Mechanical Stress Effects on 550 °C Hot Corrosion Propagation Rates in Precipitation Hardened Ni-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS

Combinations of temperature, stress and hot corrosion may cause environmentally-assisted cracking in precipitation-hardened Ni-base superalloys, which is little understood. This research aims to increase current understanding by investigating the effects of mechanical stress on the hot corrosion propagation rate during corrosion-fatigue testing of CMSX-4, CM247LC DS and IN6203DS. The parameters used during the tests included a high R-ratio, high frequency, and a temperature of 550 °C. The results showed CMSX-4 experienced a predictable increase in the hot corrosion rate, CM247LC DS also experienced increased rates, but no obvious trend was apparent; whilst IN6203DS showed no evidence of an increased rate. These different behaviours appear to be a result of an interaction between the mechanical stress and microstructural features, which include gamma-prime volume fractions in both the matrix and eutectic regions, along with the distribution of the eutectic structure. The different behaviours in the hot corrosion propagation rate subsequently affected the respective corrosion fatigue results, with both CMSX-4 and CM247LC DS experiencing fracture but with significantly more scatter involved in the CM247LC DS results. All IN6203DS corrosion-fatigue specimens completed the respective tests without fracture and showed no evidence of cracking. It, therefore, appears that precipitation hardened Ni-base superalloys, which are susceptible to environmentally-assisted cracking, also experience increased hot corrosion propagation rates.

3D characterisation of hydrogen environmentally assisted cracking during static loading of AA7449-T7651

In this investigation, synchrotron X-ray microtomography was used to perform 3D in situ observations of crack initiation and growth during hydrogen environmentally assisted cracking (HEAC) in tensile samples of AA7449-T7651. Two smooth tensile samples with a 1 mm diameter gauge section were held at a fixed displacement ($$\approx 30$$ ≈ 30 % of yield stress) in warm, moist air ($$\approx 76\,^\circ $$ ≈ 76 ∘ C, 73% relative humidity). The samples were then imaged repeatedly using X-ray tomography until they fractured completely. The tomograms showing the nucleation and evolution of intergranular cracks were correlated with electron microscopy fractographs. This enabled the identification of crack initiation sites and the characterisation of the crack growth behaviour relative to the microstructure. The samples were found to fracture within an environmental exposure time of 240 min. Some cracks in both samples nucleated within an exposure time of 80 min (33–40% of the total lifetime). Many cracks were found to nucleate both internally and at the sample surface. However, only superficial cracks contributed to the final fracture surface as they grew faster owing to the direct environmental exposure and the larger crack opening. HEAC occurred prominently via brittle intergranular cracking, and cracks were found to slow down when approaching grain boundary triple junctions. Additionally, crack shielding from nearby cracks and the presence of coarse Al–Cu–Fe particles at the grain boundaries were also found to temporarily reduce the crack growth rates. After prolonged crack growth, the HEAC cracks displayed ductile striations and transgranular fracture, revealing a change in the crack growth mechanism at higher stress intensity factors.

Effect of Material Heterogeneity on Environmentally Assisted Cracking Growth Rate of Alloy 600 for Safe-End Welded Joints

Environmentally assisted cracking (EAC) is essential in predicting light water reactors’ structural integrity and service life. Alloy 600 (equivalent to Inconel 600) has excellent corrosion resistance and is often used as a welding material in welded joints, but material properties of the alloy are heterogeneous in the welded zone due to the complex welding process. To investigate the EAC crack growth behavior of Alloy 600 for safe-end welded joints, the method taken in this paper concerns the probability prediction of the EAC crack growth rate. It considers the material heterogeneity, combining the film slip-dissolution/oxidation model, and the elastic-plastic finite element method. The strain rate at the crack tip is a unique factor to describe the mechanical state. Still, it is challenging to accurately predict it because of the complicated and heterogeneous material microstructure. In this study, the effects of material heterogeneity on the EAC crack growth behavior are statistically analyzed. The results show that the material heterogeneity of Alloy 600 can not be ignored because it affects the prediction accuracy of the crack growth rate. The randomness of yield strength has the most influence on the EAC growth rate, while Poisson’s ratio has the smallest.

Reflections on Early Stages of Environmentally Assisted Cracking from Corrosion Pits

A perspective is presented on the evolution of damage due to environmentally assisted cracking (EAC), from crack precursor development through to long crack growth. The variable nature of crack precursors is highlighted with an observation that uncontrolled chemistry excursions or fabrication defects could eliminate any significant delay associated with that step in the damage evolution process. Specimen preparation by machining and grinding can be critical in determining the apparent susceptibility of the metal to EAC and corrosion, and an example for 316L stainless steel is given to show how physical defects generated by the grinding wheel can become the dominant site for pitting attack relative to MnS inclusions. Corrosion pits are the most commonly observed precursor to cracks in aqueous chloride environments. The loci of sites of crack initiation around a pit are discussed and the inherent challenges in quantifying the growth of cracks smaller than the pit depth described with implications for modelling of the pit-to-crack transition. The remarkably enhanced stress corrosion crack growth rate data for short and small cracks in a 12Cr steam turbine blade in a simulated condensate environment are discussed in the context of crack electrochemistry modelling and the implications for engineering integrity.

Crevice corrosion and environmentally assisted cracking of high-strength duplex stainless steels in simulated concrete pore solutions

This paper presents a study of crevice corrosion and environmentally assisted cracking (EAC) mechanisms in UNS S32205 and S32304 which were cold drawn to tensile strengths of approximately 1300 MPa. The study utilized a combination of electrochemical methods and slow strain rate testing to evaluate EAC susceptibility. UNS S32205 was not susceptible to crevice corrosion in stranded geometries at Cl⁻ concentrations up to 1.0 M in alkaline and carbonated simulated concrete pore solutions. UNS S32304 did exhibit a reduction in corrosion resistance when tested in a stranded geometry. UNS S32205 and S32304 were not susceptible to stress corrosion cracking at Cl⁻ concentrations up to 0.5 M in alkaline and carbonated solutions but were susceptible to hydrogen embrittlement with cathodic overprotection.

Progress and opportunities in modelling environmentally assisted cracking

Environmentally assisted cracking phenomena are widespread across the transport, defence, energy and construction sectors. However, predicting environmentally assisted fractures is a highly cross-disciplinary endeavour that requires resolving the multiple material-environment interactions taking place. In this manuscript, an overview is given of recent breakthroughs in the modelling of environmentally assisted cracking. The focus is on the opportunities created by two recent developments: phase field and multi-physics modelling. The possibilities enabled by the confluence of phase field methods and electro-chemo-mechanics modelling are discussed in the context of three environmental assisted cracking phenomena of particular engineering interest: hydrogen embrittlement, localised corrosion and corrosion fatigue. Mechanical processes such as deformation and fracture can be coupled with chemical phenomena like local reactions, ionic transport and hydrogen uptake and diffusion. Moreover, these can be combined with the prediction of an evolving interface, such as a growing pit or a crack, as dictated by a phase field variable that evolves based on thermodynamics and local kinetics. Suitable for both microstructural and continuum length scales, this new generation of simulation-based, multi-physics phase field models can open new modelling horizons and enable Virtual Testing in harmful environments.

The role of nano-sized intergranular phases on nickel alloy 725 brittle failure

Although precipitation-hardened nickel alloys, such as Alloy 725, are extensively used in the most aggressive oil and gas production conditions given their a priori superior environmentally assisted cracking resistance, recent failures associated with hydrogen embrittlement (HE) cast doubts about their performance in H-bearing environments. These high-profile incidents have led to extensive research to elucidate the failure mechanism and develop methods to reduce risks. However, the microstructural features that made only specific heats of Alloy 725 susceptible to HE have yet to be agreed upon by the scientific community. Herein, our results showed that the abundant precipitation of F phase, i.e., a recently discovered nano-sized and embrittling phase, at grain boundaries increased cracking susceptibility. A continuous F phase network offered a low-energy path for crack opening by matrix-precipitate interfacial decohesion. Additionally, we correlated Alloy 725 microstructure with HE resistance, which is essential for thermomechanical processing optimization and the design of alternative qualification tests to obtain embrittlement-resistant nickel alloys.