This study investigated the root cause of severe corrosion damage observed on G-X4CrNi13-4 martensitic stainless steel turbine blades at the Brežice hydroelectric power plant on the Sava River in Slovenia. The investigation employed a comprehensive analytical approach, including on-site visual inspections, non-destructive testing with a portable microscope (Struers, Ballerup, Denmark), X-ray fluorescence spectroscopy (Thermo Fisher Scientific, Waltham, MA, USA), and 3D topographical mapping (Bruker Alicona, Graz, Austria). Laboratory analyses utilized scanning electron microscopy (ZEISS, Oberkochen, Germany) and energy-dispersive X-ray spectroscopy (EDAX, Pleasanton, CA, USA) to examine surface deposits, biofilms, and corrosion products, complemented by physical and chemical water quality assessments. A visual inspection revealed a dense layer of biological deposits, approximately 0.3 mm thick, with stochastic pitting damage located beneath the biofilm. The analytical results confirmed that the blade material met the specifications; however, EDS analysis (EDAX, Pleasanton, CA, USA) revealed significant localized manganese enrichment within the pits (~1.4 wt%) and in the biofilm (>10 wt%). Although water analysis showed relatively low manganese concentrations (7–18 µg/L), these values may be sufficient to support the metabolic activity of manganese-oxidizing microorganisms. Historical hydrological data suggest that extreme drought conditions in 2022, stagnant water, and elevated temperatures facilitated the attachment of microorganisms and the formation of a corrosive biofilm. These findings highlight the impact of changing environmental conditions and low-flow periods on the integrity of stainless steel components in hydroelectric facilities.