High Stress Corrosion Cracking Resistance Research Articles

In vitro evaluation of stress corrosion cracking susceptibility of PEO-coated rare-earth magnesium alloy WE43

Controlling material degradation in complex mechanobiological environments is the current challenge in advancing biodegradable magnesium implants for orthopaedic applications. Understanding stress corrosion cracking (SCC) mechanisms of magnesium alloy-coating systems in physiological fluids facilitates the design of magnesium implants with excellent biomedical performance. In this study, we investigated the SCC behaviour of a medical grade PEO-coated magnesium alloy WE43 under different loading conditions. Constant loadings tests (CLT) and slow strain rate tests (SSRT) were conducted to evaluate the SCC susceptibility of this alloy-coating system under in vitro conditions. Fracture surfaces of the non-coated and PEO-coated specimens were characterised by scanning electron microscopy to reveal the SCC mechanisms under different loading configurations. The experimental results showed that the alloy-coating system exhibits a high SCC resistance in SSRT conditions. However, high stress levels and plastic deformations lead to a significant acceleration of the SCC process. Moreover, the fracture surface analysis demonstrated that the brittle nature of the PEO coating deteriorates the mechanical integrity of the alloy under critical mechanical loadings, which results in an increased susceptibility towards stress corrosion cracking of the coated WE43.

Determining SCC resistance of stainless steel claddings in high- temperature water by constant load crack growth tests and slow strain rate tests

Stress corrosion cracking (SCC) behaviors of 309 L and 308 L weld overlays are characterized in oxygen-free and oxygen-bearing high-temperature water using slow strain rate tests (SSRTs) and crack growth rate (CGR) tests with compact tension specimens. There is no apparent SCC indication on 308 L, while local SCC is observed on 309 L after SSRT in oxygen-free water. The SCC sensitivities of 308 L and 309 L increase with the increase of dissolved oxygen (DO) content, while 308 L shows relatively higher SCC resistance than 309 L in water with 200 ppb and 8000 ppb DO. SSRT and CGR test results show a similar effect of DO concentration on SCC of 309 L and 308 L. The fracture surface of 309 L exhibits extensive intergranular SCC, while 308 L shows only local interdendritic SCC after CGR tests. The crack tip blunting decreased the CGR in the low alloy steel. The highly island-shaped δ-ferrites and moderate strain hardening in 308 L contribute to its SCC resistance over 309 L.

Effect of thermal aging on the environmental assisted cracking behavior of 308L weld metal in high temperature water

The effect of thermal aging on environmental assisted cracking (EAC) of 308 L weld metal was investigated. Experimental results reveal that thermal aging promotes corrosion fatigue (CF) crack growth with extensive inter-dendritic cracking, but has a limited effect on the stress corrosion cracking (SCC) growth under constant loads. The higher crack tip strain rate and the enhanced preferential oxidation of deteriorated δ-ferrite are responsible for the enhanced CF susceptibility after thermal aging. The dispersed δ-ferrites and the decreased crack tip strain rate account for the high SCC resistance under constant load regardless of the thermal aging.

High stress corrosion cracking resistance of in-situ nanoparticle strengthened steel

Stress corrosion cracking (SCC) is extremely detrimental to the long-term applications of high-strength low-alloy steels. In this work, the critical effects of the dimension and morphology of the non-metallic inclusions on the SCC susceptibility and pitting corrosion were investigated. With the trace-element regional supply method, the inclusions are transformed from the detrimental sharp-edged (Ti, V)N and enormous MnS into globular or elliptical Al2O3 and multicomponent inclusions with small dimensions (3.41 ± 2.36 μm) and high equivalent shape factor (0.93 ± 0.02) in the in-situ nanoparticles strengthened steel. As a result, the corrosion current, depth and diameter of pits, and roughness of the surface were reduced with suppression of pitting corrosion. The superior SCC resistance in the in-situ nanoparticles strengthened steel was obtained by inclusion refinement. The strategy of inclusion refinement by in-situ nanoparticles is beneficial for designing SCC resistant steels.

Effect of Organizational Evolution on the Stress Corrosion Cracking of the Cr-Co-Ni-Mo Series of Ultra-High Strength Stainless Steel.

Different microstructures were obtained under various thermal conditions by adjusting the heat treatment parameters of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel. The effect of organizational evolution on the stress corrosion cracking (SCC) of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel was investigated using potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and other test methods in combination with slow strain rate tensile tests (SSRTs). The results show that the Mo- and Cr-rich clusters and precipitation of the Laves phase reduce the corrosion resistance, while increasing the austenite content can improve the corrosion resistance. The Cr-Co-Ni-Mo series of ultra-high strength stainless steel has a high SCC resistance after quenching at 1080 °C and undergoing deep cooling (DC) treatment at −73 °C. With increasing holding time, the strength of the underaged and peak-aged specimens increases, but the passivation and SCC resistance decreases. At the overaged temperature, the specimen has good SCC resistance after a short holding time, which is attributed to its higher austenite content and lower dislocation density. As a stable hydrogen trap in steel, austenite effectively improves the SCC resistance of steel. However, under the coupled action of hydrogen and stress, martensitic transformation occurs due to the decrease in the lamination energy of austenite, and the weak martensitic interface becomes the preferred location for crack initiation and propagation.

Correlating oxidation resistance to stress corrosion cracking of 309L and 308L stainless steel claddings in simulated PWR primary water

Stress corrosion cracking (SCC) growth behaviors of 309L and 308L stainless steel claddings in simulated pressurized water reactor primary water at 325 °C and 350 °C were investigated by compact tension specimens and correlated to the oxide film properties in terms of microstructure. SCC growth rates of 308L were much lower than those of 309L. 309L showed typical intergranular SCC characteristics. The high SCC resistance of 308L was related to the low oxidation rate and crack tip retardation due to the presence of highly networked ferrites. The lower Cr content, as well as less and sparsely distributed δ-ferrite phases in 309L than in 308L, led to more severe local oxidation penetration beneath the metal/inner oxide interface and more porous inner oxide, therefore, contributed to its enhanced SCC growth in 309L. The SCC growth rate results by compact tension specimens are consistent with those by slow strain rate tests.

Grain boundary character and stress corrosion cracking behavior of Co-Cr alloy fabricated by selective laser melting

In this work, we used the selective laser melting (SLM) fabricated Co-Cr alloy with prominent residual strain, extremely non-equilibrium microstructures, and low stacking fault energy as a precursor to fabricate materials with the optimal grain boundary character distribution. The grain boundary engineering (GBE) of the Co-Cr alloy was achieved by a simple heat treatment of the SLM-fabricated Co-Cr alloy. The obtained GBE Co-Cr alloy exhibited 81.47% of special grain boundaries (Σ3n n = 1, 2, 3)), while it substantially disrupted the connectivity of the random high-angle boundaries, successfully reducing the propensity of intergranular degradation. Slow strain rate tests (SSRTs) showed that the GBE Co-Cr alloy possessed lower stress corrosion cracking (SCC) susceptibility and higher ductility in the corrosive environment (0.9% NaCl solution) than in the air. The high fraction of special boundaries, coupled with the stress-induced martensitic transformation (SIMT) in the GBE Co-Cr alloy yielded these results, which unique and rarely simultaneously satisfied for common structural materials. The current “SLM induced GBE strategy” offers a novel approach towards customized GBE materials with high SCC resistance and ductility in the corrosive environment, shedding new light on developing high-performance structural materials.

Reversion and re-aging of a peak aged Al-Zn-Mg-Cu alloy

High-strength Al-Zn-Mg-Cu alloys are highly susceptible to stress corrosion cracking (SCC) which severely limits their lifetime. Reversion and re-aging (RRA) temper provides a higher SCC resistance at no loss in strength, yet the microstructural origins of these enhanced properties remain elusive. In an Al-Zn-Mg-Cu alloy, we show that the fine precipitate dispersion in the grain interiors is similar in the peak aged and RRA tempers. However, upon RRA, precipitates inside the grains are enriched in Cu, lowering the Cu matrix content, and reducing the relative difference in the Cu precipitate composition between bulk and grain boundaries. This study enriches the current understanding on the critical role of Cu related to SCC resistance in Al-Zn-Mg-Cu alloys.

Stress-Corrosion Cracking Behaviour of Lean-Duplex Stainless Steels in Chloride/Thiosulphate Environments

The stress-corrosion cracking (SCC) behaviour of two lean-duplex stainless steels (DSS 2304 and LDSS 2404) was studied by slow strain-rate tests (SSRT) in 20% NaCl solution at 80 °C (pH about 6) and in NACE TM-0177 solution at 25 °C (pH 2.7), both in the absence and in the presence of thiosulphate ions (S2O32−). The SCC susceptibility of the two alloys was compared to that of LDSS 2101 investigated in a previous study. LDSS 2404 was always immune to SCC, while DSS 2304 (and LDSS 2101) suffered this corrosion form at specific concentrations. The high SCC resistance of DSS 2404 in both environments was connected to its high Mo content, while the significant SCC susceptibility of LDSS 2101 in NACE TM-0177 solution was likely due to the high Mn content of the alloy.

Two-stage double peaks ageing and its effect on stress corrosion cracking susceptibility of Al-Zn-Mg alloy

Different artificial two-stage ageing behaviors and their effect on stress corrosion cracking (SCC) susceptibility of Al-Zn-Mg alloy have been investigated. The experimental results show that two hardness peaks present on the second-stage ageing-hardening curve when the first-stage ageing is dealt with comparatively lower temperature than the conventional one. The first peak is caused by dispersive and evenly distributed G.P. zones, while η′ phases and coarsened G.P. zones contribute to the second peak. Tensile strength of experimental alloy raises 9.6% (33.2MPa) and SCC susceptibility decreases 38.9% by applying the second peak ageing regime instead of conventional T73. Al-Zn-Mg alloy obtains high strength and SCC resistance due to its finely dispersive matrix precipitates (MPts), coarsened and discontinuous grain boundary precipitates (GBPs), as well as the narrow precipitate free zone (PFZ) in the second peak ageing condition.

Stress corrosion cracking behaviour of 316L stainless steel exposed to CO2–H2S–Cl− environments in the absence and presence of methyldiethanolamine (MDEA)

Stress corrosion cracking (SCC) behaviour of 316L stainless steel in CO2–H2S–Cl− environments with and without methyldiethanolamine (MDEA) was investigated by slow strain rate testing and scanning electron microscopy (SEM). The results show that elongation ratio, reduction in area ratio (RAR) and time to failure ratio (TTFR) of 316L stainless steel were low in CO2–H2S–Cl− environments. The corresponding fractography exhibited flat brittle fracture with quasi-cleavage pattern, indicative of high SCC susceptibility. Hydrogen penetration and corrosion pits could be responsible for the high SCC susceptibility of 316L stainless steel in this condition. For the CO2–H2S–Cl− environments in the presence of MDEA, 316L stainless steel possessed high ER, RAR and TTFR (nearly 100%). High SCC resistance of 316L stainless steel could be associated with MDEA induced removal of H2S/CO2 and absorption on the steel surface.

Effect of Post-forging Heat Treatment on Stress Corrosion Cracking of Nuclear Grade 316LN Stainless Steel in Boiling MgCl2 Solution

s: Effect of post solutionand stress relieftreatment on the stress corrosion cracking (SCC) resistance via U-bend specimens test in 42% boiling MgCl2 solution, as well as the microstructure, residual strain and mechanical properties of the forged 316LN stainless steel was studied. Results showed that the yield stress was reduced and the residual strain was eliminated through post solution-treatment for the forged steel. After immersion in boiling MgCl2 solution for 24, 48 and 72 h, respectively, all the U-bend specimens of either the solution-treated or the stress relieftreated steels suffered from clearly transgranular stress corrosion cracking (TGSCC). Furthermore, of which all the stress relief-treated specimens were entirly cracked, while the solution-treated specimens were only locally cracked after immersion for 72 h, suggesting higher SCC resistance for the forged steel after a proper post solution-treatment. Finally, the mechanism of the effect of post-heat 定稿日期:2014-11-05 基金项目:国家科技重大专项项目 (2011ZX06004-009)资助 作者简介:郭跃岭,男,1990年生,硕士生 通讯作者:韩恩厚,E-mail:ehhan@imr.ac.cn,研究方向为苛刻环境中材料的腐蚀与防护 (核电高温高压水、石油化工环境等等) DOI:10.11902/1005.4537.2014.223 6期 郭跃岭等:锻后热处理对核级316LN不锈钢在沸腾MgCl2溶液中应力腐蚀行为的影响 treatments on the SCC resistance was discussed in terms of the residual strain and yield stress of the forged steel.

Environmentally Assisted Cracking of Alloy 7050-T7451 Exposed to Aqueous Chloride Solutions

The stress corrosion cracking (SCC) behavior of 7050-T7451 plate material was investigated in short-transverse direction performing constant load and constant extension rate tests. Smooth and notched tensile specimens were permanently immersed in substitute ocean water and in an aqueous solution of 0.6 M NaCl + 0.06 M (NH4)2SO4. Alloy 7050-T7451 exhibited high SCC resistance in both synthetic environments. However, conducting cyclic loading tests, environment-induced cracking was observed. Applying a sawtooth waveform, notched tensile specimens were strained under constant load amplitude conditions at constant displacement rates ranging from 2 × 10−6 to 2 × 10−4 mms−1. The stress ratio R = σ min/σ max was 0.1 with maximum stresses of 300 and 400 MPa. When cyclically loaded in substitute ocean water, notched specimens failed predominantly by transgranular environment-induced cracking. Striations were observed on the cleavage-like facets. The number of cycles-to-failure decreased with decreasing displacement rate. A slope of 0.5 was obtained by fitting the logarithmic plot of number of cycles-to-failure vs nominal loading frequency, indicating a hydrogen embrittlement mechanism controlled by diffusion.

Improvement of Stress Corrosion Cracking (SCC) Resistance of a 7150 Al-Zn-Mg-Cu Alloy by Retrogression and Reageing (RRA) Treatment

A 7150 Al-Zn-Mg-Cu alloy is artificially aged at 120 oC for varying time. The peak hardness (T6 temper) is obtained at about 24 h at that temperature. Further, the T6 temper is subjected to retrogression and reageing (RRA) treatment. Slow strain rate test (SSRT) has been carried out on the T6 and RRA tempers. SSRT results indicated that the RRA temper have higher resistance to SCC compared to that of T6 temper. SCC behaviour of the alloy tempers have been explained with the help of microstructural features studied by transmission electron microscope (TEM). The large, discrete and discontinuous grain boundary precipitates observed in the microstructure of the RRA temper are believed to be the responsible factor for achieving higher SCC resistance. Further, SEM fractographs and crack morphology have also been analyzed to evaluate the SCC behaviour of the alloy tempers.

Effect of laser shock processing on stress corrosion cracking behaviour of AZ31 magnesium alloy at slow strain rate

An investigation was performed to evaluate the effect of laser shock processing (LSP) on AZ31 magnesium alloy using the slow strain rate tensile test. Microstructure revealed that a fine grained structure was obtained by LSP. X-ray diffraction showed that high compressive residual stress occurred at the surface after LSP. Meanwhile, the tensile strength of the LSP samples was improved. The stress corrosion cracking behaviour was examined, including both the base material and the LSP samples in air and 3·5%NaCl solution respectively. Test results demonstrated that the AZ31 Mg alloy after LSP exhibits higher stress corrosion cracking resistance than the base material.

Hydrogen embrittlement susceptibility of over aged 7010 Al-alloy

Slow strain rate testing (SSRT) was carried out on over aged 7010 Al-alloy in laboratory air, glycerin and 3.5 wt.% NaCl solution with and without cathodic charging to study the hydrogen embrittlement susceptibility of the alloy in over aged condition. It was found that the over aged alloy exhibited high resistance to stress corrosion cracking (SCC) than hydrogen embrittlement (HE). The high SCC resistance is due to the modification in the grain boundary precipitate morphology and chemistry due to over aging, however it is suggested that the dislocations in the alloy are not completed annealed during over aging to arrest HE.

Stress corrosion cracking and hydrogen embrittlement of an Al–Zn–Mg–Cu alloy

The age hardening, stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of an Al–Zn–Mg–Cu 7175 alloy were investigated experimentally. There were two peak-aged states during ageing. For ageing at 413 K, the strength of the second peak-aged state was slightly higher than that of the first one, whereas the SCC susceptibility was lower, indicating that it is possible to heat treat 7175 to high strength and simultaneously to have high SCC resistance. The SCC susceptibility increased with increasing Mg segregation at the grain boundaries. Hydrogen embrittlement (HE) increased with increased hydrogen charging and decreased with increasing ageing time for the same hydrogen charging conditions. Computer simulations were carried out of (a) the Mg grain boundary segregation using the embedded atom method and (b) the effect of Mg and H segregation on the grain boundary strength using a quasi-chemical approach. The simulations showed that (a) Mg grain boundary segregation in Al–Zn–Mg–Cu alloys is spontaneous, (b) Mg segregation decreases the grain boundary strength, and (c) H embrittles the grain boundary more seriously than does Mg. Therefore, the SCC mechanism of Al–Zn–Mg–Cu alloys is attributed to the combination of HE and Mg segregation induced grain boundary embrittlement.

Stress corrosion cracking susceptibility of Al–Mg alloy sheet with high Mg content

Slow strain rate testing (SSRT) was used to study the effect of the microstructure on the stress corrosion cracking (SCC) susceptibility of Al–Mg alloy sheet containing 6.8 wt.% Mg. In the cold-rolled and fully annealed conditions, high SCC susceptibility was experienced. In those cases the ductility was strongly affected by the presence of corrosive environment (for hard temper: El air=13.6%, El SCC=0.6%; for annealed condition: El air=24.1–25.3%, El SCC=3.2–4.2%) and the elongation loss was great, El loss=81.7–95.6%. It is supposed that the high SCC susceptibility results from a continuous network of the β-phase (Mg 5Al 8) precipitate at grain boundaries for the annealed temper, and heavy precipitation of β-phase along the planes of localized deformation for the hard temper. High SCC resistance attained after thermal exposure at the temperature range 225–285 °C (stabilized condition). The ductility was almost unaffected by the presence of corrosive environment (El air=12.8–23.2%, El SCC=12.8–22%) and the elongation loss was small, El loss<7%. High SCC resistance was related to the stabilized structure, which causes discontinuous β-phase (Mg 5Al 8) precipitation in a globular form, uniformly distributed throughout the structure.

Effects of Step-Quench and Aging on Mechanical Properties and Resistance to Stress Corrosion Cracking of 7050 Aluminum Alloy

The influence of microstructural characteristics on the mechanical properties and resistance to stress corrosion cracking (SCC) of the 7050 aluminum alloy was investigated. The higher fraction of recrystallization would cause a larger amount of incoherent Al3Zr dispersoids and high-angle grain boundaries, which increases quench sensitivity and SCC susceptibility of the alloy. Therefore, both strength and SCC resistance of the cold-rolled alloy are low. On the other hand, the hot-rolled alloy has the highest strength and superior SCC resistance, as a result of the lowest fraction of recrystallization. The SCC resistance can be significantly enhanced owing to the fine grain structure obtained with the intermediate thermomechanical treatment (ITMT); however, the strength cannot be effectively improved. The proposed treatment, step-quench and aging (SQA), can significantly improve SCC resistance and attain optimum strength by controlling the grain boundary and matrix microstructures. Higher SCC resistance and strength together with much reduced aging time are obtained simultaneously with the proposed SQA(220°C/10 s or 200°C/30 s) after hot-rolling, as compared to those with the conventional T73 treatment.

Microalloying of Sc, Ni, and Ce in an advanced Al-Zn-Mg-Cu alloy

Using transmission electron microscopy (TEM), scanning electron microscopy, X-ray diffraction (XRD), and optical microscopy, the effects of microalloying elements of Sc, Ni, and Ce on the microstructure of a new super-high-strength ingot metallurgy (IM)/Al-Zn-Mg-Cu alloy (C912) have been correlated with mechanical properties and stress corrosion cracking (SCC) behavior. Using microalloying with Sc, Ni, and Ce, the C912 alloy can exhibit very high strength and good SCC resistance. Compared to the baseline C912 alloy, Sc refines the microstructure and retards recrystallization, Ni promotes the development of matrix precipitates, which enhance the strength and SCC resistance, and Ce has little effect on alloy strengthening in the three microalloying additions studied. The Sc-containing alloy (C912S) is the most attractive and even exhibits higher strength (ultimate tensile strength (UTS) greather than 660MPa) than the new Alcoa aluminum alloy 7055 and the Russian alloy B96, which have the highest strengths of the commercial IM/Al-Zn-Mg-Cu alloys.