Crack Tip Oxidation Research Articles

Fatigue crack tip corrosion processes and oxide induced closure

This paper reviews the corrosion and hydrolysis reactions that occur inside a fatigue crack at ambient temperatures and pressures in humid air and aqueous solutions. This study examines the individual reaction steps, intermediate phases formed, and the time required for the metal ions to transition into hard oxide phases. This study also finds that when a fatigue crack closes at ambient temperatures, the corrosion products inside the crack should consist of thin (≈2 nm) hard oxide films on the fracture surfaces and viscous layers of water with metal ions in various states of the transition from aqueous ions to stable end-product phases, but with insufficient strengths to resist load transfer at the crack tip. This analysis assumes that metal ion complexes form at the crack tip and transition toward the end-product phases in the wake of the crack in a manner analogous to that reported for plane surfaces exposed to humid air and aqueous solutions. It is concluded that the contribution of the viscous layers to an oxide induced crack closure (OICC) mechanism should be negligible. The impact of this finding on the interpretation of R-ratio effects on fatigue crack threshold and oxide thickness measurements is discussed. The OICC mechanism should be possible at high temperatures and in the near threshold fatigue crack growth region where hard oxides can grow to sizes relevant to the cyclic crack opening displacement (COD).

Influence of Crack Tip Oxide Film Morphology on Crack Tip Creep Field of Austenitic Stainless Steel

Influence of Crack Tip Oxide Film Morphology on Crack Tip Creep Field of Austenitic Stainless Steel

TEM investigation of SCC crack tips in high Si stainless steel tapered specimens

ABSTRACT The stress corrosion cracking (SCC) mechanism is investigated in high Si duplex stainless steel in a simulated PWR environment based on TEM analysis of FIB-extracted SCC crack tips. The microstructural investigation in the near vicinity of SCC crack tips illustrates a strain-rate dependence in SCC mechanisms. Detailed analysis of the crack tip morphology, that includes crack tip oxidation and surrounding deformation field, indicates the existence of an interplay between corrosion- and deformation-driven failure as a function of the strain rate. Slow strain-rate crack tips exhibit a narrow cleavage failure which can be linked to the film-induced failure mechanism, while rounded shaped crack tips for faster strain rates could be related to the strain-induced failure. As a result, two nominal strain-rate-dependent failure regimes dominated either by corrosion or deformation-driven cracking mechanisms can be distinguished.

A study on the surface and crack tip oxidation of alloy 600 through high-resolution characterization

Surface, stagnant grain boundary (GB), and crack tip oxidation of Alloy 600 were studied. The mirror-finished surface oxide has a duplex structure and the formation mechanisms of each layer are discussed. A more compact oxide film is observed on the cold-worked surface, while the nano-GBs are preferentially oxidized. The crack tip oxidation rate is around four orders of magnitude faster than that of the stagnant GB, and their oxidation mechanisms are believed to be the same. Diffusion-induced grain boundary migration (DIGM) is observed around the crack and oxidized GB tips. The role of Ni expulsion on the DIGM is discussed.

Effect of zinc on the environmentally-assisted fatigue behavior of 316 stainless steels in simulated PWR primary environment

In a simulated pressurized water reactor (PWR) environment with 30 ppb of Zn and peak holding, the low-cycle fatigue (LCF) life of 316 SS increased about 3 times compared to that without them. For the specimens tested with Zn and strain holding, the crack tip oxide layer was enriched with Zn. Electrochemical analysis showed the oxide layer had higher impedance and less defect density, indicating less corrosion compared to oxide layers from non-Zn-injection conditions. Thus, the improvement in LCF life of 316 SS was attributed to crack tip oxide modification by the combined effect of Zn-addition and peak strain holding.

Effect of crack tip oxidation on small fatigue crack growth in a single crystal Ni-base superalloy under out-of-phase thermo-mechanical fatigue

Effect of crack tip oxidation on small fatigue crack growth in a single crystal Ni-base superalloy under out-of-phase thermo-mechanical fatigue

An insight into PWR primary water SCC mechanisms by comparing surface and crack oxidation

Oxidation and stress corrosion cracking (SCC) of 316L stainless steel were studied in simulated pressurized water reactor primary water. Surface, crack flank, and crack tip oxides were analyzed and compared by high-resolution characterization, including oxidation state mapping. All oxides were found to have a triplex structure, although of different dimensions and composition, revealing the effects of local water chemistry and applied stress. The higher oxidation rate at the crack tip could be explained due to the existence of a higher dislocation density, higher level of stress and cation unavailability from the environment. The implications to SCC mechanisms are discussed.

High-Temperature Creep-Fatigue Behavior of Alloy 617

This paper presents the high-temperature creep-fatigue testing of a Ni-based superalloy of Alloy 617 base metal and weldments at 900 °C. Creep-fatigue tests were conducted with fully reversed axial strain control at a total strain range of 0.6%, 1.2%, and 1.5%, and peak tensile hold time of 60, 180, and 300 s. The effects of different constituents on the combined creep-fatigue endurance such as hold time, strain range, and stress relaxation behavior are discussed. Under all creep-fatigue tests, weldments’ creep-fatigue life was less than base metal. In comparison with the low-cycle fatigue condition, the introduction of hold time decreased the cycle number of both base metal and weldments. Creep-fatigue lifetime in the base metal was continually decreased by increasing the tension hold time, except for weldments under longer hold time (>180 s). In all creep-fatigue tests, intergranular brittle cracks near the crack tip and thick oxide scales at the surface were formed, which were linked to the mixed-mode creep and fatigue cracks. Creep-fatigue interaction in the damage-diagram (D-Diagram) (i.e., linear damage summation) was evaluated from the experimental results. The linear damage summation was found to be suitable for the current limited test conditions, and one can enclose all the data points within the proposed scatter band.

Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism

Aeroengine manufacturers have to demonstrate that critical components such as turbine disks, made of DA Inconel 718, meet the certification requirements in term of fatigue crack growth. In order to be more representative of the in service loading conditions, crack growth under hold time conditions is studied. Modelling crack growth under these conditions is challenging due to the combined effect of fatigue, creep and environment. Under these conditions, established models are often conservative but the degree of conservatism can be reduced by introducing the crack growth threshold in models. Here, the emphasis is laid on the characterization of crack growth rates in the low ?K regime under hold time conditions and in particular, on the involved crack growth mechanism. Crack growth tests were carried out at high temperature (550 °C to 650 °C) under hold time conditions (up to 1200 s) in the low ?K regime using a K-decreasing procedure. Scanning electron microscopy was used to identify the fracture mode involved in the low ?K regime. EBSD analyses and BSE imaging were also carried out along the crack path for a more accurate identification of the fracture mode. A transition from intergranular to transgranular fracture was evidenced in the low ?K regime and slip bands have also been observed at the tip of an arrested crack at low ?K. Transgranular fracture and slip bands are usually observed under pure fatigue loading conditions. At low ?K, hold time cycles are believed to act as equivalent pure fatigue cycles. This change in the crack growth mechanism under hold time conditions at low ?K is discussed regarding results related to intergranular crack tip oxidation and its effect on the crack growth behaviour of Inconel 718 alloy. A concept based on an “effective oxygen partial pressure” at the crack tip is proposed to explain the transition from transgranular to intergranular fracture in the low ?K regime.

Effects of microstructure on high temperature dwell fatigue crack growth in a coarse grain PM nickel based superalloy

The influence of microstructure on the dwell fatigue crack growth behaviour of an advanced nickel-based superalloy was investigated at a temperature of 700°C. Microstructural variations were induced by heat treatment variables: different cooling rates of quenching from super-solvus solution heat treatment, 0.7 and 1.8°Cs−1, and an addition of a high temperature stabilisation heat treatment (2h at 857°C) between the solution treatment and the final ageing treatment. With a one hour dwell introduced at the peak load of the fatigue cycle, such different microstructural conditions can lead to a difference of up to two orders of magnitude in crack growth rates in air, when compared to those obtained under baseline fatigue loading. By performing such dwell fatigue and baseline fatigue tests in vacuum, it is confirmed that such increases in crack growth rates under dwell fatigue loading in air are mainly environmentally related. Transmission electron microscopy (TEM) was utilised to analyse both crack tip oxides and associated deformation mechanisms in the matrix. A novel mechanism taking into account competing interactions of crack tip oxidation (leading to increases in crack growth rates) and stress relaxation (leading to decreases in crack growth rates) is outlined.

Stress Corrosion Cracking Behavior at Inconel and Low Alloy Steel Weld Interfaces

Three-dimensional microstructure observations, macro- to micro-scopic residual stress measurements by three methods and creviced bent beam SCC tests were performed for Inconel/low alloy steel (LAS) weld samples. The possible reasons for the suppression of SCC crack propagation near the weld interface found at a nuclear power plant were estimated to include the crack branching at the grain boundary (GB) parallel to the interface, i.e., Type II GB, compressive residual stresses in the LAS region and crack tip oxidation in the LAS at the interface. The formation mechanism of Type II GB and stress gradient in individual grains in the Inconel are also discussed.

Grain size effects in a Ni-based turbine disc alloy in the time and cycle dependent crack growth regimes

The fatigue crack growth (FCG) behaviour in a Ni-based turbine disc alloy with two grain sized variants, in a low solvus high refractory (LSHR) superalloy has been investigated under a range of temperatures (650–725°C) and environments (air and vacuum) with trapezoidal waveforms of 1:1:1:1 and 1:20:1:1 durations at an R=0.1. The results indicate that a coarse grained structure possesses better FCG resistance due to the enhanced slip reversibility promoted by planar slip as well as the reduction in grain boundary area. The fatigue performance of the LSHR superalloy is significantly degraded by the synergistic oxidation effect brought about by high temperature, oxidising environment and dwell at the peak load, associated with increasingly intergranular fracture features and secondary grain boundary cracking. Secondary cracks are observed to be blocked or deflected around primary γ′, carbides and borides, and their occurrence closely relates to the roughness of the fracture surface, FCG rate and grain boundary oxidation. The apparent activation energy technique provides a further insight into the underlying mechanism of the FCG under oxidation–creep–fatigue testing conditions, and confirms that oxidation fatigue is the dominant process contributing to the intergranular failure process. At high enough crack growth rates, at lower temperatures, cycle dependent crack growth processes can outstrip crack-tip oxidation processes.

Intergranular crack tip oxidation in a Ni-base superalloy

High-temperature intergranular crack tip oxidation under a single 600 s long sustained tensile load at 700°C was studied for the Ni-base superalloy Allvac 718Plus. High-resolution analytical techniques showed oxidation to take place at and immediately ahead of the tip of an open crack, forming a closed but layered oxide structure about the prior (now oxidized) grain boundary. Near the prior grain boundary the oxide is Ni-rich, with a Co-enriched layer furthest away from the metal and a Fe-enriched region below this. A Cr-rich oxide is present below the outer Ni-rich oxides throughout the crack, also in the direction of crack growth. This is believed to have a hindering effect on oxidation ahead of the crack. Ni3(Nb,Al) γ′ precipitates close to the grain boundaries were found to oxidize and form regions of near-stoichiometric NiO within the oxide layers. Remaining constituents of γ′ (e.g. Al and Nb) were found to be enriched in the surrounding oxidized matrix and also to produce thin oxide layers near the interface between the unoxidized metal and the Cr-rich oxide. The formation of the crack tip oxides is discussed with regard to thermodynamics, kinetics and the influence of applied mechanical load.

Deterministic Formulation of the Effect of Stress Intensity Factor on PWSCC of Ni-Base Alloys and Weld Metals

The fundamental correlations such as crack growth rate (CGR) versus K for primary water stress corrosion cracking (PWSCC) of nickel-base alloys in simulated pressurized water reactor environments are quantified with the theoretical model based on the combination of crack tip mechanics and oxidation kinetics. Materials reliability program (MRP) proposed a CGR disposition curve in a report MRP 55 for PWSCC of thick-section Alloy 600 materials. This deterministic CGR equation has been adopted by Section XI Nonmandatory Appendix O of the ASME Boiler and Pressure Code for flaw evaluation. MRP also proposed a CGR disposition curve in a report MRP 115 for PWSCC of Alloy 82/182/132 weld metals. Stress intensity factor (K), temperature and thermal activation energy are included in both MRP 55 and MRP 115 reports. Both MRP 55 and MRP 115 are engineering-based. The results of mechanism-based modeling are compared with the screened experimental data for typical PWSCC systems of nickel-base alloys and the consistence is observed.

Effects of water chemistry and loading conditions on stress corrosion cracking of cold-rolled 316NG stainless steel in high temperature water

The effects of electrode potential, stress intensity factor and loading history on stress corrosion cracking growth of a cold-rolled 316NG stainless steel in 288 °C pure water were investigated. Crack branching and intergranular stress corrosion cracking along random grain boundaries were observed by electron-back scattering diffraction. A strong dependence of crack growth rate on stress intensity factor is observed. A single-cycle overloading produced a retarded transient cracking growth period. The mild inhibiting effect of decreasing electrode potential on crack growth of cold-rolled 316NG SS is analyzed based on the interaction between crack tip mechanics and crack tip oxidation kinetics.

Effect of Variable Stress Intensity Factor on Hydrogen Environment Assisted Cracking

Fitness-for-service evaluations of engineered components that are subject to environment assisted cracking (EAC) often require analyses of potentially large crack extensions through regions of variable stress intensity. However, there are few EAC data and models that directly address the effects of variable stress intensity factor on EAC crack growth. The model developed here is used to evaluate stress corrosion cracking (SCC) data that were obtained on a high-strength beta-titanium alloy under conditions of variable crack mouth opening displacement (CMOD) rate. SCC of this Ti alloy in ambient temperature, near-neutral NaCl aqueous solution is thought to be due to hydrogen environment assisted cracking (HEAC). As the model equations developed here do not admit to a closed form solution for crack velocity as a function of applied stress intensity factor, K, a semiquantitative graphical solution is used to rationalize the crack growth data. The analyses support a previous suggestion that the observed crack growth rate behavior can be attributed to the effect of crack tip strain rate on rates of mechanical disruption and repair of an otherwise protective crack tip oxide film. Model elements introduced here to HEAC modeling include (1) an expression relating corrosion-active surface area to crack tip strain rate and repassivation rate, (2) an expression relating the critical grain boundary hydrogen to the applied stress intensity factor, and (3) an expression relating CTSR to both applied and crack advance strain rate components. Intergranular crack advance is modeled assuming diffusive segregation of corrosion-generated hydrogen to grain boundary trap sites causing embrittlement of the fracture process zone (FPZ). The model equations developed here provide a quantitative basis for understanding the physical significance of K-variation effects and, with additional development, will provide an engineering tool for analysis of crack growth in a variable K field.

Formulating stress corrosion cracking growth rates by combination of crack tip mechanics and crack tip oxidation kinetics

A theoretical equation for stress corrosion crack growth rate of austenitic alloys in high temperature water is reformulated based on crack tip asymptotic fields and crack tip transient oxidation kinetics. A general oxidation kinetic law is introduced, emphasizing the role of mass transport through solid oxide film at the crack tip. The effects of several parameters on crack growth rate are evaluated. The results are compared with available experimental data and other equations. A good prediction of the effect of K on stress corrosion cracking growth rate of typical austenitic alloys in simulated light water reactor environments has been achieved.

Unified Interpretation of Crack Growth Rates of Ni-base Alloys in LWR Environments

Primary water stress corrosion cracking (PWSCC) of vessel penetrations (VP) fabricated from nickel based alloys such as alloy 600 and alloy 182 weld metal has created a great demand for elucidation of the cracking mechanism and for development of life prediction technologies. The generalized FRI crack growth rate (CGR) formulation was proposed, based on a deformation/oxidation mechanism and a theoretical crack tip strain rate equation derived by the authors. The effects of crack tip oxidation and crack tip mechanics and of their interactions on crack growth can be quantified. Experimental and actual plant data of CGR for alloy 600 in PWR primary water, which are sometimes scattered in CGR-K diagrams, are interpreted with the generalized CGR formulation, emphasizing the effects of temperature, K, yield strength and variations of K with time. It is suggested that it is essential to determine the type of dependency of CGR on K for accurate flaw disposition. The generalized formulation provides a unique parameter for interpreting CGRs as well as a unified method for predicting CGRs within a narrow scattered band even under various testing parameters, which is the basis for accurately predicting component life.

Influence of Crack-Surface Oxidation on Creep-Fatigue Crack Behavior of 1Cr- and 10Cr-Steels

Abstract High temperature components with notches, defects, and flaws can introduce crack initiation and crack propagation under service conditions. Fracture mechanics procedures are needed to study crack problems and to support an advanced remnant life evaluation. Since a more flexible service mode of steam power plants causes a higher number of start-up and shut-down events, creep-fatigue crack behavior is decisive for life assessment and integrity of components. Usually, fracture mechanics experiments are carried out under air conditions, although in cases of internal cracks they are not in contact with air. Therefore, it is of interest to realize the degree to which environmental conditions, e.g., crack-tip oxidation, can influence crack initiation and crack growth behavior. In order to reveal problems related to high temperature components, the crack initiation time and crack growth rate were determined in air environment and in a gas with controlled atmosphere on 1CrMo(Ni)V- and 10CrMoWVNbN-steels. Crack initiation and propagation under creep-fatigue conditions have been described with the fracture mechanics parameters C*, KI, and ΔKI and C*. A modified “Two-Criteria-Diagram” was used to describe creep-fatigue crack initiation.

Environmental and frequency effects on fatigue crack growth rate and paths in aluminium alloy

ABSTRACTThe environmental and frequency effects on fatigue crack growth in aluminium alloys are studied theoretically and experimentally. 2024‐T351 and 7075‐T651 tested in corrosive environments (humid air or technically purified nitrogen) show a constant crack growth rate (da/dN) at low values of the effective stress intensity range (ΔKeff). Typical well‐known fits of this curve (da/dN vs ΔKeff) do not reflect the plateau‐like region. A new model of crack growth is presented, which physically attributes this region to the formation and subsequent fracture of a crack tip oxide layer. The thickness of this layer is measured with X‐ray photon electron spectroscopy. At higher loads, other mechanisms are understood to be active. The model parameters are determined from constant amplitude tests, and are valid for a given material and environment. In 7075‐T651 tested in nitrogen, with R= 0.1 and 83 Hz, unexpected macroscopical crack branching is observed when ΔKeff reaches approximately 3.0 MPa √m.