Dissolution Behaviors of Corrosion Products on 316LN Stainless Steel in Simulated Shutdown Acid-Reducing Water Chemistry

In order to enhance corrosion resistance of stainless steel (SS) 316LN at high temperature environments, surface modification was carried out by Si deposition and subsequent heat treatment at 900 °C for 1 h. This resulted in the formation of Fe5Ni3Si2 phase on the surface region. The surface-modified alloy was exposed to high temperature S-CO2 (650 °C, 20 MPa) and steam (650 °C, 0.1 MPa) for 500 h and evaluated for its corrosion behavior in comparison to the as-received alloy. In S-CO2 environment, the as-received SS 316LN showed severe oxide spallation and thick Fe-rich oxide formation, while the surface-modified alloy formed a continuous and adherent Si- and Cr-rich oxide layer. In steam, as-received SS 316LN formed very thick duplex Fe- and Cr-rich oxide layers. On the other hand, surface-modified SS 316LN formed notably thinner oxides, which could be attributed to the formation of Si-rich oxide under outer Fe-rich oxides on the surface-modified alloy. Thus, in view of the weight changes, oxide thickness, and morphologies of the two conditions, it was found that Si diffusion coating was effective in improving the corrosion resistance of SS 316LN in both S-CO2 and steam environments.

The specific activated flux has been developed for enhancing the penetration performance of TIG welding process for autogenous welding of type 304LN and 316LN stainless steels through systematic study. Initially single-component fluxes were used to study their effect on depth of penetration and tensile properties. Then multi-component activated flux was developed which was found to produce a significant increase in penetration of 10-12 mm in single-pass TIG welding of type 304LN and 316LN stainless steels. The significant improvement in penetration achieved using the activated flux developed in the present work has been attributed to the constriction of the arc and as well as reversal of Marangoni flow in the molten weld pool. The use of activated flux has been found to overcome the variable weld penetration observed in 316LN stainless steel with <50 ppm of sulfur. There was no degradation in the microstructure and mechanical properties of the A-TIG welds compared to that of the welds produced by conventional TIG welding on the contrary the transverse strength properties of the 304LN and 316LN stainless steel welds produced by A-TIG welding exceeded the minimum specified strength values of the base metals. Improvement in toughness values were observed in 316LN stainless steel produced by A-TIG welding due to refinement in the weld microstructure in the region close to the weld center. Thus, activated flux developed in the present work has greater potential for use during the TIG welding of structural components made of type 304LN and 316LN stainless steels.

Comparative study on the fatigue crack growth behavior of 316L and 316LN stainless steels: effect of microstructure of cyclic plastic strain zone at crack tip

For the future sodium-cooled fast reactors (SFRs), which are envisaged with a design life of 60 years, nitrogen-enhanced 316LN austenitic stainless steel (SS) with improved high-temperature properties is being developed. To optimize the enhanced nitrogen content in 316LN SS, the effect of nitrogen on its tensile, creep and low cycle fatigue behavior has been investigated. For different heats of 316LN SS containing 0.07-0.22 wt% nitrogen, the tensile and creep properties increased with increase in nitrogen content, while low cycle fatigue properties peaked at 0.14 wt% nitrogen. Finally, based on the evaluation of the hot cracking susceptibility of the different heats of 316LN SS with varying nitrogen content, using the Varestraint and Gleeble hot-ductility tests, the nitrogen content for the nitrogen-enhanced 316LN SS has been optimized at a level of 0.14 wt%. The 0.14 wt% nitrogen content in this optimised composition shifts the solidification mode of the weld metal to fully austenitic region, including that due to dilution of nitrogen from the base metal, thereby increasing its hot cracking susceptibility. This necessitated development and qualification of welding electrodes for obtaining weld metal with 0.14 wt% nitrogen by optimising the weld metal chemistry so as to obtain the requisite delta ferrite content, tensile properties, and very importantly impact toughness both in the as-welded and aged conditions. Studies on localised corrosion behaviour of nitrogen-enhanced 316LN SS indicated the beneficial effect of nitrogen addition to sensitization, pitting, intergranular corrosion, stress corrosion cracking and corrosion fatigue.

Effects of Zn injection into boric acid and lithium hydroxide solutions on the corrosion behaviors of 316LN stainless steel in simulated hot functional test high-temperature pressurized water

Nitrogen-alloyed 316LN stainless steel is used as a structural material for high temperature fast breeder reactor components. With a view to increase the design life of the components up to 60 years and beyond, studies are being carried out to develop nitrogen alloyed 316LN stainless steel with superior tensile, creep and low cycle fatigue properties. This paper presents the results from studies on the influence of nitrogen on the high temperature creep properties of this material. The influence of nitrogen on the creep behaviour of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt%. Creep tests were carried out at 923 K at stress levels 140, 175, 200 and 225 MPa. Creep rupture strength increased substantially with increase in nitrogen content. The variation of steady state creep rate with stress showed a power law relationship. The power law exponent varied between 6.4 and 13.7 depending upon the nitrogen content. Rupture ductility was generally above 40% at all the test conditions and for all the nitrogen contents. It was observed that the internal creep damage and surface damage decreased with increase in nitrogen content. Fracture mode was found to generally shift from intergranular failure to transgranular failure with increasing nitrogen content.

A286 alloy and 304LN stainless steel were chosen for support struts used in the construction of a mirror fusion testing facility, which is subjected to the cryogenic temperature. Electroslag remelting technology (ESR) was adopted to manufacture A286 alloy. A program was initiated to investigate the cryogenic mechanical properties with emphasis on the effect of the grain size of A286 alloy.The test results revealed that: (1) A286 alloy showed little effect of grain size on the tensile properties, while the fracture toughness properties were affected by the grain size, especially at 4 K; (2) 304LN stainless steel produced almost the same tensile and fracture toughness properties as A286 alloy.The temperature dependence of the mechanical properties of 304LN stainless steel and A286 alloy also is discussed. The support strut forgings produced excellent quality, and the proof load test results were indicative of the no plastic deformation during applications.

Fatigue fracture surfaces and crack morphologies of 316LN stainless steel that test in a simulated AP1000 first-loop water and air environment were investigated by SEM, LSCM and EBSD. The results showed that, the fatigue crack initiated at persistent slip band, impurities and grain boundary, and then propagated in a trans-granular manner with typical fatigue striations. Characteristics of corrosion fatigue, such as brittle fatigue striation, rhomboid corrosion product and the trace of corrosions were found on the fracture surface of first-loop water environment specimen. The strain on first-loop water environment specimen is unevenly distributed surrounding the crack, and the gradient is not obvious, while that on air environment ones is evenly distributed , and the distribution gradient is associated with the distance of crack from . The fatigue crack propagation was accelerated in the first-loop water environment, and the EAC mechanism is most likely to be HIC.

Designing multi-element alloy compositions to achieve target performance was the first step in the development of modern materials, but the traditional trial-and-error experiment seriously restricted the development of new materials due to its low efficiency. In this investigation, the composition design of 316LN austenitic stainless steel for enhanced liquid-hydrogen storage using multi-crucible synchronous metallurgy in a high-flux experiment was proposed. Sixteen groups of as cast austenitic stainless steel samples with different compositions were smelted using high-flux material preparation. Then, through the observation of their microstructures, the chemical composition that best matched the target performance was finally selected. The results show that high-throughput experiments can greatly improve the efficiency of composition design optimization of new stainless steel products. At the same time, this investigation also analyzed the elemental composition of δ-ferrite and the method for effectively controlling the δ-ferrite content. In 316LN stainless steel, Cr and Mo were easily enriched in ferrite grains or grain boundaries, forming Cr and Mo enriched regions. This resulted in a gradient transition of Cr and Mo from the ferrite region to the austenite region, forming galvanic corrosion. In this study, the distribution of Cr, Mo and other elements in 316LN stainless steel was studied by means of a metallographic microscope, electron probe microanalysis, transmission electron microscope and scanning electron microscope. In addition, the relationship between the ferrite content and chemical composition was explored. Finally, it was determined that high-temperature, long-term sensitization treatment is an effective method for controlling the δ-ferrite content.

Mechanical property comparisons between CrCoNi medium-entropy alloy and 316 stainless steels

The aim of this report is to examine the influence of sensitization on the mechanical properties of AISI grade 304LN stainless steel with special emphasis on its fracture toughness. A series of stainless steel samples has been sensitized by holding at 1023 K for different time periods ranging from 1 to 100 hours followed by water quenching. The degree of sensitization (DOS) for each type of the varyingly heat-treated samples has been measured by an electrochemical potentiodynamic reactivation (EPR) test. The microstructures of these samples have been characterized by optical metallography, scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) analyses, together with measurements of their hardness and tensile properties. The fracture toughness of the samples has been measured by the ball indentation (BI) technique and the results are validated by conducting conventional J-integral tests. It is revealed for the first time that the fracture toughness and ductility of AISI 304LN stainless steel deteriorate significantly with increased DOS, while the tensile strength (TS) values remain almost unaltered. The results have been critically discussed in terms of the depletion of solid solution strengtheners, the nature of the grain boundary precipitations, and the strain-induced martensite formation with the increasing DOS of the 304LN stainless steel.

Effect of grain boundary engineering on corrosion fatigue behavior of 316LN stainless steel in borated and lithiated high-temperature water

The corrosion behaviors of two materials 316L Stainless Steel and 316LN Stainless Steel have been investigated for use as biomaterials. These samples were electrophoretically coated with Dicalcium phosphate dihydrate, and dip coated with polyvinyl alcohol. Time, current, concentration and voltage were the variables during electrophoresis. Dip coating was done for the same periods of time as was done during electrophoresis. Corrosion resistance properties were measured in Ringer’s solution by Gamry Potentiostat. The ICORR and ECORR values were estimated using Gamry Echem Software and Tafel’s extrapolation method. Coated samples were immersed in SBF solution for different periods of time, viz., 1 second, 24hours, 72hours and 1week and then further ICORR and ECORR values were estimated in Ringer’s solution. For coated samples Electrochemical Impedance Spectroscopy were also done. Different parameters like Rp, alpha, Wd of EIS were used to evaluate the effectiveness of the coatings. Comparison of corrosion resistance among the coated samples revealed a few interesting characteristics. While DCPD coated Stainless Steel showed considerable improvement in corrosion resistance compared to as received sample, dip coated samples did not show appreciable improvement. Coated 316L shows better corrosion resistance than 316LN. Dip coated 316LN shows better corrosion resistance than 316L. So Electrophoretic Deposition gave much better coating in comparison to Dip coating. Coated samples were further studied by The Scanning Electron Microscope and Energy Dispersive X-Ray Spectroscopy. While SEM was done to ascertain uniformity of coating, EDAX was done to see the variation of calcium deposition as a function of different deposition parameters. Electrophoretic deposition gave much better coating and uniform variation of calcium compared to dip coating.

Intergranular corrosion (IGC) resistance of types 304LN and 316LN stainless steels (SS) thermally aged at 823, 873, and 923 K for various durations was assessed by ASTM A262 practice A test (electrolytic etch test) and electrochemical potentiodynamic reactivation (EPR) test. The results indicated that the type 316LN SS has significantly improved IGC resistance compared to 304LN SS. Based on the results of these tests, time-temperature-sensitization (TTS) diagrams were developed for both alloys. The secondary precipitates formed during thermal aging treatments were electrochemically extracted and analyzed by X-ray diffraction (XRD) to determine the types of precipitates formed during the aging treatments. The results indicated that the precipitates were mostly of M23C6 carbides.

Nano composite coatings are used to modify the substrate properties for a better life of the structures exposed to saline environments. Using a method known as spin coating, the authors of this study coated 304LN stainless steel with a mixture of TiO2 and CeO2 nano particles. The ratio of TiO2 to CeO2 in the coatings was 1:5:10:20 by weight percent. Stainless steel 304 was used as a substrate for the deposition of coatings since it is reasonably affordable. The effect that 3.5 weight percent NaCl solution had on the wett ability of the coating and its resistance to corrosion was investigated. The end effect is improved corrosion resistance in line with the rising coating weight percentage. After being exposed to the solution for 600 hours, the corrosion was significantly decreased. X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled to energy dispersive spectroscopy were used to investigate the deposits (EDS). The nano composites’ inherent physical features ensure their continued presence on the surface. The ratio of surface area to volume is satisfactory. According to the contact angle, the amount of water that is spreading out is extremely little. The photo produced cathodic protection feature of the TiO2-CeO2 coating is shown by the results of spin coating methods applied on 304LN stainless steel that was previously coated with TiO2-CeO2. After being subjected to the corrosion test, the coating made of 10% TiO2-CeO2 exhibited no signs of pitting or pinholes. The structural equation modeling (SEM) studies backed this conclusion. When it comes to the resistance to corrosion, the results produced by the contact angle are better. As a consequence of this finding, it was concluded that the coating could withstand the NaCl attack.