James T. Burns

The Effect of Microstructural Variation on the Hydrogen Environment-Assisted Cracking of Monel K-500

The influence of microstructural variation on hydrogen environment-assisted cracking (HEAC) of Monel K-500 was evaluated using five nominally peak-aged lots of material tested under slow-rising stress intensity loading while immersed in NaCl solution under cathodic polarizations. Minimal variation in HEAC resistance among material lots was observed for an applied potential of −950 mVSCE (Eapp, vs saturated calomel), whereas lot-to-lot variability in the fracture morphology demonstrates a significant difference in the HEAC resistance at the less negative potential of −850 mVSCE, suggesting that relatively severe H environments produce sufficient crack-tip H to minimize the impact of metallurgical differences. Sensitivity analyses accomplished by varying the inputs used in decohesion-based, micromechanical models imply significant variations in HEAC resistance are possible for realistic changes in grain boundary toughness, hydrogen uptake behavior, and yield strength. Grain size, impurity segregation (including the effects of gettering elements), grain boundary character/connectivity, and crack path tortuosity are also considered in the context of HEAC susceptibility. Yield strength, global hydrogen content, as well as impurity segregation to grain boundaries, especially boron and sulfur, are speculatively considered to be the dominant contributions in determining HEAC resistance. Modifications that would incorporate the effects of grain boundary segregation are proposed for the KTH model; detailed validation of such changes require high-fidelity and quantitative inputs for the degree of grain boundary segregation. Regardless, fracture mechanics-based HEAC results, detailed microstructural characterization, and micromechanical modeling were successfully coupled to gain insights into the influences governing the microstructure-dependent HEAC susceptibility of Monel K-500.

Galvanic Corrosion-Induced Fatigue Crack Initiation and Propagation Behavior in AA7050-T7451

Electrochemical experiments were used to create morphologies representative of galvanically-induced corrosion between aluminum alloy (AA) 7050-T7451 (UNS A97050) and Type 316 steel (UNS S31600) in a fastener configuration. The effects of these corrosion damage morphologies on the fatigue behavior were investigated. Results showed that severe macroscale metrics considered for discrete pits and general corrosion with surface recession do not correlate with the location of fatigue crack initiation. However, for intergranular corrosion, the plane that had higher number of fissure and higher total fissure depth tended to correlate to the location of fatigue crack formation.

The interaction of corrosion fatigue and stress-corrosion cracking in a precipitation-hardened martensitic stainless steel

Fracture mechanics-based testing was used to quantify the stress-corrosion cracking and corrosion fatigue behavior of a precipitation-hardened martensitic stainless steel (Custom 465-H950) in full immersion chloride-containing environments at two applied electrochemical potentials. A plateau in the cycle-based crack-growth kinetics (da/dN) was observed during fatigue loading at low ΔK and [Cl−] at and above 0.6 M. Evaluation of the fracture morphology and frequency dependence of this plateau behavior revealed an intergranular fracture surface morphology and constant time-dependent growth rates. These data strongly support a controlling stress-corrosion cracking mechanism occurring well below the established KISCC for quasi-static loading. Low-amplitude cyclic loading below ΔKTH (i.e., “ripple loads”) is hypothesized to enable time-dependent intergranular-stress-corrosion cracking to occur below the KISCC via mechanical rupturing of the crack-tip film and enhancement of the H embrittlement-based SCC mechanism.Stainless steels: quantifying environmental fatigue failureModern high-strength stainless steels offer combined mechanical and corrosion resistance, but are susceptible to environmental cracking in aqueous chloride solutions, typical of coastal and marine environments. New insight into environmentally enhanced fatigue behavior is thus of practical value. A team led by James Burns at University of Virgina has now quantitatively characterized the stress-corrosion cracking and fatigue fracture behavior of a precipitation-hardened martensitic stainless steel in various aqueous chloride solutions and elucidated the relative contribution of each factor to overall crack growth. The obtained data indicates an interesting mechanism that governs intergranular-stress-corrosion cracking. The present understanding may enable rational design and prognosis of engineering materials for use in the field.

Erratum to: Measurement and Modeling of Hydrogen Environment-Assisted Cracking in Monel K-500

Fig. 17—(Color online) Model predicted (Eq. [2]) and measured Stage II crack growth rate (at elastic K = 50 MPa m, Table III; Fig. 9) vs crack tip CH-Diff (bottom axis) and stress enhanced CHr (top axis) for aged Monel K-500 (ATI Allvac, j; Table I) stressed in 3.5 pct NaCl at EA between 1100 and 700 mVSCE. The rH/rYS is 12.0 in Eq. [3]. Measured da/dtII are included for a Special Metals lot of Monel K-500 (m) using the corresponding H uptake law (Footnotes * and **). Fig. 18—(Color online) Model predicted (Eq. [2]) and measured Stage II crack growth rate (at elastic K = 50 MPa m, Table III; Fig. 9) vs applied cathodic potential for aged Monel K-500 (ATI Allvac, j; Table I) stressed in 3.5 pct NaCl. The rH/rYS equals 12.0 in Eq. [3]. Measured da/dtII values are included for a Special Metals lot of Monel K-500 (Footnote *, m).