Predicting the Effect of Applied Potential on Crack Tip Hydrogen Concentration in Low-Alloy Martensitic Steels Available to Purchase

The dependency of hydrogen environment embrittlement on applied potential of an ultrahigh-strength steel was explained. This is accomplished through (1) definition of crack chemistry and potential; (2) development of a database of diffusible hydrogen concentration (C_H,diff) as a function of local chemistry and potential; (3) prediction of crack tip hydrogen concentration (C_H,tip) as a function of material, geometry, and chemistry of the bulk environment; and (4) use of damage models that predict the threshold stress intensity for cracking (K_TH) as a function of C_H,tip. In this paper the primary material selected for study is UNS K92580, which is characterized by high-purity (i.e., low sulfur and phosphorus content) and nanoscale strengthening solute zones and coherent precipitates. The experimental relationship between applied potential and the threshold stress intensity for cracking (K_TH) has been established previously for UNS K92580 through slow strain rate testing in an aqueous environment over a range of cathodic and anodic applied potentials. The results reveal a window of applied potentials where there is reduced susceptibility to hydrogen environment embrittlement (HEE). Damage models for initiation and growth can justify this behavior based on the potential dependent value of C_H,tip. However, the applied potential itself does not yield insight as to the exact conditions at the crack tip. Ohmic potential (IR) drop and electrochemical/ chemical reactions in the crack can lead to a significantly different environment at the crack tip than on the bulk surface. Potential-pH conditions in the crack environment are defined as a function of material, geometry, and chemistry, and then connected to C_H,tip, which can then serve as inputs to damage models for HEE initiation and growth.