Vivekanand Kain

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.

Electrochemical Investigations of Pitting Corrosion in Nitrogen-Bearing Type 316LN Stainless Steel

Abstract Nitrogen (N) alloying was found to inhibit active dissolution and to introduce a secondary loop with fluctuating currents in the anodic polarization curve of type 316LN stainless steel (SS) (UNS S31653) in 1 N hydrochloric acid (HCl) solution. Potentiostatic tests in this potential range confirmed the occurrence of current transients as a result of metastable pits, resulting in secondary loop formation. Higher minimum chloride (Cl−) concentration and low acidic pH were shown to be required for stable pit formation in type 316LN SS compared to similar alloys without N alloying. Results showed no selective anodic dissolution of any of the alloying elements in actively growing pits in type 316LN SS. Although ammonium ions (NH4+) were found within pits under suitable applied potentials in 1 N HCl and under natural corrosion in ferric chloride (FeCl3) solutions, the more anodic the potential, the less was its yield. The formation of NH4+ ions was found to be greater at more active potentials under uni...

Influence of sigma-phase formation on the localized corrosion behavior of a duplex stainless steel

Because of their austenitic-ferritic microstructures, duplex stainless steels offer a good combination of mechanical and corrosion resistance properties. However, heat treatments can lower the mechanical strength of these stainless steels as well as render them susceptible to intergranular corrosion (IGC) and pitting corrosion. In this study, a low-carbon (0.02%) duplex stainless steel is subjected to various heat treatments at 450 to 950 °C for 30 min to 10 h. The heat-treated samples then undergo ASTM IGC and pitting corrosion tests, and the results are correlated with the microstructures obtained after each heat treatment. In the absence of Cr23C6 precipitation, σ-phase precipitates render this duplex stainless steel susceptible to IGC and pitting corrosion. Even submicroscopic σ-phase precipitates are deleterious for IGC resistance. Longer-duration heat treatments (at 750 to 850 °C) induce chromium diffusion to replenish the chromium-depleted regions around the σ-phase precipitates and improve IGC resistance; pitting resistance, however, is not fully restored. Various mechanisms of σ-phase formation are discussed to show that regions adjacent to σ-phase are depleted of chromium and molybdenum. The effect of chemical composition (pitting resistance equivalent) on the pitting resistance of various stainless steels is also noted.

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.

Testing Sensitization and Predicting Susceptibility to Intergranular Corrosion and Intergranular Stress Corrosion Cracking in Austenitic Stainless Steels

Abstract In this paper, degree of sensitization (DOS) of austenitic stainless steels (SS) is defined in terms of the characteristic parameters of the chromium depletion zones: coverage (proportion of the grain boundary length covered by chromium depletion zones), width, and depth (the minimum level of chromium in the depletion zones). A sample matrix was developed that provides heat-treated samples of Type 304 SS (UNS S30400) having the same coverage developed at four different temperatures of heat treatments. The coverage was measured by quantitative metallography. The absolute value of depth was measured by the quantitative potentiostatic electrochemical test. Other electrochemical (potentiodynamic and potentiostatic) techniques were used to assess the depth of chromium depletion zones. New parameters have been developed to express the results of the potentiodynamic, potentiostatic, and electrochemical potentiokinetic reactivation (EPR) tests that reflect the contributions attributable to either coverag...

Mechanism of improved corrosion resistance of type 304L stainless steel, nitric acid grade, in nitric acid environments

Materials for critical components in nuclear fuel reprocessing plants are required to have low corrosion rates and long designed life because access for repairs is not possible. Stainless steel type 304L, nitric acid grade (NAG), is the new material suitable for such applications. It has guaranteed low corrosion rates and is not susceptible to intergranular corrosion (IGC) in nitric acid environments. The corrosion behavior of type 304L stainless steel, NAG, and type 304L stainless steel, commercial purity (CP), in nitric acid environments is investigated in detail. Studied are: microstructural mapping in the three directions (longitudinal, long transverse, and short transverse), effect of sensitization heat treatment, resolution annealing and sensitization heat treatment for the as-received and cold-worked samples of the two varieties on the resultant microstructures. The anodic polarization characteristics along the three directions for both varieties in 1N HNO3 are compared. The susceptibility of both varieties to end grain corrosion in 9N HNO3 + 1 g Cr+6/liter boiling solution is assessed, and microstructural examination of the exposed sample is carried out to compare the degree of end grain corrosion. Their susceptibility to IGC due to segregation of impurity elements to grain boundaries is also compared. It is shown that controlled microstructure (fine grain size, retained cold work, and discrete precipitation at grain boundaries) along with controlled chemical composition is responsible for improved corrosion resistance of the NAG variety. The NAG variety has much less susceptibility to corrosion along the long- and short-transverse directions and, therefore, less susceptibility to end grain corrosion. The means and consequences of controlling chemical composition are also discussed.

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.

Detecting Classical and Martensite-Induced Sensitization Using the Electrochemical Potentiokinetic Reactivation Test

Abstract The electrochemical potentiokinetic reactivation (EPR) test is commonly used to evaluate the degree of sensitization of austenitic stainless steels. It is shown in this study that the EPR test can be used to distinguish between classical sensitization, i.e., chromium depletion formation resulting from the precipitation of chromium-rich precipitates at grain boundaries and martensite-induced sensitization that takes place at lower temperatures of 450°C to 500°C. The reactivation from classical sensitization starts early, at more noble potentials than that for martensite-induced sensitization. This has been brought out in EPR results of cold-worked, warm-worked, and sensitized samples of Type 304 (UNS S30400) stainless steel.

Laser Surface Treatment for Enhancing Intergranular Corrosion Resistance of AISI Type 304 Stainless Steel

Abstract A novel laser surface modification approach to suppress sensitization in AISI 304 (UNS S30400) austenitic stainless steel is described. Surface modification of austenitic stainless steel w...

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).