Lakshman Neelakantan

The microstructure and corrosion behavior of Al-0.5Mg-0.08Sn-0.08Ga (wt%) anode material solutionized at different temperatures ranging from 400 to 550 °C is studied. Thermal analysis reveals that Sn is soluble in the Al alloy in this temperature range. Microstructural observation shows maximum solubility of Sn above 450 °C. Electrochemical investigations in 3.5 wt% NaCl and 9 M NaOH solution reveals lower corrosion and hydrogen evolution rates for the specimen solutionized at 450 °C. Though Sn is soluble above 450 °C, the corrosion slightly increases at higher solutionizing temperatures due to the segregation Ga at the surface. The corrosion and hydrogen evolution rate are higher when Sn is present as precipitate and causes localized (pitting) type of corrosion. On the contrary, presence of Sn in solid solution improves the corrosion resistance and decreases the hydrogen evolution rate. Hence, it can be concluded that the presence of Sn in solid solution is essential for reducing the corrosion rate and the distribution of the Sn plays a major role in the corrosion behavior of Al-Mg-Sn-Ga alloys.

Ball milled SnCoC composites are an attractive commercial anode material to conventional graphite due to their higher specific capacity and low temperature performance. The effect of ball milling time on the structure and electrochemical properties of the (Sn71Co29)50C50 wt% composite anodes are studied to understand the reasons for the non-realization of the theoretical capacity. Structural analysis reveals the damage of graphite structure with increasing ball milling time from 10 h to 60 h. The cyclic voltammetry and differential capacity measurements indicate the decreasing contribution to capacity from graphite and increasing contribution from Sn with increase in the milling time. The charge-discharge cycling of the anodes at different C rates indicates that though the specific capacity does not improve with longer milling time, the rate capability improves significantly. The damage in the graphite structure during high energy ball milling is found to reduce the capacity of the SnCoC anodes. Based on the investigations, it can be concluded that 10 h of milling time is optimum to realize high specific capacity, whereas longer durations of milling are desirable for high rate discharge characteristics.

A novel route of synthesizing (Sn71Co29)50C50 wt% powders by melt spinning SnCo alloy followed by high energy ball milling with natural graphite is explored for the first time. The specific capacity of the composite prepared using melt spun alloy and conventionally melt alloy routes are evaluated by charge-discharge cycling CR2032 type coin cells. In the conventionally made alloy, higher amount of unalloyed Sn is present which reduces the capacity of the cell. Melt spinning reduces the amount of unalloyed Sn and also increases the Sn2Co phase which enhances the capacity. Ball milling for 10 h reduces the crystallite size to nano scale. Electrochemical measurements confirm, higher capacity of the cells made by combination of melt spinning and ball milling over the conventionally made alloy or graphite system and this route is much faster and commercially viable.

Powder particles of polymer derived SiHfCN(O) ceramics were pulsed electric current sintered at 1300 and 1500 °C to produce amorphous and partially crystalline ceramic pellets for corrosion studies in salt (NaCl or Na2SO4) and acid (HF) environments. While, sodium dramatically accelerated phase transformation and catalyzed the crystallization process, the open porosity acted as the main cause for sodium penetration in these materials. The samples, however, were completely disintegrated during fluoride acid tests. The cristobalite and HfO2 crystalline phases were severely corroded and it was found that the SiC grains were relatively stable in comparison with other phases in the system.

The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of fatigue on corrosion and vice versa. This can be achieved by performing ex situ andor in situ measurements. The versatility of the combined electrochemicalmechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9 NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-fatigue on the localized corrosion resistance, or the influence of hydrogen on fatigue life. Ex situ experiments showed that a pre-fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the fatigue failure. © 2013 American Institute of Physics.

The current study investigates the effect of crystallite size and surface morphology of TiO 2 nanotubes on their wettability and electrochemical properties. Self-organized amorphous TiO 2 nanotubes were synthesized by anodization process in an acidic (0.5 wt% HF) and a neutral electrolyte (1 M Na 2 SO 4 + 0.5 wt% NaF). Subsequently, the nanotubes were annealed at 450 °C to achieve crystalline phase. Scanning electron microscope micrographs revealed that nanotubes formed from the neutral bath are four times longer (1.2 μm) than the ones synthesized from the acidic bath (325 nm). The charge consumed during anodization is greater under the acidic conditions implying the severity of the attack on the nanotubes by the electrolyte. X-Ray diffraction analysis showed that after annealing TiO 2 crystallizes in the tetragonal lattice as anatase structure. Peak fitting method for line profile analysis was employed to estimate the crystallite size and the micro strain. The oxide nanotubes formed in neutral medium showed smaller crystallite size (28.91 nm) than the one formed in acidic medium (43.37 nm). Wettability measurements showed wetting angles <60°, indicating hydrophilic nature of the anatase nanotubes. Further, both the dimensional aspect (i.e., length and diameter of nanotubes) and the crystallite size have significant effect on the hydrophilic behavior. Electrochemical impedance spectroscopy in a simulated body fluid environment confirmed that structural changes in the oxide layer influence the electrochemical properties. Polarization studies demonstrated that crystallite size affects the passive behavior of the nanotubes. Smaller crystallite size (28.91 nm) lowers the passive current density (0.11 μA cm -2 ), indicating the good protectiveness.

In this study, TiO2 nanotubes were fabricated using anodic oxidation in fluoride containing weak organic acid for different durations (0.5 h, 1 h, 2 h and 3 h). Scanning electron microscope (SEM) micrographs reveal that the morphology of titanium oxide varies with anodization time. Raman spectroscopy and X-ray diffraction (XRD) results indicate that the as-formed oxide nanotubes were amorphous in nature, yet transform into crystalline phases (anatase and rutile) upon annealing at 600 °C. Wettability measurements show that both as-formed and annealed nanotubes exhibited hydrophilic behavior. The electrochemical behavior was ascertained by DC polarization and AC electrochemical impedance spectroscopy (EIS) measurements in 0.9% NaCl solution. The results suggest that the annealed nanotubes showed higher impedance (105-106 Ω cm2) and lower passive current density (10- 7 A cm- 2) than the as-formed nanotubes. In addition, we investigated the influence of post heat treatment on the semiconducting properties of the oxides by capacitance measurements. In vitro bioactivity test in simulated body fluid (SBF) showed that precipitation of Ca/P is easier in crystallized nanotubes than the amorphous structure. Our study uses a simple strategy to prepare nano-structured titania films and hints the feasibility of tailoring the oxide properties by thermal treatment, producing surfaces with better bioactivity.

Nowadays, various concrete systems with fly ash, slag, limestone calcined clay, etc. exhibiting high ionic resistivity are used to enhance the resistance against chloride-induced corrosion. This study deals with the corrosion assessment of steel in three cementitious systems, namely (i) Ordinary Portland Cement (OPC), (ii) OPC + 30% fly ash, and (iii) limestone calcined clay cement (LC3) exhibiting ‘low to moderate’, ‘moderate to high’, and ‘very high’ resistivities, as per AASHTO T358 (2017). Results from the ASTM G109 and impressed current corrosion (ICC) tests were evaluated. It was found that LC3 systems have excellent resistivity against the ingress of chlorides and provide better corrosion resistance. It was also found that the corrosion products formed on steel in LC3 systems are different and less expansive than that found in the OPC systems.

Influence of composition, specifically manganese and nitrogen content, on the microstructure associated corrosion resistance property of newly developed stainless steel has been studied. The developed steels have been characterised for their microstructure, mechanical and electrochemical properties. The results indicate that the addition of manganese and nitrogen as a substitute for nickel favours the austenite microstructure, higher yield strength (>350 MPa), tensile strength (>700 MPa), elongation and superior Charpy V-notch impact toughness properties. The results obtained from electrochemical tests such as potentiodynamic polarisation and electrochemical impedance spectroscopy of manganese stainless steel show remarkable improvement (about 4 times) in corrosion resistance exhibiting passivity behaviour like that of commercial stainless steel (316L).