U. Kamachi Mudali

Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel

Hydroxyapatite (HAP) coatings were developed on type 316L stainless steel (SS) by electrophoretic deposition at various deposition potentials from 30 to 90 V using the stoichiometric HAP (Ca/P ratio 1.67) powder in a suspension of isopropyl alcohol. The optimum coating parameters were established at 60 V and 3 min, after vacuum sintering at 800 C. The phase purity of the coated surface was confirmed by XRD and secondary ion mass spectrometry confirmed the presence of both Ca and P on the coated layers. The electrochemical corrosion parameters Ecorr (open circuit potential) and pitting potentials, evaluated in Hank’s solution shifted towards noble direction for the HAP coated specimens in comparison with uncoated type 316L SS. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values after immersing the coated specimens in Hanks solution for 200 h. This indicates the stable nature of the coatings formed. 2002 Elsevier Science Ltd. All rights reserved.

Corrosion and microstructural aspects of dissimilar joints of titanium and type 304L stainless steel

To link titanium and zirconium metal based (Ti, Zr-2, Ti-5%Ta, Ti-5%Ta-1.8Nb) dissolver vessels containing highly radioactive and concentrated corrosive nitric acid solution to other nuclear fuel reprocessing plant components made of AISI type 304L stainless steel (SS), high integrity and corrosion resistant dissimilar joints between them are necessary. Fusion welding processes produce secondary precipitates which dissolve in nitric acid, and hence solid-state processes are proposed. In this work, various dissimilar joining processes available for producing titanium-304L SS joints with adequate strength, ductility and corrosion resistance for this critical application are highlighted. Developmental efforts made at IGCAR, Kalpakkam are outlined. The possible methods and the microstructural-metallurgical properties of the joints along with corrosion results obtained with three phase (liquid, vapour, condensate) corrosion testing are discussed. Based on the results, dissimilar joint produced by the explosive joining process was adopted for plant application.

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.

Electrochemical and electrophoretic deposition of hydroxyapatite for orthopaedic applications

Abstract The basic calcium phosphate mineral, hydroxyapatite (HAP) (Ca10(PO4)6(OH)2), is the prototype of one of the major constituents of bone and teeth. Thin layers of HAP were coated on the surface of type 316L stainless steel by electrophoretic deposition (EPD) from a 2·5% suspension in isopropyl alcohol, and this was followed by vacuum sintering at 800°C for 1 h. The development of HAP coatings was affected by the applied potential and time. In addition, HAP coatings were synthesised on pure Ti by electrochemical deposition and fully characterised to validate their use in orthopaedic implants. Electrodeposition was carried out from a bath containing low concentrations of Ca(NO3)2 and NH4H2PO4 at pH 6·0 by cathodic polarisation. Both types of HAP coating were fully characterised, including with respect to corrosion resistance. The advantages of these techniques include: (i) control over the composition and structure of the coating; and (ii) the ability to coat irregular surfaces easily.

Sintering atmosphere and temperature effects on hydroxyapatite coated type 316L stainless steel

Abstract Electrophoretically deposited hydroxyapatite (HAP) coatings on type 316L SS was developed at the optimum coating parameters of 60 V and 3 min. Sintering of the coating enhances the metal–ceramic bond strength, but HAP structure is sensitive to temperature as it decomposes to other calcium phosphate phases. Sintering of HAP coatings in air at 900 °C for 1 h indicate the formation of a composite surface containing oxides of the alloy and decomposition products of HAP, mainly tricalcium phosphate. Open circuit potential–time measurements, potentiodynamic cyclic polarisation and electrochemical impedance experiments performed in Ringer’s solution indicate that the corrosion performance of HAP coatings were severely affected by the sintering atmosphere and temperature. Higher capacitance and low polarisation resistance values obtained from electrochemical impedance spectroscopic studies further indicate that the coatings are more prone to dissolution on comparison with the pristine type 316L SS. The sintering of the coatings in vacuum at 600, 800 and 900 °C for 1 h did not alter the phase purity of the coatings, and shifted the electrochemical parameters towards noble direction. Sintering of the coatings in vacuum lead to the formation of an adherent, stoichiometric HAP coating with enhanced corrosion resistance.

Corrosion studies on materials of construction for spent nuclear fuel reprocessing plant equipment

Abstract Corrosion studies on specimens of commercial Type 304L stainless steel (SS), nuclear grade type 304L SS, extra low-carbon nitric acid grade (NAG) Uranus-16 SS, NAG Uranus-65 SS, Ti, Ti-5% Ta, Ti-0.25% Pd, Zircaloy-2, weldments of Ti and of Ti-5% Ta, and surface-modified (thermally oxidised and anodised) Ti were carried out to assess their corrosion resistance in nitric acid medium. The results indicated that Zircaloy-2, Ti-5% Ta, Uranus-16 SS and Uranus-65 SS have excellent corrosion resistance in boiling nitric acid solution. Specimens of Zircaloy-2, Ti-5% Ta and thermally-oxidised Ti showed excellent corrosion resistance also in a simulated uranium-containing reprocessing medium in a concentrated nitric acid solution. SEM and XRD analyses were carried out on the tested specimens to examine the scale morphology and phases present on the surface.

Nitrogen ion implantation and in vitro corrosion behavior of as-cast Ti-6Al-7Nb alloy

Abstract In the present investigation, surface modification of Ti–6Al–7Nb alloy with nitrogen ions is considered as a method to improve its performance with respect to corrosion. Nitrogen ion was implanted on Ti–6Al–7Nb alloy at an energy of 70 and 100 keV using a 150 keV accelerator at different doses between 1×10 16 and 3×10 17 ions/cm 2 . Gracing incidence X-ray diffraction was employed on the implanted specimens to understand the phases formed with increasing doses. The implanted samples were subjected to electrochemical study in Ringers solution in order to determine the optimum dose that can give good corrosion resistance in a simulated body fluid condition. The OCP of the implanted specimens were found to shift in the noble direction in comparison with unimplanted specimen. The passive current density and area of the repassivation loop were found to decrease as the dose values increased. The electrochemical impedance spectroscopic results indicate that the polarization resistance was higher for the dose of 2.5×10 17 ions/cm 2 implanted at both energy of 70 and 100 keV. Nitrogen ion implantation enhanced the passivability and reduces the corrosion kinetics of the alloy surface with increasing tendency for repassivation. Nature of the surface and reason for the variation and improvement in corrosion resistance are discussed in detail.

Synthesis of hydroxyapatite nanoparticles by a novel ultrasonic assisted with mixed hollow sphere template method.

Hydroxyapatite (HAP) is the main inorganic component of bone material and is widely used in various biomedical applications due to its excellent bioactivity and biocompatibility. In this paper, we have reported the synthesis of hydroxyapatite nanoparticles by a novel ultrasonic assisted mixed template directed method. In this method glycine-acrylic acid (GLY-AA) hollow spheres were used as an organic template which could be prepared by mixing of glycine with acrylic acid. The as-synthesized HAP nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) and tunnelling electron microscope (TEM) to investigate the nature of bonding, crystallinity, size and shape. The thermal stability of as-synthesized nanoparticles was also investigated by the thermo gravimetric analysis (TGA). The effect of ultrasonic irradiation time on the crystallinity and size of the HAP nanoparticles in presence of glycine-acrylic acid hollow spheres template were investigated. From the inspection of the above results it is confirmed that the crystallinity and size of the HAP nanoparticles decrease with increasing ultrasonic irradiation time. Hence the proposed synthesis strategy provides a facile pathway to obtain nano sized HAP with high quality, suitable size and morphology.

Meeting the challenges related to material issues in chemical industries

Reliable performance and profitability are two important requirements for any chemical industry. In order to achieve high level of reliability and excellent performance, several issues related to design, materials selection, fabrication, quality assurance, transport, storage, inputs from condition monitoring, failure analysis etc. have to be adequately addressed and implemented. Technology related to nondestructive testing and monitoring of the plant is also essential for precise identification of defect sites and to take appropriate remedial decision regarding repair, replacement or modification of process conditions. The interdisciplinary holistic approach enhances the life of critical engineering components in chemical plants. Further, understanding the failure modes of the components through the analysis of failed components throws light on the choice of appropriate preventive measures to be taken well in advance, to have a control over the overall health of the plant. The failure analysis also leads to better design modification and condition monitoring methodologies, for the next generation components and plants. At the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, a unique combination of the expertise in design, materials selection, fabrication, NDT development, condition monitoring, life prediction and failure analysis exists to obtain desired results for achieving high levels of reliability and performance assessment of critical engineering components in chemical industries. Case studies related to design, materials selection and fabrication aspects of critical components in nuclear fuel reprocessing plants, NDT development and condition monitoring of various components of nuclear power plants, and important failure investigations on critical engineering components in chemical and allied industries are discussed in this paper. Future directions are identified and planned approaches are briefly described

Electrophoretic deposition of hydroxyapatite coatings and corrosion aspects of metallic implants

Metallic materials such as stainless steels, cobalt-chromium alloys, titanium and its alloys, have been used extensively for centuries in restoration of anatomical structures due to their superior mechanical properties. The most important considerations for their selection in the human body are their biocompatibility, corrosion resistance, tissue reactions, surface conditions, and osseointegration (a bone bed formed through direct attachment to bone). Metallic implants are unique in that they are exposed to living cells, tissues and biological fluids, which are not only dynamic but also form a hostile environment for the implant. Clinical experience has shown that metallic implants are susceptible to localized corrosion in the human body, releasing metal ions into the surrounding tissues. Numerous failures of such implants have led to the application of biocompatible and corrosion resistant coatings, as well as to surface modification of the alloys. Ceramic materials that are based on hydroxyapatite (HAP), the principal constituent of dentine, bone and other hard tissues, are considered promising for osteo-implants and as a means of aiding the regeneration of bone. There are indications that chemical bonding may occur between HAP and bone. However, the poor mechanical properties of HAP limit its use in implantation. Bioceramic coatings have been intensively studied in order to obtain implant materials which have both biological affinity and high mechanical strength. This article reviews various aspects of corrosion resistance and biocompatibility of metallic implants. Surface modification of metallic materials by electrophoretic deposition (EPD) of HAP and the corrosion performance of these coatings in simulated body fluid conditions are also discussed.