Showing papers on "Stress corrosion cracking published in 2013"

Current understanding of radiation-induced degradation in light water reactor structural materials

Abstract: Current phenomenological knowledge and understanding of mechanisms are reviewed for radiation embrittlement of reactor pressure vessel low alloy steels and irradiation assisted stress corrosion cracking of core internals of stainless steels. Accumulated test data of irradiated materials in light water reactors and microscopic analyses by using state-of-the-art techniques such as a three-dimensional atom probe and electron backscatter diffraction have significantly increased knowledge about microstructural features. Characteristics of solute clusters and deformation microstructures and their contributions to macroscopic material property changes have been clarified to a large extent, which provide keys to understand in the degradation mechanisms. However, there are still fundamental research issues that merit study for long-term operation of reactors that requires reliable quantitative prediction of radiation-induced degradation of component materials in low-dose rate high-dose conditions.

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Abstract: Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (Ea) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys ( ~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.

Effect of electro discharge machining (EDM) on the AISI316L SS white layer microstructure and corrosion resistance

Abstract: The localised corrosion resistance of austenitic stainless steels is strongly influenced by the quality of finished surface. EDM machining induces substantial changes by the high thermal gradients generated by electric sparks. Experimental techniques such as roughness measurement, scanning electron microscopy (SEM), energy dispersive microanalysis (EDX) and X-ray diffraction technique, reveal micro-geometrical, microstructural, chemical and mechanical changes. These changes lead to white and heat-affected layers with a depth less than 100 μm. The white layer is a melted material characterised by dendritic structure and constituted by austenite, chromium carbide and e-carbide. The heat-affected layer is characterised by very large grain size comparatively to the bulk material. Electrochemical test coupled with metallographic examinations using SEM reveals a weakening of the resistance to pitting and intergranular corrosion comparatively to diamond polished surface. This weakening is correlated to differences in structure and chemical composition of white layer. Susceptibility to stress corrosion cracking has been attributed to the field of tensile residual stresses resulting from thermal effects. The removal of the white layer material by polishing or wire brushing restores the corrosion resistance of the AISI316L SS.

Hydrogen Embrittlement of Magnesium and Magnesium Alloys: A Review

Abstract: Pure magnesium is inherently susceptible to stress corrosion cracking (SCC) 1–3 and many of its alloys suffer SCC in environments considered innocuous for most other engineering alloys, e.g. distilled water. 4,5 In order to prevent SCC, some authors suggest that the applied stress has to be kept below 50% of the yield strength (YS), 6,7 reducing the attractiveness of Mg alloys for structural applications. Pure magnesium 1–3,8 and magnesium-aluminum alloys 5,9–21 have received special attention in the literature. The susceptibility of magnesium alloys to SCC increases with increasing aluminum content, 17 a trend that is opposite to the beneficial effect of Al in stress-free corrosion rates. Precipitation of β phase Mg17Al12 occurs in alloys with more than 2.1 wt% Al. 16 Since Mg17Al12 is nobler than the Mg-Al matrix, 11,22 precipitation of Mg17Al12 phase along grain boundaries 11,16,20 is thought to promote intergranular SCC (IGSCC) caused by preferential galvanic dissolution of the surrounding matrix. 16 The effect of second phase particles is discussed in detail in a separate section. Extremely fast crack growth rates (i.e. in the order of 10 −5 m/s) have been often reported for pure Mg 2 and Mg-Al alloys. 14,23 If cracking was entirely controlled by faradaic anodic dissolution, such crack growth rates would require anodic current densities in the order of 50 A/cm 2 . 14 Therefore, most SCC models include some contribution

Stress corrosion cracking of high-strength steels

Abstract: The mechanisms of stress corrosion cracking (SCC) and hydrogen embrittlement were recently reviewed by Lynch in this journal. The present review, in contrast, focuses on the rate-limiting step of the SCC of low-alloy high-strength steels in water and particularly focuses on the influence of the applied stress rate on the SCC of lowalloy high-strength steels. Linearly increasing stress tests of low-alloy high-strength steels in distilled water indicated that the stress corrosion crack velocity increased with increasing applied stress rate until the maximum crack velocity, corresponding to v in fracture mechanics tests in distilled water. Moreover, the crack velocity was dependent only on the applied stress rate and was not influenced by the steel composition. The rate-limiting step could be the rupture of a surface film, which would control the rate of metal dissolution and/or the production and transport of hydrogen to the crack tip or to the regions ahead of the crack tip.

Stress corrosion cracking susceptibility of ultrafine grained Al–Mg–Sc alloy

Abstract: General corrosion and stress corrosion cracking (SCC) behavior of coarse grained, fine grained and ultrafine grained (UFG) AA5083 and Al–Mg–Sc–Zr alloy in different thermomechanical conditions were studied in the present work. Friction stir processing (FSP) was carried out to refine the grain size. The average grain size achieved after FSP for AA5083 was 7±3 μm, whereas that for Al–4Mg–0.8Sc–0.08Zr was 0.39±0.16 μm with 100% UFG microstructure. Linear polarization resistance and cyclic polarization techniques were used to study general corrosion behavior of these alloys in 3.5 wt% NaCl solution. The UFG microstructure showed the highest polarization resistance ( R p ) of ∼93 kΩ which increased to ∼160 kΩ after 24 h of exposure in chloride solution. The peak aged Al–Mg–0.8Sc–0.08Zr showed the most positive breakdown potential with passivity in the range of −800 to −600 mV vs SCE. A power law type relationship was observed between grain size and R p for Al–Mg alloys. SCC susceptibility was estimated in 3.5 wt% NaCl solution using slow strain rate testing (SSRT) at initial strain rate of 10 −6 s −1 .The parent AA5083 alloy showed moderate susceptibility while the FSP AA5083 microstructure showed no susceptibility. The UFG Al–Mg–0.8Sc–0.08Zr condition showed >55% loss in ductility when tested in chloride solution.

Oxidation of austenitic and ferritic/martensitic alloys in supercritical water

Abstract: Supercritical Water Reactors (SCWRs), one of the concepts considered by the Generation IV International Forum, are an attractive option due to their high thermal efficiency, 45% vs. 33% for current Light Water Reactors (LWR) and their more simple design. The reference design for the European SCWR is a direct cycle system operating at 25 MPa with core inlet temperatures of 280 and average core outlet temperature of 500 °C. In this range of temperatures, shifting from subcritical to supercritical conditions, there is a sharp change in water density as well as in chemical properties, such as dielectric constant and ionic product. These changes in properties could influence the behavior of austenitic alloys and other materials to oxidation and stress corrosion cracking, providing unexpected responses in the light of the available knowledge for LWRs. Oxide layer characteristics are relevant to both stress corrosion cracking and corrosion product transport.Oxidation experiments on austenitic alloys, 316 L SS, Alloy 600 and Alloy 625, and F/M T91 were performed at two temperatures, 400 °C and 500 °C, and 25 and 30 MPa in order to explore the influence of the chemical properties of supercritical water on the oxide layers characteristics. Results from the characterization of the oxide layers by optical, SEM/EDX and Auger spectroscopy as well as the oxidation kinetics are discussed.

TEM investigations of the oxide layers formed on a 316L alloy in simulated PWR environment

Abstract: Atomic scale observations of the oxide formed on stainless steels, under simulated nuclear reactor conditions, are performed to estimate the oxide layer contribution on stress corrosion cracking (SCC) mechanisms. A duplex oxide composed of a chromium enriched inner layer (Fe1.5Cr1.5O4) and an outer layer composed of magnetite crystallites (Fe3O4) is found. The oxide layer structure evolves from amorphous, for oxidation times of 1 min, to nano-crystalline at 2 min and mono-crystalline after 5 h. IFFT images, calculated from Cs-corrected HRTEM images recorded on grains oriented in the 〈111〉 direction, highlight a double network of dislocations with ½ 〈10-1〉 and ½ 〈−110〉 Burgers vectors. This network leads to the decrease in non-relaxed deformation and favors an epitaxial growth between steel and oxide. Both crystal structure transformations and epitaxial relations between metal and oxide have provided relevant information which contributed to progress on SCC modeling.

Linking Grain Boundary Microstructure to Stress Corrosion Cracking of Cold-Rolled Alloy 690 in Pressurized Water Reactor Primary Water

Abstract: Grain boundary microstructures and microchemistries are examined in cold-rolled Alloy 690 (UNS N06690) materials and comparisons are made to intergranular stress corrosion cracking (IGSCC) behavior in pressurized water reactor (PWR) primary water. Chromium carbide precipitation is found to be a key aspect for materials in both the mill-annealed and thermally treated conditions. Cold rolling to high levels of reduction was discovered to produce small IG voids and cracked carbides in alloys with a high density of grain boundary carbides. The degree of permanent grain boundary damage from cold rolling was found to depend directly on the initial IG carbide distribution. For the same degree of cold rolling, alloys with few IG precipitates exhibited much less permanent damage. Although this difference in grain boundary damage appears to correlate with measured SCC growth rates, crack tip examinations reveal that cracked carbides appeared to blunt propagation of IGSCC cracks in many cases. Preliminary results su...

Microstructure effect on hydrogen-induced cracking in TM210 maraging steel

Abstract: Hydrogen embrittlement (HE) of TM210 maraging steel was studied by slow strain rate tensile and constant load tests. The over-aged sample exhibited the best resistance to HE, since HE susceptibility of the maraging steel does not depend on the strength, but rather on the reverted austenite content. The hydrogen concentration, observed by scanning Kelvin probe force microscopy, was enriched in the reverted austenite at the grain boundaries and martensite lath boundaries, resulting in hydrogen-induced cracks propagating along the grain boundaries and martensite lath boundaries.

Study of the SCC Behavior of 7075 Aluminum Alloy After One-Step Aging at 163 °C

Abstract: For the past many years, 7075 aluminum alloys have been widely used especially in those applications for which high mechanical performances are required. It is well known that the alloy in the T6 condition is characterized by the highest ultimate and yield strengths, but, at the same time, by poor stress corrosion cracking (SCC) resistance. For this reason, in the aeronautic applications, new heat treatments have been introduced to produce T7X conditions, which are characterized by lower mechanical strength, but very good SCC behavior, when compared with the T6 condition. The aim of this study is to study the tensile properties and the SCC behavior of 7075 thick plates when submitted to a single-step aging by varying the aging times. The tests were carried out according to the standards and the data obtained from the SCC tests were analyzed quantitatively using an image analysis software. The results show that, when compared with the T7X conditions, the single-step aging performed in the laboratory can produce acceptable tensile and SCC properties.