Pitting corrosion of steel tubes in an air preheater

Severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw firing. Approximately 1450 tubes were removed from Stage 3 of the preheater (air inlet and flue gas outlet) due to corrosion and local bursting. Samples from Stage 2 (where corrosion was low) and Stage 3 (severe corrosion) were taken and subjected to visual inspection, scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness measurement, and chemical and microstructural analysis. It was determined that extended non-operation of the plant resulted in the settlement of corrosive species on the tubes in Stage 3. The complete failure of the tube occurred due to diffusion of these elements into the base metal and precipitation of potassium and chlorine compounds along the grain boundaries, with subsequent dislodging of grains. The nonmetallic inclusions acted as nucleating sites for local pitting bursting. Nonuniform heat transfer in Stage 3 operation accelerated the selective corrosion of front-end tubes. The relatively high heat transfer in this stage resulted in condensation of some corrosive gases and consequent corrosion. Continuous operation of the plant with some precautions during assembly of the tubes reduced the corrosion problem.     

POD Evaluation: The Key Performance Indicator for NDE 4.0

The magnified images of thin wires, plastic, steel and lead plates which were obtained using linear microfocus hard bremsstrahlung generated through interaction of an 18 MeV electron beam with a 13 μm thick Ta foil oriented along the internal beam of a B-18 betatron are presented. The images indicate high absorption contrast of the objects due to a small horizontal size of the radiation source the width of which is 115-fold smaller than the diameter of the electron beam. Some results have shown a few peculiarities in the images which were not earlier observed. Several results were compared with the ones obtained earlier using the microfocus bremsstrahlung generated by the 18 MeV electron beam of B-18 in a narrow Si target.

     

Analysis of high contact resistance in silver impregnated graphite (SIG) contacts

Silver impregnated graphite (SIG) contacts are used in high safety applications of railway signaling, traffic signaling, and numerous other applications due to high silver content that eliminates the risk of welding in the presence of graphite. High contact resistance (>200 mΩ) was observed in SIG contacts in a railway signaling operation after a few thousand operations of the relay. The high contact resistance was discovered to be a result of loose dust/foreign particles residing between the two contacts. Silver had been preferentially removed from the contacting surfaces during switching operations by an abrading effect of the dust/foreign particles, leaving only graphite on the contact area. Wear was accentuated by the presence of porosity and low hardness of the contacts, leading to improper interaction between the mating surfaces.     

Emission properties of colloidal quantum dots on polyelectrolyte multilayers

We present steady state and time-resolved photoluminescence (PL) characteristics of differently charged CdTe quantum dots (QDs) adsorbed onto a polyelectrolyte (PE) multilayer. The PE multilayer is built up using a layer-by-layer assembly technique. We find that the diffusion of the QDs into the PE multilayer is an important factor in the case of 3-mercapto-1, 2-propanediol stabilized QDs (neutral surface charge), resulting in a ～31-fold enhancement in PL intensity accompanied by a blue shift in the PL spectra and an increase in decay lifetime from 3.74?ns to a maximum of 11.65?ns. These modified emission properties are attributed to the enhanced surface related emission resulting from the interaction of the QD's surface with the PE. We find that diffusion does not occur for thioglycolic acid (TGA) stabilized QDs (negative surface charge) or 2-mercaptoethylamine stabilized QDs (positive surface charge), indicating localization of the QDs on top of the PE multilayer. However, the PL lifetime of the TGA stabilized QDs decreases from 9.58 to 5.78?ns with increasing PE multilayer thickness. This provides evidence for increased intrinsic exciton recombination relative to surface related emission, which results in an overall reduction in the average lifetime. Our studies indicate the importance of the QD surface charge in determining the interaction with the PE multilayers and the subsequent modification of the QD emission properties.     

Wear Performance of Laser Processed Tantalum Coatings

This first generation investigation evaluates the in vitro tribological performance of laser-processed Ta coatings on Ti for load-bearing implant applications. Linear reciprocating wear tests in simulated body fluid showed one order of magnitude less wear rate, of the order of 10(-4)mm(3)(N.m)(-1), for Ta coatings compared to Ti. Our results demonstrate that Ta coatings can potentially minimize the early-stage bone-implant interface micro-motion induced wear debris generation due to their excellent bioactivity comparable to that of hydroxyapatite (HA), high wear resistance and toughness compared to popular HA coatings.     

Encapsulation of enzymes within polymer spheres to create optical nanosensors for oxidative stress

We describe the fabrication and characterization of poly(ethylene glycol) (PEG) hydrogel spheres containing the enzyme horseradish peroxidase (HRP) for application as optical nanosensors for hydrogen peroxide. HRP was encapsulated in PEG hydrogel spheres by reverse emulsion photopolymerization, yielding spheres with a size range from 250 to 350 nm. Encapsulated HRP activity and sensitivity to hydrogen peroxide were investigated by the Amplex Red assay based on the fluorescence response as a function of H2O2. These HRP-loaded spheres were then introduced to murine macrophages with Amplex Red in the culture media. After phagocytosis, the biocompatibility of spheres was determined by live cell staining using calcein AM (5 microM). The HRP-loaded PEG hydrogel spheres were activated (i.e., fluorescent oxidized Amplex Red produced within the spheres) by oxidative stresses such as exogenous H2O2 (100 microM) and lipopolysaccharide (1 microg/mL), which induced the production of endogenous peroxide inside macrophages. The results presented here indicate that after polymerization, the enzyme activity of HRP was still maintained and that using these HRP-containing nanospheres, peroxide production could be sensed locally within cells.     

Experimental investigation on the performance of nanoboric acid suspensions in SAE-40 and coconut oil during turning of AISI 1040 steel

Machining is one of the most fundamental and indispensable processes in manufacturing industry. The heat generated in the cutting zone during machining is critical in deciding the workpiece quality. Though cutting fluids are widely employed to carry away the heat in machining, their usage poses threat to ecology and the health of workers. Hence, there arises a need to identify eco-friendly and user-friendly alternatives to conventional cutting fluids. The present work features a specific study on the application of nanosolid lubricant suspensions in lubricating oil in turning of AISI 1040 steel with carbide tool. SAE-40 and coconut oil are taken as base lubricants and boric acid solid lubricant of 50 nm particle size as suspensions. Variation of cutting tool temperatures, average tool flank wear and the surface roughness of the machined surface with cutting speed and feed are studied with nanosolid lubricant suspensions in lubricating oil.     

In vitro cytotoxicity and ion release of multi-ion doped hydroxyapatite

Multi-ion doped hydroxyapatite (HA) is gaining more attention due to its potential in enhancing multifunctional biological, structural, and mechanical properties for orthopedic and dental applications. In this study, HA doped with multiple cations (Sr⁺², Zn⁺², Ag⁺) and anion (F⁻) was prepared by high-energy ball milling. Sintered HA samples were evaluated for their in vitro cytocompatibility, ion release, and bioactivity. The composition of multi-ion doped HA was optimized using Design of experiments (DOE). Our analysis showed that the contribution of each dopant on cell proliferation changes with culture duration. During first 3 days, F⁻ exhibited strongest influence and during 7-day proliferation Sr⁺² and Ag⁺ had maximum influence. Binary ion doping found to have strong interaction on cell proliferation, while the ternary and quaternary ion doping did not show any interactions. In general, up to twofold increase in the cell viability was achieved with ternary and quaternary ion doping consisting of Sr⁺², Zn⁺², Ag⁺ and F⁻. Although large number of compositions has been identified to exhibit better in vitro cell viability than pure HA, for enhanced long-term cytocompatibility the compositions of multi-ion doped HA would be 2.5Sr-2.5Zn-2.5Ag, 2.5Sr-5Zn-2.5Ag, and 5Sr-2.5Zn-2.5Ag with up to 5 wt% F.     

Influence of Powder Morphology and Microstructure on the Cold Spray and Mechanical Properties of Ti6Al4V Coatings

The powder microstructure and morphology has significant influence on the cold sprayability of Ti6Al4V coatings. Here, we compare the cold sprayability and properties of coatings obtained from Ti6Al4V powders of spherical morphology (SM) manufactured using plasma gas atomization and irregular morphology (IM) manufactured using the Armstrong process. Coatings deposited using IM powders had negligible porosity and better properties compared to coatings deposited using SM powders due to higher particle impact velocities, porous surface morphology and more deformable microstructure. To evaluate the cohesive strength, multi-scale indentation was performed and hardness loss parameter was calculated. Coatings deposited using SM powders exhibited poor cohesive strength compared to coatings deposited using IM powders. Images of the residual indents showed de-bonding and sliding of adjacent splats in the coatings deposited using SM powders irrespective of the load. Coatings deposited using IM powders showed no evidence of de-bonding at low loads. At high loads, splat de-bonding was observed resulting in hardness loss despite negligible porosity. Thus, while the powders from Armstrong process lead to a significant improvement in sprayability and coating properties, further optimization of powder and cold spray process will be required as well as consideration of post-annealing treatments to obtain acceptable cohesive strength.