Passivity & Breakdown of Super Duplex Stainless Steel
Studied with Electrochemical & Synchrotron Techniques
Time: Fri 2020-09-18 10.00
Subject area: Chemistry
Doctoral student: Marie Långberg , Yt- och korrosionsvetenskap, Yt och korrosionsvetenskap
Opponent: Professor Sannakaisa Virtanen, Friedrich-Alexander-Universität
Supervisor: Professor Jinshan Pan, Materialvetenskap, Materialvetenskap, Kemi, Yt- och korrosionsvetenskap; Doktor David Lindell, Swerim; Doktor Ulf Kivisäkk, AB Sandvik Materials Technology; Professor Edvin Lundgren, LU
Super duplex stainless steel consists of two phases: austenite and ferrite, and is a highly corrosion resistant material, with a wide range of applications. The corrosion resistance of stainless steels is determined by the performance of a spontaneously formed thin Cr oxide rich film with a thickness of 1-3 nm. This film makes the stainless steel passive towards further oxidation processes, corrosion. The passive film is a dynamic system where degradation and formation reactions occur simultaneously.
Super duplex stainless steel is a multi-phase, multielement material which has a high corrosion resistance and the question is whether its microstructure and alloying elements are affecting its passive film properties.
This thesis contains two parts: real time synchrotron measurements to study the degradation mechanism during anodic polarization which pushes the material toward oxidation processes; and ex-situ measurements to map the lateral thickness and microscopic elemental distribution in the passive film.
The in-situ/operando synchrotron experiments combined several experimental techniques, including X-ray reflectivity (XRR), X-ray diffraction (XRD), X-ray fluorescence (XRF) and electrochemical impedance spectroscopy (EIS), to simultaneously characterize the structure and chemical/electrochemical properties, as well as their changes of the surface region of the samples. It was found that the oxide film became more defective with increasing potential, leading to a decreased density of the passive film. On the other hand, the Ni rich alloy surface layer below the oxide film showed an increasing density, indicating an increased concentration of heavy elements (Mo and Ni). The XRD data showed evidence of nanocrystalline passive film whereas the amorphousness of the passive film increased with increasing potential.
The ex-situ investigations employed by hard X-ray photoelectron emission microscopy (HAXPEEM), providing chemical data from individual grains with different crystallographic orientations parallel to the sample plane. The Cr content was higher on the ferrite than the austenite, whereas the thickness was rather uniform. The grain orientation has a small but detectable influence on the thickness and Cr content of the native passive film.