Initiation of stress corrosion cracking for pipeline steels in a carbonate-bicarbonate solution

The linearly increasing stress test (LIST) was used to study the stress corrosion cracking (SCC) behavior of a range of pipeline steels in carbonate-bicarbonate solution under stress rate control at different applied potentials. Stress corrosion cracking, at potentials below -800 mV(SCE), was attributed to hydrogen embrittlement. Stress corrosion cracking, in the potential range from about-700 to -500 mV(SCE), was attributed to an anodic dissolution mechanism. In the anodic potential region, the SCC initiation stress was larger than the yield stress and was associated with significant plastic deformation at the cracking site. The relative SCC initiation resistance decreased with in-creasing yield strength. In the cathodic potential region, the SCC initiation stress was smaller than the yield stress of steel; it was approximately equal to the stress at 0.1 pct strain(@#@ Σ_0.1pct) for all the steels. The original surface was more susceptible to SCC initiation than the polished surface.     

Role of Mg in the stress corrosion cracking of an Al-Mg alloy

The corrosion and stress corrosion cracking (SCC) susceptibility of an Al-Mg alloy, AA5083, has been shown to depend on the precipitation of the Mg-rich β phase, (Al₃Mg₂), but not the enrichment of elemental Mg at grain boundaries to an enrichment ratio of 1.4. These results were determined by measuring the progress of Mg enrichment at grain boundaries, for increasing thermal-treatment times, using auger electron spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by analytical electron microscopy (AEM) of thin foils. The progress of the β phase precipitation was followed by AEM and scanning electron microscopy (SEM), for the same thermal-treatment times. The lack of a Mg-segregation effect on SCC was demonstrated by results obtained with X-ray photoelectron spectroscopy (XPS) analysis of Mg-implanted Al following in-situ electrochemical tests and SCC tests, while the dominance of β phase precipitation was demonstrated by electrochemical analysis and SCC testing. Crack-growth tests of alloy AA5083 demonstrated faster cracking at potentials anodic to the open circuit potential (OCP) with no increase at potentials cathodic to the OCP.     

300M超高强度钢电化学性能及应力腐蚀开裂

采用动电位扫描技术和慢应变速率拉伸试验研究了超高强度钢300M在3.5%NaCl溶液中的应力腐蚀行为,并利用扫描电镜观察了不同外加电位下的断口形貌.300M钢在3.5%NaCl溶液中开路电位下的应力腐蚀开裂机制为阳极溶解型,Cl-的存在明显地增加了材料的应力腐蚀开裂敏感性.阳极电位-600 mV下300M钢溶解速率加快,表现出较高的应力腐蚀开裂敏感性,断面收缩率损失由开路电路下的52.6%升高至99.5%,裂纹起源于表面点蚀坑处,应力腐蚀开裂为阳极溶解型机制.阴极电位-800 mV下材料处于阴极保护电位范围,表现出较低的应力腐蚀开裂敏感性,强度和韧度与空气中拉伸的数值相近,开裂机制为阳极溶解和氢致开裂协同作用.在更低电位(低于-950 mV)下,300M钢的应力腐蚀开裂机制为氢致开裂,在氢和拉应力的共同作用下表现出很大的应力腐蚀开裂敏感性.      

Environmental aspects of near-neutral pH stress corrosion cracking of pipeline steel

The severity of four different soil environments toward the development of near-neutral pH stress corrosion cracking (SCC) of pipeline steel was evaluated using slow strain-rate testing (SSRT). These soils were collected from pipeline sites where near-neutral pH SCC has been observed. It was demonstrated in this investigation that SSRT can differentiate the severity of various soil electrolytes to near-neutral pH SCC. For different soils, the relative susceptibility was found to be determined by the pH values of the soil electrolytes in equilibrium with a given CO₂/N₂ gas mixture. The higher the pH value up to ∼7, the more conducive the soil electrolyte was to near-neutral pH SCC. The pH value in a soil electrolyte was found to depend on the level of CO₂ in the soil solution and the initial HCO ₃ ⁻ concentration before the introduction of CO₂. For a given soil, the susceptibility depends on the actual level of CO₂ in the soil electrolyte. Higher levels of CO₂ lower the pH in the soil electrolyte and tend to increase the susceptibility to SCC. In laboratory tests, cathodic polarization was found to increase the susceptibility to failure, possibly by inhibiting general corrosion, which otherwise removed discrete stress-raising pits and defects from the specimen surface that acted as crack initiation sites or by increasing the extent of hydrogen-induced crack initiation or propagation. In the field, cathodic polarization is likely to prevent near-neutral pH SCC by increasing the pH at the pipe surface to values greater than 7.5. The pH was maintained near-neutral in the lab tests by continuous purging of the test solution with CO₂/N₂. A method is proposed for assessing the relative aggressiveness of various soil extracts to near-neutral pH SCC. Aggressive soil extracts appear to exhibit a narrower variation in pH between solutions purged with N₂ and with CO₂ than that for less-aggressive soil extracts purged with the same gases.     

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (h _i), secondary dendrite arm spacing (λ₂), corrosion potential (E _Corr), corrosion rate (i _Corr), polarization resistance (R ₁), capacitances values (Z _CPE), ultimate tensile strength (UTS, σ _u ), yield strength (YS, σ _y ), and elongation. It is shown that σ _U decreases with increasing λ₂ while the CR increases with increasing λ₂, for both alloys experimentally examined. A combined plot of CR and σ _U as a function of λ₂ is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.     

Caustic Stress Corrosion Cracking of Mild Steel

The stress corrosion cracking (SCC) behavior of cold worked mild steel in hot, aqueous, 33 pct NaOH solutions was studied with prefatigue cracked double cantilever beam specimens. SCC kinetics were studied under freely corroding potentials (E _corr ≈ −1.00 V_SHE) and potentiostatic potentials of −0.76 V_SHE near the active-passive transition. The pH of the liquid within the crack was determined and fractography was studied by scanning electron microscopy. Cracking was transgranular atE _corr, intergranular at −0.76 V_SHE, and produced no detectable change in crack liquid pH from that of the bulk solution. Crack rates were dependent upon temperature, potential, and stress intensity (K ₁). The apparent activation energy in Region II, where crack growth rate was independent ofK, was ∼ 24kJ/mol for both cracking modes. This was considered to be due to mixed rate control involving activation polarization and mass transport processes. The mechanism of cracking was entirely consistent with metal dissolution at –0.76 V_SHE and may involve hydrogen embrittlement and/or dissolution effects atE _corr. DOUGLAS SINGBEIL, formerly Research Student, University of British Columbia, is Research Scientist, Pulp and Paper Research Institute of Canada, 570-Blvd. St. Jean, Pointe Claire, Quebec, Canada H9R 3J9.     

A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel

A new test specimen configuration, designated the T-notch double cantilever beam (TNDCB), was developed, calibrated and employed for a fracture mechanics study of stress corrosion cracking (SCC) of cold worked Type-316 austenitic stainless steel exposed to hot aqueous solutions of 44.7 wt pct MgCl₂. The effects of stress intensity (K _I ), temperature (T) and electrochemical potential (E) upon the crack velocity (v) and fractography were investigated. The stress intensity (K _ISCC ) below whichv became immeasurably small was ∼12 MN·m^−3/2. Above this value, three regions of behavior were observed. Region I exhibitedK _I dependent cracking followed by Region II which exhibitedK _I independent cracking and an apparent activation energy of 63 to 67 kJ/mol, followed by Region III where cracking again became dependent uponK _I . The relative proportions of intergranular and transgranular crack paths were markedly dependent upon bothK _I andE, and less sensitive toT. Crack velocity was insensitive to small changes inE with respect to the free corrosion potentials (E _corr), but could be terminated by an applied active potential of ∼−0.35 V_SCE. The pH within the propagating crack was estimated to be <1.0 atE _corr, rising to ∼4.5 at −0.35 V_SCE. The mechanism of SCC was discussed with respect to film rupture events caused by crack tip plastic deformation, adsorption controlled processes on the metal surface, and hydrogen diffusion in the metal lattice. Alan J. RUSSELL, formerly Research Student, University of British Columbia     

Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel

The tensile properties, fracture toughness and stress corrosion cracking (SCC) behavior of HY-180 M steel at 22 °C were studied after final 5 h overaging treatments >510 ≤650 °C. SCC tests were conducted for 1000 h with compact tension specimens in aqueous 3.5 pct NaCl solutions at a noble (anodic) potential of −0.28 V_SHE ( −0.48 V_Ag/AgC1) and a cathodic protection potential of −0.80 V_SHE (−1.0 V_Ag/AgC1). The SCC resistance improved at aging temperatures >565 °C, the most significant improvement being at −0.80 V_She, especially after 650 ° aging whereK _ISCC was raised to at least 110 MPa · m^1/2. However, this was at the expense of mechanical properties. Provided low crack propagation rates of ∼3 X 10⁻¹¹ m/s at −0.80V _SHEmay be tolerated, the best compromise between strength, toughness, and SCC resistance was obtained after 594 °C aging. Under these conditions, stress intensities as high as ∼ 110 MPa · m^1/2 can be used, with a yield strength of ∼ 1150 MPa and fracture toughness of ∼ 170 MPa · m^1/2. The retained austenite content after aging increased with aging temperature up to 25 pct by vol at 650 °C. It appeared to correlate with improved SCC resistance, but other microstructural effects associated with aging may be involved. Formerly Research Associate with theDepartment of Metallurgical Engineering , University of BritishColumbia     

Stress corrosion cracking of high strength HY-180M steel in 3.5 Pct NaCl

Stress corrosion cracking of HY-180M steel was studied at 22°C in an aqueous solution of 3.5 pct NaCl (pH = 6.5). The steel had a nominal weight percentage composition of 10Ni-14Co-2Cr-lMo-0.16C and was heat treated to yield a fracture toughness value ofK _Ic ≃ 160 MPa . m^1/2. The SCC velocity (v) was studied as a function of stress intensity (K _I) and electrochemical potential (E) using precracked compact tension specimens, a Ag/AgCl reference electrode and a 1000 h exposure test. Also, the polarization behavior, microstructure, fractography and corrosion products were studied. The results showed that SCC was markedly dependent uponE, and did not occur whenE =-0.52 V_SHE (-0.72 V_Ag/AgCl), which corresponded closely to the thermodynamically reversible potential of iron. However, SCC occurred at a more noble potential of-0.28 V_SHE (-0.48 V_Ag/AgCl ) and at a less noble potential of-0.80 V_SHE (-1.00 V_Ag/AgCl). The stress intensity below which SCC was not observed was K_ISCC ≃ 5.5 MPa . m^1/2 at -0.28 V_SHE and K_ISCC ≃ 60 MPa . m^1/2 at -0.80 V_SHE . Also, Region I behavior (v dependent uponK ₁) and Region II behavior (v independent ofK ₁) were observed. Cracking was considered to occur solely by hydrogen embrittlement at -0.80 Vshe, whereas anodic dissolution processes played a necessary role, either directly or indirectly, in SCC at -0.28 V_SHE . The indirect effects were discussed in relation to hydrolysis effects in the crack promoting hydrogen embrittlement and/or corrosion product wedging stresses.     

On stress corrosion cracking in aluminum-lithium alloys

The stress corrosion cracking (SCC) susceptibility of two aluminum-lithium alloys, a binary AlLi and a ternary AlLiCu alloy, in 0.5 M NaCl solution was investigated using the constant elongation rate technique (CERT). Susceptibility increased with decreasing strain rate and with aging. The alloys were susceptible under both anodic and cathodic applied potentials. The susceptibility dependence of the alloys as a function of applied potential correlates well with published hydrogen permeability data. The susceptibility increased dramatically when hydrogen was charged into the specimen using a hydrogen re-combination “poison” during CERT testing. These experiments suggest that hydrogen plays a major role in the SCC of these alloys. A brittle hydride having the composition LiAlH₄ forms in the AlLi system under conditions of severe SCC susceptibility. The brittleness of the hydride is explained. The formation of the hydride is a sufficient condition for SCC of AlLi alloys. A process of SCC in AlLi alloys is proposed wherein hydrogen causes damage by the formation of a hydride.     

300M超高强度钢的应力腐蚀开裂

利用慢应变率拉伸试验技术研究了３００Ｍ超高强度钢的应力腐蚀开裂（ＳＣＣ），结果表明，３００Ｍ钢在３．５％ＮａＣｌ水溶液和蒸馏水中均对ＳＣＣ敏感，随温度增加和拉伸应力变速率的降低钢的ＳＣＣ敏感性增加，并且阴极极化和阳极极化均提高钢的ＳＣＣ敏感性，除氧和降低溶液ｐＨ值均提高钢的ＳＣＣ敏感性。     

Effect of temperature upon stress corrosion cracking of HY-180M steel in 3.5 Pct NaCI

A fracture mechanics and fractographic study of stress corrosion cracking (SCC) of heat treated HY-180 M steel was undertaken over the temperature range 22 to 95 °C at applied potentials of −0.28 V_SHE (−0.48 V_Ag/AgCl) and −0.80 V_SHE (−1.0 V_Ag/Agcl). Particular attention was directed toward Region II behavior, where crack propagation rates were independent of stress intensity(K _l). Region II rates were always higher at the less noble potential of −0.80 V_SHE than at the more noble potential of — 0.28 V_SHE. However, fractography studies suggested that the basic mechanism of cracking at both potentials was the same, and involved hydrogen embrittlement. An Arrhenius analysis of Region II rates showed that crack propagation was under the control of more than one process. Consequently, the mechanistic details remained obscure. Formerly Research Associate in theDepartment of Metallurgical Engineering, University of BritishColumbia.     

Near-Neutral pH Stress Corrosion Cracking Susceptibility of Plastically Prestrained X70 Steel Weldment

The application of strain-based design for pipelines requires comprehensive understanding of the postyield mechanical behavior of materials. In this article, the impact of plastic prestrain on near-neutral pH stress corrosion cracking (SCC) susceptibility of welded X70 steel was investigated with a slow strain rate tensile (SSRT) test. Generally, plastic prestrain reduces the SCC resistance in various welded zones. The SCC susceptibility of the test materials can be put in the following order: heat-affected zone (HAZ) > weld metal (WM) > base metal (BM). Fractographic analysis indicates that there are two cracking modes, mode I and mode II, during SSRT tests. Mode I cracks propagate along the direction perpendicular to the maximum tensile stress, and mode II cracks lie in planes roughly parallel to the plane where the maximum shear exists. The SCC of the BM is governed by mode I cracking and fracture of the HAZ, and the WM is dominated by mode II cracking. Damage analysis shows that the detrimental impact of plastic prestrain on the residual SCC resistance cannot be evaluated with the linear superposition model. A plastic prestrain sensitivity, a material constant independent of plastic prestrain, is proposed to characterize the susceptibility of SCC resistance to plastic prestrain, and it increases with the SCC susceptibility of the steels. The enhanced SCC susceptibility caused by plastic prestrain may be related to an increase in yield strength. The correlation of the ratio of the reduction in area in NS4 solution to that in air (RA _SCC/RA _air) with the yield strength is microstructure dependent.     

Stress Corrosion Cracking of HY-180 Steel in Aqueous 3.5 Pct NaCI

Stress corrosion cracking of HY-180 steel (Fe-10 Ni-2 Cr-1 Mo-8 Co-0.12 C) was studied in aqueous 3.5 pct NaCI (pH = 6.5) at 22 °C. The alloy was austenitized, water quenched and aged at 510 °C for 5 h. Specimens were of the precracked, double cantilever beam (DCB) variety and exposure times extended up to 1000 h. The crack propagation rates (v) were studied as a function of stress intensity(K,) under both freely corroding potentials(E ≈-0.36 V_SHE) and potentials produced by coupling to Zn(E ≈ -0.82 V_SHE. Crack fractography was studied by scanning electron microscopy and corrosion products were identified by electron diffraction analysis. The stress intensity, K_ISCC, below which SCC could not be detected was ~45 MPa m^1/2 for both freely corroding and Zn-coupled conditions. Analysis of the results showed that cracking was consistent with a hydrogen embrittlement mechanism, irrespective of potential. Furthermore, comparison of the data with previous studies on a similarly heat treated and closely related alloy (HY-180 M), containing 14 Co-0.16 C, showed no significant difference in SCC behavior, provided comparison was made at similar electrochemical potentials.     

Study on the relationship between intermediate-temperature embrittlement and triaxial stress induced by the ellipsoidal graphite in hot-rolled ferritic spheroidal graphite cast iron

The intermediate-temperature embrittlement of a hot-rolled ferritic spheroidal graphite cast iron was studied with the consideration that triaxial stress is induced by the ellipsoidal graphite particles of three unequal axial radii. The graphite shape was changed by various rolling reductions, and the tensile tests were performed at 673 K. The results show that the elongation and flow stress are independent of rolling reduction, and intergranular fracture occurs in all specimens. In the plasticity analysis, the triaxiality ratio (σ _m σ_eq) at a point in the ferrite matrix center can be expressed in terms of graphite shape ratio (b _d ) and graphite interparticle spacing (2a _d ) as σ _m /σ_eq = 1/3 +a _d /(2b _d ) where σ _m is the hydrostatic tensile stress and σ_eq is the equivalent stress. Accordingly, the average triaxiality ratio in the matrix center region is independent of rolling reduction and greater than one, a result that is consistent with the fact that the elongation is about constant, and all specimens undergo intergranular fracture. Moreover, the rolling reduction independent flow-stress behavior can be rationalized by the analytical result that the average σ _m /σ_eq is unchanged with rolling reduction, where σ _z is the internal stress along the tensile direction.     

The influence of deformation path on the slow strain-rate stress corrosion cracking of admiralty brass sheet

The slow strain-rate stress corrosion cracking (SCC) of Admiralty brass sheet in an aqueous 0.1M CuSO₄ solution has been studied over a range of strain paths from uniaxial to equibiaxial tension. At a constant bulk solution pH and open circuit potential, the brass undergoes transgranular SCC characterized by cleavage-like cracks propagating on a macroscopic plane normal to the maximum principal strain axis for all strain paths. The average crack growth velocity is also independent of deformation path. It is thus concluded that the mechanism of transgranular SCC in this system does not depend on multiaxial strain path for the range of stress states examined. However, the fracture strain data show that the slow strain-rate SCC of the brass sheet results in ductility losses which are much larger in equibiaxial tension than in either uniaxial or plane strain tension. This behavior is attributed to stress corrosion cracks acting as linear imperfections whose presence causes failure of the uncracked ligament by a form of localized necking and whose influence is dependent not only on time but also on strain path.     

Limitations of potentiostatic control in stress corrosion crack growth measurements

The electrode potential distribution along a crack in a potentiostatically polarized specimen has been derived analytically by including polarization behavior and solution conductance considerations. The analysis has been applied to the stress corrosion cracks within low alloy steels in an 8M sodium hydroxide solution at 373 K and shows that the electrode potential at the tip falls to the normal equilibrium corrosion potential as the crack length increases. These results show that potentiostatic control at the tip of a stress corrosion crack is subject to large varying systematic errors. Consequently the validity of stress corrosion mechanisms based on potentiostatically controlled crack growth measurements which do not take into account such errors should be reexamined. List of Symbolsα the Tafel constant for the anodic dissolution reaction on a natural logarithm scale, V,β the Tafel constant for the cathodic reduction reaction on a natural logarithm scale, V,C the specific conductance of a solution, Cl'1 mδ the crack opening displacement, m,E _c the free corrosion potential, V,E _app the potentiostatically applied potential, V,E _x the potential at position x within a crack, VE _y Young's modulus of elasticity, MNm^-22,i _c the corrosion current density at the free corrosion potential, Am^-2,i _app the net anodic current density supplied to polarize the specimen to a potential E _app , Am^-2,i ₁ the cathodic current density at potential E_app, Am^-2,i ₂ the anodic current density at potential E, Am-2,i _x the net anodic current density at potential Ex, Am-2,i f the current flow per unit length along the crack at position x, Am-2,ρ _y the yield stress, MNm-2,K the stress intensity, MNm-3/2, w the width of the stress corrosion crack, m, and x the length of the stress corrosion crack, m.     

Aqueous environmental crack propagation in high-strength beta titanium alloys

The aqueous environment-assisted cracking (EAC) behavior of two peak-aged beta-titanium alloys was characterized with a fracture mechanics method. Beta-21S is susceptible to EAC under rising load in neutral 3.5 pct NaCl at 25 °C and −600 mV_SCE, as indicated by a reduced threshold for subcritical crack growth (K _TH ), an average crack growth rate of up to 10 μms, and intergranular fracture compared to microvoid rupture in air. In contrast, the initiation fracture toughness (K _ICi ) of Ti-15-3 in moist air is lower than that of Beta-21S at similar high σ_YS (1300 MPa) but is not degraded by chloride, and cracking is by transgranular microvoid formation. The intergranular EAC susceptibility of Beta-21S correlates with both α-colonies precipitated at β grain boundaries and intense slip localization; however, the causal factor is not defined. Data suggest that both features, and EAC, are promoted by prolonged solution treatment at high temperature. In a hydrogen environment embrittlement (HEE) scenario, crack-tip H could be transported by planar slip bands to strongly binding trap sites and stress/strain concentrations at α colony or β grain boundaries. The EAC in Beta-21S is eliminated by cathodic polarization (to −1000 mV_SCE), as well as by static loading for times that otherwise produce rising-load EAC. These beneficial effects could relate to reduced H production at the occluded crack tip during cathodic polarization and to increased crack-tip passive film stability or reduced dislocation transport during deformation at slow crack-tip strain rates. High-strength β-titanium alloys are resistant, but not intrinsically immune to chloride EAC, with processing condition possibly governing fracture. Formerly Graduate Research Associate, University of Virginia Formerly Graduate Research Associate, University of Virginia     

The strain dependence of the flow stress-grain size relation for 70:30 brass

The grain size dependence of the stress-strain behavior of annealed 70:30 brass was evaluated using room temperature tensile tests. The resulting data, which covered × 10^-5 to 4 × 10^-1, were analyzed in terms of the conventional Hall-Petch stress-grain size equation, σ_∈ = σ_O∈ +k∈l-1/2, and, also, in terms of the extended Hall-Petch equation previously proposed for 70:30 brass, σ_є = σ0y+ A Є _p + β(є _p /l ^1/2 +kl ^−1/2 The lattice friction stress, σ0_∈, increased linearly with plastic strain over nearly the full strain range. The lattice friction stress for the initiation of plastic flow, σ0_y, was evaluated using two alternative double extrapolation procedures. Both extrapolation techniques, which involved the macrostrain behavior, gave the same σ0_y value of 3.4 kg/mm², which agreed with the σ0_∈ value determined directly in the microstrain region (∈ <-10^-3). Large grain size specimens, which yielded homogeneously, exhibited a kx2208; value of only 0.2 kg/mm^3/2 at a plastic strain of 1 × 10^-5; however this small kx2208; increased rapidly with increasing microstrain. For the small grain size specimens, which yielded via a Luders extension,kε was essentially constant at 0.8 kg/mm^3/2 for all microstrains; however, kx03B5; did increase in the macrostrain region to a maximum value of 1.6 kg/mm^3/2. When consideration was given to a grain size dependent increase in dislocation density, an intrinsic grain boundary resistance to plastic flow of approximately 0.7 kg/mm^3/2 was obtained. This paper is based upon a thesis submitted by W. L. Phillips in partial fulfillment of the requirements of the degree of Doctor of Philosophy at University of Maryland.