N. Parvathavarthini

Influence of metallurgical variables on sensitisation kinetics in austenitic stainless steels

Abstract Austenitic stainless steels are the most favoured construction materials for various components required in chemical, petrochemical, fertiliser and nuclear industries. However, these steels are prone to sensitisation. In the sensitised condition, the steels are quite susceptible to intergranular corrosion and intergranular stress corrosion cracking in chloride and caustic environments resulting in the premature failure of the fabricated components during precommissioning and service periods. The topic of sensitisation has been of interest in studies worldwide. The studies included mechanism of sensitisation, test methods, data generation, and influence of several variables and protection methods. This article covers a review of important contributions to this topic with special emphasis on the influence of metallurgical variables. The present state of understanding is discussed in the following areas: mechanisms, test methods, sensitisation diagrams, influence of chemical composition, cold work and grain size, low temperature sensitisation, modelling and laser surface treatment. The need for further investigation in certain areas is highlighted.

Surface modification of type 304 stainless steel with duplex coatings for corrosion resistance in sea water environments

Abstract An attempt was made to improve the localised corrosion resistance of AISI type 304 stainless steel by modifying the surface with duplex coating (titania over ceria). Simple chemical immersion method was adopted for ceria coating, whereas sol–gel route with dip coating method was followed for titania coating. The adherence of the coating was evaluated by ASTM D3359 standard, and the composition of the duplex coatings was evaluated by X-ray photoelectron spectroscopy. The experimental parameters, namely, bath composition, temperature, immersion time and withdrawal speed of the specimens, were optimised to get homogeneous adherent coating. The localised corrosion resistance of ceria, titania and duplex coatings in sea water was studied by potentiodynamic anodic polarisation experiments. These coatings were characterised also by optical and scannning electron microscopes for analysing their pit morphologies. Duplex coated 304 SS exhibited superior corrosion resistance, and the probable reason for the improvement is discussed in detail in this investigation.

A silver nanoparticle loaded TiO2 nanoporous layer for visible light induced antimicrobial applications.

A nanoporous TiO2 layer was formed on commercially pure titanium by a simple anodization method in aqueous hydrofluoric acid (HF) medium. Silver nanoparticles (AgNP) were loaded into the nanoporous TiO2 layer by UV light irradiation. The morphology, chemical composition and photocatalytic activity of the modified titanium surfaces were characterized by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and UV-vis absorption spectroscopy techniques. The redox behavior of the AgNP loaded TiO2 layer was analyzed by cyclic voltammetry (CV) studies. The impedance behavior of the nanoporous TiO2 layer with and without AgNP was investigated by electrochemical impedance spectroscopy (EIS). The antibacterial effect of the AgNP loaded TiO2 layer was evaluated using Pseudomonas sp. and Bacillus sp. cultures. The efficacy of this modified layer to act as an antibacterial agent to minimize biofouling of titanium is demonstrated in this investigation.

Electrochemical techniques for estimating the degree of sensitization in austenitic stainless steels

Abstract The thermal exposure of austenitic stainless steels in the temperature range of 500°C–800°C results in the precipitation of chromium-rich M23C6 carbides along the grain boundaries and concomitant chromium depletion. When chromium level falls below 12% at such depleted zones, the material is said to be sensitized. When the sensitized material is exposed to corrosive media, intergranular corrosion (IGC) and intergranular stress corrosion cracking take place. ASTM standard A262-13 is available for detecting the susceptibility to IGC. However, the extent of chromium depletion of shop-fabricated or field-constructed components is required to determine the presence or absence of sensitization for critical applications. In this review, various electrochemical techniques that can be employed to assess the degree of sensitization (DOS) in austenitic stainless steels are critically reviewed. Techniques such as electrochemical potentiokinetic and potentiostatic reactivation with recent modifications for the accurate estimation of the DOS are analyzed. The possibility of using AC impedance technique and electrochemical noise technique for assessing the DOS is also presented. The merits and demerits of each technique and areas where further research should be focused are presented in this review.

Corrosion characteristics of sulfate-reducing bacteria (SRB) and the role of molecular biology in SRB studies: an overview

Abstract Sulfate-reducing bacteria (SRB), an anaerobic bacterial group, are found in many environments like freshwater, marine sediments, agricultural soil, and oil wells where sulfate is present. SRB derives energy from electron donors such as sulfate, elemental sulfur or metals, and fermenting nitrate. It is the major bacterial group involved in the microbiologically influenced corrosion (MIC), souring, and biofouling problems in oil-gas-producing facilities as well as transporting and storage facilities. SRB utilizes sulfate ions as an electron acceptor and produce H2S, which is an agent of corrosion, causing severe economic damages. Various theories have been proposed on the direct involvement of H2S and iron sulfides in corrosion; H2S directly attacks and causes corrosion of metals and alloys. Many reviews have been presented on the aforementioned aspects. This review specifically focused on SRB corrosion and the role of molecular biology tools in SRB corrosion studies viz. cathodic and anodic depolarization theories, corrosion characteristics of thermophilic SRB and influence of hydrogenase, temperature, and pressure in thermophilic SRB corrosion, SRB taxonomy, molecular approaches adopted in SRB taxonomical studies, sulfate and citrate metabolism analyses in completed SRB genomes, and comparative studies on SRB’s dissimilatory sulfite reductase structures.

Influence of Prior Deformation on the Sensitization Kinetics of Nitrogen Alloyed 316L Stainless Steels

This paper presents the influence of prior deformation on the sensitization kinetics of nitrogen-alloyed 316L stainless steels. Systematic investigations were carried out for two varieties of 316L SS containing (i) 0.025% C and 0.14% N; (ii) 0.033% C and 0.11% N. Using ASTM standard A262 Practice A and E tests, time-temperature-sensitization diagrams were constructed for as-received as well as 5-25% cold-worked materials. Using these TTS diagrams, critical cooling rates (CCR) above which there is no risk of sensitization were calculated. TTS diagrams established for these two stainless steels will be useful for avoiding time-temperature combinations that may result in sensitization and susceptibility to IGC. These CCR obtained can be used to optimize heating rates/cooling rates to be followed which will not lead to sensitization during solution annealing, stress-relieving, and dimensional stabilization of critical components for fast breeder reactors.