Double-loop Electrochemical Potentiokinetic Reactivation Tests Research Articles

Effect of phosphorus content on stress corrosion cracking susceptibility of shielded metal arc weld metals for 600 type alloy in high temperature pressurised pure water*

The intergranular stress corrosion cracking (IGSCC) susceptibility of shielded metal arc weld metals for Inconel alloy 600 has been investigated with particular reference to the influences of P and Nb contents on the basis of a creviced bent beam (CBB) test in pressurized hot water (corresponding to the service condition of boiling water reactor nuclear power plant). The IGSCC susceptibility of the weld metals in the as-welded state exhibited a tendency to increase with P content. When the post-weld heat treatment stress relief+low-temperature ageing (SR+LTA) of SR annealing at 893 K followed by ageing at 673 K was applied, however, the influence of P content on the IGSCC susceptibility of weld metals depended on the Nb content; i.e. the IGSCC susceptibility of the weld metals containing 1.2 mass% Nb was almost independent of P content, whereas the weld metal containing 4.4 mass% Nb showed much decreased IGSCC susceptibility by increasing the P content from 0.012 to 0.031 mass%. Since the double-loop electrochemical potentiokinetic reactivation test suggested that the grain boundary corrosion resistance of the weld metals of 4.4 mass% Nb decreased with the rise in P content, the improved IGSCC susceptibility of the weld metals of 4.4 mass% Nb and 0.031 mass% P cannot be explained in terms of the corrosion resistance of the grain boundary. The weld metal containing 4.4 mass% Nb was hardened significantly by the precipitation of the γ′ phase (Ni3Nb) in interdendritic areas during the SR+LTA treatment. The increase in the hardness brought about by the SR+LTA treatment was depressed significantly by the rise in P content probably due to the reduction in the precipitation area of the γ′ phase. It can be considered that the decreased hardness contributes to the improvement in the IGSCC susceptibility with increasing the P content, since the tensile stress applied to the specimen during the CBB test increases with the hardness.

Microstructural and electrochemical characterisation of heat-treated 347 stainless steel with different phases

Type 347 stainless steel was solution annealed at three different temperatures (1100, 1250 and 1300°C) followed by ageing at 650 and 750°C. This was done to study the phase transformations that may occur in stainless steel 347 during welding and/or subsequent ageing. The microstructural examination showed chromium carbide and σ phase formation at the grain boundaries after ageing. The electrochemical behaviour was evaluated by double-loop electrochemical potentiokinetic reactivation test and anodic polarisation in HCl solution. Chromium carbide precipitation caused large increase in the degree of sensitisation, while σ phase had the major effect in decreasing the pitting resistance.

The role of niobium carbide in radiation induced segregation behaviour of type 347 austenitic stainless steel

The effect of niobium carbide precipitates on radiation induced segregation (RIS) behaviour in type 347 stainless steel was investigated. The material in the as-received condition was irradiated using double-loop 4.8 MeV protons at 300 °C for 0.43 dpa (displacement per atom). The RIS in the proton irradiated specimen was characterized using double-loop electrochemical potentiokinetic reactivation (DL-EPR) test followed by atomic force microscopic examination. The nature of variation of DL-EPR values with the depth matched with the variation of the calculated irradiation damage (dpa) with the depth. The attack on grain boundaries during EPR tests was negligible indicating absence of chromium depletion zones. The interface between niobium carbide and the matrix acts as a sink for point defects generated during irradiation and this had reduced point defect flux toward grain boundaries. The attack was noticed at a few large cluster of niobium carbide after the DL-EPR test at the depth of maximum attack for the irradiated specimen. Pit-like features were not observed within the matrix indicating the absence of chromium depletion regions within the matrix.

Corrosion behavior of Eurofer 97 and ODS-Eurofer alloys compared to traditional stainless steels

In the present work, the corrosion resistance of ferritic-martensitic EUROFER 97 and ODS-EUROFER steels was tested in solutions containing NaCl or H2SO4 and KSCN, both at 25 °C. The results were compared to those of AISI 430 ferritic and AISI 410 martensitic conventional stainless steels. The as-received samples were tested by electrochemical techniques, specifically, electrochemical impedance spectroscopy, potentiodynamic polarization curves, and double-loop electrochemical potentiokinetic reactivation tests. The surfaces were observed by scanning electron microscopy after exposure to corrosive media. The results showed that EUROFER 97 and ODS-EUROFER alloys present similar corrosion resistance but lower than ferritic AISI 430 and martensitic 410 stainless steels.

Evaluation of aged Incoloy 800 alloy sensitization to intergranular corrosion by means of double loop electrochemical methods and image analysis

In this paper, the sensitization of aged Incoloy 800 alloy to intergranular corrosion has been systemically investigated by double loop electrochemical potentiokinetic reactivation (DL-EPR) technique in combination with oxalic acid etching test and microstructure observation. The DL-EPR results show that the specimens aged at 650 °C and 700 °C for 4 h were intensely sensitized with I r: I a value greater than 30% while there was no sensitization phenomenon for the specimens aged at 800 °C for 4 h. It was also found that the degree of sensitization increased gradually with the aging time in the range of 0–10 h at 650 °C, and I r: I a value reached the maximum −46% after an aging time of 10 h. However, further increasing aging time decreased the sensitization due to the healing effect incurred by the diffusion of chromium from adjacent grains to chromium-depleted zones. Comparison between two evaluating techniques (the DL-EPR and oxalic acid etching test) has also been conducted.

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 (Ir/Ia 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 Ir/Ia > 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.

EVALUATION OF THE SUSCEPTIBILITY TO INTERGRANULAR ATTACK OF 2205 DUPLEX STAINLESS STEEL BY DL-EPR METHOD

Duplex stainless steels (DSS), consisted of ferrite and austenite, are widely used in oil, chemical, petrochemical, nuclear and marine industries because of their attractive combination of higher mechanical strength and corrosion resistance in various aggressive environments. However, precipitations of detrimental phases, such as chromium carbides, nitrides, intermetallic phases (χ, σ )i n- evitably occur when manufacturing conditions and welding procedure heat these duplex stainless steels to temperature ranging from 300 to 1000 , which results in a reduction in corrosion properties due to the presence of chromium-depleted zones around these precipitates. In this paper, the microstructure evolution of 2205 DSS was studied qualitatively at 800 by using Thermo-Calc coupled with optical microscope. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) was optimized through studying the influences caused by four factors (scan rate, temperature, the concentration of the electrolyte and the surface finish). Then the susceptibility to intergranular attack (IGA) of 2205 DSS was tested by optimized DL-EPR. The results indicated that σ phase will be precipitated in the ferrite phase when 2205 DSS aged at 800 , and the volume fraction would increase with the aging time increasing. The optimized DL-EPR test can evaluate the susceptibility to IGA of 2205 DSS both quantitatively and qualitatively, which is proved by the morphologies of the sample after DL-EPR test.

Low temperature embrittlement of duplex stainless steel: Correlation between mechanical and electrochemical behavior

The low temperature embrittlement behavior of duplex stainless steel 2205 was investigated on the basis of changes in mechanical and electrochemical properties after aging for 5000 h at 335, 365 and 400 °C. Aging leads to increase in the hardness of δ-ferrite phase. The ferrite hardening was very rapid in the initial stages of aging; thereafter the hardness increase was more gradual at all the aging temperatures. Charpy impact test of the aged samples showed drastic decrease in impact toughness. The value of Charpy impact energy saturated after aging to 5000 h at all the aging temperatures. The embrittlement in the material is known to be caused by spinodal decomposition reaction in which the δ-ferrite decomposes into iron-rich α and chromium-enriched α′. For the purpose of non-destructive evaluation of thermal aging embrittlement, double loop electrochemical potentiokinetic reactivation (DL-EPR) test and anodic polarizations in acetic acid and HCl solution were carried out. The peak current density during the anodic scan in DL-EPR test and the peak anodic current density for secondary passivation during polarization in acetic acid increased with increase in aging time. A good linear correlation was observed between the peak anodic current density for secondary passivation during polarization in acetic acid and the microhardness of δ-ferrite phase.

Impact of sensitization on dissolution process of AISI 304 stainless steel during intergranular corrosion evaluated using DEIS technique

The paper presents the results of instantaneous impedance changes measurements vs. reactivation potential performed by means of dynamic electrochemical impedance spectroscopy (DEIS) technique for AISI 304 stainless steel (SS) dissolution process during intergranular corrosion (IG) in 0.5 M H2SO4 + 0.01 M KSCN solution. With the use of DEIS method, it was possible to estimate dynamic changes of the examined system’s impedance in conditions of proceeding IG process. Furthermore, the paper proposes an alternative way of evaluating AISI 304 stainless steel dissolution rate during intergranular corrosion based on approximation to theory of iron dissolution in sulfuric acid medium. Simultaneously, based on the DEIS measurements, information about the degree of sensitization of the examined material were obtained. Performed research revealed the advantages of the DEIS technique over the classical double-loop electrochemical potentiokinetic reactivation tests when investigating intergranular corrosion process.

Evaluation of toughness deterioration by an electrochemical method in an isothermally-aged N-containing austenitic stainless steel

This work presents the results of an evaluation of the deterioration of cryogenic toughness by means of an electrochemical method in a N-containing austenitic stainless steel (JK2) aged at temperatures of 700, 800 and 900 °C for times from 10 to 1000 min. The aging process at 700 and 800 °C caused the decrease in the Charpy V-Notch impact energy at − 196 °C because of the intergranular precipitation of carbides. Scanning electron micrographs of the Charpy V-Notch test specimens showed the presence of intergranular brittle fracture. The degree of sensitization was determined by the ratio of the maximum current density generated by the reactivation scan to that of the anodic scan, I r/ I a, using the double-loop electrochemical potentiokinetic reactivation test. The Charpy V-Notch impact energy decreased with increase in the I r/ I a ratio. This relation permits an estimate of the deterioration of cryogenic toughness due to thermal aging in this type of steel.

Quantitative Evaluation of Aged AISI 316L Stainless Steel Sensitization to Intergranular Corrosion: Comparison Between Microstructural Electrochemical and Analytical Methods

The evaluation of the degree of sensitization (DOS) to intergranular corrosion (IGC) of a commercial AISI 316L austenitic stainless steel aged at temperatures ranging from 550 °C to 800 °C during 100 to 80,000 hours was carried out using three different assessment methods. (1) The microstructural method coupled with the Strauss standard test (ASTM A262). This method establishes the kinetics of the precipitation phenomenon under different aging conditions, by transmission electronic microscope (TEM) examination of thin foils and electron diffraction. The subsequent chromium-depleted zones are characterized by X-ray microanalysis using scanning transmission electronic microscope (STEM). The superimposition of microstructural time-temperature-precipitation (TTP) and ASTM A262 time-temperature-sensitization (TTS) diagrams provides the relationship between aged microstructure and IGC. Moreover, by considering the chromium-depleted zone characteristics, sensitization and desensitization criteria could be established. (2) The electrochemical method involving the double loop–electrochemical potentiokinetic reactivation (DL-EPR) test. The operating conditions of this test were initially optimized using the experimental design method on the bases of the reliability, the selectivity, and the reproducibility of test responses for both annealed and sensitized steels. The TTS diagram of the AISI 316L stainless steel was established using this method. This diagram offers a quantitative assessment of the DOS and a possibility to appreciate the time-temperature equivalence of the IGC sensitization and desensitization. (3) The analytical method based on the chromium diffusion models. Using the IGC sensitization and desensitization criteria established by the microstructural method, numerical solving of the chromium diffusion equations leads to a calculated AISI 316L TTS diagram. Comparison of these three methods gives a clear advantage to the nondestructive DL-EPR test when it is used with its optimized operating conditions. This quantitative method is simple to perform; it is fast, reliable, economical, and presents the best ability to detect the lowest DOS to IGC. For these reasons, this method can be considered as a serious candidate for IGC checking of stainless steel components of industrial plants.

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.

Assessment of intergranular corrosion in AISI Type 316L stainless steel weldments

This paper deals with the applicability of various techniques for the assessment and quantification of sensitisation in AISI Type 316L welds. Welded joints of AISI Type 316L stainless steel were aged at 973 K for periods of up to 200 h. The base and weld metal components of the aged joints were then assessed for susceptibility to sensitisation and intergranular corrosion (IGC) by using various tests specified by ASTM A262, Practices A and E, and ASTM G108 (the electrochemical potentiokinetic reactivation (EPR) test). The possibility of using eddy current testing (ECT) to detect sensitisation and IGC was also assessed. The use of ASTM A262 Practice A and E tests indicated sensitisation in base metal aged for 20 h and above. Aged weld metals showed no failure in these tests. Tensile tests on the weld joints before and after exposure to Cu–CuSO4-H2SO4 solution did not indicate any differences in the tensile properties. Double loop EPR tests indicated a significant increase in the ratios of charge and peak current densities on reactivation to activation after aging the welded joint for 20 h and above. However, weld metal showed no change in the ratios of the above two parameters. Following doubts about the suitability of the EPR test for on line corrosion monitoring, the ECT technique was investigated in order to assess its suitability for the detection and quantification of sensitisation. The ratio of eddy current amplitudes after and before exposure to Cu–CuSO4-H2SO4 solution was used as an assessment criterion. A significant increase in this ratio was observed on aging the base metal for more than 20 h. No significant change was observed in the ratios of eddy current amplitudes for weld metal. The ECT results correlated very well with the findings of the EPR and ASTM Practice E tests. This indicates that ECT holds promise as an on line monitoring tool for sensitisation and IGC.

Influence of laser surface melting on the susceptibility to intergranular corrosion of sensitized Alloy 600

The effects of laser surface melting (LSM) on the intergranular corrosion of the sensitized Alloy 600 were studied by the double loop electrochemical potentiokinetic reactivation (DL-EPR) method in de-aerated 0.01 M H 2SO 4+20 ppm KSCN at a scan rate of 0.5 mV/s at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM, and the sensitized Alloy 600 after LSM also exhibited a relatively low DOS, comparing with that of the sensitized but not laser treated Alloy 600. From the microscopic examination, it was found that the microstructural changes induced by the laser treatment, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement in resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to intergranular stress corrosion cracking of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination.

Double loop electrochemical potentiokinetic reactivation test of Nickel-base Alloy 600 surface-melted by a CO2 laser beam

Ni-base Alloy 600 has been widely used as a steam generator (S/G) tubing material in nuclear power plants because of its good mechanical and corrosion properties at high temperatures. However, degradations of S/G tubes due to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) during normal operation have been frequently reported. In particular, Alloy 600 can be very susceptible to IGA/IGSCC in some sulfur-bearing environments by sensitization. In this paper, the beneficial effects of laser surface melting (LSM) on intergranular corrosion of the sensitized Alloy 600 is presented from the results of the double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The DL-EPR test was performed in de-aerated 0.01 M H2SO4+20 ppm KSCN at a scan rate of 0.5 m V/sec at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM. The sensitized Alloy 600 after LSM also exhibited a relatively low DOS, compared with that of the sensitized but not laser treated Alloy 600. From the microscopic observation, it was found that the microstructural changes brought about by the LSM process, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement of resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to IGSCC of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination.

Carbon and Nitrogen Effects on Sensitization Resistance of Type 347 Stainless Steels

Abstract The sensitization resistance of the stabilized Type 347 (UNS S34700) stainless steels (SS) having different carbon and nitrogen contents was evaluated using the double-loop electrochemical potentiokinetic reactivation (DL-EPR) and ASTM A262, practice E methods. Samples were heat-treated to simulate the coarse grain region of the heat-affected zone (HAZ) during welding and subsequently aged at 948 K for different times. For comparison, DL-EPR tests also were conducted for the unstabilized steels, Type 316 (UNS S31600) and Type 304 (UNS S30400). For Type 347 SS, a criterion for sensitization in accordance with the ASTM A262, practice E test corresponded to a condition of > 10% reactivation ratio (peak reactivation current [Ir] /peak activation current [Ia]) from DL-EPR test. The carbon and nitrogen effects on the sensitization resistance of the Type 347 SS were explained in view of the observed microstructures. By introducing a resistance parameter based on the chemical composition, sensitization r...

Evaluation of the σ-phase degradation for cast stainless steel CF8M by the electrochemical method

The present investigation is concerned with the degradation characteristics of cast stainless steel(CF8M), exposed to the σ-phase degradation at 700℃. In the present paper, the degradation of CF8 M at 700℃ is evaluated by a non-destructive test, DL-EPR(double loop electrochemical potentiokinetic reactivation). The surface of specimens is observed by using scanning electron microscopy after DL-EPR test. Also. chromium contents of matrix, grain boundary and ferrite phase are analyzed by electron probe X-ray micro analyzer. Through the experiments, the following results are obtained 1) The degree of sensitization(DOS) of CF8M aged up to 15hr at 700℃ is increased with acing time while that is decreased with aging time from 15hr to 150hr. 2) The impact energy decreases with increase of σ-phase while DOS increases with σ-phase until aging time reaches to 15hr. After the aging time. 15hr, the σ-phase and the rate of impact energy with respect to aging time decreases. Therefore the degradation behavior of the CF8M can be evaluated by comparing SEM micrographs and the value of DOS.

Technical noteEffect of ceramic heat insulating paste on degree of sensitisation of AISI type 304 stainless steel

Abstract Double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests and oxalic acid etch tests were conducted to evaluate the effect of a ceramic heat insulating paste on the degree of sensitisation of AISI type 304 stainless steel. Steel strips in the as received condition, partially and totally protected with the ceramic heat insulating paste, were heat treated in a furnace at 700°C for intervals of 15, 30, and 60 min. Measurements of the ratios of the reactivation current to the anodic current indicated that the ceramic heat insulating paste effectively prevented sensitisation of the steel for the first 15 min at 700°C and reduced the degree of sensitisation for specimens heat treated for 30 and 60 min. Microstructural examination of specimens after DL-EPR tests and oxalic acid etch tests provided additional evidence for the effectiveness of the ceramic paste. The decreased degree of protection at 30 and 60 min was due to dehydration and subsequent cracking of the protective paste layer on...

Some aspects of electron beam welded and laser welded ferritic Fe-22Cr-3Mo-5Ni-0.3Ru alloy

Ferritic Fe-22Cr-3Mo-5Ni-0.3Ru alloy was welded by electron beam (EB) and laser techniques. Tensile tests, impact tests, oxalic acid etch, electrochemical potentiokinetic reactivation (EPR) and potential-time measurements were employed to investigate the mechanical properties, microstructural features and corrosion behaviour of the EB and laser welded metals. The oxalic acid etch tests revealed that the microstructure of the EB and laser welded metals consisted of columnar grains and cells, respectively. These structures were formed directly from the substrate by epitaxial solidification. Furthermore, the oxalic acid tests also showed that the EB and laser welded metals were immune to intergranular corrosion. Similar results were obtained by double loop EPR tests.