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Narayanan, T.S.N. Sankara,Lee, Min-Ho ELSEVIER SCIENCE 2016 JOURNAL OF ALLOYS AND COMPOUNDS Vol.687 No.-
<P><B>Abstract</B></P> <P>Microarc oxidation (MAO) of magnesium is performed in a single stage using an alkaline silicate electrolyte and acidic zirconate electrolyte as well as in two stages, sequentially using the alkaline silicate followed by the acidic zirconate electrolyte. The morphological characteristics, surface roughness, phase content, corrosion behaviour and bioactivity were compared. The surface morphology, thickness, average surface roughness (R<SUB>a</SUB>), and phase composition of the MAO coatings show a strong dependence on the type of electrolytes used and the methodology employed for deposition. Based on the corrosion protective ability in Hank’s balanced salt solution, the uncoated and MAO coated Mg are ranked as follows: Uncoated Mg < MAO coated Mg prepared using alkaline silicate electrolyte < MAO coated Mg prepared using acidic zirconate electrolyte < MAO coated Mg prepared in two stages, sequentially using the alkaline silicate followed by the acidic zirconate electrolyte. Poorly crystalline calcium deficient apatite is formed on all samples after immersion in SBF at 37 °C for 240 h.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Characteristics of MAO coating depend on electrolyte type and deposition methodology </LI> <LI> MAO coating by dual electrolytes in two stages offered better corrosion resistance. </LI> <LI> Poorly crystalline calcium deficient apatite is formed after immersion in SBF. </LI> </UL> </P>
Fretting corrosion of lubricated tin-plated contacts
Narayanan, T.S.N. Sankara,Park, Young Woo,Lee, Kang Yong Emerald Group Publishing Limited 2008 Industrial lubrication and tribology Vol.60 No.5
<B>Purpose</B> - The objective of this paper is to study the effect of a commercial lubricant, which contains a 50-50 mixture of zinc diamyldithiocarbamate and petroleum oil, on the fretting corrosion of tin-plated copper alloy contacts. <B>Design/methodology/approach</B> - The change in contact resistance as a function of fretting cycles was used to assess the effectiveness of the lubricant in preventing the fretting corrosion of tin-plated contacts. The surface profile, surface roughness, extent of fretting damage and extent of oxidation of the contact zone were assessed by a laser scanning microscope and surface analytical techniques to correlate the change in contact resistance with fretting cycles. <B>Findings</B> - The lubricant film provides a surface coverage of 6.76±1?mg/cm<SUP>2</SUP> and it easily establishes metallic asperity contact between the mated tin-plated contacts. The contact resistance of lubricated contacts remains stable for several thousand fretting cycles. Lubricated contacts reach a threshold value of 0.1?O around 100,000 cycles, whereas unlubricated contact reaches this value around 13,500 cycles itself. For lubricated contacts, the extent of mechanical wear of the tin coating is significantly reduced. As a result, they experience a lesser damage at the contact zone and exhibit a smoother profile. The formation of tin oxide is not appreciable and there is no oxide accumulation at the contact zone even at 380,000 cycles. The lubricant is very effective in delaying the fretting wear during the initial stages and in preventing the oxidation and accumulation of oxidation products at the contact zone in the later stages. <B>Originality/value</B> - Metallic dialkyldithiocarbamates are useful anti-wear and extreme pressure additives for lubricating oils. Dithiocarbamates improve the antioxidant properties of the lubricants and are effective in reducing the wear and increasing the friction-reducing and load-carrying ability of the base stock. The use of molybdenum dithiocarbamate as a grease additive is found to be effective in reducing fretting corrosion of ball bearings under random rotary vibrating conditions. The effect of dithiocarbamate containing lubricant oils or greases on the fretting corrosion of electrical contacts has not far been studied. The paper explores the effect of a lubricant that contains a 50-50 mixture of petroleum oil and zinc diamyldithiocarbamate on the fretting corrosion of tin-plated contact.
Sankara Narayanan, T. S. N.,Park, Il Song,Lee, Min Ho The Royal Society of Chemistry 2014 Journal of Materials Chemistry B Vol.2 No.21
<P>The methodology of deposition of fluoride conversion coatings is modified with the use of galvanic coupling, agitation of the electrolyte solution, and addition of K2CO3, which helps to provide a better understanding of the mechanism and new avenues to tailor the composition of the coating. A very good correlation exists between the F/O ratio of the coatings prepared under varying experimental conditions and their <I>i</I>corr, |<I>Z</I>| and phase angle maximum; the higher the F/O ratio, the better the corrosion protective ability of the coatings in Hank's balanced salt solution. The corrosion behaviour of the coatings of the present study suggests that fluoride conversion coatings show much promise for their use for biomedical applications, as long as their uniformity is improved and the composition is tailored to enrich the MgF2 phase, encompassing a higher F/O ratio.</P>
Deposition of zinc–zinc phosphate composite coatings on steel by cathodic electrochemical treatment
Kavitha, C.,Sankara Narayanan, T. S.,Ravichandran, K.,Park, I. S.,Lee, M. H. Springer Science + Business Media 2014 Journal of coatings technology and research Vol.11 No.3
The present work aims at the development of an energy-efficient and eco-friendly approach for the deposition of zinc phosphate coatings on steel. The study describes the possibility of preparing zinc-zinc phosphate composite coatings by cathodic electrochemical treatment using dilute phosphoric acid as an electrolyte and zinc as an anode. The methodology enables the preparation of coatings with different proportions of zinc and zinc phosphate by suitably varying the applied current density, pH, and treatment time. Adhesion of the coating on mild steel and adhesion of paint film on the phosphate coating were found to be good. The surface morphology of the coatings exhibited platelet-type features and small white crystals (agglomerated at some places) which represented zinc and zinc phosphate, respectively. An increase in current density (from 20 to 50 mA/cm(2)) increased the size of the zinc crystals, and coatings prepared at 40 and 50 mA/cm(2) resembled that of electrodeposited zinc. Since the proportions of zinc and zinc phosphate could be varied with applied current density, pH, and treatment time, it would be possible to use this methodology to prepare coatings that would offer different degrees of corrosion protection.
Balusamy, T.,Sankara Narayanan, T. S. N.,Ravichandran, K.,Lee, Min Ho,Nishimura, T. The Electrochemical Society 2015 Journal of the Electrochemical Society Vol.162 No.6
<P>The present paper addresses the influence of surface mechanical attrition treatment on the characteristic properties of EN8 steel and, to analyze the correlation between the change in properties of the treated steels with their corrosion behavior in 0.6 M NaCl and borate buffer, evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy studies. Surface mechanical attrition treatment of EN8 steel induced plastic deformation, enabled surface nanocrystallization, increased the surface roughness, microstrain and defects/dislocations and, imparted compressive residual stresses. The change in properties of EN8 steel following treatment has caused a deleterious influence on its corrosion resistance in 0.6 M NaCl but an improvement in corrosion behavior in borate buffer. The possible reasons for the change in corrosion behavior of the treated EN8 steel are suggested. Pictorial models are proposed to explain the evolution of surface roughness and the mechanism of corrosion attack of the treated EN8 steels in 0.6 M NaCl and borate buffer.</P>
Thangaraj, Balusamy,T. S. Nellaiappan, Sankara Narayanan,Kulandaivelu, Ravichandran,Lee, Min Ho,Nishimura, Toshiyasu American Chemical Society 2015 ACS APPLIED MATERIALS & INTERFACES Vol.7 No.32
<P>The study addresses how surface nanostructuring of AISI 304 stainless steel (SS) by surface mechanical attrition treatment (SMAT) influences its characteristic properties and corrosion behavior in Ringers solution. SMAT of 304 SS induced plastic deformation, enabled surface nanocrystallization, refined the grain size, transformed the austenite phase to strain induced alpha'-martensite phase, increased the surface roughness, induced defects/dislocations, imparted compressive residual stresses at the surface, decreased the contact angle, and increased surface energy. The change in properties of 304 SS following treatment using 5 and 8 mm empty set balls for 15, 30, 45, and 60 min has caused a deleterious influence on its corrosion resistance in Ringers solution, while an improvement in corrosion behavior is observed for those treated using 2 mm ? balls. The increase in surface roughness, transformation of the austenite to a'-martensite phase, a higher extent of deformation, and the presence of larger number of defects/dislocations are main factors responsible for the lower corrosion resistance observed for 304 SS treated using 5 and 8 mm empty set balls in Ringers solution. In spite of having these attributes with a relatively lower extent, 304 SS treated using 2 mm empty set balls offered a better corrosion resistance and exhibits a better passivity. For those treated using 2 mm empty set balls, the ability of the nanocrystalline surface to promote passivation outweighs the deleterious influences caused by the limited amount of deformation and defects/dislocations. Based on the findings of this study, it is recommend that SMAT of 304 SS using 2 mm empty set balls for 1530 min is the optimum condition to achieve the suitable surface profile, surface characteristics with better corrosion resistance.</P>