Surface Alloying of Nitrogen to Improve Corrosion Resistance of Steels and Stainless Steels

It is well known that the addition of nitrogen to steels and stainless steels enhances the passivity and localized corrosion resistance, in addition to improving the mechanical properties. Selective alloying of surfaces of steels and stainless steels with nitrogen could also enhance the corrosion resistance and improve the mechanical properties without affecting the bulk properties. Techniques like ion implantation, laser alloying, nitriding, etc. can be effectively used to introduce very high levels of nitrogen. In addition, these techniques can also produce modified surfaces with novel microstructures to further improve the properties. The surface alloying methods also provide an opportunity to selectively nitrogenate the surface of finished components in order to obtain better properties. The review highlights the techniques, modifications and the properties obtained further.     

Influence of nitrogen addition on microstructure and pitting corrosion resistance of austenitic weld metals

A study to investigate the role of nitrogen in improving the pitting corrosion resistance of types 316 and 304 stainless steel weld metals has been attempted. Weld metals were prepared by autogenous TIG welding process with argon-nitrogen mixture as the shielding gas. Delta-ferrite measurements, made with Magne-Gage, showed the absence of delta-ferrite in all the samples of type 316 ss weld metal whereas for type 304 ss it decreased from 3.8 to 0 FN with increase in the levels of nitrogen. Potentiodynamic anodic polarization studies carried out on weld metal samples in deaerated neutral chloride medium showed improved resistance to pitting attack with increase in the amount of nitrogen. This improvement in pitting resistance could be due to the dissolution of nitrogen and formation of inhibiting compounds at the pit sites. The ESCA results of the anodically polarized weld metal samples showed the presence of nitrogen in a compound form. SEM and optical microscopic studies of the pitted samples showed that the initiation sites for pitting attack were triple points, austenite centres and delta-ferrite/austenite interfaces.     

Influence of shape and size on residual stress in ceramic/metal joining

The effects of shape and size on the residual stress on the surfaces of silicon nitride/Invar alloy joints have been examined by means of the strain gauge method. The highest residual stress perpendicular to the interface appeared near the corners in the rectangular bond face joint. It was tensile in the silicon nitride and compressive in the Invar alloy. The highest tensile stress in the rectangular bond face joint was larger than that in the circle bond face joint. The larger the diameter of the cylindrical joint used, the larger was the tensile stress induced. The residual stress parallel to the interface was compressive in silicon nitride while that in the Invar alloy was tensile.     

Corrosion of type 316L stainless steel in molten LiCl–KCl salt

Pyrochemical reprocessing in molten chloride salt medium has been considered as one of the best options for the reprocessing of spent metallic fuels. The AISI 316L stainless steel (SS) is envisaged as a candidate material for the fabrication of components for various unit operations like salt preparation vessel, electro‐refiner and cathode processor, on which ceramic coatings with metallic bond coat will be applied by the thermal plasma spraying. The unit operation like electro‐refining is carried out in the molten lithium chloride–potassium chloride (LiCl–KCl) eutectic salt at 773 K in argon atmosphere. The corrosion behaviour of the container vessel in molten chloride salts is therefore important, hence corrosion tests were carried out in a molten salt test assembly under argon gas atmosphere. The present paper discusses the corrosion behaviour of 316L SS in the molten LiCl–KCl eutectic salt at 873 K. The 316L SS samples were immersed in the molten LiCl–KCl eutectic for 25, 100 and 250 h, while 316L SS with yttria stabilized zirconia coating was exposed for 1000 h. The exposed samples were examined by optical and scanning electron microscope for corrosion attack. The X‐ray mappings of the cross‐section of the degraded layer onto the 316L SS indicated that the mechanism of corrosion corresponds to the selective diffusion of Cr to the surface with the formation of voids below, and the formation of chromium compounds at the surface. The results of the present study indicated that the yttria stabilized zirconia coating onto the 316L SS exhibits a better corrosion resistance in molten chloride salt than with uncoated 316L SS.     

Synthesis and Characterization of Nanostructured Platinum Coated Titanium as Electrode Material

The present paper discusses the synthesis and characterization of nanostructured platinum coated titanium as electrode material. The electrodeposition of Pt particles on the Ti substrate was carried out from chloroplatinic acid at room temperature. Based on the cyclic voltammetric studies and UV-visible spectroscopy, direct reduction of Pt(IV) to Pt (0) was confirmed. The morphology of Pt particles was observed in scanning electron microscopy which revealed that the Pt particles with desired size were deposited on Ti from 5 mM electrolyte concentration at 60 s duration. The electrodeposited Pt particle on the Ti substrate subjected to hydrothermal reduction achieved complete coverage. Further microscopic observations and the energy dispersive spectroscopy results showed that the electrodeposited Pt particles acted as nuclei and enhanced further growth and coverage of particles during hydrothermal reduction. Also, morphology and surface coverage of Pt particle after hydrothermal reduction were found to be significantly influenced by the applied potential of the electrodeposition step. The electrochemical activity of the synthesized electrode was tested by conducting hydrogen adsorption-desorption study and oxygen reduction reaction.     

Corrosion Behavior of Alloy 600 in Simulated Nuclear High Level Waste Medium

Nickel-based alloys are being considered as candidate materials for the storage of high level waste. In the present investigation, Alloy 600 was assessed by potentiodynamic anodic polarization technique for its corrosion behavior in the as-received, solution annealed, and sensitized condition in 3 M HNO₃ and 3 M HNO₃ containing simulated high level waste. From the results of the investigation, it was found that the solution annealed specimen possesses superior corrosion resistance compared to the as-received and sensitized specimen. Double loop electrochemical potentiokinetic reactivation test was carried out to study the degree of sensitization. The effect of different concentrations of chloride ions in 3 M HNO₃ at 25 °C indicated tendency for pitting as the concentration of chloride ions was increased. Microstructural examination was carried out by optical microscope and scanning electron microscope after electrolytic etching. X-ray photoelectron spectroscopy study was carried out to investigate the passive film formed in 3 M HNO₃ and 3 M HNO₃ simulated high level waste.     

Characterisation of microstructural damage due to corrosion fatigue in AISI type 316 LN stainless steels with different nitrogen contents

Corrosion fatigue (CF) behaviour of AISI type 316 LN stainless steels (SS) with three different nitrogen contents was evaluated in a boiling aqueous solution of 5?M NaCl+0·15?M Na₂SO₄+2·5?ml?l^?1 HCl at a stress ratio of 0·5 and a frequency of 0·1?Hz. After the CF tests, the specimens were observed under a field emission gun scanning electron microscope (FEG-SEM) as well as an atomic force microscope (AFM) to understand the deformation mechanism which led to the failure. Slip character could be explained based on the surface deformation features observed using FEG-SEM and AFM. A slip irreversibility relation has been proposed which when applied could explain the CF behaviour of these steels with varying nitrogen contents. Increase in the nitrogen content increased the slip reversibility up to 0·14?wt-% nitrogen; however, further increase in nitrogen content had no beneficial effect on the slip reversibility.     

On the pitting corrosion resistance of nitrogen alloyed cold worked austenitic stainless steels

Pitting corrosion studies were carried out on cold worked (5%, 10%, 15%, 20%, 30% and 40%) nitrogen-bearing (0.05%, 0.1% and 0.22% N) type 316L austenitic stainless steels in neutral chloride medium. Potentiodynamic anodic polarisation study revealed that cold working up to 20% enhanced the pitting resistance, and thereafter a sudden decrease in pitting resistance was noticed at 30% and 40% cold working. Increase in nitrogen content was beneficial up to 20% cold work in improving the pitting corrosion resistance, beyond which it had a detrimental effect at 30% and 40% cold working. The role of nitrogen in influencing the deformation band width and dislocation configuration is explained based on the results of transmission electron microscopy investigations. Scanning electron microscopy observation of the pitted specimens indicated decreasing size and increasing density of pits, along the deformation bands with increasing nitrogen for 40% cold worked specimens. The macrohardness values increased as the cold working increased from 0% to 40%. X-ray diffraction studies revealed the increased peak broadening of austenite peak {0 2 2} with increase in cold working. The relationship between pitting corrosion and deformation structures with respect to nitrogen addition and cold working is discussed.     

Microstructural studies on lattice imperfections in irradiated titanium and Ti-5 Pct Ta-2 Pct Nb by X-ray diffraction line-profile analysis

Microstructural parameters such as the average domain size, effective domain size at a particular crystallographic direction, and microstrain within the domains of titanium and Ti-5 pct Ta-2 pct Nb, irradiated with 116 MeV O⁵⁺ ion, have been characterized as a function of dose by X-ray diffraction line-profile analysis (XRDLPA) using different model-based approaches. The dislocation density and stacking-fault probabilities have also been estimated from the analysis. The analysis revealed that there was a significant decrease of the average domain size with dose as compared to the unirradiated sample. The estimated values of dislocation density increased significantly for the irradiated samples and were found to be an order of magnitude more as compared to the unirradiated one. However, the dislocation density became saturated with an increase in dose. The deformation (stacking-fault) probabilities were found to be negligible even with the increase in dose of irradiation.