H.S. Khatak

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.

Diffusion behaviour of hydrogen in nitrogen containing austenitic alloys

Abstract Electrochemical permeation technique and thermal desorption spectroscopy were used to evaluate the hydrogen diffusion, solubility, and trapping behaviour in nitrogen containing austenitic alloys. The hydrogen diffusion of all alloys obeyed Arrhenius relationships in the studied experimental temperature range. The apparent diffusion coefficients were determined and the relevant activation energies were estimated. Nitrogen at higher concentration (>0.38 wt.%) as well as chromium decrease the hydrogen diffusion whereas nickel has opposite effect. The thermal analysis of all alloys except Cr18Mn18N0.57 shows a main single peak at different heating rates indicating no significant hydrogen interaction with microstructural defects. The alloy Cr18Mn18N shows a shoulder in addition to the peak which is attributed to nitrogen–hydrogen interaction. The activation energy estimated from the thermal analysis is nearly equal to the activation energy for diffusion obtained from the Arrhenius plots of the corresponding alloys.

Failure analysis of a T-joint of AISI type 316 L stainless steel

Abstract The flow of sodium through the various circuits of a PFBR is controlled by centrifugal pumps. T-joints, made of type 316 LN stainless steel, will be used as pipe fittings to connect sodium pumps in the secondary side of the PFBR. However, two such T-joints received from the manufacturer were found to have developed fine cracks on the surface, which were seen after the pickling and passivation operation was carried out at the users site. Visual, metallographic and fractographic examinations suggested that the failure occurred due to fatigue, which initiated because of surface roughening caused by the fabrication process. Initial large grain size of the material caused surface roughening to occur during fabrication. The rough surface led to initiation of fatigue cracks during fabrication, which was not carried out in one stretch as recommended by the user. Absence of corrosion product on the surface suggested no role of corrosion in the failure. The high hardness of the material indicated non-compliance with the users specifications by the manufacturer vis-a-vis post-fabrication annealing. To avoid future failures, it was recommended that proper selection of material with respect to starting grain size should be made; qualified fabrication procedures should be adopted and followed so as to avoid cyclic loading on the component during fabrication; and appropriate solution annealing at 1323 K should be carried out to relieve cold work.

Localized corrosion studies on materials proposed for a safety-grade sodium-to-air decay-heat removal system for fast breeder reactors

The present investigation was carried out to assess the localized corrosion resistance of materials proposed for the construction of the safety-grade sodium-to-air decay-heat removal system for fast breeder reactors. The materials, such as Alloy 800,9Cr-lMo steel, and type 316LN stainless steel, in different microstructural conditions were assessed for pitting and stress-corrosion cracking resistances in a chloride medium. The results indicated that 9Cr-lMo steel in the normalized and tempered condition can be considered for the above application from the standpoint of corrosion resistance.

Improved pitting corrosion resistance of cold worked and thermally aged AISI type 316 L(N) SS by laser surface modification

Abstract Laser surface melting has been conducted on the surface of prior cold worked and aged AISI type 316 L(N) stainless steel specimens to homogenise the composition and as a result improve the corrosion resistance. Although the top layers of surface melted specimens did not show a marked improvement in corrosion resistance, the subsequent layers exposed after mild polishing showed a dramatic increase in pitting corrosion resistance and an appreciable improvement in intergranular and general corrosion. This is attributed to the dissolution of secondary phases in the melted and heat affected zones.

Corrosion Behaviour of Nitrogen Bearing Austenitic Stainless Steel Surface Diffusion Alloyed with Ti/Al Precoated Films

Abstract The present work investigates the aqueous corrosion behaviour of type 316L stainless steel (SS) containing various matrix nitrogen contents (0·015, 0·1, 0·2 and 0·56%N), surface modified by diffusion annealing of a precoated film of titanium/aluminium. Type 316L SS specimens were precoated with a Ti/Al multilayer by the electron beam deposition method and surface diffusion annealed at 1173 K for 1 h in vacuum. X-ray diffraction analysis indicated the formation of Ti3Al, Al5Ti2, Al2Ti and Al13Fe4 intermetallic phases. Nitrides such as Ti2N were also observed, particularly in high nitrogen steels. The interaction between the titanium/aluminium coating and the matrix constituents, particularly with nitrogen, was characterised by secondary ion mass spectrometry (SIMS). The nitrogen content at the modified surface increased with increase in the nitrogen content of the substrate matrix. SEM observation of cross-sectionally mounted surface modified alloys indicated the formation of thick adherent layers. The role of such intermetallic phases in corrosion resistance in both 0·5 M H2SO4 and 0·5 M NaCl is discussed in detail based on open circuit potential-time measurements, potentiodynamic polarisation studies and electrochemical impedance spectroscopy (EIS) investigations. The role of matrix nitrogen in the formation of intermetallic coatings and its role in corrosion resistance in acidic and chloride media are investigated.

Pitting induced failure of austenitic cryogenic condenser coil for acetone service

Abstract A vertical condenser made of type 304 SS material and 304L SS finned coils for acetone–methanol vapour condensation using brine solution (70:30 methanol–water) developed leakage through pinholes formed in the coils. Detailed investigations of the failed coil were made by visual and dye penetrant examination, stereomicroscopic and scanning electron microscopic observation, XRD analysis of the corrosion products, and intergranular corrosion and electrochemical investigations on the coil specimens in brine solution containing chloride. The analysis indicated that carbon steel parts in the process streams corroded and transportation of rust from them lead to the deposition of corrosion products on the stainless steel coil. Under-deposit attack in the presence of chloride ions led to severe pitting corrosion and subsequent failure. Wrong use of carbon steel in the system and ingress of chloride ions resulted in the failure. High temperatures at the bottom of the condenser led to more failures of the coils at such locations.