moreover, due to thepartitioning of the key alloying elements of ferrite (Cr andMo) and austenite (N and Ni), the local interactions betweenthe phases must also be considered.The aim of this doctoral study was to increase the knowledgeabout the local dissolution behavior of DSS in acidic-chlorideenvironments. The recent developments of new local probingtechniques have opened a new frontier in corrosion science,providing valuable local information not accessible in thepast. The local techniques used include electrochemicalscanning tunneling microscopy (EC-STM), scanning probe forcemicroscopy (SKPFM), magnetic force microscopy (MFM), andscanning Auger electron Spectroscopy (SAES), all withmicrometer or sub-micrometer resolution.With EC-STM, it was possible to monitor local dissolutionprocesses on DSS in situ, and in real time. MFM was capable ofimaging the phase distribution in DSS without the need of thetraditional surface etching, while SKPFM revealed that theVolta potential difference between the two phases wasmeasurable and significant. SAES showed that the compositiongradient at the phase boundaries is narrower than 2µm.Different types of DSSs have been studied, from low-alloyedDSS to superduplex. Higher contents of Cr, Mo and Nstrengthened both phases as well as the phase boundaries,resulting in phases having similar corrosion resistance thatshowed a more uniform dissolution behavior. However, the Voltapotential difference between the phases proved to be of thesame order for all the DSSs studied. Austenite was in generalassociated to regions displaying a more noble Volta potentialthan ferrite, resulting in a higher dissolution rate of theferrite next to the austenite phase.Key words:In situ, local dissolution, electrochemical,STM, SKPFM, MFM, SAES, duplex stainless steel, acidic-chloridesolution.