应力比及载荷频率对腐蚀疲劳断口形貌的影响

本工作研究了应力比和载荷频率对高强度低碳硅锰钢在蒸馏水及3.5％NaCl溶液中腐蚀疲劳断口形貌的影响。试验结果表明,在介质影响疲劳裂纹扩展行为的范围内的断口上,随着应力比的增加或者载荷频率的减小,应力腐蚀开裂的断口形貌所占比例逐渐增加。根据多视域的观测结果,建立起了腐蚀疲劳断裂机制示意图。     

Ni-P镀层对海洋结构钢A_(537)CL_1腐蚀疲劳寿命的影响

本文研究了Ni-P化学镀A_(587)CL_1在人工海水溶液中的腐蚀疲劳寿命。结果表明,在不同疲劳载荷和不同频率下,镀层提高了试样的腐蚀疲劳寿命,在343MPa应力和0.3Hz频率时,提高到1.8倍左右,在147MPa应力时,提高到2.5倍(0.3Hz频率)和4.3倍(1.7Hz频率)。文章还根据疲劳试样及其断口形貌分析,讨论了镀层抑制疲劳过程的腐蚀行为和进一步提高腐蚀疲劳寿命的途径。     

电站锅炉水冷壁管腐蚀疲劳断口分析

对锅炉水冷壁开裂管样断口特征和断裂机理进行分析,结果表明裂纹源区于腐蚀坑,在扩展区可见疲劳弧线、腐蚀产物和腐蚀坑,局部区域可见疲劳条带和二次裂纹;在锅炉启停过程中,准解理型和韧窝型断口形貌特征交替出现;水冷壁管的腐蚀疲劳失效为交变温差应力与腐蚀介质共同作用的结果.     

外加电位对奥氏体不锈钢应力腐蚀与腐蚀疲劳交互作用的影响SCIEI

本文研究了0Cr18Ni9Ti奥氏体不锈钢在42%沸腾MgCl_2中,在低频,高平均应力循环载荷下的环境敏感断裂行为,研究了应力腐蚀疲劳与应力腐蚀在裂纹扩展规律及断口形貌上的差别与联系。从断裂特征上分析了应力腐蚀与腐蚀疲劳的交互作用,并着重研究了外加极化电位对这种交互作用的影响。     

奥氏体不锈钢应力腐蚀和氢致开裂的机理

通过金相跟踪观察、力学测量及断口分析,研究了奥氏体不锈钢氢致开裂和应力腐蚀的机理.结果表明,无论是不稳定型(321)还是稳定型(310)奥氏体不锈钢,电解充氢时先产生塑性变形,当它发展到临界状态时就导致氢致裂纹的形核.但在42%MgCl_2沸腾溶液中应力腐蚀时,裂纹的形核和滞后塑性变形无关.两种(321)輿氏体不锈钢应力腐蚀的门槛值K_(ISCC)远比严重充氢时氢致开裂的门槛值K_(IH)要低.两者的断口形貌也不同,应力腐蚀是解理断口,且和K_I无关.而氢致开裂断口和K_I有关,K_I高是韧窝,K_I低则获得准解理断口.实验表明,氢在奥氏体不锈钢应力腐蚀(沸腾MgCl_2介质)过程中并不起主要作用.     

MECHANISM OF SLOW CRACK GROWTH AND STRESS CORROSION CRACKING IN AUSTENITIC STAINLESS STEEL

通过金相跟踪观察、力学测量及断口分析,研究了奥氏体不锈钢氢致开裂和应力腐蚀的机理.结果表明,无论是不稳定型(321)还是稳定型(310)奥氏体不锈钢,电解充氢时先产生塑性变形,当它发展到临界状态时就导致氢致裂纹的形核.但在42%MgCl_2沸腾溶液中应力腐蚀时,裂纹的形核和滞后塑性变形无关.两种(321)輿氏体不锈钢应力腐蚀的门槛值K_(ISCC)远比严重充氢时氢致开裂的门槛值K_(IH)要低.两者的断口形貌也不同,应力腐蚀是解理断口,且和K_I无关.而氢致开裂断口和K_I有关,K_I高是韧窝,K_I低则获得准解理断口.实验表明,氢在奥氏体不锈钢应力腐蚀(沸腾MgCl_2介质)过程中并不起主要作用.     

应力分量对黄铜Ⅱ型缺口试样应力腐蚀的影响

用Ⅰ型缺口试样研究丁黄铜在含5 g/l cucl_2的1 N NH_4OH 水溶液中应力腐蚀时应力分量的作用。由门槛值的测量、交变载荷的作用、室温蠕变的影响、断裂过程的金相观察及断口形貌等结果,可以确认;黄铜不论是在氨水中应力腐蚀或腐蚀疲劳,还是在空气中疲劳,裂纹都从Ⅱ型缺口试样的最大正应力处(0=-110°)萌生,并沿该处的最大剪应力方向(α=-35°))扩展。这表明,正应力在这种应力腐蚀体系中起主导作用,这和正应力能促进阳极溶解过程有关。     

Al-Zn-Cu-Mg及Al-Cu-Mg在氯化物水溶液中应力腐蚀开裂的扫描电镜断口形貌

本文研究了Al—Zn—Cu—Mg及Al—Mg—Cu在氯化物水溶液中用慢应变速率方法得到的应力腐蚀开裂的扫描电镜断口形貌。结果发现两种铝合金在氯化物水溶液中的应力腐蚀开裂断口复盖着—层腐蚀产物,这层腐蚀产物可以呈现不同的扫描电镜断口形貌,如类似解理的“平滑”形貌,“龟裂泥巴状”形貌等等。遮盖了应力腐蚀开裂固有的真实形貌;应力腐蚀开裂固有的真实形貌是相同的,呈典型的沿晶开裂形貌,盖有腐蚀产物层的形貌则取决于介质pH和电位。因此,应该根据铝合金应力腐蚀开裂固有的真实扫描电镜断口形貌来鉴别铝合金的应力腐蚀。将断口进行化学去膜处理可以得到铝合金应力腐蚀开裂的固有的真实的扫描电镜断口形貌。     

夯实钢管混凝土桩环境中X60管线钢的腐蚀行为研究

研究了X60管线钢在含水土壤和NS4溶液中的应力腐蚀行为。结果表明,在含水土壤中,当应变速率小于或者等于0.67×10~(-7)s~(-1)时,管线钢的应力腐蚀敏感性分别为27.9和25.7;在NS4溶液中,应变速率大于或者等于1.33×10~(-5)s~(-1)时不会发生应力腐蚀。恒载荷应力腐蚀断口形貌与慢应变应力腐蚀断口形貌类似,都具有准解理区和细小二次裂纹形态。     

热处理对7075铝合金应力腐蚀及断口形貌的影响

采用预裂纹双悬臂梁(DCB)试样研究了7075铝合金的应力腐蚀行为.结果表明,四种不同热处理状态合金的应力腐蚀敏感性由高向低的排列顺序为T6>G180>RRA>T7351.不同热处理状态的7075铝合金试样在干燥空气中的SSRT拉伸断口形貌都是由韧窝剪切台阶组成,属于韧窝型延性断裂.在3.5%NaCl溶液中,应力腐蚀敏感性较高的T6状态铝合金,其断口形貌是由塑性韧窝和腐蚀平坦区组成的,而应力腐蚀敏感性较轻微的7075-G180铝合金,属韧性断裂.     

EFFECT OF POTENTIAL ON INTERACTION BETWEEN STRESS CORROSION CRACKING AND CORROSION FATIGUE OF AUSTENITIC STAINLESS STEEL IN BOILING 42% MgCl_2 SOLUTION

The environment-sensitive fracture behaviour of 0Cr18Ni9Ti austenitic stainless steel inboiling 42% MgCl_2 under the specific load of low frequency and high mean stress was inves-tigated from the relations and differences of crack growth rates and fractographs betweenstress corrosion fatigue and stress corrosion cracking.The interaction between stress corro-sion cracking and corrosion fatigue was also studied from fracture characteristics with empha-sis on the effects of applied potential on the interaction.     

Characterising the early stages of crack development in environment-assisted cracking

An overview is given of recent NPL research to characterise the early stages of crack development in environment-assisted cracking, focusing on the nature of the crack precursors, the pit-to-crack transition, and the impact of solution chemistry on growth rate in the small and long crack growth rate regime. The impact on corrosion and cracking of surface grinding-induced changes in near-surface material and mechanical properties is highlighted, noting that awareness of this aspect in the corrosion community still remains limited. A major feature is the remarkable insight into the evolution of small surface cracks, from dealloyed layers and from corrosion pits, provided by advanced 3D imaging. Finally, further evidence for the concept of the solution conductivity-dependent crack-size effect in stress corrosion cracking and corrosion fatigue is put forward.     

SCF FEATURE OF AUSTENITIC STAINLESS STEEL IN BOILING MgCl_2SOLUTION

Corrosion fatigue under the load of low frequency and bigh mean stress has been generallydefined as stress corrosion fatigue(SCF).It is a specific failure process due to the inter-action between stress corrosion cracking(SCC) and corrsion fatigue(CF),the effectsof which on fracture characteristics,including crack initiation and propagation.servicelife and cracking mode have not been investigated systematically.The purpose of this pa-per was to study the environment-sensitive fracture behaviour of OCr18Ni9Ti austeniticstainless steel under the load of different fiequencies and high mean stress in boilingMgCl_2 solution.The interaction between SCC and CF would be emphasized.     

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。     

Modelling the crack propagation rate for corrosion fatigue at high frequency of applied stress

The application of standard mathematical techniques for the solution of mass transport equations, in the case of advection that is caused by the pulsating movement of crack walls in the case of corrosion fatigue, can be very time consuming. This problem arises, due to the requirement that the time step that must be employed, when solving the non-stationary equations numerically, must be significantly smaller than the period of oscillation. For overcoming these time-consuming limitations, a simple algorithm, which is based on eliminating the convective term from the equations of mass transfer in the pulsating slab by a suitable change of variables, was developed. The estimation of the advection effect on the rate of corrosion fatigue has been performed for the cases of diffusion and mixed kinetic control at high frequencies of applied stress. It is shown that, in many cases, it is possible to use codes that were developed for describing stress corrosion cracking, i.e. for the case of mass transfer without advection at zero loading frequency, to predict corrosion fatigue crack propagation rate, by simply substituting an effective crack length. Numerical calculations that have been performed in this work also show that the method developed here yields results that are applicable not only to the elevated frequencies, but to the any arbitrary frequency from 0 to ∞ for estimating corrosion fatigue crack propagation rate.     

Numerical modeling of general cracks from the viewpoint of eddy current simulations

This study discusses numerical modeling of fatigue and stress corrosion cracking in eddy current simulations. Ten fatigue crack specimens and another 10 stress corrosion crack specimens are prepared for this purpose. The specimens are made of type 316 stainless steel and measure 10 mm in thickness for a general evaluation of the model. Eddy current inspections of the specimens are performed using a differential type plus point probe; the specimens then undergo destructive tests to confirm the true profiles of the cracks. Subsequent numerical simulations are conducted to evaluate the equivalent conductivity and width of the cracks. The simulations demonstrate that a fatigue crack can be modeled as a non-conductive region, and it is not necessary to know exactly how wide the opening of a fatigue crack is. They also revealed, in contrast, that stress corrosion cracking needs to be modeled as a conductive region with a certain width.     

新型含钪Al-Mg-Cu合金的抗应力腐蚀开裂特性

对3.5%NaCl溶液中新型含钪Al-Mg-Cu合金的应力腐蚀开裂宏观性能进行测试,并对裂纹尖端的成分与微观形貌进行分析。根据线弹性断裂力学理论,预制疲劳裂纹试样裂纹尖端处于平面应变状态,得到裂纹匀速扩展时的扩展速率、裂纹尖端应力强度因子以及应力腐蚀开裂强度因子的门槛值。扫描电镜及EDS分析表明:应力腐蚀开裂主要是沿晶扩展,预制裂纹与腐蚀介质中的溶解氧生成Al2O3,产生楔入力促使裂纹扩展;裂纹尖端基体主要发生阳极溶解反应,腐蚀产物以氯化铝为主。     

The characteristics of hydrogen diffusion and concentration around a crack tip concerned with hydrogen embrittlement

Previously, we proposed a physical model for hydrogen diffusion and accumulation around the crack tip and performed accurate numerical analysis which takes account of the effects of both hydrogen diffusion and accumulation due to the stress gradient. Based on this analysis, the characteristics of hydrogen accumulation around the crack tip were clarified.Since the characteristics of stress corrosion cracking and corrosion fatigue are dominated by chemical anodic reaction, hydrogen embrittlement and dislocation mechanism, to perform the analysis on the competitive phenomenon by these mechanism and to relate the sensitivity of hydrogen embrittlement to the characteristics of corrosion fatigue, it is necessary to construct a exact physical law on the characteristics of hydrogen diffusion and concentration and to formulate the characteristics as a simple function such as diffusion constant, D, yield stress σ_ys, and stress intensity factor, K. The effect of stress field such as plane strain and plane stress on the hydrogen embrittlement is necessary to be clarified as the effect of specimen thickness on the hydrogen embrittlement.In this paper, based on this view point, the effect of D, σ_ys, and K on hydrogen embrittlement were investigated and formulated. A quantitative parameter which characterize hydrogen embrittlement was proposed for both cases of plane strain and plane stress conditions as the effect of specimen thickness on the hydrogen embrittlement.     

Corrosion failure of marine steel in sea‐mud containing sulfate reducing bacteria

The corrosion failure behavior of marine steel is affected by stress, which exists in offshore structures at sea‐mud region. The sulfate reducing bacteria (SRB) in the sea‐mud made the steel more sensitive to stress corrosion cracking (SCC) and weaken the corrosion fatigue endurance. In this paper, a kind of natural sea‐mud containing SRB was collected. Both SCC tests by slow strain rate technique and corrosion fatigue tests were performed on a kind of selected steel in sea‐mud with and without SRB at corrosion and cathodic potentials. After this, the electrochemical response of static and cyclic stress of the specimen with and without cracks in sea‐mud was analyzed in order to explain the failure mechanism. Hydrogen permeation tests were also performed in the sea‐mud at corrosion and cathodic potentials. It is concluded that the effect of SRB on environment sensitive fracture maybe explained as the consequences of the acceleration of SRB on corrosion rate and hydrogen entry into the metal.