Project L2-50060

The research project aims to understand the reasons and mechanisms behind microbiologically induced corrosion (MIC) in rivers and to develop a new grade of MIC-resistant martensitic stainless steel. MIC is present in natural water environments and all steels, even stainless steels are prone to it to some degree. These steels are used for hydroelectric power generation (in our case specifically for hydroelectric turbine blades) and also as parts of cooling systems (in our case, specifically the secondary cooling system of a nuclear power plant). The experimental part will involve testing for MIC damage of different standard stainless steel grades (austenitic, duplex, and martensitic) in the laboratory and in the Sava river. Additionally, a new martensitic stainless steel with improved MIC resistance will be developed and produced, which will be subjected to the same testing conditions in order to show improvement.

The main objective of the proposed research project is to understand the reasons and mechanisms behind MIC in rivers, with specifics to the Sava river in Slovenia, and to develop a new grade of MIC resistant martensitic stainless steel for casting. In addition, to better understand conditions for biofilm formation in relation to MIC and material response of different standard stainless steel grades (different microstructures and chemical compositions) will be tested. The results will give us valuable guidelines for improving MIC resistance of steel in the natural environment of Sava river near two hydroelectric power plants and near a water source for nuclear power plant cooling.

Other socioeconomic impacts include a better understanding of the MIC process, material susceptibility to MIC, better understanding of the maintenance of steels in fresh waters, resilience to changing environment due to human activity, better reliability and safety of steel components in aqueous environment. The results will be applicable to other freshwater environments across the globe and will be used to improve climate change resilience.

Project goals will be achieved through thermodynamic modeling, composition modification through alloying additions, alloying elements such as Cr, Cu, Mo, W, and others. To achieve specific mechanical properties required for turbine blades the steel matrix will have to be martensitic, but with improved corrosion resistance, mainly higher PREN. Based on the subsequent pitting corrosion phenomenon, the best measure of MIC resistance is the PREN (Pitting Resistance Number), estimated from the chemical composition. However, higher PREN is related to the occurrence of delta ferrite in MSS that impairs both mechanical and corrosion properties. Laboratory experimental steel grade will be made in an induction melting furnace for the production of castings. Heat treatments will be planned and corresponding mechanical, electro corrosion, technological properties and microstructures will be analyzed. The anti MIC potential of the manufactured new martensitic stainless steel will be determined in the laboratory under optimal conditions for MIC development. As MIC occurrence is closely related to the environmental conditions, we will monitor microbiologically relevant parameters in the upstream of hydroelectric power plants and near water sources for nuclear power plant cooling systems. In addition, we will monitor naturally occurring MIC formation in Sava river by placing test coupons of the new martensitic stainless steel and the reference stainless steel materials in the Sava river upstream of the hydro and nuclear power plants.