A theoretical evaluation of the influence of mechanical variables on the concentration of oxygen in a corrosion fatigue crack

The oxygen concentration in a corrosion fatigue crack has been evaluated theoretically assuming that oxygen was consumed by cathodic reduction on the walls of the crack and mass transport occurred by diffusion and advection (forced convection) with the latter resulting from the sinusoidal variation of the displacement of the crack walls. Using parameters relevant to a compact tension specimen the time-dependent distribution of the oxygen concentration in the crack was calculated as a function of ΔK (the range of the stress intensity factor), R-value (minimum load/maximum load), frequency, crack length and electrode potential (within the constraint of no significant potential drop in the crack). The influence of advection was to enhance significantly the mass transport of oxygen in the crack compared to “diffusion-only” even at low frequencies and low values of ΔK. Regions in the crack were identified in which advection dominance or diffusion dominance of the mass transport of oxygen occurred.     

Factors controlling crack velocity in 7000 series aluminium alloys during fatigue in an aggressive environment

Results obtained for the corrosion fatigue of the aluminium-zinc-magnesium alloy 7017-T651 in seawater at frequencies from 0.1 to 70 Hz are examined on the basis of existing models for predicting crack growth rates. The enhanced growth rates in seawater, compared with dry air, are not compatible with the simple superposition model that may be applied to 7079-T651, unless some secondary process produces several orders of magnitude increase over the stress corrosion rates observed under static loading. The changes in fracture mode observed at certain critical crack velocities, and their dependence upon the square root of the reciprocal of the frequency of loading, are shown to be consistent with an environment-enhanced crack growth rate involving diffusion of hydrogen ahead of the crack tip during each fatigue cycle. Examination of the reported frequency dependence of crack growth rates during corrosion fatigue in steels and other aluminium alloys indicates a need for further work to identify the rate-determining steps for crack propagation during corrosion fatigue. Theories proposed to date are found to be inadequate.     

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.     

腐蚀疲劳裂纹扩展的断裂模型

将修正的疲劳裂纹扩展静态断裂模型和裂尖腐蚀溶解相结合,提出了腐蚀疲劳裂纹扩展(CFCP)的腐蚀-钝化-断裂模型.根据该模型和断裂力学原理,导出了CFCP速率的定量表达式,它揭示了CFCP速率与力学条件,裂尖表面腐蚀率、加载频率、假设的裂尖材料元临界断裂应力之间的定量关系,并能说明氢脆对CFCP率的影响规律。实验结果表明,应用所提出的模型可很好地描述铝合金在35％NaCI中CFCP的一般规律。     

Zur Riß- und Bruchausbildung bei austenitischen Cr?Ni-Stählen unter spezifischen Bedingungen der Spannungs- und Schwingungsrißkorrosion

Crack and fracture formation in austenitic CrNi steels in the specific conditions of stress corrosion cracking and corrosion fatigue Even in media producing stress corrosion craking the fatigue component dominates at frequencies around 50 Hz and is noticable at frequencies as low as 30 Hz. A typical feature of such failures is the appearance of intercrystalline fracture regions in the range of low crack propagation velocities. With decreasing tolerable load the fatigue component effect gradually disappears and crack formation is initiated by localized attack from the surrounding medium.     

An alternative to the Shoji crack tip strain rate equation

Fundamentally derived crack tip strain rate (CTSR) equations are being sought for use in modelling stress corrosion crack growth. The CTSR equation derived by Shoji and coworkers is gaining prominence in modelling the effects of cold work on stress corrosion cracking of nuclear reactor materials due to an ability to model the effects of yield stress and strain hardening on CTSR. However, the Shoji equation is compromised by assumptions that are inconsistent with the Gao-Hwang (GH) crack tip strain equation that was used in derivation of Shoji’s equation. Moreover, the GH equation appears to have been incorrectly derived. As a result, the Shoji equation cannot be considered fundamentally supported. An alternative to the Shoji equation is developed here by extending the Rice, Drugan and Sham CTSR equation to include strain hardening.     

Einfluß von Prüffrequenz und Salzgehalt des Korrosionsmediums auf die Korrosionsermüdung von Stählen bei betriebsähnlichen Lastabläufen

Influence of test frequency and NaCl-concentration on the fatigue life of steels under a realistic load sequence Because of the size and fatigue life of offshore structures the corrosion fatigue behaviour of those structures cannot be determined under service conditions. Also for the relevant corrosion fatigue tests in the laboratory using welded joints a compromise has to be found to reduce testing time to an economic justifiable amount. This compromise may be a higher stress used in the test, a higher test frequency or a higher concentration of NaCl in the seawater. The influence of test frequency and NaCl concentration on the fatigue life of fillet welded V-shaped specimens made from Fe E 355 KT was determined in artifical seawater and under a realistic load sequence. The fatigue life was separated into the crack initiation phase and the crack propagation phase as well. Mean frequencies of 0.2 Hz, 1.0 Hz and 10 Hz did not affect the fatigue life of welded joints until crack initiation, while low test frequencies of 0.2 Hz and 1.0 Hz accelerated the crack propagation. Higher NaCl-concentration of the seawater, however, had no effect on the fatigue life in both phases.     

The role of environmental exposure in the fatigue behaviour of an aluminium alloy

This paper reports measurements of fatigue crack velocities for an aluminium alloy 7017-T651 in high purity gaseous environments with different values of water vapour pressure and frequency. Also a detailed fractographical analysis by SEM is presented and the fracture surface morphology is correlated with the testing parameters and with the crack stress intensity level. Within a range of environmental exposure (defined as the product between the pressure and the time available for surface reaction during one fatigue cycle) crack propagation rates depend linearly on the exposure. The experimental fatigue crack propagation data are satisfactorily explained in terms of a corrosion-fatigue model for gaseous environments.     

Corrosion fatigue crack growth behaviour of naval steels

Fatigue crack growth behaviour of two varieties of HSLA steels used in naval structural applications have been evaluated in air and 3.5% NaCl solution. In air both the HSLA steels showed similar resistance to fatigue crack growth. However, in 3.5% NaCl, the fatigue crack growth resistance of HSLA-80 steel was superior to that of HSLA-100. The apparent inferiority of HSLA-100 to corrosion fatigue crack growth resistance is attributed to rapid film formation and rupture, and occurrence of planar modes of failure. Effect of R-ratio on air fatigue and corrosion fatigue crack growth behaviour is rationalised by the concept of crack closure. Effect of cyclic frequency on corrosion fatigue behaviour is examined. It is noted that the mechanism of corrosion fatigue crack growth for the two HSLA steels changes with attendant change in the Paris slope. This leads to increase or decrease of crack growth rates, depending up on the ΔK range of interest.     

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Prestressing steels occasionally fail by a process named “stress corrosion cracking”. This process has not been fully elucidated and several theories exists in order to explain the cases in which real structures have collapsed. This paper briefly mentions the different theories and identifies the progress in understanding whether it is necessary to use a testing method, which is able to separate the different steps and mechanisms contributing to the failures.This paper presents the methodology used for inducing controlled localized attack to study the susceptibility of the high strength steels resistance to stress corrosion cracking (SCC). The method is designed to study the growth of cracks initiated from a mechanical notch; the crack is not produced by fatigue.It consists of several stages: coating of the bar with epoxy resin, generation of a small notch, constant load and controlled potential test in the media, mechanical test in air and fractographic study. It allows us to calculate the crack propagation rate and the fracture toughness in the same test.Finally, it has been possible to apply the surface mobility mechanism (SMM) in order to identify the SCC mechanism that operates.     

Effect of preloading on corrosion fatigue of 70/30 brass

The effects of a single tensile overload and precycling load in air on subsequent corrosion fatigue cracking in 70/30 brass exposed to 1 M sodium nitrite solution were studied. A single tensile overload caused a decrease in subsequent corrosion fatigue crack propagation rate. Precycling load also showed retardation effects, depending on the load conditions for precycling and corrosion fatigue. A transsition from a higher mean stress for precycling to a lower mean stress for corrosion fatigue decreased corrosion fatigue crack growth rate, while an opposite transition did not appreciably change the corrosion fatigue crack propagations. The responses of bulk mechanical properties to a presingle overload and precycling load in air were measured, showing hardening which may probably be responsible for the observed retardation.     

Comments on the “Stress corrosion cracking of zirconium and zircaloy-4 in halide aqueous solutions” by S.B. Farina, G.S. Duffo, J.R. Galvele

The above paper [Corros. Sci. 45 (2003) 2497] declares a general approach to describing stress corrosion cracking (SCC) mechanism and despite of the criticism of the so-called “surface mobility mechanism” (SMM) e.g., [Corros. Sci. 36 (1994) 669, Corros. Sci. 45 (2003) 2105]. The paper [Corros. Sci. 45 (2003) 2497] demonstrates inner contradictions and mutually excluding statements of SMM when fitted calculated and measured values of crack propagation rate. As shown in these Comments, the crack propagation rate calculated using SMM is only the Nabarro-Herring creep deformation rate, which can never be numerically equal to the crack propagation rate. Thus, tests undertaken by the author of SMM cannot be accepted as experimental evidence of the SMM.     

Monitoring of corrosion fatigue cracking using harmonic analysis of current responses induced by cyclic stressing

The response of a current induced by sinusoidal stress within an elastic deformation was measured during corrosion fatigue tests to monitor the course of corrosion fatigue cracking and to evaluate corrosion fatigue damage. The test material was an iron bar subjected to a passive potential in borate buffer solution containing 5 mM NaCl. Harmonic analysis was used to analyze the current response and to extract valuable parameters that were associated with crack initiation and corrosion fatigue damage. The current response before crack initiation consisted of only the fundamental current without any harmonics. Higher harmonic amplitudes of the current response, especially the second and third harmonics, appeared and increased after crack initiation. In addition, the phase shift between the strain and the fundamental current response to the sinusoidal stress decreased from nearly 90° towards 0° as harmonic amplitudes of the current response increase. The results indicate that the amplitudes of the harmonic current response and the phase shift are the optimal parameters for detecting crack initiation and evaluating corrosion fatigue damage.     

The corrosion fatigue and stress corrosion cracking of α-brass in ammoniacal solutions

A study of the corrosion fatigue and stress corrosion cracking under a constant strain-rate in 70 Cu?30 Zn brass as a function of pH has been made. Correlation between the two has been discussed. Corrosion fatigue was very dependent on pH both with respect to the lifetime and to the nature of cracks as well as those of stress corrosion cracking. A minimum in fatigue life and a large fatigue crack growth rate were observed in neutral tarnishing solutions of pH 6.5–7.0. The fatigue crack propagation rate in neutral tarnishing solution was about twice as great as that in an acidic non-tarnishing solution. Fractographic examination on the fracture surfaces of corrosion fatigue and stress corrosion cracking showed that intergranular cracking predominated both in neutral tarnishing and acidic non-tarnishing solutions, and transgranular cracking predominated in alkaline solution. It is concluded that the tarnish film is not a primary factor controlling the cracking mode in the acidic to neutral pH range and that intergranular cracking may result from the localized dissolution of alloy at grain boundaries.     

腐蚀疲劳裂纹扩展的机理

针对高强度低合金钢、钛合金和镁合金进行了腐蚀疲劳裂纹的扩展FCG、外加电压对于腐蚀疲劳裂纹扩展速率的影响以及断裂表面的研究。在外加电压对于腐蚀疲劳裂纹扩展速率影响的研究过程中,在一段时间内发生极化,可以根据此期间内的开路电压记录裂纹扩展速率,并测量极化情况下的裂纹增长速率。由于裂纹扩展测量技术的进步,测量的时间很少超过300s,这使观测非独立模式阴极极化对于腐蚀疲劳裂纹扩展速率的影响成为可能。当最大应力强度（Kmax）超过给定材料--溶液组合的特定临界特征值时,阴极极化会加速裂纹的扩展。当Kmax低于临界值,而所有其他条件（试件、溶液、pH值、载荷频率、应力比率、温度等）不变时,同样的阴极极化会妨碍裂纹扩展,或者对于裂纹扩展无影响。断口显微分析结果显示,阴极极化下加速裂纹的扩展是由于氢致腐蚀(HIC)。因此,根据氢致腐蚀机理以及KHIC和△ KHIC的显示,Kmax的临界值,以及应力范围（△ K）是由相应的腐蚀疲劳裂纹扩展的症状所确定的。当Kmax > KHIC（△ K > △ KHIC）时,腐蚀疲劳裂纹扩展的主要机理是HIC。对于大多数的材料--溶液组合的研究表明,当Kmax < KHIC（△ K < KHIC）时,应力协助扩散在腐蚀疲劳裂纹扩展中起决定性作用。     

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.     

Critique of the Ford-Andresen film rupture model for aqueous stress corrosion cracking

The Ford-Andresen film rupture model for aqueous stress corrosion cracking has obtained a prominent position in the nuclear reactor industry. The model is said to have superior predictive capabilities because it is derived from a fundamental understanding of the film rupture-repassivation mechanism of crack advance. However, a critical review shows that there are conceptual and mathematical problems with the Ford-Andresen model development; there are inconsistencies among the stated and implied assumptions, the crack tip current density expression lacks the necessary dependence on crack tip strain rate and the fundamental proportionality that exists between crack tip strain rate and crack growth rate is overlooked and omitted from the model development. Consequently, the Ford-Andresen model must be considered neither phenomenologically nor fundamentally supported.     

CORROSION FATIGUE OF HSLA STEELS IN SALT SPRAY

The corrosion fatigue propagation behaviour of high strength low alloy Cr-Ni and Si-Mnsteels has been investigated in 3.5 %NaCl solution spryly at the frequencies of 0.1 and 5.5 Hz.It was shown that the fatigue crack initiation resistances of the two steels are significantly re-duced in salt spray;the fatigue crack growth rates of steels increase with the decrease of fre-quency and are much higher in salt spray than in air within low ΔK range.A critical stress in-tensity factor was observed for each steel and the crack growth stoppage will occur if the ΔKvalues are lower than it.It was found that the active dissolution makes great contribution tothe fatigue crack growth within low ΔK range.     

Experimental investigation on gas-liquid two-phase slug flow enhanced carbon dioxide corrosion in vertical upward pipeline

The carbon dioxide corrosion behavior of API N80 grade steel enhanced by gas-liquid two-phase vertical upward slug flow has been both mechanistically and experimentally investigated. It is found that the hydrodynamic characteristics of slug flow, such as the direction alternated wall shear stress, the fluctuation of wall normal stress, and the mass transfer near the wall, have significant effects on the carbon dioxide corrosion process. It is difficult to form dense, compact, and protective corrosion product film in the corrosion process, which is dominated by general corrosion, and can develop into pitting and mesa attack due to localized corrosion. An empirical correlation is suggested to predict slug flow enhanced carbon dioxide corrosion. It is found that the mass transfer in corrosion product film can be neglected and the slug flow enhanced carbon dioxide corrosion is dominantly controlled by mass transfer or by both of mass transfer and electrochemical corrosion reaction.     

马氏体与贝氏体组织GC—4超高强度钢的腐蚀疲劳裂纹扩展

本文研究了马氏体组织与贝氏体组织４０ＣｒＭｎＳｉＭｏＶＡ（ＧＣ－４）超高强度钢的腐蚀疲劳（ＣＦ）裂纹扩展特性及机理。结果表明，不显微组织状态下，ＧＣ－４钢在３．５ＮａＣｌ溶液中的ＣＦ裂纹扩展曲线上，都出现了类似于应力腐蚀的平台区，而且马氏体组织ＧＣ－４钢的平台区裂纹扩展速度远大于贝氏体组织。断口分析与理论研究表明，氢脆在ＧＣ－４钢的腐蚀疲劳中起重要作用。