pH estimation at corrosion fatigue crack tip in 13Cr-4Ni martensitic stainless steel

The blades of hydraulic turbines experience repeated loading during operation, promoting initiation and propagation of fatigue cracks in a corrosive environment. To identify the environmental damage mechanism—anodic dissolution or hydrogen embrittlement—potential (E) and pH must be measured locally at the crack tip, as values for these parameters measured at the crack tip differ from those measured in the bulk electrolyte. Direct measurement of potential drop (∆E=E_exterior - E_interior) and pH at the crack tip is, however, challenging. This study focuses on estimating local pH at the fatigue crack tip using thermodynamic analysis combined with ∆E measurements at the crack tip. The methodology was applied to the tip of cracks propagating in a martensitic stainless steel compact tension specimen (CT). ∆E was measured as the crack propagated in a simulated crack environment (deaerated synthetic river water). The potential dropped from 0.075 V_SHE in the synthesized river water to -0.09 V_SHE in the deaerated synthesized river water. XPS analysis of the corrosion product found on the fracture surface after testing revealed it consisted of Fe₂O₃ and FeCr₂O₄. Based on a Pourbaix diagram, E = -0.09 V_SHE coupled with presence of Fe₂O₃ and FeCr₂O₄ as corrosion products yields a thermodynamically stable solution with a pH ranging from 4.4 to 4.6.