Effect of Retained Austenite on the Corrosion Behaviour of Q&P Processed Martensitic Stainless Steels

The fuel consumption of cars has become an important issue in the development of new materials and. These developments resulted in the generation of new grades of steel for the automotive industry. In particular, new advanced high strength steels (AHSS) have been developed for the automotive industry to lower fuel consumption (with weight reduction) by combining strength with formability. Especially, quenching and partitioning (Q&P) steels from 3rd generation AHSS can exhibit significant strength and ductility balance by combining martensite with retained austenite. Recently, there has been a new interest in applying the Q&P treatment to stainless steels, in particular martensitic stainless steel. For the automotive industry, the development in Q&P treated martensitic stainless steel can be a game-changer. The mechanical properties of Q&P treated commercial martensitic stainless steels have been widely researched. Unlike mechanical behaviour, the corrosion behaviour of Q&P treated martensitic stainless steel has not been investigated deeply. The effect of environmental factors or the effect of microstructure on the corrosion performance of Q&P processed martensitic stainless steel needs to be studied. This master thesis aims to identify the effect of microstructural constituents (retained austenite, primary and fresh martensite) on the corrosion response of Q&P treated martensitic stainless steels. To this end, an experimental approach is taken for this project. This work aims to create a relationship between heat treatment, microstructure, and the resulting corrosion properties. As an experimental path, open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and Mott-Schottky experiments were carried out in 3.5 wt.% NaCl solution to reveal the corrosion response and passive film properties of the Q&P microstructures. In addition, X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical composition and fractions of oxide layers in the passive layer. Results demonstrate a phase dependency for the corrosion performance of Q&P treated martensitic stainless steels.Materials Science and Engineerin