Crack initiation model for sensitized 304 stainless steel in high temperature water

To investigate the initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel in high temperature water, a constant load SCC test method combined with in situ crack observation technique was employed. The in situ crack observation system allowed us to detect small cracks of at least 100 μm. As a result, a fracture time decreased with an increase in an applied stress. The first cracks were observed at most 3 h before the specimen was fractured under all the stress conditions. After that, many cracks were initiated in a short time to fracture. The fracture was caused by coalescence of multiple cracks rather than by growth of some primary cracks. The simulation model for surface crack initiation was newly developed using a Monte Carlo method, which was based on damage mechanics and stress analysis around the existing cracks. The simulation could represent the empirical results of changes in the crack distribution and the cumulative number of cracks during the SCC tests. It was concluded, therefore, that the crack initiation process should be considered in simulating the life prediction of the material in this SCC system.     

Monte Carlo simulation of stress corrosion cracking on a smooth surface of sensitized stainless steel type 304

Stress corrosion cracking (SCC) on a smooth surface of structural metal materials occurs by initiation and coalescence of micro cracks, subcritical crack propagation and multiple large crack formation or final failure under combination of material, stress and corrosive environment. In this paper, a Monte Carlo simulation of the SCC process is proposed based on stochastic properties for micro crack initiation and concepts in fracture mechanics for crack coalescence and propagation. The procedure is as follows: The possible number of grain-sized micro cracks which can be initiated is set for a given space and initiation times for all cracks are assigned by random numbers based on exponential distribution. Sites and sizes of cracks are assigned by uniform random numbers and normal random numbers, respectively. Coalescence and propagation of cracks are determined based on fracture mechanics. The emphasis in the model is put on the influence of semi-elliptical surface cracks. Numerical simulations are carried out based on the results of creviced-bent-beam tests for sensitized stainless steel type 304 under high-temperature and high-purity water containing dissolved oxygen and the influence of micro crack initiation rate and coalescence condition on the simulation results is discussed.     

Evaluation of stress corrosion cracking phenomenon in an AISI type 316LN stainless steel using acoustic emission technique

This paper deals with the analysis of the acoustic emission (AE) signals to determine the micro-process during stress corrosion cracking (SCC) of AISI type 316LN stainless steel that cause the AE, and thus the mechanism of the SCC process. AE with amplitudes ranging from 27.6 to 46.5 dB with different counts, energy and rise times occurred during SCC of type 316LN stainless steel in 45% MgCl₂ at 413 K. The analysis of the AE signals in conjunction with fractography indicated that a surge in the AE counts and energy indicated initiation of SCC. AE was found to be continuous prior to the initiation. The time gap between AE events increased during initiation. AE events occurred in bursts during crack growth. Plastic deformation ahead of the crack tip was determined to be the major source of AE during propagation of SCC in type 316LN stainless steel. The cracking was found to initiate and propagate in the transgranular mode.     

Discontinuous surface cracks during stress corrosion cracking of stainless steel single crystal

Single crystal 321 stainless steel stress corrosion cracking was studied in a 42 wt.% MgCl₂ solution. Cracks propagated macroscopically in the maximum tensile stress plane regardless of the notch orientation with respect to the applied tensile load direction. Some stress corrosion cracks nucleated discontinuously at the intersection of the two slip bands. Most cracks, however, were not related to the slip bands. Cleavage-like fracture was observed, and the river-markings exhibited microshear facets along the {1 1 1} plane. Interaction between the main crack and the discontinuous microcracks increased the calculated stress intensity factor by 17 times and promoted crack coalescence, resulting in mechanical fracture of the ligaments between the cracks.     

Transient and steady state crack growth kinetics for stress corrosion cracking of a cold worked 316L stainless steel in oxygenated pure water at different temperatures

The stress corrosion cracking (SCC) growth kinetics for a cold worked 316L stainless steel was continuously monitored in high purity water at different temperatures and dissolved oxygen (DO) levels under a K (or K_max) of 30 MPa m^0.5. The total SCC test time was more than 8000 h to make sure the steady state crack growth rate under each test condition could be reached. Crack growth rate (CGR) increases with increasing temperature in the range 110-288 °C. A typical intergranular-cracking mode is identified. Depending on the previous test condition, especially the temperature, three kinds of crack growth kinetics, i.e., increasing with testing time then becoming steady, being constant during the whole period, or decreasing with test time then becoming steady, are identified and discussed. Time-dependent and testing history-dependent crack growth modes were confirmed in two series of tests in 2 ppm DO and 7.5 ppm DO pure water. The apparent activation energies are calculated and compared with other data in different environments under different applied loading levels for understanding the cracking mechanism.     

Effect of inclusions on initiation of stress corrosion cracks in X70 pipeline steel in an acidic soil environment

The effect of inclusions on the initiation of stress corrosion cracking (SCC) X70 pipeline steel was investigated in an acidic soil solution using slow strain rate test, scanning electron microscopy and energy-dispersive X-ray techniques. The results demonstrated that stress corrosion cracks are not initiated in X70 steel when it is under anodic polarization. At cathodic polarization, hydrogen evolution is enhanced, and hydrogen is actively involved in SCC processes. Two types of inclusions exist in the steel and play different role in crack initiation. The inclusions enriching in Al are brittle and incoherent to the metal matrix. Microcracks and interstices are quite easily to be resulted in at the boundary between inclusions and metal. There is no crack initiating at inclusions containing mainly Si.     

The role of TiN inclusions in stress corrosion crack initiation for Alloy 690TT in high-temperature and high-pressure water

Microchemistry and microstructure of TiN in Alloy 690TT were analyzed using SEM and TEM. Constant loading test was conducted on Alloy 690TT in 325 °C water to study effect of TiN on SCC initiation. It was found that TiN-inclusion stringers existed along the longitudinal axis. Numerous micron-size TiN with Al₂O₃ and/or MgO nuclei and submicron-size TiN existed in this alloy. After holding a constant load equal to 180% of the yield strength for 40 days, cracks about 1-3 μm initiated at micron-size TiN and TiN-inclusion stringers in the near surface. The mechanisms of Alloy 690TT crack initiation at these TiN are discussed.     

Effect of cations on stress corrosion cracking: Ag-40Cd alloy in silver ion aqueous solutions containing a variety of foreign cations

Uhlig showed that cupric ions accelerated the stress corrosion cracking (SCC) of α-brass in ammoniacal solutions. When adding foreign metal cations (cadmium, zinc, cobalt and nickel) to the solutions, it was found that only the copper ion had an effect on the fracture time. The aim of the present work was to study if this is a more general phenomenon in SCC. To that purpose, a Ag-40Cd (at/o) alloy was strained in silver nitrate aqueous solutions containing a variety of foreign metal cations, at the equilibrium potential of the reaction: Ag⁺ + e⁻ = Ag. SCC was found in all cases. Silver ions specifically accelerated the crack propagation rate and the addition of foreign cations showed no significant effect on the phenomenon. Most of the cracks found were intergranular, and the specific effect of the cations could be explained through the surface mobility SCC mechanism.     

Stress corrosion cracking under low stress: Continuous or discontinuous cracks?

Two-dimensional and three-dimensional crack morphologies of stress corrosion cracking (SCC) were studied by serial-sectioning and synchrotron-based X-ray computed tomography. Discontinuous surface cracks were actually continuous inside the specimen, which matched typical river-like fractographs and finite element simulations. A low stress SCC model was created, where a main crack continuously grew along the main propagation direction (MPD) due to anodic dissolution; then, discontinuous secondary microcracks emanated from MPD, angularly extending to the two sides of MPD. Finally, some of the secondary microcracks reached the sample surface, resulting in the formation of discontinuous surface cracks.     

Influence of the applied stress rate on the stress corrosion cracking of 4340 and 3.5NiCrMoV steels under conditions of cathodic hydrogen charging

Stress corrosion cracking (SCC) of as-quenched 4340 and 3.5NiCrMoV steels was studied under hydrogen charging conditions, with a cathodic current applied to the gauge length of specimens subjected to Linearly Increasing Stress Test (LIST) in 0.5 M H₂SO₄ solution containing 2 g/l arsenic trioxide (As₂O₃) at 30 °C. Applied stress rates were varied from 20.8 to 6 × 10⁻⁴ MPa s⁻¹. Both the fracture and threshold stress decreased with decreasing applied stress rate and were substantially lower than corresponding values measured in distilled water at 30 °C at the open circuit potential. The threshold stress values correspond to 0.03–0.08 σ_y for 4340 and 0.03–0.2 σ_y for the 3.5NiCrMoV steel. SCC velocities, at the same applied stress rate, were an order of magnitude greater than those in distilled water. However, the plots of the crack velocity versus applied stress rate had similar slopes, suggesting the same rate-limiting step. The fracture surface morphology was mostly intergranular, with quasi-cleavage features.     

Steric hindrance as a rate controlling step in stress corrosion cracking

The effect of steric hindrance in the stress corrosion cracking (SCC) of Ag-15Pd (a/o) in AgI forming solutions was studied at room temperature. The solutions used were 1 M KI at a potential higher than that of AgI formation, and iodine dissolved in different n-alcohols. It was found that Ag-15Pd (a/o) is susceptible to intergranular stress corrosion cracking in those solutions. The SCC susceptibility measured as percentage of elongation to rupture and crack propagation rate is related to the size of the active species. While in KI aqueous solutions, as well as in iodine saturated benzene or iodine saturated toluene, the rate controlling step (RCS) was the reaction at the tip of the crack, in alcoholic-iodine solutions, the diffusion rate of the iodine containing molecules became the RCS.     

Crack tip chemistry and electrochemistry of environmental cracks in AA 7050

The role of the crack environment in establishing environment-assisted crack (EAC) propagation in AA 7050 alloys is elucidated. A suite of mini-electrodes provided real-time in situ measurements of the crack potential, pH, and chloride concentration during stage II cracking in a chromate-chloride electrolyte under electrochemical control. For material aged to an EAC-susceptible condition, crack growth during an incubation period is characterized by tip polarization to near the applied electrode potential (E_App) and bulk-like chemistry near the crack tip. In contrast, establishment of high-rate crack growth coincided with the development of an acidic, high chloride concentration tip environment and tip depolarization. During steady state high rate crack growth, the tip potential was ∼−0.85V_SCE; near-tip potential gradients were ∼1 V/cm. Large ohmic potential drop within fast-growing cracks is indicative of net anodic current in the near tip region and increased mass transport resistance within the crack due to solid corrosion products and/or hydrogen bubble formation. Microinjection of a corrosion-inhibiting or corrosion-promoting solution at the tip suppresses or prompts, respectively, the transition from incubation to high-rate cracking, highlighting the intimate dependence of the crack growth kinetics on the local chemistry. The exceptional EAC resistance of over-aged AA 7050 is intrinsic; injection of an acidic aluminum chloride solution at the tip of a crack of this material while polarized to a high E_App failed to induce brittle crack advance.     

New insights into the stress corrosion cracking of carbon steel in ethanolic media

Potentiostatic slow strain rate testing was conducted on stress corrosion cracking (SCC) test specimens exposed to ethanolic environments prepared from pure dehydrated ethanol. The mechanism of SCC in such environments is not well-understood. Cracks of various types—intergranular and transgranular open cracks, and sharp closed transgranular cracks—were found by altering several testing parameters. The presence of chloride was found to be essential for crack initiation. A scanning electron microscope examination indicated that an “anodic” cracking mechanism, not necessarily slip dissolution, was most likely operating at high elongations. Sharp, closed transgranular cracks, with a maximum depth of 4 µm, were detected at elongations below 3% in ethanol solutions containing 2.5-mM LiCl. A focused ion beam was used to extract such a transgranular crack tip for analytical transmission electron microscopy using electron energy loss spectroscopy, which confirmed that the crack was in a ferrite grain. The sharp closed transgranular cracks seem to ally with the cracks observed in CO–CO₂–H₂O and anhydrous ammonia environments, which are proposed to grow by unique cleavage mechanisms. The possibility of embrittlement by carbon interstitials produced by ethanol electro-oxidation within the crack is discussed.     

Characterization and perspective of stress corrosion cracking of austenitic stainless steels (type 304 and type 316) in acid solutions using constant load method

The stress corrosion cracking (SCC) of the commercial austenitic stainless steels, type 304 and type 316 has been extensively investigated as functions of applied stress, sensitizing temperature, sensitizing time and the environmental factors such as pH, anion concentration, anion species (chloride ions and sulfate ions), test temperature, applied potential and inhibitor concentrations of chromate and molybdate by using a constant load method. We have found that the steady state elongation rate obtained from corrosion elongation curve becomes a relevant parameter for predicting time to failure and also for criterion on assessment of whether SCC takes place or not. The value of t_ss/t_f is also found to become an indicator for assessment of whether SCC takes place or not. Furthermore, from the results obtained, it is deduced that a unified SCC mechanism is qualitatively proposed to explain both of transgranular SCC (TGSCC) and intergranular SCC (IGSCC), where the unified SCC mechanism is basically based on a film rupture- formation event at crack tips.     

Effect of carbon on stress corrosion cracking and anodic oxidation of iron in NaOH solutions

Anodic behaviour of decarburised iron and of quenched Fe-C materials with up to 0.875 wt% C was examined in 8.5 M NaOH at 100 °C to explain the role of carbon in caustic stress corrosion cracking (SCC) of plain steels. Removal of carbon from Armco iron strongly reduced its intergranular SCC. Slip steps on grains did not initiate cracks. It has been shown that carbon at low contents deteriorates the passivation of iron, whereas at high contents it promotes the formation of magnetite. High resistance to SCC of high carbon steels can be explained by an intense formation of magnetite on these steels.     

AE response of α-brass during stress corrosion crack propagation

The present work was aimed at characterizing the acoustic emission (AE) response of α-brass during SCC propagation. For that purpose, slow strain rate tensile tests (SSRT) under potentiostatic control were performed in a 1 M NaNO₂ solution and in Mattsson’s solution. The differences between the AE signals generated by transgranular and intergranular stress corrosion cracking (SCC) were analysed. Results show that the AE activity during the propagation of TGSCC is various orders of magnitude higher than the AE activity during the IGSCC propagation. However, the mean amplitude and rise time of the AE signals registered during the propagation of TGSCC are similar to the AE parameters of the signals registered during the propagation of IGSCC. The measured AE activity was associated with the mechanical tearing of the metallic ligaments left behind by the propagating crack.     

Effect of carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.     

Effect of corrosion severity on fatigue evolution in Al-Zn-Mg-Cu

The effect of existing-localized corrosion on fatigue cracking of 7075-T6511 was established using crack surface marker-band analysis and a fracture mechanics model. The substantial reduction of fatigue life due to EXCO solution L-S surface pre-corrosion is nearly independent of exposure time after initial-sharp degradation, scaling with the evolution of pit-cluster size and initial stress intensity range with exposure time. Independent of exposure time, formation of a resolvable fatigue crack (∼10 μm) accounts for a similar-low (∼5%) fraction of total fatigue life at low stress range (σ_max = 150 MPa, R = 0.1). Crack formation occurs at microscopic protrusions into the corroded volume. A corrosion-modified-equivalent initial flaw size (CM-EIFS); predicted with the AFGROW tool using measured initial aspect ratio, initiation cycles, and total fatigue life inputs; accurately represents the corrosion damage effect on fatigue for a range of exposures. The similar deleterious effect of several corroding environments for various-exposed surfaces is described by a lower-bound CM-EIFS with a 300 μm depth and 1200 μm surface length suggesting fatigue is governed by a microscopic pit-based topography. Either an approximate lower-bound, or specific CM-EIFS calibrated by limited measurements of fatigue life for service-environment exposed specimens, can be used to assess the impact of corrosion in a damage tolerant framework. Complexities (e.g., local H embrittlement, 3D pit geometry, topography dependent initiation, and microstructure sensitive small-crack growth) do not compromise the CM-EIFS estimation, but must be better understood for refined modeling.     

Intergranular stress corrosion cracking of copper in nitrite solutions

Semi-hard tubes of deoxidized high phosphorous copper with different levels of tangential residual stresses have been exposed to nitrite solutions in a laboratory heating circuit. After characterization of investigated materials influence of temperature, location of heating, concentration of solution, electrochemical potential, and atmosphere on stress corrosion cracking susceptibility of those copper tubes has been investigated. Threshold stress for crack initiation has been determined. Maximum duration of experiments was 1 month. Breakthrough time of tubes has been measured as criterion for susceptibility to SCC.To vary stress level in a wider range constant load tests on tubes with different degrees of cold working (annealed, semi-hard, hard) were done by mounting them in a steel frame.Stress corrosion cracks were always intergranular. A tenorite layer covered surface of cracked copper tubes. A reaction scheme for ammonia formation is presented. Necessary conditions for formation of stress corrosion are shown and critically discussed.Results show that intergranular cracking takes place at much lower stress levels below yield strength when compared to literature data on transgranular cracking above yield strength. For transgranular cracking cross slipping and cleavage formation as cracking mechanism is confirmed while for intergranular cracking chemical dissolution of grain boundaries plays a more important role.     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.