Electrochemical Potentiokinetic Reactivation Technique Research Articles

Sensitization Behavior of Alloy 800H as Characterized by the Electrochemical Potentiokinetic Reactivation (EPR) Technique

Abstract The need for a nondestructive test to evaluate levels of sensitization in alloy 800H (UNS N08810) led to modification of the electrochemical potentiokinetic reactivation (EPR) test techniq...

Electrochemical and microstructural characterization of an austenitic stainless steel irradiated by heavy ions at 515°C

Abstract The electrochemical and microstructural behavior of a solution-annealed, heavy-ion-irradiated, austenitic stainless steel. designated LS1A, have been investigated at 515°C after doses of 1.10 and 30 displacements per atom (dpa). Changes in electrochemical properties due to radiation-induced segregation in thin radiation-affected layers of the material were detected by the electrochemical potentiokinetic reactivation (EPR) technique using TEM disk specimens. At all doses, the Flade potential and reactivation charge were greater than those measured for thermally-aged control specimens. Grain face etching, similar to that found on EPR-tested neutron irradiated austenitic stainless steels, was observed on all specimens after testing. Duplicate heavy ion irradiated specimens were also examined by high resolution analytical electron microscopy (AEM). The 1 dpa specimen showed only a high density of small faulted dislocations (~ 10 nm), and no grain boundary precipitation or grain boundary segregation was detected. AEM confirmed chromium and molybdenum depletion at grain boundaries as measured by EPR for the 10 and 30 dpa specimens.

Stress Corrosion Cracking of Candidate Structural Materials in Simulated First-Wall/Aqueous Coolant Environments

Stress corrosion cracking (SCC) susceptibility of Types 316NG, 316, and 304 stainless steels (SS) was investigated in slow-strain-rate tests (SSRTs) in oxygenated water that simulates important parameters anticipated in first-wall/blanket systems. The water chemistry was based on a computer code which yielded the nominal concentrations of radiolytic species produced in an aqueous environment under ITER-type conditions. Actual SSRTs were performed in a less benign, more oxidizing reference environment at temperatures from 52 to 150{degree}C. Predominantly ductile fracture was observed in Type 316NG and nonsensitized Types 316 SS and 304 SS SSRT specimens strained to failure in a reference ITER water chemistry. The failure behavior of Type 304 SS specimens heat-treated to yield sensitization values of 2, 3, or 20 Coulomb (C)/cm{sup 2} by the electrochemical potentiokinetic reactivation (EPR) technique, demonstrated that the degree of sensitization had a dramatic effect on intergranular stress corrosion cracking (IGSCC) susceptibility. Ranking for resistance to SCC in simulated ITER water by electron microscopy and SSRT parameters, i.e., failure time, ultimate strength, total elongation and stress ratio is 304 SS (EPR = 20 <2 C/cm{sup 2}) < 316NG SS. 11 refs., 6 figs., 7 tabs.

Influence of Prior Deformation on The Sensitisation of AISI Type 304 Stainless Steel and Applicability of EPR Technique

The influence of prior plastic deformation on the degree of sensitisation (DOS) of AISI 304 stainless steel has been studied for various levels of cold work ranging from 0 to 25% using the ASTM standard A262 practices A and E, and the electrochemical potentiokinetic reactivation (EPR) technique. Peak current density and reactivation charge density were determined from single loop EPR experiments and the ratio of the peak current during reactivation to that during activation was determined from double loop EPR experiments. The feasibility of using these techniques for determining the DOS in cold worked samples was examined. The reproducibility of the EPR results is rather poor. There appears to be no well defined systematic trend between the degree of cold work and the DOS as estimated from the EPR parameters. EPR parameters were found to be dependent on the temperature of aging and the degree of prior cold work. Threshold values above which a sample can be treated as sensitised cannot therefore be determined from EPR tests without being confirmed independently by conventional ASTM standard methods.

Evaluation of Radiation-Induced Sensitization of Austenitic Stainless Steels Using Electrochemical Potentiokinetic Reactivation Technique

Abstract The electrochemical potentiokinetic reactivation (EPR) test technique was applied to the determination of radiation-induced sensitization in a neutron-irradiated (420°C, 9 displacements per atom [dpa]) titanium-modified, austenitic stainless steel. Miniaturized specimens (3 mm diam by 0.25 mm thick) in both the solution-annealed and 25% cold-worked conditions were irradiated and tested by the single-loop EPR test technique. The degree of sensitization (DOS) was calculated in terms of the reactivation charge (Pa). The Pa value was increased by the irradiation as compared to control specimens thermally aged at the irradiation temperature. Grain boundary etching was observed on the specimen surface after EPR testing. Subsequent x-ray microanalysis by analytical electron microscopy (AEM) revealed that chromium depletion at grain boundaries had occurred. These results suggest the occurrence of radiation-induced sensitization associated with chromium depletion at grain boundaries. However, the post-EPR...

Intergranular corrosion studies on warm worked type 316 austenitic stainless steel

Though warm working offers several technological advantages, warm working of austenitic stainless steels might lead to intergranular corrosion since the temperature range of warm working coincides with that of sensitization. In the present study, warm working of AISI Type 316 stainless steel (SS) was carried out between 823–1123 K up to 30% strain level, and subjected to intergranular corrosion tests as per ASTM A 262 practices A and E, and electrochemical potentiokinetic reactivation technique. The results indicated that the warm working process employed did not lead to intergranular corrosion. Additionally, after a sensitization aging treatment given to the warm worked specimens, they still showed a reduced tendency to intergranular corrosion as compared to the solution annealed and aged material. Optical and TEM observations showed copious intragranular precipitates suggesting that the growth of such precipitates formed during warm working would have reduced the density and distribution of intergranular carbides resulting in lesser chromium depleted grain boundary regions.

The influence of nitrogen and molybdenum on the sensitization properties of low-carbon austenitic stainless steels

The electrochemical potentiokinetic reactivation technique has been applied to the quantitative assessment of the influence of nitrogen and molybdenum on the sensitization properties of low-carbon austenitic stainless steels. The resulting isosensitization curves show that the addition of nitrogen (up to 0.123%) significantly improves the resistance of molybdenum-containing austenitic stainless steels to sensitization. This effect also occurs in conventional low carbon (0.03%) steels, as well as in steels with a higher carbon content (0.05%). Furthermore, it is shown that additions of molybdenum (up to 2.3%) to nitrogen-containing steels improve their resistance to sensitization by retarding the precipitation of chromium carbides.

An EPR Study on the Influence of Prior Cold Work on the Degree of Sensitization of AISI 304 Stainless Steel

Abstract The influence of prior cold work on the degree of sensitization of AISI 304 stainless steel (SS) has been studied for a wide range of cold work (up to 55%) by using a modified version of the conventional Strauss test as well as by using the more recently developed electrochemical potentiokinetic reactivation technique (EPR). It has been found that with up to approximately 20% of prior cold work, the degree of sensitization (DOS) increases with increasing cold work. However, for prior deformation levels exceeding 35%, a reverse trend has been noticed. It has also been observed that, irrespective of the degree of cold working (0 to 55%), the electrochemical technique can yield a reliable estimation of the degree of sensitization only when the integrated charge (Q) values are considered.

A Modified Electrochemical Technique (Electrochemical Potentiokinetic Reactivation) for Evaluating the Susceptibility of Inconel 600 to Intergranular Corrosion

Abstract The electrochemical potentiokinetic reactivation (EPR) method has recently become a popular test for evaluating the intergranular attack (IGA) of high-nickel alloys and different types of stainless steels (SSs). The EPR method was used in a study on the IGA and intergranular stress corrosion cracking (IGSCC) of a high-nickel alloy (Inconel 600). A number of experimental test conditions were optimized to (1) cause selective grain boundary attack and (2) suppress other types of corrosion. It was found that the EPR curve is composed of different peaks. Microstructural analysis showed that the peak at − 30 mV/SCE corresponds with the IGA of the sample, and the peak at − 150 mV/SCE corresponds with a pitting-like attack. The IGA of Inconel 600 was tested with the modified EPR technique and compared with the results from chemical (Huey and Streicher) tests. Good agreement was found for sensitized samples. Mill-annealed samples showed moderate sensitization in the chemical tests. The modified EPR techni...

On working iron and steel

Sigma phase is a deleterious phase which can be formed in duplex stainless steels (DSSs) during heat treatment, manufacturing, or welding processes, leading to a decrease of mechanical properties and corrosion resistance. This issue is highly concerning for numerous industries because the presence of σ–phase in the austenite/ferrite microstructure of DSSs reduces their lifetime and promotes a brittle fracture in service. This review focuses on the non-destructive techniques (NDT) for the detection of σ–phase and their application in engineering failure analysis and field study cases in recent years. The most used NDT as inspection tools are ultrasound (US) and magnetic techniques; there are further alternative methods such as electrochemical potentiokinetic reactivation (EPR) technique and replication metallography (RM) analysis. Their advantages and drawbacks are discussed and exemplified with case studies. The current trend for NDT applied to the σ–phase is thoroughly studied and represented, showing the key mechanisms and factors influencing σ–phase formation and its consequences on DSSs. In addition, a detailed description of important failure cases wherein σ–phase has been responsible is presented.