Double loop electrochemical potentiokinetic reactivation test optimization in checking of duplex stainless steel intergranular corrosion susceptibility

The double loop electrochemical potentiokinetic reactivation (DL-EPR) test using an electrolyte of 33 pct H₂SO₄ solution with 0.3 pct HCl, at room temperature and at a potential scan rate dE/dt of about 2.5 mV/s, was chosen to evaluate the sensitization of austeno-ferritic duplex stainless steels (DSS). Reproducible and optimal test responses and high test selectivity in detecting integranular corrosion (IGC) susceptibility were verified for four DSS differing in their method of fabrication (cast or wrought) and their ferrite phase content (44 to 57 pct). The test was successfully used to analyze the interactions between precipitation, chromium depletion, and IGC sensitization of the UNS S31250 steel, which was aged between 6 minutes and 120 hours at temperatures varying from 500 °C to 900 °C. The eutectoid decomposition of the ferrite, at different aging temperatures, was investigated using various techniques. The chromium depletion was analyzed qualitatively by X-ray mapping in a scanning transmission electronic microscope (STEM) and quantitatively by analytical calculation based on the chromium diffusion in the ferrite. It was shown that the chromium content in the ferrite can decrease from 30 to 7.5 pct by weight during aging before total decomposition occurs. The interactions between precipitation and IGC sensitization during DSS aging were clearly shown by superimposing the time-temperature-start of precipitation (TTP) and time-temperature-sensitization (TTS) diagrams obtained from the DL-EPR tests performed for various levels of sensitization.     

Chromium depletion in the vicinity of carbides in sensitized austenitic stainless steels

The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M₂₃C₆ carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M₂₃C₆ carbides.     

Deterioration of electromotive force of chromel-alumel thermocouples in reducing atmospheres at high temperatures

The deterioration of electromotive force (emf) of Chromel-Alumel (CA) thermocouples in 80 pct H₂ + 15 pct CO + 5 pct CO₂ has been analyzed in terms of the corrosion behavior of Chromel. Emf of the CA thermocouple deteriorated drastically in 80 pct H₂ + 15 pct CO + 5 pct CO₂. After exposure for about 1000 hours at 900 °C, the decrease of emf was about 16 mV. The deterioration process could be separated into three terms. The first term, which has the smallest time constant of about 20 hours, was attributed to carbon deposition on the Chromel surface in the temperature range of 600 to 700 °C. The second term, which has a time constant of about 100 hours, was attributed to the severe internal oxidation of chromium in the temperature range of 500 to 800 °C. The third term, having the largest time constant of several thousand hours, might be attributed to the moderate and gradual preferential oxidation of chromium in Chromel in the range 800 to 900 °C. Boron nitride (BN) coating on CA thermocouples could reduce this deterioration of emf; the decrease of emf was improved to about 3 °C during 700 hours test at 900 °C.     

Effect of minor alloying element variation on the properties of alloy 800

Application of Alloy 800 in steam generator tubing of fast reactors, where continuous service temperature of the order of 550° is experienced, has been analyzed with respect to small variations in its chemical composition. Several laboratory melts of Alloy 800 have been prepared and their microstructural and mechanical property changes during simple aging and creep tests at 500 to 600° have been studied. It has been found that in the above temperature range precipitation of M₂₃C₆on the grain boundaries is independent of the Ti : C ratio generally specified for Alloy 800. Gamma prime precipitation occurred in alloys containing as low as 0.5 pct Ti+ Al after 1000 h of aging and was accompanied with a creep ductility decline. Upon γ precipitation creep rate was retarded and its reacceleration for test times up to 8500 h at 550° was not observed. Based on the findings, increased Ti concentration at the expense of Al within the specified chemical composition range with carbon content of 0.030 to 0.050 has been suggested. A. A. Tavassoli is on sabbatical leave from the Arya-Mehr University of Technology, Tehran, Iran.     

The influence of prior ageing on creep damage development in two variants of Alloy 800

The influence of high temperature thermal ageing treatments on the development of intercrystalline creep damage in two variants of Alloy 800 has been investigated. Ageing up to 3000 h and creep testing were carried out at 800 and 900°C. The high temperature behaviour of the 800HT variant is discussed with reference to the effect of heat treatments on the microstructure. The metallographic methods by which the creep damage was quantitatively determined are described. The growth rate of intercrystalline microcracks was described using a statistical model and the dependence of crack growth rate on the thermal history for both 800HT and 800H was determined. The carbide precipitation and growth processes were determined as functions of the exposure temperature and duration. The results showed the three characteristic stages, precipitation, growth and coarsening (Ostwald ripening). The largest increase in the intergranular creep damage was found in Alloy 800HT within the first 1000 h, in which the precipitation of chromium carbides on the grain boundaries is substantially completed. The comparison between the two variants showed significant differences in the damage development. 800HT exhibited at 800 and 900°C higher creep crack growth rates with increasing exposure duration, but in 800H this effect was not clearly identified.     

Sensitization of 12 Wt Pct chromium,titanium-stabilized ferritic stainless steel

The sensitization behaviors of 19 heats of titanium stabilized, 12 Wt Pct chromium, ferritic stainless steel were evaluated by measuring the intergranular corrosion resistance of samples given a two-step heat treatment. The latter consisted of a one-hour high temperature exposure followed by a water quench and one of 19 different low temperature treatments. The results indicate that heat treating at temperatures ≳ 1000 °C produces a microstructure which can be sensitized by subsequent aging at temperatures ≲ 600 °C. The amount of carbon and not the amount of nitrogen nor the amount of carbon plus nitrogen dictated the sensitization resistance of the titanium-stabilized, 12 Wt Pct chromium alloy. The proposed mechanism of sensitization in these alloys suggests that during the high temperature exposure, the titanium carbonitrides decompose, freeing carbon or carbon and nitrogen into interstitial solid solution. During subsequent heat treating at temperatures ≲600 °C, chromium-containing carbides precipitate intergranularly, and a chromium-depleted zone is formed along the grain boundaries. The presence of the chromium-depleted zone results in susceptibility to intergranular corrosion, α′ precipitates form after the intergranular precipitates during heat treating at 600 °C. The presence of α′ enhances the intergranular corrosion rate.     

Influence of deformation on the structure and mechanical and corrosion properties of high-nitrogen austenitic 07Kh16AG13M3 steel

The correlation has been studied between the structure of a high-nitrogen austenitic Cr-Mn-N steel formed in the process of combined hardening treatment, including cold plastic deformation (CPD), and its mechanical and corrosion properties. The structure and properties of commercial high-nitrogen (0.8% N) 07Kh16AG13M3 steel is analyzed after rolling by CPD and aging at 500 and 800°C. It is shown that CPD of the steel occurs by dislocation slip and deformation twinning. Deformation twinning and also high resistance of austenite to martensitic transformations at true strains of 0.2 and 0.4 determine the high plasticity of the steel. The contribution of the structure imperfection parameters to the broadening of the austenite lines during CPD is estimated by X-ray diffraction. The main hardening factor is stated to be lattice microdistortions. Transmission electron microscopy study shows that heating of the deformed steel to 500°C leads to the formation of the intermediate CrN phase by a homogeneous mechanism, and the intermtallic χ phase forms along the austenite grain boundaries in the case of heating at 800°C. After hardening by all investigated technological schemes, exception for aging at 800°C, the steel does not undergo pitting corrosion and is slightly prone to a stress corrosion cracking during static bending tests, while aging at 800°C causes pitting corrosion at a pitting formation potential E _pf = ?0.25 V.     

Influence of halide ions and alloy microstructure on the passive and localized corrosion behavior of alloy 22

Alloy 22 (N06022) is the current candidate alloy used to fabricate the external wall of the high-level nuclear waste containers for the Yucca Mountain repository. It was of interest to study and compare the general and localized corrosion susceptibility of Alloy 22 in fluoride and chloride solutions at 90 °C. Standard electrochemical tests such as cyclic potentiodynamic polarization, amperometry, and electrochemical impedance spectroscopy were used. Studied variables included the solution pH and the alloy microstructure (thermal aging). Results show that Alloy 22 is highly resistant to general corrosion in all the solutions tested. Thermal aging is not detrimental and even seems to be slightly beneficial for general corrosion at the higher solution pHs. Pitting corrosion was never observed. Crevice corrosion was found only for high chloride-containing solutions after anodic polarization. The presence of fluoride ions together with chloride ions seems to increase the susceptibility of Alloy 22 to crevice corrosion compared to pure chloride solutions. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

Evaluation by the Double Loop Electrochemical Potentiokinetic Reactivation Test of Aged Ferritic Stainless Steel Intergranular Corrosion Susceptibility

An experimental design method was used to determine the effect of factors that significantly affect the response of the double loop–electrochemical potentiokinetic reactivation (DL-EPR) test in controlling the susceptibility to intergranular corrosion (IGC) of UNS S43000 (AISI 430) ferritic stainless steel. The test response is expressed in terms of the reactivation/activation current ratio (I _r /I _a pct). Test results analysed by the analysis of variance (ANOVA) method show that the molarity of the H₂SO₄ electrolyte and the potential scanning rate have a more significant effect on the DL-EPR test response than the temperature and the depassivator agent concentration. On the basis of these results, a study was conducted in order to determine the optimal operating conditions of the test as a nondestructive technique for evaluating IGC resistance of ferritic stainless steel components. Three different heat treatments are considered in this study: solution annealing (nonsensitized), aging during 3 hours at 773 K (500 °C) (slightly sensitized), and aging during 2 hours at 873 K (600 °C) (highly sensitized). The aim is to find the operating conditions that simultaneously ensure the selectivity of the attack (intergranular and chromium depleted zone) and are able to detect the effect of low dechromization. It is found that a potential scanning rate of 2.5 mV/s in an electrolyte composed of H₂SO₄ 3 M solution without depassivator, at a temperature around 293 K (20 °C), is the optimal operating condition for the DL-EPR test. Using this condition, it is possible to assess the degree of sensitization (DOS) to the IGC of products manufactured in ferritic stainless steels rapidly, reliably, and quantitatively. A time–temperature–start of sensitization (TTS) diagram for the UNS S43000 (France Inox, Villepinte, France) stainless steel was obtained with acceptable accuracy by this method when the IGC sensitization criterion was set to I _r /I _a  > 1 pct. This diagram is in good agreement with the time–temperature–start of precipitation (TTP) diagram that delineates the domain of low dechromization consecutive to chromium carbide precipitation.     

Development of athermal and isothermalε-martensite in atomized Co-Cr-Mo-C implant alloy powders

In this work, CoCr-Mo compacted powders were sintered at 900°C to 1300°C for 1 to 2 hours and conditions for total carbide dissolution in fcc cobalt were determined. Accordingly, it was found that sintering at temperatures between 900°C to 1100°C led to removal of the dendritic structure and to carbide precipitation at the grain boundaries (gbs), as well as in the bulk. Moreover, recrystallization and grain growth were always found to occur during powder sintering. At temperatures above 1100°C, no carbide precipitation occurred indicating that carbides were not stable at these temperatures. Hence, compact powders were annealed at 1150°C to promote the development of a single-phase fcc solid solution. This was followed by rapid cooling to room temperature and then aging at 800°C for 0 to 18 hours. Rapid cooling from 1150°C promoted the development of up to 64 pct athermal ε-martensite through the face-centered cubic (fcc) → hexagonal crystal structure (hcp) martensitic transformation. The athermal martensite was associated with the development of a network of parallel arrays of fine straight transgranular markings within the fcc matrix. Moreover, aging at 800°C for 15 hours led to the development of 100 pct isothermal hcp ε-martensite. From the experimental outcome, it is evident that isothermal ε-martensite is the most stable form of the hcp Co phase. Apparently, during aging at 800°C, the excess defects expected in athermal martensite are removed by thermally activated processes and by the development of isothermal ε-martensite, which has the appearance of “pearlite.”     

The microstructural response of mill-annealed and solution-annealed INCONEL 600 to heat treatment

Samples of INCONEL* 600 were examined in the mill-annealed and solution-annealed states, and after isothermal annealing at 400 °C and 650 °C. The corrosion behavior of the samples was examined, analytical electron microscopy was used to determine the microstructures present and the chemistry of grain boundaries, and Auger electron spectroscopy was used to measure grain boundary segregation. Samples of different alloys in the mill-annealed state were found to have quite different microstructures, with Cr-rich M₇C₃ carbides occurring either along grain boundaries or in intragranular sheets. The corrosion behavior of the samples correlated well with the occurrence of grain boundary chromium depletion. Solution annealing at 1190 °C caused dissolution of all carbides, whereas at 1100 °C the carbides either dissolved or the grain boundaries moved away from the carbides, depending upon alloy carbon content. Low-temperature annealing at 400 °C had little effect on millannealed or fully solutionized samples, but in samples with intragranular carbides present, the grain boundaries moved until intersecting or adjacent to the carbides. Isothermal annealing at 650 °C caused carbide nucleation and growth at grain boundaries in fully solutionized samples. Chromium depletion at grain boundaries accompanied carbide precipitation, with a minimum chromium level of 6 wt pct achieved after 5 hours. Healing was found to occur after 100 hours. Solution-annealed samples with intragranular carbides present had more rapid corrosion kinetics since the grain boundaries moved back to the existing carbides. Thermodynamic analysis of the chromium-depletion process showed good agreement with experimental measurements. The Auger results found only boron present at grain boundaries in the mill-annealed state. Aged samples had boron, nitrogen, and phosphorus present, with phosphorus and nitrogen segregating to the greatest extent. The kinetics of phosphorus segregation are much slower at 400 °C compared with 650 °C.     

Low temperature sensitization of type 304 stainless steel pipe weld heat affected zone

Large-diameter Type 304 stainless steel pipe weld heat-affected zone (HAZ) was investigated to determine the rate at which low temperature sensitization (LTS) can occur in weld HAZ at nuclear reactor operating temperatures and to determine the effects of LTS on the initiation and propagation of intergranular stress corrosion cracks (IGSCC). The level of sensitization was determined with the electrochemical potentiokinetic reactivation (EPR) test, and IGSCC susceptibility was determined with constant extension rate tests (CERT) and actively loaded compact tension (CT) tests. Substructural changes and carbide compositions were analyzed by electron microscopy. Weld HAZ was found to be susceptible to IGSCC in the as-welded condition for tests conducted in 8-ppm-oxygen, high-purity water at 288 °C. For low oxygen environments (i.e., 288 °C/0.2 ppm O₂ or 180 °C/1.0 ppm O₂), IGSCC susceptibility was detected only in weld HAZ that had been sensitized at temperatures from 385 °C to 500 °C. Lower temperature heat treatments did not produce IGSCC. The microscopy studies indicate that the lack of IGSCC susceptibility from LTS heat treatments below 385 °C is a result of the low chromium-to-iron ratio in the carbide particles formed at grain boundaries. Without chromium enrichment of carbides, no chromium depleted zone is produced to enhance IGSCC susceptibility.     

Tensile properties and microstructure of haynes 25 alloy after aging at elevated temperatures for extended times

The tensile properties of Haynes 25 alloy have been measured after various aging treatments, time, and temperature: as received; and aged at 600 °C for three months; 800 °C for 6 and 12 months; and 1000 °C for 3 and 6 months. Contour plots in temperature-ln (time) space were constructed based on the literature and our own data, detailing changes in yield strength, ultimate tensile strength, and tensile elongation. Scanning electron microscopy and transmission electron microscopy observations of the Haynes 25 alloy microstructure provided an explanation of why the properties changed with aging. Intense lattice distortions after aging at 600 °C, the presence of an α-Co₃W, a L1₂-ordered, fcc phase, a=0.357 nm, after aging at 800 °C, and the nucleation and growth of W₃Co₃C carbides from aging at 800 °C and 1000 °C produced the changes in tensile properties. We did not observe either the Co₂W Laves phase or Co₇W₆ γ phase in any of the material conditions we examined, using TEM of thin foils: as received and aged at 600 °C, 800 °C, and 1000 °C. Other researchers believe these phases cause a loss of ductility in the Haynes 25 alloy with prolonged high-temperature exposure.     

The influence of thermal treatment on the chemistry and structure of grain boundaries in inconel 600

Although it is widely accepted that certain heat treatments result in carbide precipitation accompanied by chromium depletion at the grain boundaries, no direct evidence of this phenomenon exists for Inconel 600. Using the Scanning Transmission Electron Microscope (STEM), the extent of grain boundary chromium depletion is quantitatively determined as a function of thermal treatment time at 700 °C following a 30 min solution anneal at 1100 °C. Results confirm the presence of grain boundary chromium depletion that varies in extent with time at temperature, the chromium concentration falling to values as low as 3 wt pct. The chromium depletion volume is characterized by a depletion parameter which is correlated with intergranular corrosion test results to determine a self-healing (desensitization) chromium concentration of 9 wt pct. Trace element segregation at grain boundaries is measured by Auger Electron Spectroscopy (AES) as a function of aging treatment. Results show that after thermally treating samples for various times at 700 °C, phosphorus is always present at the grain boundaries. Intergranular corrosion behavior as a function of thermal treatment appears to be governed more strongly by chromium depletion than trace element segregation. G. S. WAS, formerly Research Assistant, Nuclear Engineering Dept., Massachusetts Institute of Technology H. H. TISCHNER, formerly Postdoctoral Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Experimental Investigation and Analytical Prediction of σ-Phase Precipitation in AISI 316L Austenitic Stainless Steel

The phase precipitation in industrial AISI 316L stainless steel during aging for up to 80,000 hours between 823 K and 1073 K (550 °C and 800 °C) has been studied using transmission electron microscopy, scanning transmission electron microscopy, and carbon replica energy-dispersive X-ray microanalysis. Three phases were identified: Chromium carbides (M₂₃C₆), Laves phase (η), and σ-phase (Fe-Cr). M₂₃C₆ carbide precipitation occurred firstly and was followed by the η and σ-phases at grain boundaries when the aging temperature is higher than 873 K (600 °C). Precipitation and growth of M₂₃C₆ create chromium depletion zones at the grain boundaries and also retard the σ-phase formation. Thus, the σ-phase is controlled by the kinetic of chromium bulk diffusion and can appear only when the chromium reaches, at grain boundaries and at the M₂₃C₆/γ and M₂₃C₆/η/γ interfaces, content higher than a critical value obtained by self-healing. An analytical model, based on equivalent chromium content, has been established in this study and successfully validated to predict the time–temperature–precipitation diagram of the σ-phase. The obtained diagram is in good agreement with the experimental results.     

Corrosion Behavior of Alloy 625 in PbSO4-Pb3O4-PbCl2-ZnO-10?Wt?Pct CdO Molten Salt Medium

Corrosion behavior and degradation mechanisms of alloy 625 under a 47.288 PbSO₄-12.776 Pb₃O₄-6.844PbCl₂-23.108ZnO-10CdO (wt pct) molten salt mixture under air atmosphere were studied at 873?K, 973?K, and 1073?K (600?°C, 700?°C, and 800?°C). Electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) measurements, and potentiodynamic polarization techniques were used to evaluate the degradation mechanisms and characterize the corrosion behavior of the alloy. Morphology, chemical composition, and phase structure of the corrosion products and surface layers of the corroded specimens were studied by scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray map analyses. Results confirmed that during the exposure of alloy 625 to the molten salt, chromium was mainly dissolved through an active oxidation process as CrO₃, Cr₂O₃, and CrNbO₄, while nickel dissolved only as NiO in the system. Formation of a porous and nonprotective oxide layer with low resistance is responsible for the weak protective properties of the barrier layer at high temperatures of 973?K and 1073?K (700?°C and 800?°C). There were two kinds of attack for INCONEL 625, including general surface corrosion and pitting. Pitting corrosion occurred due to the breakdown of the initial oxide layer by molten salt dissolution of the oxide or oxide cracking.     

Delamination of Sensitized Al-Mg Alloy During Fatigue Crack Growth in Room Temperature Air

Fatigue crack growth experiments in air at 295?K (22?°C) were conducted on 6.35?mm thick plate samples of a commercial Al-Mg alloy machined from the L-T orientation. Thermal exposures for times up to 2000?hours at 343?K, 353?K, 373?K, and 448?K (70?°C, 80?°C, 100?°C, and 175?°C) produced sensitization and delamination in the S-T plane during fatigue testing, dependent on the level of thermal exposure and K _max employed. Identical tests conducted in a ??dry?? environment produced no delamination, indicating that environment may enhance the phenomena.     

Relative stress corrosion susceptibilities of alloys 690 and 600 in simulated boiling water reactor environments

The relative susceptibilities of alloys 600 and 690 to intergranular stress corrosion cracking (IGSCC) in pure water and a simulated resin intrusion environment at 288 °C were evaluated. A combination of creviced and noncreviced slow-strain-rate, and precracked fracture mechanics tests were employed in the evaluation. Susceptibility was determined as a function of dissolved oxygen content, degree of sensitization, and crevice condition. The results indicated that alloy 600 was susceptible to various degrees of IGSCC in oxygen containing pure water when creviced, and immune to IGSCC when uncreviced. Alloy 690 was immune to IGSCC under all pure water conditions examined. Alloy 600 and alloy 690 were both susceptible to cracking in the simulated resin intrusion environment. Alloy 690, however, exhibited the greatest resistance to SCC of the two alloys.