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.     

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.     

Oxide properties and stress corrosion cracking behaviour for Alloy 600 in leaded caustic solutions at high temperature

The stress corrosion cracking behaviour of Alloy 600 in caustic solutions with and without PbO at 315 °C was investigated by means of slow strain rate tension tests. The characterisation of the oxide that formed on Alloy 600 was derived from transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. Lead was incorporated into the oxide in a metallic lead state and a lead oxide state, which degraded the passivity and induced PbSCC susceptibility. NiB was used as an inhibitor. It reduced the lead incorporation level in the oxide layer and decreased PbSCC susceptibility.     

Effect of AC on stress corrosion cracking behavior and mechanism of X80 pipeline steel in carbonate/bicarbonate solution

The influence of various AC current densities on stress corrosion cracking behavior and mechanism of X80 pipeline steel was investigated in carbonate/bicarbonate solution by polarization curves and slow strain rate tensile tests. With the increasing AC current density, the SCC susceptibility of the steel increases, especially at high AC current density. A significant difference in the SCC behavior and mechanism is found for the steels with or without AC application. In the absence of AC, the fracture mode is intergranular and the mechanism is attributed to anodic dissolution. Under AC application, the cracks propagation is transgranular, and the mechanism is mixed controlled by both anodic dissolution and hydrogen embrittlement.     

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.     

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 high gradient magnetic fields on the anodic behaviour and localized corrosion of iron in sulphuric acid solutions

The influence of high gradient magnetic fields on the anodic dissolution of iron in sulphuric acid solutions and the localization of the corrosion attack is investigated by means of potentiodynamic and potentiostatic polarization experiments and subsequent surface profile analysis. A localization of the material loss is observed in every potential region of the anodic Fe dissolution except from the passive region. The impact of the magnetic field on the anodic current density and the localization of the corrosion attack are explained by the action of the Lorentz force and the magnetic field gradient force.     

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.     

Intergranular stress corrosion cracking of copper - A case study

The objective of this work was to identify the conditions and mechanisms for stress corrosion cracking (SCC) of a hard copper tube employed in a cooling system. The fractured tube was made of deoxidized high phosphorous copper (Cu-DHP). The identification was performed on the ground of fractography, metallography, residual stress measurements and corrosive environment analysis. It was found that humidity and environment containing ammonium and trace amounts of nitrite and nitrate ions were responsible for initiation of SCC yielding to breakdown of oxide surface layer by pitting. Cracking was found intergranular and perpendicular to circumferential stresses. Stress corrosion crack propagation appeared the most consistent with the oxide rupture mechanism. The findings were discussed in relation to the literature data in order to get a better understanding of cracking behaviour.     

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 of grain size on corrosion of nanocrystalline copper in NaOH solution

Effect of grain size on corrosion of bulk nanocrystalline copper was investigated using potentiodynamic polarization measurements in 0.1 M NaOH solution. Bulk nanocrystalline copper was prepared by inert gas condensation and in situ warm compress (IGCWC) method. The grain sizes of all bulk nanocrystalline samples were determined to be 48, 68 and 92 nm using X-ray diffraction (XRD). Results showed that bulk coppers displayed an active-passive-transpassive behaviour with duplex passive films. From polycrystalline to nanocrystalline, grain size variation showed little effect on the overall corrosion resistance of copper samples.     

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.     

Passivity breakdown and stress corrosion cracking of α-brass in sodium nitrite solutions

The stress corrosion cracking (SCC) susceptibility of pure copper and four α-brasses of different zinc alloy concentration in NaNO₂ 1 M solution, at various pH values, was studied by means of potentiodynamic polarisation curves and constant potential slow strain rate experiments. The results confirmed that passivity rupture was a necessary condition for SCC of α-brass and copper in nitrite solutions. Susceptibility to SCC was observed only when the potential was equal to or higher than a certain critical value at which passivity breakdown was triggered by the slow dynamic straining of the metal. Cracks were found only in those metal areas where passivity rupture had taken place.     

Zr-based bulk glassy alloy with improved resistance to stress corrosion cracking in sodium chloride solutions

In order to evaluate stress corrosion cracking (SCC) susceptibility of Zr-based bulk glassy alloys and develop the BGAs with low susceptibility to SCC, the SCC behaviour of Zr₅₀Cu₄₀Al₁₀, Zr₅₀Cu₃₀Al₁₀Ni₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ BGAs in various environments including sodium chloride solution has been investigated using a slow strain rate technique at an initial strain rate of 5 × 10⁻⁶ s⁻¹. It is found, for the first time, that the Zr₇₀Cu₆Al₈Ni₁₆ BGA has no susceptibility to SCC in a 0.5 M NaCl solution. On the other hand, Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₀Al₁₀Ni₁₀ BGAs are highly susceptible to SCC in the NaCl solution, although they are not susceptible to SCC in de-ionized water, phosphate buffer, 0.5 M Na₂SO₄ and 0.5 M NaNO₃ solutions. The possible cause of the high susceptibility to SCC in the NaCl solution for the Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₀Al₁₀Ni₁₀ BGAs is discussed.     

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

An investigation has been undertaken to establish the effect of surface preparation method on the susceptibility of a 304 stainless steel to stress corrosion cracking under simulated atmospheric corrosion conditions. MgCl₂ was deposited onto four-point bend specimens, which were then placed in a chamber with a relative humidity of 45% and temperature of 60 °C. These test conditions were designed to reflect external exposure of stainless steel components in industrial plant, including nuclear reactor components, situated in a coastal region, but with the severity of the exposure conditions enhanced to allow discrimination of the effect of surface preparation in a short timescale (up to 1500 h). Four surface preparation methods were evaluated: transverse grinding, longitudinal grinding, transverse dressing using an abrasive flap wheel, and transverse milling. For each case, surface topography, surface defect mapping, near-surface microhardness mapping, residual stress and electron back-scattered diffraction measurements were undertaken. Stress corrosion cracks were observed for the ground and milled specimens but not for the dressed specimens, with cracks apparently originating at corrosion pits. The density of cracks increased in the order: transverse ground, milled and longitudinal ground, with the cracks notably much smaller in length for the transverse ground condition. The propensity for cracking could be linked to the high residual stress and apparent nanocrystalline microstructure at the surface. There was a greater propensity for pitting to initiate at local defect sites on the surface (laps, deeper grooves). However, the tendency was not overwhelming, suggesting that other factors such as more general roughness or the distribution of MnS inclusions had an influence, perhaps reflecting the severity of the environment.     

Pit to crack transition in stress corrosion cracking of a steam turbine disc steel

Long term exposure tests have been carried out on a 3 NiCrMoV steam turbine disc steel in the form of cylindrical tensile test specimens self-loaded to 90% of σ_0.2 and exposed to three environmental conditions, viz. deaerated pure water, aerated pure water, and aerated water containing 1.5 ppm of chloride ion. Pitting occurred in all environments but the density and depth of pits in the chloride-containing medium was markedly greater. No cracking was observed in deaerated pure water but cracks initiated in aerated water between 13 and 19 months and in less than 7 months in aerated 1.5 ppm Cl⁻ solution. The probability of a crack initiating from a pit of specific depth in aerated solution could be described well by a Weibull function. Profiling of pits and cracks in the disc steel tested in aerated 1.5 ppm Cl⁻ solution showed that there while there were many cracks with a depth greater than that of the corresponding pit the depth of some cracks was smaller than that of the corresponding pit, suggesting that cracks do not necessarily initiate from the bottom of the pits. The growth rate of short cracks emerging from pits appeared greater than that of long cracks in fracture mechanics specimens.     

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.     

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.     

Effect of plasma electrolytic oxidation coating on the stress corrosion cracking behaviour of wrought AZ61 magnesium alloy

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10⁻⁶ s⁻¹ and 1 × 10⁻⁷ s⁻¹, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.     

Quantitative characterization by micro-electrochemical measurements of the synergism of hydrogen, stress and dissolution on near-neutral pH stress corrosion cracking of pipelines

Occurrence of stress corrosion cracking of pipelines under a near-neutral pH condition depends on the synergism of stress, hydrogen and anodic dissolution at the crack tip of the steel. In this work, micro-electrochemical techniques, including localized electrochemical impedance spectroscopy and scanning vibrating electrode technique, were used to characterize quantitatively the synergistic effects of hydrogen and stress on local dissolution at crack-tip of a X70 pipeline steel in a near-neutral pH solution. Results demonstrate that, upon hydrogen-charging, the anodic dissolution of the steel is enhanced. The resistance of the deposited corrosion product layer depends on the charging current density. There is a non-uniform dissolution rate on the cracked steel specimen, with a highest dissolution current density measured at crack-tip. For a smooth steel specimen, the synergistic effect factor of hydrogen and stress is equal to 5.4, and the total effect of hydrogen and stress on anodic dissolution of the steel is 7.7. In the presence of a crack, the hydrogen effect factor, stress effect factor and the synergistic effect factor are approximately 4.3, 1.3 and 4.0, respectively. The total effect factor is up to 22.4, which is very close to the 20 times of difference of crack growth rate in pipelines in the presence and absence of the hydrogen involvement recorded in the field.