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.     

The fractography of stress corrosion cracking in carbon steels

The stress corrosion cracking of carbon steels in a number of environments (nitrates, hydroxides, carbonates, chlorides and liquid ammonia) has been studied by scanning electron microscopy. The crack path depends on both heat treatment and environmental conditions. In annealed steels it appears that solutions which produce thick corrosion products induce intergranular cracking, while in solutions where adsorption induced inhibition occurs or thin films form, cracking is transgranular. Although the available fractographic evidence is ambiguous, it appears to rule out either hydrogen embrittlement or brittle films as being important in crack propagation. However, the fracture surface morphologies are consistent with the cracks propagating by anodic dissolution, possibly aided by mechanical tearing.     

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.     

The correlation between mechanical and electrochemical parameters of stress corrosion cracking of austenitic stainless steels

Extension measurements of exposed specimens of austenitic stainless steels in hot magnesium chloride solutions are interpreted with the potential-time curves. For austenitic stainless steels, it is very difficult to determine the yield point; it is necessary to study the stress-elongation curves at different elongation rates and to known the creep behaviour of the steels. In dead load stress corrosion tests the elongation-time curves allow the incubation time of stress corrosion cracks to be distinguished from the propagation time. The propagation time is more important than incubation time for the classification of the susceptibility of austenitic stainless steels to stress corrosion cracking, because the incubation time is more dependent on experimental procedure than is the propagation time. The stainless steel classification obtained was compared with a new test in which a load is applied and immediately taken off; after a rapid fall the potential-time behaviour provides information about the crack velocity.     

The austenitic manganese-chromium steels resistant to stress corrosion cracking

The basic type of MnCr steels (composition 0.05Mn19Cr13) was found to be highly resistant to chlorides. Additions of titanium, niobium and vanadium did not impair resistance to stress corrosion cracking, but addition of nickel in excess of 0.5% appreciably impaired the resistance. The chemical composition of the stabilized steels was selected in such a way as to make the ferrite content as low as possible. The two stabilized steel grades ability to passivate was limited, also because of the presence of manganese; this makes these grades suitable only for less aggressive environments. The ferrite content can be controlled, or the chromium content increased for better corrosion resistance, only by adding another strong austenite-former. Considering the adverse effect of nickel on the resistance to stress corrosion cracking, attention was given to manganese-chromium grade modified with nitrogen and molybdenum.     

The role of oxide films in stress corrosion cracking initiation

In the low-potential transition range, passivatable alloys undergo electrolytic crevice corrosion i.e. localized anodic attack at any area prevented from passivating by shielding effects. A possible role of stress in this range may be seen, accordingly, in producing at every film rupture uncontrollable localized attack in critical underfilm crevices at the feet of emerging slip steps. The fundamental condition for s.c.c. would be that the incubation time, i.e. the time of attack required to form a critical notch, must be shorter than the film lifetime. The resulting model for s.c.c. initiation at the active-passive transition fits in with the mostly accepted slip-dissolution mechanism. A simple mathematical treatment is given for such model and applied to three typical systems, that are the combination of steel with K₂CO₃KHCO₃, NaOH and NH₄NO₃ solutions, respectively. Calculated incubation times are consistent with experimental times to rupture and cracking rates from s.c.c. tests.     

The role of oxide films in stress corrosion cracking initiation

In the low-potential transition range, passivatable alloys undergo electrolytic crevice corrosion i.e. localized anodic attack at any area prevented from passivating by shielding effects. A possible role of stress in this range may be seen, accordingly, in producing at every film rupture uncontrollable localized attack in critical underfilm crevices at the feet of emerging slip steps. The fundamental condition for s.c.c. would be that the incubation time, i.e. the time of attack required to form a critical notch, must be shorter than the film lifetime. The resulting model for s.c.c. initiation at the active-passive transition fits in with the mostly accepted slip-dissolution mechanism. A simple mathematical treatment is given for such model and applied to three typical systems, that are the combination of steel with K₂CO₃-KHCO₃, NaOH and NH₄NO₃ solutions, respectively. Calculated incubation times are consistent with experimental times to rupture and cracking rates from s.c.c. tests.     

The predictability of stress corrosion cracking susceptibility of steels in acetate solutions from potentiodynamic polarization curves

Stress corrosion cracking susceptibilities of mild and low alloy steels in acetate solutions, assessed using slow strain-rate testing techniques, correlate well with those predicted on the basis of data from fast and slow sweep rate potentiodynamic polarization curves. Accurate prediction of cracking susceptibility necessitates allowances being made for changes of solution pH which occur during stress corrosion and polarization tests. A predictive technique which allows for these pH changes has been developed, and when implemented yields polarization data that indicates the influence of potential, steel composition and solution temperature on cracking susceptibility. The stress corrosion cracking of steels in acetate solutions is intergranular and occurs only when the electrochemical conditions correspond to those of a predicted potential-pH domain that is associated with the Fe²⁺/Fe₃O₄ transition.     

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.     

Environmental effects on the stress corrosion cracking susceptibility of 3.5NiCrMoV steels in high temperature water

Constant elongation rate tests (CERTs) were carried out to investigate the effects of environmental factors of dissolved oxygen and temperature on the stress corrosion cracking (SCC) susceptibility of 3.5NiCrMoV turbine steels. Tests were conducted in pure water of various dissolved oxygen concentrations at temperatures of 50 °C-200 °C in the range of strain rates from 5 × 10⁻⁸/s to 1 ×  10⁻⁶/s. Dissolved oxygen significantly affected the SCC susceptibility of turbine steels in water. The SCC susceptibility of the turbine steels increases as the dissolved oxygen concentration in water increases. The elongation of the turbine steels tested in aerated water at 150 °C at a strain rate of 1 × 10⁻⁷/s decreased to half of that of the steels tested in deaerated water in the same test condition. And the SCC susceptibility of the steels increased with decreasing strain rate, and with increasing temperature. The increase of the SCC susceptibility of the turbine steels in the higher dissolved oxygen environment is considered to be due to the higher content of dissolved oxygen enhancing the reduction reactions of oxygen on the metal surfaces (cathode) and accelerating the dissolution rate at the crack tips (anode) by galvanic attack of an aeration cell.     

Activation pH and susceptibility to hydrogen assisted stress corrosion cracking of martensitic stainless steels in sulphate solutions

The investigation of two different high strength stainless steels by the slow strain-rate technique has shown the existence of critical pH values (activation pH) for stress corrosion cracking in sulphate-containing media which depend on the steel composition. The slow strain-rate test indicates a sharp decrease of specimen ductility at the activation pH the value of which can also be determined by independent electrochemical methods.     

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.     

The effect of low-frequency cyclic stresses on the initiation of stress corrosion cracks in X60 line pipe steel in carbonate solutions

The initiation and early stages of growth (< 100 μm) of stress corrosion cracks in X65 line pipe steel have been studied with superimposed low frequency cyclic stresses. The effects of frequency, cyclic amplitude and the level of background tensile stress have been examined.     

Microstructure and stress corrosion cracking in simulated heat-affected zones of duplex stainless steels

The effects of nitrogen content and the cooling rate on the reformation of austenite in the Gleeble simulated heat-affected zone (HAZ) of 2205 duplex stainless steels (DSSs) were investigated. The variation of stress corrosion cracking (SCC) behavior in the HAZ of 40 wt% CaCl₂ solution at 100 °C was also studied. Grain boundary austenite (GBA), Widmanstatten austenite (WA), intergranular austenite (IGA) and partially transformed austenite (PTA) were present in the HAZ. The types and amounts of these reformed austenites varied with the cooling rate and nitrogen content in the DSS. U-bend tests revealed that pitting corrosion and selective dissolution might assist the crack initiation, while the types and amounts of reformed austenite in the HAZ affected the mode of crack propagation. The presence of GBA was found to promote the occurrence of intergranular stress corrosion cracking. WA, IGA and PTA were found to exhibit a beneficial effect on SCC resistance by deviating the crack propagation path.     

The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part I: Pitting and cracking occurrence

In this investigation, tensile test specimens were fabricated with increasing levels of compressive and tensile residual stress on the surface and through the thickness of the specimen. These residual stresses were then measured by neutron diffraction at multiple points along the length and through the depth of the specimens. The specimens were then exposed to a neutral pH aqueous soil environment in combination with an applied cyclic stress for various lengths of time in order to initiate and propagate stress corrosion cracking (SCC). The formation of micro-pitting was found to occur preferentially in areas where the tensile residual stresses were the highest (approximately 300 MPa), while SCC initiation occurred with a 71% normalized frequency in areas where the surface residual stress was in the range 150–200 MPa. The difference between residual stress levels occurring at SCC locations versus pitting locations resulted from both the change of residual stress during cyclic stress application during SCC testing and the residual stress gradient in the depth direction.     

The stress corrosion cracking of welded austenitic stainless steels in MgCl2 solutions in the presence of NaI additions

The influence of Nal additions on the stress corrosion cracking (SCC) of Type 304L and 316L welded stainless steels in 42 wt% aqueous MgCl₂ solution at 154°C has been investigated. The results indicate clearly that addition of 1 N NaI to the boiling MgCl₂ solution prevents SCC of the welded steels. The I⁻ can act as an effective cathodic inhibitor. The correlation between mechanical properties, fracture morphology and the mechanism of the inhibition behaviour of I⁻ on SCC is discussed.     

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.     

Anion effects on the stress corrosion cracking behaviour of aluminium alloys

The stress corrosion cracking behaviour of plate material of the aluminium alloys 2024‐T351, 8090‐T8171, 7475‐T651, and 7075‐T7351 was investigated performing constant load tests. Short transverse tensile specimens were permanently immersed in aerated aqueous 0.6 M Na₂Cl solutions with additions of Na₂SO₄, NaNO₃, NaHCO₃, NH₄HCO₃, Na₂HPO₄, Na₂SO₃ or Na₂CO₃. The concentration of the added salts was 0.06 M. The applied stress was 100 MPa, except with 7075‐T7351 specimens, which were loaded at 300 MPa. Environment induced failure was not observed in neutral 0.6 M NaCl solution. The various salts added promoted intergranular stress corrosion cracking with the alloys 2024‐T351, 8090‐T8171, and 7475‐T651. Threshold stresses were generally below 100 MPa. For 8090‐T8171 exposed to chloride containing electrolytes with additions of sulfate, hydrogen phosphate, or sulfite, threshold stresses were approximately 100 MPa or higher. Similar results were obtained for 7475‐T651 plate when immersed in chloride‐hydrogen phosphate and chloride‐carbonate solutions. Alloy 7075‐T7351 was resistant against intergranular stress corrosion cracking. Specimens suffered pitting corrosion during immersion in the corrosive environments. Failure observed with 7075‐T7351, in particular when exposed to the chloride‐nitrate solution, was associated with reduction of cross‐sectional area due to pitting and transgranular stress corrosion cracking.