| The environment assisted cracking (EAC) of nickel-based superalloy 718 was characterized in acidic chloride solutions under hydrogen-producing conditions using a rising-load fracture mechanics method. The stress intensity at the onset of crack growth (KTH) was used to measure EAC susceptibility as a function of applied electrode potential and solution chemistry. For all test conditions KTH was reduced from the air fracture initiation toughness (KICi). EAC susceptibility depended on both the electrode potential and solution pH. When the electrode potential was constant, susceptibility increased as the solution pH decreased. When the solution pH was constant, there was a minimum in KTH at intermediate electrode potentials.;The appearance of the fracture surface gradually changed from voids and transgranular facets to voids with transgranular and intergranular facets as KTH decreased. The amount of plasticity associated with the voids and transgranular facets decreased as KTH decreased. Transgranular cracking dominated the onset of crack growth and occurred primarily by slip band fracture. A ductile fracture model, based on a critical fracture strain as measured by void growth, accurately predicted KTH and microstructure effects, suggesting that absorbed hydrogen lowered KTH from K ICi by promoting secondary microvoid nucleation which lead to intravoid strain localization and transgranular cracking.;An empirical model of hydrogen production and absorption, based on a local crack chemistry that was less acidic than the bulk, was developed and used to predict the pH dependence of KTH at -1.0 VSCE . Gaseous hydrogen embrittlement data from the literature, hydrogen charging results, potentiostatic and potentiodynamic polarization data, and data from a buffered solution were combined to predict KTH of Alloy 718 as a function of solution pH at -1.0 VSCE in acidic chloride environments. The model accurately predicted KTH over the pH range studied. |
