Intergranular Corrosion and Chromium Depletion in Low-Medium Chromium Martensitic Stainless-Steel Turbine Blades: A Thermodynamic and Metallographic Analysis

Abstract

Geothermal turbine blades manufactured from low-medium chromium stainless steel (11.5–13.5% Cr) exhibit unexpected intergranular corrosion (IGC) despite compliance with material specifications. This study investigates the mechanisms of chromium depletion and sensitization in stainless steel blades operating under geothermal conditions. Six failed blades and three operational blades from Ol-Karia II Geothermal Power Station were analysed using scanning electron microscope (SEM) fitted with energy dispersive spectrometry (EDS), X-ray diffraction (XRD), and Calculation of Phase Diagram (CALPHAD) thermodynamic modelling. Results revealed localized chromium depletion zones adjacent to grain boundaries, carbide precipitation patterns, and intergranular corrosion pathways. Thermodynamic phase diagrams predicted carbide formation temperatures and chromium diffusion profiles. Electrochemical stability diagrams showed that geothermal steam conditions (high chloride and sulphide concentrations) exceed the passivity boundaries of the tested material. This work demonstrates that intergranular corrosion in spec-compliant stainless-steel results from sensitization during manufacturing or service, exacerbated by aggressive geothermal fluid chemistry.