Stress corrosion cracking of cold-worked austenitic stainless steels

Stress corrosion cracking (SCC) of AISI 304L and AISI 316L stainless steels, cold-worked under various conditions (i.e. at different degrees of deformation obtained by drawing and rolling at room temperature and at liquid nitrogen temperature) has been carried out in H₂O containing 1000 ppm Cl^? at 250°C and in a boiling MgCl₂ solution. The effect of heat treatments at 400 and 900°C on the SCC of previously cold-worked steels has also been studied. Particular attention was directed towards heat treatment at 400°C. In steels deformed at room temperature, it increases the SCC resistance. By contrast, for steels deformed at liquid nitrogen temperature, heat treatment at 400°C reduces the SCC resistance if carried out for short periods of time (1–6 h). Hardness measurements, structural analyses via X-rays, scanning and transmission electron microscopy (SEM and TEM), as well as modified Strauss tests, seem to prove that reduced stress corrosion resistance is not to be related to the chromium-rich carbides precipitation which could have been accelerated by the presence of α′-martensite. Instead, they tend to suggest that perhaps this phenomenon is connected to an increase in the level of internal micro-stresses which are generated by a reciprocal re-ordering of the α′ and γ structural phases.     

High voltage electron metallography of stress corrosion cracking of austenitic stainless steels

High voltage electron metallographic studies of stress corrosion cracks nucleated on miniature U-bend specimens of austenitic stainless steels support an anodic dissolution mechanism of cracking. For all the alloys examined crack nucleation occurs at slip traces and a characteristic slotted dissolution morphology is observed in crack tip regions. The crystallography of this attack as determined by stereo-microscopy and trace analysis, reveals the slots to be bounded by {110} planes whilst growing in 〈111〉 directions.Cracking occurs through the network of corrosion slots at the crack tip and the well known features of stress corrosion fracture surfaces can be interpreted by this mechanism.     

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.     

Controlling grain boundary energy to make austenitic stainless steels resistant to intergranular stress corrosion cracking

Intergranular corrosion and intergranular stress corrosion cracking are the two localized corrosion mechanisms that are of concern to the typical applications of austenitic stainless steels in industries. Until recently, the common understanding was that a higher frequency of random boundaries increases the susceptibility, caused by a sensitization heat treatment or by operating temperatures, of austenitic stainless steels to both intergranular corrosion and intergranular stress corrosion cracking. A recent study demonstrated that extreme randomization of grain boundaries leads to a considerable improvement of resistance to both sensitization and intergranular corrosion. This work is a continuation of Ref. 1 and relates the effects of grain boundary randomization to intergranular stress corrosion cracking: the results show a trend consistent with earlier observations on intergranular corrosion. It is shown that there is improvement in resistance to intergranular stress corrosion cracking with extreme randomization of grain boundaries.     

Effect of alloy element contents on caustic stress corrosion cracking of several stainless steels

The stress corrosion cracking behavior in caustic solutions (200 g/l sodium hydroxide, 10 g/l sodium chloride) of three austenitic (18Cr-10Ni-2.5Mo, 20Cr-25Ni-4.5Mo, 27Cr-31Ni-3.5Mo) and three duplex (23Cr-4Ni, 22Cr-5Ni-3Mo, 25Cr-7Ni-4Mo-N) stainless steels was examined. U-bend and Slow Strain Rate (SSR) tests were performed at 200–250°C. The negative influence of nickel in the lower range content for the 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo has been shown; when the nickel content is significantly increased (>30%), as in the case of the steel 27Cr-31Ni-3.5Mo, an increase of SCC resistance has been detected. The negative effect of molybdenum, mainly on the behaviour of duplex stainless steels, has also been evidenced. The duplex stainless steels show better caustic SCC resistance than austenitic stainless steels type 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo. The best behaviour has been found for the less-alloyed steel 23Cr-4Ni.     

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.     

On the pitting corrosion resistance of nitrogen alloyed cold worked austenitic stainless steels

Pitting corrosion studies were carried out on cold worked (5%, 10%, 15%, 20%, 30% and 40%) nitrogen-bearing (0.05%, 0.1% and 0.22% N) type 316L austenitic stainless steels in neutral chloride medium. Potentiodynamic anodic polarisation study revealed that cold working up to 20% enhanced the pitting resistance, and thereafter a sudden decrease in pitting resistance was noticed at 30% and 40% cold working. Increase in nitrogen content was beneficial up to 20% cold work in improving the pitting corrosion resistance, beyond which it had a detrimental effect at 30% and 40% cold working. The role of nitrogen in influencing the deformation band width and dislocation configuration is explained based on the results of transmission electron microscopy investigations. Scanning electron microscopy observation of the pitted specimens indicated decreasing size and increasing density of pits, along the deformation bands with increasing nitrogen for 40% cold worked specimens. The macrohardness values increased as the cold working increased from 0% to 40%. X-ray diffraction studies revealed the increased peak broadening of austenite peak {0 2 2} with increase in cold working. The relationship between pitting corrosion and deformation structures with respect to nitrogen addition and cold working is discussed.     

Effects of surface preparation on pitting resistance, residual stress, and stress corrosion cracking in austenitic stainless steels

Surface finishing treatments such as shot blasting and wire brushing can be beneficial in improving the integrity of machined surfaces of austenitic stainless steels. These operations optimize in-service properties such as resistance to pitting corrosion and stress corrosion cracking (SCC). In this study, ground steel surfaces were subjected to a series of sand blasting and wire brushing treatments. The surfaces were then characterized by their hardness, surface residual stress state, and resistance to stress corrosion and pitting corrosion. Some samples were selected for depth profiling of residual stress. It is found that surface hardening and the generation of near-surface compressive residual stress are the benefits that can be introduced by sand blasting and brushing operations.     

Investigations into the stress corrosion cracking of stainless steel alloyed with nitrogen

The resistance to stress corrosion of austenitic stainless steel alloyed with nitrogen (ASS N25) was determined in comparison with AISI 316L steel. The research into stress corrosion cracking was performed by using corrosion elongation curves in a 50% CaCl₂ solution, at 100°C, under axial tensile loadings defined as various percentages of Rp_0,2, and with anodic polarization at a current density of 1.0 mA/cm². The increased resistance of the nitrogen‐alloyed steel to stress corrosion is based on the occurrence of NH₄⁺ ions, and the repulsive action of the negatively charged nitrogen atoms towards the Cl⁻ ions on the interface between the passive layer and the corrosion medium. Additionally, the interstitially dissolved nitrogen is able to accelerate the local deformation hardening of the material at the crack tip, for which reason the critical stress needed for stress corrosion to occur, or for further growth of the crack if it has already occurred, is increased.     

In situ TEM study of stress corrosion cracking of austenitic stainless steel

A constant deflection device designed for use within a transmission electron microscope (TEM) was used to investigate the change in dislocation configuration ahead of a crack tip during stress corrosion cracking (SCC) of type 310 austenitic stainless steel in a boiling MgCl₂ solution, and the initiation of stress corrosion microcracking. Results showed that crack tip corrosion processes during SCC-enhanced dislocation emission, multiplication and motion. SCC microcracks initiated when the corrosion-enhanced dislocation emission and motion had fully develop.A passive film formed during corrosion of austenitic stainless steel in the boiling MgCl₂ solution generated a tensile stress. During SCC, the additive tensile stress generated at the metal/passive film interface assists the applied stress to enhance dislocation emission and motion.     

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.     

The inhibition of stress corrosion cracking of stainless steels in chloride solutions

The inhibition of s.c.c. n AISI 316S stainless steel in 2N HCl by aromatic and heterocyclic substances has been studied. The evaluation of the inhibitors in reducing the s.c.c. of U-bend specimens has been carried out by micrographic measurements.The determination of anodic and cathooic polarization curves shows that the inhibiting action on s.c.c. is related to the effect of the organic compounds on the anodic dissolution process of the metal. An interpretation of s.c.c. in the chloride solutions is hypothesized involving an adsorbed layer of chloride ions. Tests performed with scratched electrodes seem to confirm the proposed interpretation.     

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.     

Localised corrosion of highly alloyed stainless steels in an ammonium chloride and diethylamine chloride aqueous solution

This paper studies the pitting corrosion of highly alloyed stainless steels (SS) immersed in an ammonium chloride and diethylamine chloride aqueous solution at temperatures of 25, 40, 60 and 80°C. Four materials were tested: UNS S 31726 SS, UNS S 31254 SS, UNS S 32550 SS and titanium, for comparative purposes. Gravimetric, after immersion up to 103 days, polarisation curves, impedance, SEM, EDX and optical microscopy were the techniques used. UNS S 31726 SS shows the lowest corrosion resistance. UNS S 31254 and UNS S 32550 SSs show similar behaviour. Titanium presents impedance data three orders higher than the other SS materials. The SS materials present a pseudoinductive behaviour on impedance data, associated with an adsorption process of the amine compound which decreases as the temperature increases.