The effect of acetic acid on the stress corrosion cracking of 3.5NiCrMoV turbine steels in high temperature water

The effects of acetic acid on the stress corrosion cracking (SCC) susceptibility of ASTM A470/471 turbine disk/rotor steels (3.5NiCrMoV) were evaluated. Thus, this study investigated the possibility of corrosion damage to turbine components due to acidified, early condensates containing acetic acid, which is a common impurity in steam cycle systems. Constant extension rate tests (CERTs) were carried out in aqueous solutions containing acetic acid at various concentrations (pH 3-5) at 150 °C to ascertain the susceptibility in terms of strain to failure and reduction in area upon fracture. Acetic acid significantly affects the SCC susceptibility of the turbine steels with the susceptibility increasing with increasing acetic acid concentration, except at the very highest concentration at which excessive dissolution led to crack blunting. The SCC susceptibility test results were analyzed using the CEFM (Coupled Environment Fracture Model), and it is shown that this model accounts for the experimentally-observed relationships between the SCC growth rate and the concentration of the acetic acid in terms of the conductivity of the solution.     

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.     

The influence of applied stress rate on the stress corrosion cracking of 4340 and 3.5NiCrMoV steels in distilled water at 30 °C

Linearly Increasing Stress Tests conducted in 30 °C aerated distilled water using as-quenched 4340 and 3.5NiCrMoV turbine rotor steels indicated that stress corrosion cracking occurred at all applied stress rates for 4340 steel, whilst only at applied stress rates less than or equal to 0.002 MPa s⁻¹ for the turbine rotor steel. The crack velocity increased with increasing applied stress rate for both steels with the maximum crack velocity for 4340 steel corresponded to v_II in fracture mechanics tests in room temperature water. The fracture surface morphology was mixed mode consisting of intergranular and transgranular fracture regions.     

Impact of temperature excursion on stress corrosion cracking of stainless steels in chloride solution

The impact of a temperature excursion on the subsequent stress corrosion crack growth at the normal operating temperature has been investigated for 321 stainless steel (UNS32100) and 316L stainless steel (UNS31603) using precracked compact tension specimens. Although the data are preliminary the indication is that once crack growth has initiated in 321 SS at the elevated temperature, 130 °C in this study, the crack growth may be sustained at the lower temperature (40 °C), at least over the exposure time of about 700 h. However, the growth rate of 316L SS at the lower temperature was significantly lower than for 321 SS and tended to zero after 2000 h. For the 316 SS a temperature transient should not impact on structural integrity, provided it is short in duration.     

Effect of temperature on the stress corrosion cracking of zircaloy-4 in iodine alcoholic solutions

Zircaloy-4 is susceptible to stress corrosion cracking (SCC) in solutions of iodine dissolved in different alcohols (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol and 1-octanol). The crack propagation rate is known to decrease as the solvent molecular weight increases, as a consequence of steric hindrance. However, the mechanism that operates during SCC is still unknown. In the present work the effect of temperature on SCC susceptibility was evaluated in 1-butanol and 1-pentanol iodine containing solutions. The dependence of the crack growth rate with temperature follows an Arrhenius law, and the activation energy obtained from experimental data is consistent with a process controlled by volume diffusion of the active species (the iodine-alcohol complex) to the crack tip.     

Effect on marine immersion corrosion of carbon content of low alloy steels

This paper attempts to reconcile information from a number of different sources about the effect of small changes in carbon content on the immersion corrosion of specimens of normal commercial mild and low alloy steels. It does so through interpreting the data reported in the literature in terms of the recently proposed theoretically based phenomenological model for marine immersion corrosion. This model postulates different corrosion phases as corrosion progresses. When the experimental results are interpreted in terms of the model it is found that carbon content has minimal effect on the kinetically controlled corrosion phase. The next phase, when corrosion rate is controlled by oxygen diffusion, is also unaffected, in agreement with theoretical predictions. However, carbon content does affect the two anaerobic phases, with increased corrosion as the carbon content and the water temperature increase. The model allows apparently conflicting observations to be reconciled and shows that carbon content may be influential for longer-term corrosion and for corrosion in tropical waters.     

Role of second phases in the stress corrosion cracking of a nickel-aluminium bronze in saline water

We investigated the role of second phases in the stress corrosion cracking of a complex copper-aluminium alloy CuAl₉Ni₃Fe₂. We conducted slow strain rate tensile tests in synthetic sea water. Cracking is mainly intergranular. The crack path is determined by the electrochemical interactions between the α matrix and the second phases. We observe the selective dissolution of the aluminium rich areas that are anodic with respect to the α matrix. Under cathodic polarisation, the detrimental effect on mechanical properties is enhanced. The number of cracks is reduced. Crack blunting by dissolution is impeded.     

Role of Bayer solution concentration and temperature in stress corrosion cracking susceptibility of steel

To improve the efficiency of the Bayer process for the extraction of alumina from Bauxite ore, there is a push for increasing processing temperature and caustic concentrations, which has also led to an increased concern for caustic embrittlement. In this study, the caustic cracking behaviour of steel in Bayer solutions of 2.5, 5, 7.5 and 10 mol dm⁻³ “free caustic” concentrations have been studied at different temperatures using pre-cracked circumferential notch tensile specimens. It has been observed that at 100 °C, steel is susceptible to caustic cracking in each of the four Bayer solutions. Caustic cracking has also been observed at temperatures as low as 55 °C. Tests were also conducted using only the notched specimens (i.e., without pre-cracking) in a 7.5 mol dm⁻³ “free caustic” Bayer solution at 120 °C to study the stress corrosion crack formation and propagation behaviour in blunt notches.     

Effect of small compositional changes on marine immersion corrosion of low alloy steels

Small changes in the composition of mild and low alloy steels can effect their immersion corrosion behaviour. A number of comprehensive test programs for coupons immersed at different locations and recovered at different times have been reported. Comparison between them has also been attempted with modest success as well as leaving some apparent inconsistencies in the effect of some alloying elements.In this paper, a new comparative analysis of previously reported observations is reported. It employs a recently reported multi-phase phenomenological corrosion-time model, with different corrosion phases governing corrosion behaviour. Each phase is a function of time.The analysis shows that metal composition can influence the first, kinetically controlled corrosion phase and also the long-term anaerobic corrosion phases. However, during the phase controlled by oxygen diffusion through the corrosion product, metal composition is largely irrelevant, in agreement with theoretical predictions. It is shown that the several observations in the literature about the effect of particular alloys can be reconciled, including apparently conflicting observations about the effect of chromium content.     

Research of stress corrosion cracking of T225NG titanium alloy in loop water of high temperature and high pressure

Double cantilever beam （DCB） specimens were used to research the stress corrosion cracking of T225NG titanium alloy in loop water of high temperature and high pressure. DCB specimens were forced pre-stress, put into high pressure ataoclave , and the stress corrosion and crack expansion of specimens were observed and measured in 500 h, 1 000 h and 2000 h respectively. The results show that small expansion occurred along the direction of pre-cracking. According to calculation, the speed of cracking expansion is lower than 10 -9 m/s in 500 h and the value of K ISCC /KI is higher than 0. 75, which proves that T225NG has an excellent corrosion resistance in loop water. The main reason is that there is an oxide film on the surface of specimens. According to the analysis of energy dispersive spectroscopy （EDS） and X-ray diffraction （ XRD ） , the oxide film consists of TiO2. Therefore, the oxide film at the crack tip impedes the hydrogen separating out from the cathode to penetrate into titanium alloy and resists hydrogen embrittlement.     

Environmental effects in the stress corrosion cracking of turbine disc steels

The effects of environmental parameters, such as the impurity concentration, oxygen content and pH value on the stress corrosion cracking (SCC) of 3.5Ni-Cr-Mo-V and 2Cr-Ni-Mo steels were studied. SCC of these two alloys only occurs in steams containing both oxygen and contaminating ions. A concentration of 40 ppb of Cl⁻ or around 5 ppm of SO₄²⁻ in steam is found to be enough to induce SCC on both alloys when oxygen exists in the steam. Sulfate ions suppress the severity of chloride ions in causing SCC of both materials in a Cl⁻ and SO₄²⁻ containing steam. This effect was proposed to result from the iron sulfate salt deposition. SCC of these steels in steam is accomplished by corrosion of active path, while the reaction rate is determined by the rate of cathodic reaction of the SCC system. Unless the pH value is 3 or less, the change in pH of de-aerated environments induces no SCC on both materials. SCC in the pH 3 environment is a result of the change of the cathodic reaction from water decomposition to proton reduction in this SCC system.     

Role of water chemistry and microstructure in stress corrosion cracking in the fusion boundary region of an Alloy 182-A533B low alloy steel dissimilar weld joint in high temperature water

Stress corrosion cracking (SCC) in the fusion boundary (FB) region of an Alloy 182-low alloy steel (LAS) dissimilar weld joint in 288 °C water was investigated by experiments and finite element simulation. Creviced bent beam and crack growth rate (CGR) experiments showed that, while the FB was a barrier to SCC growth, further crack growth into LAS was activated by a combined effect of sulfate and dissolved oxygen in water. Finite element simulation suggested that a positive gradient of hardness as the crack approached to the FB in dilution zone caused decreased CGR. Role of microstructure and water chemistry in SCC was discussed.     

Intergranular to transgranular transition in the stress corrosion cracking of Zircaloy-4

Stress corrosion cracking susceptibility of Zircaloy-4 wires was previously studied in 1 M NaCl, 1 M KBr and 1 M KI aqueous solutions and in iodine alcoholic solutions. In all cases, intergranular attack preceded transgranular propagation. It is generally accepted that the intergranular-transgranular transition occurs when a critical value of the stress intensity factor is reached. In the present paper it was confirmed that the transition from intergranular corrosion to transgranular propagation in Zircaloy-4 wires occurs when a critical value of the stress intensity factor is reached. This critical stress intensity factor in wire samples is independent of the solution tested and close to 11 MPa m^−1/2. This value is in good agreement with those reported in the literature measured by different techniques and with different specimen geometries.     

Effects of polarisation on mechanical properties and stress corrosion cracking susceptibility of 7050 aluminium alloy

The effects of polarisation (including cathodic and anodic polarisation) on mechanical properties and stress corrosion cracking (SCC) susceptibility (I_SCC) of 7050 aluminium alloy have been investigated by means of polarisation and slow strain rate test. The results of cathodic polarisation experiments showed that the I_SCC increased with shifting negatively the polarisation potential when the cathodic potential E_C≧?1100 mV(SCE), while it decreased with shifting negatively the polarisation potential when the cathodic potential E_CI_SCC increased severely with increasing the polarisation potential. In addition, the extents for the effect of polarisation potential on I_SCC were different among the 7050 aluminium alloy under various aging states. Polarisation was of the biggest effect on the I_SCC of under aged state, the smallest effect of over aged state and the middle effect of peak aged state. The SCC mechanism of aluminium alloy was a combination of anodic dissolution and hydrogen embrittlement, and the effects of hydrogen on SCC increased with increasing the hydrogen concentration.     

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.     

Effects of water chemistry and loading conditions on stress corrosion cracking of cold-rolled 316NG stainless steel in high temperature water

The effects of electrode potential, stress intensity factor and loading history on stress corrosion cracking growth of a cold-rolled 316NG stainless steel in 288 °C pure water were investigated. Crack branching and intergranular stress corrosion cracking along random grain boundaries were observed by electron-back scattering diffraction. A strong dependence of crack growth rate on stress intensity factor is observed. A single-cycle overloading produced a retarded transient cracking growth period. The mild inhibiting effect of decreasing electrode potential on crack growth of cold-rolled 316NG SS is analyzed based on the interaction between crack tip mechanics and crack tip oxidation kinetics.     

A model for an anodic dissolution cell in connection to its dimensions for stress corrosion cracking

The anodic dissolution is modelled for pure iron for conditions simulating those of low alloy steels in high temperature water undergoing stress corrosion cracking. The dissolution is assumed to be initiated by the rupture of the protective oxide film and arrested by the repassivation. The goal of the model is to identify the relevant parameters of the anodic dissolution by a parameter sensitivity study. The cell is three-dimensional for taking into account the cathode to anode area ratio, at the price of the assumption of homogeneity of the electrolyte, and is assumed to be electrically uncoupled from the rest of the crack. The model uses the Butler-Volmer, the Ohm law, the charge conservation, and takes into account the passive current. The main results include the limited extent of the cell along the crack, depending on the values of the parameters, and non-linear and coupled effects for the occurrence and amplitude of the dissolution with as critical parameters the composition of the electrolyte, the exchange current densities and the anode dimensions.     

Effect of N and C on stress corrosion cracking susceptibility of austenitic Fe18Cr10Mn-based stainless steels

Stress corrosion cracking (SCC) susceptibility of austenitic Fe18Cr10Mn alloys with 0.3N, 0.6N and 0.3N0.3C was investigated in aqueous chloride environment using a slow strain rate test method. The SCC susceptibility of Fe18Cr10Mn alloys in 2 M NaCl solution at 50 °C under constant anodic potential condition decreased with increase in N content from 0.3 to 0.6 wt%, and with addition of 0.3 wt% C to the Fe18Cr10Mn0.3N alloys. The present study strongly suggested that the beneficial effects of N and C on the SCC behavior of Fe18Cr10Mn alloys would be associated with the resistance to pitting corrosion initiation and the repassivation kinetics.     

国产X70管线钢的硫化氢应力腐蚀性能及其焊接性

针对在含H2S的介质中，不同成分条件的国产管线钢H2S应力腐蚀开裂(Sulfide stress corrosion cracking，SSCC)性能、焊接性分别采用恒载荷法及插销试验法进行了研究；对插销试样断口进行了电子显微镜分析。试验结果表明，不同C、Mn、P含量的国产X70管线钢在相同的试验条件下具有不同的SSCC断裂时间与断裂应力。其原因主要是由于它们的Mn、P元素含量的不同引起的，而不是C元素；采用LINCOLN E6010焊条，预热70℃时的临界应力σcr=630MPa，不预热时的σcr=610MPa，即预热可以使国产X70管线钢的临界应力明显提高；插销试样的断口形貌是由解理断裂、准解理断裂和韧窝断裂构成的混合型断口。     

Effects of laser shock processing on stress corrosion cracking susceptibility of AZ31B magnesium alloy

Laser shock processing (LSP) is a new technique for strengthening metals. The effects of LSP on the stress corrosion cracking (SCC) susceptibility of AZ31B magnesium (Mg) alloy were investigated. Water-immersed specimens of AZ31B magnesium alloy were shocked by Q-switched Nd: glass laser with a wavelength of 1064 nm. A fine-grained structure with an average sub-grain size of 5.8 μm was obtained after four laser impacts. Residual stress distribution as a function of depth was assessed by using X-ray diffraction technology. It was observed that with increasing the number of laser impacts, the compressive residual stress near the surface increased. The depth of the compressive residual stress induced by LSP exceeded 0.8 mm from the surface. SCC test in 1 wt.% NaOH solution showed that LSP retarded the SCC initiation and growth in AZ31B Mg alloy.