P.K. De

Resistance to sensitization and intergranular corrosion through extreme randomization of grain boundaries

Abstract Two grades of austenitic stainless steel, type 304 and 316L, were cold rolled to different reductions by unidirectional and by cross rolling. Subsequent solutionizing of the cold rolled samples produced noticeable textural differences in type 304, but insignificant differences in type 316L. Both the solutionized materials had however the same trend in grain boundary character distribution (GBCD): an increasing fraction of random boundaries with an increasing pre-solutionizing reduction percentage. The degree of sensitization (DOS) was measured by the double loop electrochemical potentiokinetic reactivation (DL-EPR) test in both the alloys. The susceptibility to intergranular corrosion was assessed by the standard weight loss technique (practice B, A262 ASTM) in type 304 alloy. These increased with increase in random boundary concentration, but then dropped significantly beyond a ‘critical’ concentration—a pattern observed in both the grades. Such a pattern may be explained from a balance between nucleation rate of Cr-carbides and grain boundary Cr-flux, though postulating an exact model is premature at this stage. The present study, however, demonstrates a clear possibility of remarkable improvement in DOS and IGC through extreme grain boundary randomization.

High corrosion resistant Ti–5%Ta–1.8%Nb alloy for fuel reprocessing application

Abstract The conventional low carbon austenitic stainless steels display good corrosion resistance behaviour in nitric acid media. However, they are sensitive to intergranular corrosion in boiling nitric acid media in the presence of oxidizing ions like hexavalent chromium, tetravalent iron and hexavalent plutonium. The Ti–5%Ta–1.8%Nb alloy was evaluated as a candidate material for such applications of nuclear fuel reprocessing. Extensive tests were carried out to establish the superior corrosion properties in comparison to the conventional stainless steel or nitric acid grade stainless steel. The fabricability of this new alloy to various shapes like rod, sheet, wire and its weldability, which is required for making vessels, was found to be good.

Controlling grain boundary energy to make austenitic stainless steels resistant to intergranular stress corrosion cracking

Intergranular corrosion and intergranular stress corrosion cracking are the two localized corrosion mechanisms that are of concern to the typical applications of austenitic stainless steels in industries. Until recently, the common understanding was that a higher frequency of random boundaries increases the susceptibility, caused by a sensitization heat treatment or by operating temperatures, of austenitic stainless steels to both intergranular corrosion and intergranular stress corrosion cracking. A recent study demonstrated that extreme randomization of grain boundaries leads to a considerable improvement of resistance to both sensitization and intergranular corrosion. This work is a continuation of Ref. 1 and relates the effects of grain boundary randomization to intergranular stress corrosion cracking: the results show a trend consistent with earlier observations on intergranular corrosion. It is shown that there is improvement in resistance to intergranular stress corrosion cracking with extreme randomization of grain boundaries.

Controlling corrosion in the back end of fuel cycle using nitric acid grade stainless steels

The materials for use in nitric acid service have to be resistant to intergranular corrosion and have low general corrosion rates. End grain corrosion resulting from inclusions and segregation along dislocation bands needs to be minimised. The Nitric Acid Grade of stainless steel resistant to these corrosion problems has been indigenously produced. It is emphasised that proper fabrication practices have to be employed for NAG grade to take full advantage of its cleanliness, controlled chemistry and microstructure. The problems associated with fabrication of large diameter pipes, oxide embedment and welding are described with our experience in its production and quality assessment.

Assessment of hydrogen levels in Zircaloy-2 by non-destructive testing

A non-destructive assessment of Zircaloy-2 samples charged with hydrogen in the range of 50 to 1150 mg/kg has been made using ultrasonic and eddy current testing. It has been found that the ratio of the longitudinal to the shear wave velocity is a parameter which can be directly correlated with the hydrogen content up to a level of 100 to 200 mg/kg. This parameter together with the values of longitudinal and shear wave velocities can be utilized in a multi-parametric correlation approach for estimation of higher levels of the hydrogen content (up to 1150 mg/kg). The sensitivity at different ranges has been found to be acceptable. Ultrasonic attenuation measurements at higher frequencies and eddy current test parameter are also effective for estimation of hydrogen levels above 250 mg/kg in zirconium alloys. Microstructural characterization including TEM studies have been carried out for studying the influence of the type and the morphology of hydride precipitates on ultrasonic parameters.

Assessment of the corrosion behavior of SS 304L in nitric acid environments ― an alternative approach

Stainless steel (SS) 304L is widely used as a construction material in industries requiring corrosion resistance in nitric acid environments. Representative samples from 20 heats of SS 304L products were subjected to a testing procedure consisting of testing as per the rapid screening test (Practice A) and the Huey test (Practice C) of ASTM A 262, Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels. The presensitization heat treatment of 677°C for 1 h and the modified heat treatment of 677°C for 20 min were used for characterizing the microstructures in Practice A, whereas only the former presensitization heat treatment was used for Practice C. The percent duality after the two heat treatments has been correlated with the corrosion rate obtained in Practice C. A chemical composition based parameter, Cr effective = [% Cr − 0.18 (%Ni) − 100 (%C)], was calculated for all the heats and correlated with the percent duality after the two heat treatments as well as the corrosion rate in Practice C. It is observed that the SS 304L heats with Cr effective >14.0 and

Development of EPR Test Technique for Alloy 800

The degree of sensitization (DOS) of austenitic stainless steels and some nickel-based alloys (e.g., alloy 600) is evaluated by the electrochemical potentiokinetic reactivation (EPR) test. In this study a number of test solutions based on H2SO4 + KSCN composition have been evaluated to establish a reliable EPR test method for alloy 800. Different passivation (vertex) potentials are also tested. It has been shown that dilute test solutions with lower vertex potentials resulted in single loop (SL) and double loop (DL) EPR test methods that distinguished between different sensitized samples and also between sensitized and desensitized samples. It has been shown that an SL-EPR test in 1 M H2SO4 + 0.002 M KSCN (de-aerated) at 26 °C at a scan rate of 3 mV/s from a vertex potential of 700 mVSCE (180 s hold time) gave results that matched with the DOS indicated by microstructures and the Huey test results. Similarly, the DL-EPR test in 1 M H2SO4 + 0.002 M KSCN (de-aerated) at 26 °C, forward scanning from the OCP to + 700 mVSCE and then backward scanning from there to the OCP at a scan rate of 2 mV/s produced a good measure of DOS as indicated by the Huey test results. The effectiveness of the EPR test was ascertained by testing on alloy 800 containing Ti and Al (alloy 800 HT) and Nb (alloy 800 Nb).

Stress distribution and hydride orientation in ZrNbCu garter spring under complex loading

Abstract A garter spring of the tight fit design in a PHWR experience three type of loads: an axial load due to spring tension, a compressive load due to coolant tube resting on it and a torus moment due to bending. The influence of these, on the hydride orientation in Zr2.5Nb0.5Cu garter springs has been studied by stress analysis and out-of-pile simulation experiments. The stress analysis showed that the magnitude of the stress is maximum at the surface and it decreases rapidly towards the core of the spring wire of rectangular cross section. This trend has also been observed in the experimental results. The preferred orientation of hydrides was found to be confined to the surface up to a depth of 300 μm. A random distribution of hydrides was observed at the central region where the stress was evidently below the threshold value for reorientation. Further a map of the planes of maximum principal stresses at the surface layer of the spring helix, constructed by stress analysis was found to be consistent with the observed hydride distribution. The diametrical compressive load did not show much influence on the hydride morphology in the range of 0–4 kg/turn under the axial load of 1 kg. It was also seen that the axial load of 1 kg imposed by the tight fit design did not significantly alter the hydride distribution which is primarily governed by torus bending.

STRESS-CORROSION CRACKING OF ZIRCALOY-2 IN METHANOL--IODINE SOLUTIONS.

Annealed Zircaloy-2 cracks intergranularly when uniaxially stressed in boiling methanol-iodine solutions containing 0.002 to 20.0 g I21. The time to failure decreases with increasing iodine concentration. Stressing to 75% of the 0.2% proof-stress increases the dissolution rate by about 20 times. Both the corrosion rate and the time to failure under stress are independent of impressed electrochemical potential. The activation energy for dissolution, 8350 cals/mole for the unstressed condition, is reduced to 3360 cals/ mole under applied stress. This activation energy probably corresponds to the adsorption of I2 on the Zircaloy surface. The mechanism of the stress-corrosion cracking seems to be one of stress—assisted dissolution controlled by the adsorption of I2.

Effects of Overall Grain Boundary Nature on Localized Corrosion in Austenitic Stainless Steels

Thermo-mechanical processing of type 304 and type 316L stainless steels done by (a) cold rolling to a reduction in thickness of 20 to 80 percent and (b) solution annealing to obtain a medium size of grains led to a considerable improvement in resistance to both sensitization and intergranular corrosion. The nature of the resultant grain boundaries was examined in a scanning electron microscope using orientation imaging microscopy in electron back scattered diffraction mode. Fraction of random and special grain boundaries were established for each set of thermo-mechanical processing. After appropriate sensitization treatments, the degrees of sensitization of these stainless steels were evaluated by double loop electrochemical potentiokinetic reactivation tests. Standard ASTM tests were used to evaluate susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC). These studies showed that a particular combination of thermomechanical processing led to formation of over 75 percent random grain boundaries in the steels and this imparted resistance to sensitization and to IGC and IGSCC. This opens a new concept in grain boundary (GB) engineering of a high fraction of random GB increasing the resistance to localized corrosion like IGC and IGSCC. Textural studies were carried out with the help of X-ray and MTM-FHM software. It showed significant change of texture in type 304 stainless steel, while no change in the texture of type 316L stainless steel after cold rolling and annealing.