Grain Boundary Chromium Depletion Research Articles

Segregation effects on the corrosion behaviour of a phosphorus-doped AISI type 304L stainless steel

The effect of grain boundary (GB) segregation on intergranular stress corrosion cracking (IGSCC) in hot water environments at 150°C and 250°C was studied in a P-doped AISI type 304L stainless steel. The extent of segregation was measured by an exposure test in boiling 5 N HNO 3 + 8g/L K 2Cr 2O 7 solution as well as by a potentiostatic etch test at +1325 mV (SHE) in 5 N H 2SO 4 solution. Although GB segregation was detected in all the aged specimens, IGSCC was shown by only the specimens aged for 550°C/1000 h. The results suggest that it is the GB chromium depletion, rather than the segregation of phosphorus at the GBs, that controls IGSCC of stainless steels in the hot water environments studied.

Mechanisms of deformation-induced grain boundary chromium depletion (sensitization) development in type 316 stainless steels

Deformation accelerates the development of grain boundary chromium depletion (GBCD), or sensitization, in type 316 austenitic stainless steels (SS). Quantitative assessment of the degree of sensitization (DOS) using the electrochemical potentiokinetic reactivation (EPR) test indicates that the acceleration in GBCD is a function of the amount of strain in the material and temperature of isothermal sensitization treatment. A systematic increase in strain from 0 to 20 pct yields a continuous increase in EPRDOS values below 700°C, while at higher temperatures, a threshold strain of 6 to 10 pct is required to cause accelerated GBCD development. Straining SS above 20 pct also produces higher amounts of chromium depletion, though the (intergranular) sensitization susceptibility of the material could not be quantitatively evaluated due to the presence of grain matrix or transgranular corrosion. Classical C-curve precipitation-sensitization behavior was also noted for strained and unstrained materials, though strain moved the C-curves to the left. Microstructural evaluation of sensitization revealed a systematic increase in grain boundary and twin boundary corrosion on EPR attack surfaces with strain, which corroborated the deformation-induced acceleration of EPRDOS. A time-temperature-strain dependence of transgranular corrosion was also identified on EPR-etched samples strained above 20 pct. These were also reflected in transmission electron microscope (TEM) observations of higher grain boundary carbide precipitation on strainedvs unstrained specimens and site-specific carbide precipitation on deformation sites in the material. Kinetic and thermodynamic modeling of deformation effects on carbide precipitation and depletion development in type 316 SS indicated that strain induces a reduction in the activation barrier to diffusion (Q) a and thermodynamic barrier to nucleation (ΔG *) during the precipitation-depletion process. The lowering ofQ a with strain caused chromium diffusivity and depletion development to be accelerated in strainedvs unstrained materials and appears to be due to increased dislocation pipe diffusion with strain. Reduction of ΔG * with strain was related to an increase in the free energy change of the grain boundary (ΔG) gb and accelerated carbide precipitate nucleation in deformed SS. The effect of strain on the kinetics and thermodynamics of the precipitation-depletion process decreases with increasing temperature.

Deformation effects on chromium diffusivity and grain boundary chromium depletion development in type 316 stainless steels

This paper reports on austenitic stainless steels (SS) susceptible to the development of grain boundary chromium depletion (GBCD), or sensitization, during thermomechanical (TM) treatment. The depletion is induced by the precipitation of intergranular (IG) chromium-rich M23C6 carbides during the TM processes. The presence of GBCD produces localized breakdown of the chromium-rich passive oxide film that protects the SS. This renders the SS susceptible to IG corrosion and IG stress corrosion cracking (SCC) in certain aqueous environments. The development of GBCD is dependent on the thermodynamics of carbide precipitation and kinetics of chromium diffusion, both of which may be altered by thermal and strain cycles produced during TM treatment. A thermodynamic and kinetic evaluation of temperature and strain effects on GBCD phenomena can, therefore, be used to understand trends in precipitation-depletion processes occurring in SS during TM treatment. This can be extended further to develop a mechanistic understanding of TM effects on microstructure, microchemistry and property development in materials.

Quantitative mapping of time-temperature-strain effects on sensitization of type 316 stainless steels

Austenitic stainless steels (SS) are susceptible to the development of sensitization, or grain boundary chromium depletion (GBCD), during thermomechanical (TM) processing and fabrication treatments. Precipitation of chromium-rich M{sub 23}C{sub 6} carbides occurs on grain boundaries during TM treatment, leading to chromium-depletion in the vicinity of the interface. The induction of sensitization has been a recurring cause for intergranular (IG) corrosion and IG stress corrosion cracking (SCC) of primary and secondary power plant components for many years, and is also now being recognized as a potential cause for failure in SS containers used for storage and transport of nuclear and toxic waste materials. Research on sensitization development is, thus, important for understanding phenomena of IG corrosion and IGSCC in SS. Evaluation of thermal and strain cycle effects on carbide precipitation-chromium depletion development in SS also provides a key towards fundamentally understanding mechanisms of precipitate nucleation-growth in materials. While thermal effects on GBCD in SS have been studied for a range of isothermal and continuous cooling heat treatments, research on strain-induced sensitization has only been performed for limited strain and heat treatment schedules. Bain et al observed that straining of Type 304 SS about 50% caused development of accelerated, but less severe,more » sensitization due to precipitation of carbides within transgranular (TG) versus grain boundary regions. Later Tedmon, Vermilyea and Broecker found that TG depletion could also be rapidly desensitized (healed) during heat treatment. Bariant and Ritter observed TG corrosion in heavily deformed Types 304 and 316 SS during (450-800{degrees}C) heat treatment, but considered the onset of sensitization to be enhanced over annealed SS due to absence of previously reported rapid healing.« less

Quantitative Modeling of Sensitization Development in Austenitic Stainless Steel

Abstract Existing capabilities to quantitatively model carbide precipitation and sensitization development in austenitic stainless steels are assessed and critically analyzed. A theoretically based, empirically modified model is described that predicts carbide nucleation kinetics, chromium depletion characteristics, and material degree of sensitization (DOS) as measured by the electrochemical potentiokinetic reactivation (EPR) test. Individual model components are validated by correlation to the available database including direct comparisons to grain-boundary chromium depletion. The model is shown to quantitatively predict isothermal sensitization development in a large number of commercial type 304 (UNS S30400) and 316 (UNS S31600) stainless steel (SS) heats. This work represents a first step to evolve methods that realistically assess microstructural characteristics and structural reliability in stainless steel components and weldments.

Detecting Grain-Boundary Chromium Depletion in Inconel 600

Techniques to evaluate grain-boundary chromium depletion in Inconel Alloy 600 were investigated. Procedures studied were a modified Huey test, reactivation polarization, magnetic permeability measurements, and eddy current measurements. Results from these tests were correlated with susceptibility to stress-assisted intergranular cracking in polythionic acid. Thermally treated Inconel Alloy 600 steam generator tubing was the principal source of material evaluated, but experimental heats of Ni-Cr-Fe alloys with 8–18 wt.% Cr were prepared to determine the critical chromium level below which stress-assisted intergranular cracking occurs; this critical chromium content was found to be between 9.8 and 11.7 wt.%. All four techniques were considered suitable to evaluate grain-boundary chromium depletion; the modified Huey test and reactivation polarization technique showed a greater sensitivity than the magnetic permeability and eddy current measurements.

Microstructural aspects of the thermal treatment of Inconel alloy 600

It has been reported that the stress corrosion cracking (SCC) resistance of Inconel 600 can be improved by a thermal treatment in the carbide precipitation regime. Microstructural changes that occur when Inconel 600 is thermally treated in the temperature range 593–870°C (1100–1600°F) have therefore been studied in order to provide a better understanding of the factors controlling the SCC resistance of thermally treated Inconel 600. A total of 13 thermal treatments were used to provide a wide range of grain-boundary microstructures. Microstructural analysis concentrated on three aspects of grain-boundary structure: (1) grain-boundary precipitate morphology, (2) grain-boundary chromium depletion, and (3) grain-boundary segregation. There was a wide range of grain-boundary precipitate morphologies with the associated chromium depletion being prevalent in material annealed at the lower temperatures for the shorter times. Phosphorus was segregated to the grain boundaries in all the thermally treated material. These microstructural observations were correlated with previously published data on the SCC resistance in caustic and lead-doped water environments. The maximum improvement in intergranular SCC resistance correlated with the presence of a semicontinuous grain-boundary carbide precipitate.