Showing papers by "U. Kamachi Mudali published in 2016"

Load-bearing metallic implants: electrochemical characterisation of corrosion phenomena

Abstract: Surgical management with implant devices is one of the major workable solutions as the pain has to be alleviated and patients have to resume their physical activities. The growing number of implant surgeries coupled with parallel increase in the revision surgeries of hip and knee implants has brought to focus the need to predict and understand the causes of failures of implant materials. The electrochemical and mechanical properties play a critical role in determining the lifespan of an implant. The onset of any one form of corrosion effects the mechanical properties of the implant leading to removal of the implant. The present article deals with the need for orthopaedic and dental implants, the metals and alloys currently used and their electrochemical corrosion and mechanical behaviour under in vitro conditions. Surface engineering of metals and alloys with coatings offers an ideal solution to enhance the implant performance in vivo with superior osseointegration and blood compatibility.

Superhydrophobic coating on modified 9Cr – 1Mo ferritic steel using perfluoro octyl triethoxy silane

Abstract: Lotus leaves are a typical example of superhydrophobic surface. Numerous studies have confirmed that surface morphology possessing micro- and nanoscale roughness along with a low surface energy material coating leads to apparent water contact angle (WCA) ⩾150°. In nuclear power plants, modified 9Cr–1Mo ferritic steel is the favoured steam generator tubing material. During transit, storage and installation, SHP surface on modified 9Cr–1Mo ferritic steel can impart good corrosion resistance to retain the integrity of the specimen during operation. In this study, SHP surface of modified 9Cr–1Mo ferritic steel with a WCA of 150±1° was successfully achieved by polishing, etching, perfluoro octyl triethoxy silane coating and baking. The WCA and contact angle hysteresis were measured. The surface morphology and the composition were characterised by atomic force microscopy and attenuated total reflection–infrared spectroscopy respectively. Superhydrophobicity and its related theories are also discussed in this paper.

Kinetics of extraction of nitric acid into binary mixture of tri-n-butyl phosphate and normal paraffin hydrocarbon

Abstract: The extraction kinetics of nitric acid with tri- n -butyl phosphate (TBP) dissolved in normal paraffin hydrocarbon (NPH) was investigated. Experiments were carried out in a constant area stirred cell. Influences of stirring speed, temperature, specific interfacial area and concentrations of extractant and nitric acid on the extraction rate of nitric acid were studied. The objective of the present study was to identify the reaction location, extraction reaction regime and to determine intrinsic rate constant of the reaction and mass transfer coefficients. A systematic approach was adopted for the determination of these rate parameters. Based on experimental results, it is concluded that the liquid-liquid reaction takes place at the interface and the overall rate is controlled by both the intrinsic kinetics as well as the mass transfer of the species involved.

Reduced graphene oxide/nano-Bioglass composites: processing and super-anion oxide evaluation

Abstract: 45S5 Bioglass with a mean particle size in the nano regime were synthesized and fabricated with rGO sheets using three different strategies. The fabricated nanocomposites were analysed for their bond formation and defects. Morphology, size and distribution of n-BG particles on rGO were visualised. An attempt to understand the superoxide anion production by n-BG particles, rGO and its nanocomposites was made.

Corrosion behaviour of 304LN activated tungsten inert gas and flux-cored arc weld metals

Abstract: Activated tungsten inert gas (A-TIG) and flux-cored arc (FCA) weld metals were prepared using 304LN stainless steel plate. The weld metals were thermally aged at 923, 973 and 1023 K for 100 h to study the decomposition of initial δ-ferrite in A-TIG (∼10 ferrite number (FN)) and FCA (∼5 FN) weld metals into secondary phases like M23C6 carbides, χ and σ. Ferrite number is the measurement of δ-ferrite based on the principle of magnetic property using ferritescope. Preliminary microstructural studies revealed the formation of carbides in FCA weld metals aged at 923 K for 100 h, which was correlated with higher carbon content (0.04 wt-%), and also ageing at higher temperature transformed δ-ferrite into χ/σ phases. However, A-TIG weld metals showed the transformation of δ-ferrite mainly into χ/σ phases. The δ-ferrite transformation kinetics was found to be sluggish in A-TIG weld metals compared to FCA weld metals. This difference was attributed to the difference in the carbon contents of A-TIG and FCA welds. Ac...

Mass transfer and hydrodynamic studies in a 50 mm diameter centrifugal extractor

Abstract: Centrifugal extractor is a preferred device for process intensification in solvent extraction. The complexities of high radiation damage to solvent and resultant degradation of solvent performance also necessitate the need for fast contactors like centrifugal extractors in the nuclear fuel recycle operations. In this study, mass transfer performance as well as hydrodynamic characterization was reported for a 50 mm diameter annular centrifugal extractor. Mass transfer studies included extraction and stripping experiments with solvent-aqueous pair of 30% Tri- n -butyl phosphate/ n -dodecane and aqueous nitric acid solutions for different operating conditions like rotor speed, combined throughput and phase flow ratios. Flooding zone was characterized. Dispersed-phase holdups in annular as well as rotor regions and interfacial area for mass transfer were measured at different operating conditions. Non-ideal effects were studied by conducting residence time distribution analysis using aqueous phase as well as organic phase pulse tracer experiments. Various models like dispersion and tanks-in-series models were applied to experimental data. The contactor in study, was approximated as N number of tanks in series with a finite dead volume.

Equilibrium and kinetics of adsorption of ruthenium on activated charcoal from nitric acid solutions

Abstract: Equilibrium and kinetics of adsorption of ruthenium on activated charcoal were investigated experimentally using batch method under different conditions. The Freundlich isotherm model was used to describe the partitioning behaviour of ruthenium between the solution and adsorbent phase. A pore diffusion model, coupled with Freundlich isotherm equation was used to interpret the experimentally obtained adsorption kinetic curves at different concentrations of ruthenium. The model was found to fit with the experimental data fairly well. The effective diffusivity and external mass transfer coefficient were estimated to be in the range 10 −8 cm 2 s −1 to 10 −4 cm s −1 respectively.

Comparison of SCC Behavior of 304L Stainless Steels With and Without Boron Addition in Acidic Chloride Environment

Abstract: The stress corrosion cracking (SCC) behavior of 304L B4 grade borated stainless steel (SS) as well as 304L SS was investigated by constant load and slow strain rate testing (SSRT) techniques. The microstructure, pitting, and SCC behavior of borated SS in the as-received, sensitized, and solution-annealed conditions were analyzed. Potentiodynamic anodic polarization and double loop electrochemical potentiokinetic reactivation (DLEPR) experiments were carried out to find out pitting corrosion resistance and degree of sensitization (DOS). The number of boride particles (composed of Cr, Fe, and B) were highest for the specimen solution annealed at 1423 K/2 h. Solution-annealing treatment at 1423 K/4 h was found to be beneficial in improving the corrosion resistance of borated 304L SS. Although the borated 304L SS exhibited a higher DOS, it showed improved pitting corrosion resistance compared to 304L SS. Constant load experiments revealed the time to failure to be the highest for the specimen solution annealed at 1423 K/4 h. SCC susceptibility index (Iscc) values obtained from SSRT tests were lower for solution-annealed borated 304L SS compared to the as-received and sensitized conditions. The improved SCC resistance of borated 304L SS was attributed not only to the solution-annealing treatment but also the higher stacking fault energy (SFE) value compared to 304L SS.

Characterizing biofilms for biofouling and microbial corrosion control in cooling water systems

Abstract: Purpose The purpose of this paper is to study the metal-Microbe interaction playing a crucial role in microbiologically influenced corrosion (MIC) and biofouling of materials in cooling water systems. Treatment regimens should be planned based on this understanding. Design/methodology/approach Attempts were made in the past decades to characterize and understand biofilm formation on important power plant structural materials such as carbon steel (CS), stainless steel (SS) and titanium in fresh water and in seawater to achieve better control of biofouling and minimize MIC problems. Findings This report presents the results of detailed studies on tuberculation-formed CS because of the action of iron-oxidizing bacteria and the effects of algae- and bacteria-dominated biofilms on the passivity of SS. The preferential adhesion of different bacterial species on SS under the influence of inclusions and sensitization was studied in the context of preferential corrosion of SS weldments due to microbial action. Detailed characterization of biofilms formed on titanium (the likely condenser material for fast breeder reactors) after exposure for two years in Kalpakkam coastal waters revealed intense biofouling and biomineralization of manganese even in chlorinated seawater. Studies on the effectiveness of conventional fouling control strategies were also evaluated. Originality/value The detailed studies of different metal/biofilm/microbe interactions demonstrated the physiological diversity of microbes in the biofilms that were formed on different materials, coupling their cooperative metabolic activities with consequent corrosion behaviour. These interactions could enhance either anodic or cathodic reactions and exploit metallurgical features that enhance biofilm formation and/or the capacity of microbes to mutate and overcome mitigation measures.

Superhydrophobic Coating on Mod.9Cr-1Mo Ferritic Steel for Enhancing Corrosion Resistance and Antibacterial Activity

Abstract: A novel and facile approach like polishing as well as etching followed by dip coating using low surface energy material, perfluoro octyl tri ethoxy silane was attempted to create superhydrophobic (SHP) surface modification on Mod.9Cr-1Mo. A water contact angle (CA) of 150° ± 1° was achieved with advancing angle (θ adv ) as 163°, Receding Angle (θ rec ) as 148°, contact angle hysteresis (CAH) as 15o and tilting angle (TA) as 7o. Characterization of the surface by atomic force microscopy and surface profilometer revealed the micro–nano topography on the surface ranging between 63.5 nm and 4.3 µm. The enhanced corrosion performance and the antibacterial properties were confirmed using electrochemical impedance spectroscopy (EIS) and epifluorescence microscopy.

Light scattering investigation of the hydrolyzed thorium(IV) colloids and polymers

Abstract: The hydrolytic polymerization of Th(IV) has been investigated by light scattering technique in 10-20 mM and 0.05-0.4 M Th(IV) solutions. The present investigation shows that well defined colloids are formed at two different Th concentration and pH domains. The ini- tially formed colloids at lower concentration are larger than those formed at higher concentration, which may be due to more crystalline nature of colloids formed at higher con- centration. The colloidal polymers formed are of high molecular weight. The results of this work are a clear indication that Th(IV) hydrolysis do not differ from hydrolysis of Pu(IV).

Corrosion Behavior of Laser Melted Alumina–40 wt% Titania Coated High Density Graphite in Molten Salt

Abstract: Plasma sprayed alumina–40 wt% titania (A40T) coatings deposited on high density graphite were laser melted and immersed in molten LiCl–KCl salt for 24 h at 873 K to compare the corrosion behavior of the coatings before and after laser surface treatment. The as-sprayed and laser melted coatings after exposure to molten salt were characterized to evaluate their microstructure and phase composition. Uniform microstructure and homogeneous surface generated by laser melting process facilitated enhanced protection of the A40T coatings from molten salt attack. Absence of phase change in the coating after corrosion test, implied that no reaction occurred between the molten salt and coating. While keeping other beam parameters constant, increasing the laser average power from 6.4 to 8 W had not only resulted in improving the surface quality and the corrosion resistance of A40T coating, but also reduced the penetration and settling of molten salt on the coating.

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

Abstract: 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.

Experimental measurement of drop-size and estimation of specific interfacial area in a helical microbore tube based micromixer-settler with 30% TBP/nitric acid system

Abstract: Hydrodynamic experiments were conducted in a microbore helix based passive mixer for the system of HNO 3 /30% TBP/ n -dodecane under non-mass transfer conditions. Hold up and drop sizes were experimentally measured for an aqueous phase flow rate of 0.15–0.80 mL/min and organic phase flow rate of 0.10–0.75 mL/min for various A/O ratios from 0.2 to 5. Although a fine dispersion was generated in the mixer, it did not affect coalescence and clear streams emerged from exit points of a mini settler (∼1.2 mL). Due to smaller drop sizes generated, the observed specific interfacial area was huge and for the experimental conditions reported, it was ranged about 2.84 × 10 4 to 8.56 × 10 4 m 2 /m 3 . It is of the same order of that observed in a 50-mm centrifugal extractor with identical aqueous-organic pair.

Direct dissolution of g-level U metal and U-6 % Zr alloy bits by TBP-nitric acid adduct and in situ extraction at ambient pressures

Abstract: A study on direct dissolution of g-level metallic U and U-6 % Zr alloy bits by TBP-nitric acid adduct followed by in situ extraction at ambient pressures under batch and dynamic conditions was performed. The product organic solution was adjusted to 30 % TBP in dodecane and from it, U(VI) could be stripped quantitatively with 0.01 N nitric acid. Experimental results of the study are presented in this paper.

Microstructure and Corrosion Behavior of Hf-40 Wt Pct Ti Alloy in Nitric Acid Medium for Reprocessing Applications

Abstract: The Hf-40 wt pct Ti (Hf-Ti) alloy was developed for neutron poison application in the spent nuclear fuel reprocessing plant. The furnace-cooled Hf-Ti sample exhibited the microstructure comprising equiaxed-α, lamellar-α, and feathery-α. The water-quenched Hf-Ti sample confirmed the presence of lath and internally twinned martensite. In comparison to the furnace-cooled sample, low corrosion current density and passivation current density values obtained for the water-quenched Hf-Ti in 6 M HNO3 at 298 K (25 °C) indicated better passivation ability. The martensitic structure exhibited high hardness (660 HV) and negligible corrosion rate in 6 M nitric acid at 298 K (25 °C). X-ray photoelectron spectroscopic (XPS) analysis confirmed that passivation behavior of this alloy was due to the protective passive film composed of TiO2 and HfO2.

Characterization and Performance of Magnetron-Sputtered Zirconium Coatings Deposited on 9Cr-1Mo Steel

Abstract: Zirconium coatings of different thicknesses have been deposited at 773 K on 9Cr-1Mo steel substrate using pulsed DC magnetron sputtering. These coatings were heat treated in vacuum at two different temperatures (1173 and 1273 K) for one hour. X-ray diffraction (XRD) analysis of Zr-coated samples revealed the formation of α-phase (HCP structure) of Zr. XRD analysis of heat-treated samples show the presence of Zr3Fe and Zr2Fe intermetallics. The lattice parameter of these coatings was calculated, and it matches with the bulk values when the thickness reached 2µm. In order to understand this, crystallite size and strain values of these coatings were calculated from XRD plots employing Williamson-Hall method. In order to assess the performance of the coatings, systematic corrosion tests were carried out. The corrosion current density calculated from the polarization behavior showed that the corrosion current density of the uncoated 9Cr-1Mo steel was higher than the coated sample before and after the heat treatment. Studies using electrochemical impedance spectroscopy confirmed that the coated steel has higher impedance than the uncoated steel. The corrosion resistance of 9Cr1Mo steel had improved after Zr coating. However, the corrosion resistance of the coating after heat treatment decreased when compared to the as-deposited coating. The microstructure and composition of the surface oxide film influence the corrosion resistance of the Zr-coated 9Cr1Mo steel.