Corrosion Fatigue Crack Initiation Research Articles

The Initiation Characteristics of Corrosion Fatigue Crack in 18Ni (250) Steel

Maraging steel is the material of deep sea pressure equipment. The characteristics of corrosion fatigue crack initiation in 18Ni (250) were investigated by performing a tensile fatigue test under different loading conditions. The mechanism of corrosion fatigue crack initiation was analyzed considering the influence of welding. The results showed that the crack initiation life decreases with increasing load and increases with the stress ratio. In a corrosive seawater environment, crack initiation is affected by both mechanical damage and electrochemical corrosion. The proportion of the crack initiation life of the total sample life is approximately 71.45–99.55%, where the larger this proportion is, the smaller the fatigue zone is. Crack initiation and microcrack propagation in maraging steel correspond to the microfracture mechanism of dimple fractures. The microfracture mechanism is strongly affected by the presence of a weld but weakly affected by the weld position. The results of this study provide a theoretical basis for predicting the corrosion fatigue life of deep-sea submersible pressure hulls.

Influence of hot corrosion on thermal fatigue behavior of Ni-based single crystal superalloy: Invisible and accelerated crack

As an important failure model for single crystal materials in service, with the increasing complexity of working environments, the thermal corrosion fatigue failure needs to be studied urgently. In this study, the thermal corrosion fatigue crack initiation and propagation behaviors of an Ni-based single crystal superalloy in both air and hot corrosion environments at 25–900 °C were studied by using a designed precise thermal fatigue test device. The microstructure characteristics and crack composition were analyzed by SEM, TEM and EDS, and the internal stress and recrystallization during thermal fatigue were characterized by EBSD. Moreover, the detailed and comprehensive crack morphology was characterized with different surface treatments. The results showed that the crack morphology was invisible in the hot corrosion environment, which is worthy of being highlighted. The initiation and propagation of thermal fatigue cracks were also accelerated in the hot corrosion environment, which contributed to the sufficient thermal stress, the accelerated material degradation of the inner/outer layers in the crack region, and the rapid forward diffusion of O2 in the inner crack layer. Finally, the influence mechanism of hot corrosion on the thermal fatigue behavior of the Ni-based single crystal superalloy is proposed.

Experimental investigation on corrosion fatigue crack initiation and growth of heat-treated U75V rail steel

Understanding the corrosion fatigue mechanism of rail materials is of great importance to guide the assessment of the structural integrity of modern railway components. To this purpose, the corrosion fatigue crack (CFC) initiation and growth behaviors of heat-treated U75V rail steel are investigated under 3.5 wt% NaCl solution in terms of high-cycle fatigue, fatigue crack growth (FCG) rate, together with surface morphology and microstructure. Experimental results reveal that the U75V rail steel is susceptible to corrosion fatigue failure. Examinations on fractured samples clearly show that the electric potential difference between ferrite and cementite triggers the preferential anodic dissolution of ferrite, leading to corrosion fatigue crack initiation. The crack initiation is not only solely determined by the size of the corrosion pit, but also strongly influenced by its local shape. In comparison with the fatigue crack growth in air, the corrosion FCG tends to grow in a relatively straight path, because of hydrogen embrittlement. Additionally, the CFC behaviors indicate a tendency to transverse through soft-oriented grains and deflect upon encountering hard-oriented grains.

Effect of 0.1 wt% Nb on the microstructure and corrosion fatigue performance of high strength steels

In this manuscript, the effect of 0.1% Nb addition on the corrosion fatigue behavior of the marine engineering steel has been studied through corrosion fatigue experiments, electrochemical hydrogen charging, and a series of microscopic characterization methods. The results show that BM steel has a high corrosion fatigue sensitivity in the marine environment. Compared to BM steel, the corrosion fatigue sensitivity of 0.1 Nb steel is reduced by 8% in the marine environment, and it reduced by 40% in the hydrogen charging environment. 0.1% Nb adding inhibits hydrogen concentration in local areas by forming irreversible hydrogen traps NbC and by improving the microstructure, and it lowers the diffusible hydrogen content and total hydrogen content inside the steel by inhibiting electrochemical hydrogen evolution reaction to hold back the corrosion fatigue crack initiation and propagation, thereby improving the ability of resisting corrosion fatigue cracking for the marine engineering steel.

A continuum damage model for prediction of crack initiation life of pitting corrosion and fatigue

Corrosion fatigue damage can lead to degradation of engineering structures and components. As a major stage of the total corrosion fatigue life, the crack initiation of pitting corrosion and fatigue is studied. A continuum damage mechanics model for predicting the crack initiation life of pitting corrosion fatigue is presented based on a pure fatigue damage model and a fracture-mechanics-based pit-to-crack transition criterion. The synergistic effect of pitting corrosion and fatigue are quantified as a time-varying amplification of the fatigue stress field. The proposed model is validated by corrosion fatigue test data of two different metallic materials. The predicted curves of corrosion fatigue crack initiation life are in good agreement with the test data. The model demonstrates a good prediction of corrosion fatigue crack initiation life, i.e., corrosion fatigue life is always lower than fatigue life, and the lower the stress, the more significant the life reduction.

Toward superior fatigue and corrosion fatigue crack initiation resistance of Sanicro 28 pipe super austenitic stainless steel

The present work deals with the fatigue and corrosion-fatigue crack initiation in cold-pilgered Sanicro 28 super austenitic stainless steel. Toward this end, the stress-controlled symmetric reverse bending fatigue tests were conducted in air and mixed acid aqueous solution under the stress amplitude of 200–570 MPa with the stress ratio of −1 and frequencies of 1 and 15 Hz at ambient temperature. The fatigue study was supplemented with electrochemical experiments and advanced microstructural analyses to explore the damage characteristics in detail. The planar character of dislocation substructure owing to the low stacking fault energy of the material, and formation of parallel slip bands led to a more homogenous distribution of the imposed strain which retarded the progress of damage accumulation. According to the X-ray photoelectron spectrometer results, the formation of the insoluble and stable MoO3 oxide increased the rate of repassivation. This sticky oxide resisted against breakdown rendered by the attack of aggressive chloride ions. The corrosion fatigue behavior of the experimented alloy under the various test frequencies was discussed relying on the synergistic effects of mechanical and environmental damages.

Study on corrosion fatigue behavior and mechanism of 6005A aluminum alloy and welded joint

Purpose The purpose of this paper is to determine the corrosion fatigue behavior and mechanism of 6005A aluminum alloy and welded joint. Design/methodology/approach Electron back-scattered diffraction (EBSD) were adopted to characterize the microstructure of 6005A aluminum alloy and welded joint. Through potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and corrosion fatigue experiments, the corrosion fatigue behavior and mechanism of 6005A aluminum alloy base metal and welded joint were studied. Findings The results show that the corrosion fatigue crack initiation of 6005A aluminum alloy base metal and welded joint is mainly caused by the preferential anodic dissolution and hydrogen concentration in the areas with inclusions and welding defects. Originality/value The research is an originality study on the corrosion fatigue behavior and mechanism of 6005A aluminum alloy and welded joint.

Corrosion kinetics study of 316 N weldment under cyclic loading in acidified chloride environment

Corrosion fatigue crack initiation (CFCI) life of AISI Type 316 N weldment in as-welded (AW) and heat treated (Solution annealed (SA), 550 °C/4h and 750 °C/1h) conditions was studied in high cycle fatigue regime (above 104 cycles) with a frequency (η) of 0.1 Hz and and R-ratio (σmin/σmaxwhere σmin and σmax signify minimum and maximum tensile stress in MPa, respectively) of 0.5 for different stress amplitude (σa) values in boiling acidified 5 M NaCl + 0.15 M Na2SO4 + 2.5 ml/l HCl solution under axial loading in load controlled mode. In-situ electrochemical measurements were employed during corrosion fatigue tests to study the passive film characteristics. SA weldment showed better corrosion fatigue (CF) resistance as compared to AW and heat treated conditions. Homogenization of microstructural and microchemical heterogeneities facilitates highest resistance to CF cracking process for SA weldment condition. Number of cycles to failure (Nf) increased with decrease in stress amplitude for all material conditions. CFCI mechanism was identified based on variation in the strain rate (ε°) due to combined effect of both emergence of persistant slip bands (PSBs) casued by rupture of passive film and initiation of microcrack caused by dissolution around depassivation site. The failure of weld metal occurred by a combination of transgranular (TG) and stress-assisted dissolution (SAD) of delta-ferrite. Crack growth zone was minimum for AW and heat treated (550 °C/4h and 750 °C/1h) conditions as compared to SA weldment.

Role of Nonmetallic Inclusions in Premature Stress-Corrosion Fractures of Drill Pipes

We analyze the causes of the in-service loss of integrity of drill pipes made of G105 steel connected with their premature stress-corrosion fractures. In this case, the mechanical characteristics of steel are satisfactory, except a somewhat lowered elongation. At the same time, both in the structure and on the fracture surfaces of impact test specimens, we reveal a large number of corrosion-active nonmetallic inclusions of the calcium-aluminate type, which may increase the rate of local corrosion by an order of magnitude, promote the formation of deep pits on the inner surface of the pipe, and play the role of sites of initiation of corrosion-fatigue cracks. This is why we assume that the stage of crack initiation, which mainly determines the total durability of drill pipes after premature failures is controlled by the presence of corrosion-active nonmetallic inclusions and their density in the structure of steel.

Mechanical and corrosion fatigue behaviors of gradient structured 7B50-T7751 aluminum alloy processed via ultrasonic surface rolling

In this work, ultrasonic surface rolling process (USRP) was utilized to produce a gradient structured layer on 7B50-T7751 aluminum alloy, and the mechanical properties and corrosion fatigue behavior of treated samples were studied. These results reveal that underwent USRP, a 425 μm thick gradient structure and a 700 μm deep compressive residual stress field are created, aluminum grain size become fine(∼ 67 nm), and the corrosion rate of treated surface reduces by 60.08% owing to the combined effect of compressive residual stress and surface nanocrystallization. The corrosion fatigue strength is enhanced to 117% of that of 7B50 Al alloys by means of USRP due to the introduced compressive residual stress, which is considered as the major favorable factor in suppressing the initiation and early propagation of corrosion fatigue cracks. Besides, the gradient structure is an important factor in providing a significant synergistic contribution to the improvement of corrosion fatigue performance.

Microstructural evolution and oxidation behaviour of CF8A austenitic stainless steel during corrosion fatigue in a simulated pressurised water reactor environment

The microstructural evolution and oxidation behaviour of CF8A stainless steel during corrosion fatigue (CF) crack initiation and propagation were investigated in a simulated pressurised water reactor environment. Under the action of alternating loads, the near-surface properties of the specimens deteriorate due to the formation of Cr-depleted zones and slip bands (SBs), and CF cracks tend to initiate from SBs on the surface of the specimens. As the CF cracks propagate, dynamic interactions are present between the stress state and the oxidation behaviour at the crack tip due to intense anodic dissolution and the redistribution of stresses and strains.

Corrosion Fatigue Crack Initiation of Type 316N Weldment Under the Influence of Cyclic Stress Amplitude

Corrosion fatigue behavior of type 316N stainless steel (SS) weldments on the damage characteristics is studied. The S–N curve of the fatigue life versus stress amplitude showed a specific two slope Basquin relationship. At low stress amplitude, the deformation microstructure consists mainly of planar slip dislocation structure and at high-stress amplitudes, planar multiple slip dislocations govern the cyclic behavior. A correlation between microstructural changes and the corresponding susceptibility of 316N SS weldment in as-welded condition towards environmental cracking in boiling acidified aqueous solution has been presented. The objective of the present study is to investigate the environmental effects on the corrosion fatigue crack initiation of type 316N weldmetal under the influence of cyclic stress amplitude. The difference in deformation micro mechanisms with stress amplitude in a corrosive environment is believed to be the major reason for the occurrence of a specific bilinear Basquin relationship in the S–N curve. A schematic of corrosion fatigue crack initiation involving the emergence of a slip step and dissolution of the step on the surface: (a): before cyclic loading (b): fatigue crack initiation and (stage I and II) crack propagation during cyclic loading.

Effect of Al⁻Zn Alloy Coating on Corrosion Fatigue Behavior of X80 Riser Steel.

This paper presents a corrosion fatigue cyclic failure test for X80 steel, which has arc sprayed with an Al–Zn coating in natural seawater under different stress levels. We found that the Al–Zn coating can significantly improve the corrosion fatigue resistance and slow the crack initiation of X80 steel. The effect of the Al–Zn coating on the corrosion fatigue crack initiation is mainly attributed to its physical isolation, cathodic protection and residual prestress while the effect on crack propagation is due to its inhibition of the formation and evolution of secondary cracks. Moreover, according to the test results, a new life prediction model for corrosion fatigue based on the damage evolution law is proposed and the effect of corrosion–fatigue coupling damage in the proposed model is also considered.

Corrosion fatigue crack initiation in ultrafine-grained near-α titanium alloy PT7M prepared by Rotary Swaging

The study focuses on corrosion fatigue processes taking place in an ultrafine-grained (UFG) near-α-titanium alloy Ti-2.5Al-2.6Zr (Russian industrial name PT7M) used in nuclear engineering. UFG structure formed with Rotary Swaging is found to increase resistance to corrosion fatigue. Parameters of the Basquin's equation are defined and the slope of the fatigue curve σa-lg(N) is shown to depend (nonmonotonic dependence) on the UFG alloy annealing temperature. This effect can be explained with the patterns of microstructural evolution in a UFG alloy PT7M during annealing: (1) reduced density of lattice dislocations, (2) precipitation and dissolution of zirconium nanoparticles, (3) release of α′′-phase particles causing internal stress fields along interphase (α-α′′)-boundaries, and (4) intensive grain growth at elevated annealing temperatures. It is shown that the fatigue crack closure effect manifested as changing internal stress fields determined using XRD method may be observed in UFG titanium alloys.

Numerical analysis of pitting corrosion fatigue in floating offshore wind turbine foundations

The mooring system of offshore floating wind foundations, which anchors the floating foundations to the seabed, sustains large dynamic loads during operation. The mooring chains are connected to the floating foundation below the water level through fairleads and chain-stoppers. The corrosive marine environment and the cyclic loading make the mooring connection prone to corrosion pitting and fatigue crack initiation and propagation from the pits, particularly in the weld zones. In this study, a finite element analysis of the crack growth from corrosion pits has been performed and the results are presented in order to provide an estimate of the extent of damage after the crack is detected. A Python script have been developed which generates the pit profiles based on a non-uniform random distribution of pit dimensions. 3D pits and elliptical cracks are embedded at critical points of weldment on the mooring point and analysed using ABAQUS XFEM. The Walker’s model has been applied in the model to examine the effect of realistic R ratios in floating structures on pitting corrosion fatigue crack propagation along with direct cyclic solver. The numerical results obtained from this study are discussed in terms of the corrosion pitting effects on fatigue crack propagation behaviour in Spar-type floating offshore wind turbine foundations.

Effects of temperature on corrosion fatigue behavior of 316LN stainless steel in high-temperature pressurized water

Influence of temperature on corrosion fatigue (CF) behavior of 316LN stainless steel in high-temperature pressurized water was investigated from 373 K to 598 K at a strain rate of 0.04%s−1. It is found that the CF life of 316LN stainless steel decreases with increasing temperature in water environment. The relationship between the reciprocal of temperature and the logarithm of fatigue life was found to be linear. Mechanisms involving temperature effects on CF crack initiation are discussed and a linear CF model to account for the above temperature effects of CF life is proposed.

Modeling for corrosion fatigue crack initiation life based on corrosion kinetics and equivalent initial flaw size theory

In present paper, a novel, effective and economical model based on corrosion kinetics and equivalent initial flaw size theory was proposed to predict the corrosion fatigue crack initiation (CFCI) life of E690 steel in simulated seawater. Comparison between the predicted S-N curves and the measured data showed that this model was of nice accuracy under high peak stresses, while non-negligible deviation existed under low peak stresses due to lack of enough measured data or consideration for the stress effect on corrosion rate. Despite of that, thought of this model still has a good practicability in predicting CFCI life of steels.

Variation of the Corrosion Behavior Prior to Crack Initiation of E690 Steel Fatigued in Simulated Seawater with Various Cyclic Stress Levels

The electrochemical process and corrosion behavior prior to crack initiation of E690 steel fatigued in simulated seawater are investigated under different cyclic stress levels by methods of electrochemical impedance spectroscopy, cyclic current response, electrochemical noise and morphology observation. The results show that the electrochemical behavior changes with the peak stress of cyclic loading and the corrosion behavior transforms as the peak stress increases over the proof strength. The charge transfer resistance (Rct) decreases with elevated peak stress. Continuous decreasing Rct with cycling number under peak stress of 1.02 σp0.2 indicates that continuous fresh metal induced by work-softening is exposed to the seawater, which is also proved by cyclic current response induced by cyclic stress and contact angle test. As a result, the corrosion type of E690 steel changes from general corrosion to pitting corrosion with the peak stress close to and above the proof strength. That induces the corrosion fatigue crack initiation mechanism to change correspondingly.

Data-Science Analysis of the Macro-scale Features Governing the Corrosion to Crack Transition in AA7050-T7451

This study applies data science approaches (random forest and logistic regression) to determine the extent to which macro-scale corrosion damage features govern the crack formation behavior in AA7050-T7451. Each corrosion morphology has a set of corresponding predictor variables (pit depth, volume, area, diameter, pit density, total fissure length, surface roughness metrics, etc.) describing the shape of the corrosion damage. The values of the predictor variables are obtained from white light interferometry, x-ray tomography, and scanning electron microscope imaging of the corrosion damage. A permutation test is employed to assess the significance of the logistic and random forest model predictions. Results indicate minimal relationship between the macro-scale corrosion feature predictor variables and fatigue crack initiation. These findings suggest that the macro-scale corrosion features and their interactions do not solely govern the crack formation behavior. While these results do not imply that the macro-features have no impact, they do suggest that additional parameters must be considered to rigorously inform the crack formation location.

3D time-resolved observations of corrosion and corrosion-fatigue crack initiation and growth in peak-aged Al 7075 using synchrotron X-ray tomography

Corrosion fatigue crack initiation and growth from corrosion pits in 7075 aluminium alloys was observed in situ using 3D x-ray synchrotron tomography. Peak-aged Al 7075 was corroded by soaking in 3.5 wt.% NaCl solution, then fatigue tested in situ in the solution using synchrotron X-ray tomography to analyse the corrosion fatigue crack initiation and growth from the pits. Hydrogen bubbles were observed within the intergranular corrosion fatigue cracks, indicating chemical changes in the sample during corrosion fatigue. In situ crack growth, the effect of microstructural constituents on dissolution and growth, and multimodal observations of the fracture surface were conducted and are discussed.