Y. Blanco

Sensitization of Powder Metallurgy Type 430L Stainless Steel Sintered in Nitrogen-Hydrogen Atmosphere

Abstract The degree of sensitization of ferritic AISI 430L (UNS S43000) stainless steel, prepared by powder metallurgy (PM), was examined by exposure methods (ASTM A763-93, Practice W and Practice Z) and electrochemical reactivation methods such as electrochemical potentiokinetic reactivation (EPR) and electrochemical potentiokinetic reactivation double-loop (EPRDL) tests. Two sintering atmospheres, vacuum and nitrogen-hydrogen, were chosen. The latter was the focus of the work and the former was used for comparison. It was seen that nitrogen played an important role in the degree of sensitization, either when dissolved or in intermetallic precipitates. Because heat treatment was necessary to control the microstructural features, a set of heat treatments was established: annealing and aging at 100, 200, 300, 375, 675, and 875°C for different times (90 min and 4, 6, 8, and 48 h). The effect of nitrogen and temperature-time aging on corrosion resistance was investigated. Experimental results showed that the...

Electrochemical behavior of polypyrrol/AuNP composites deposited by different electrochemical methods: sensing properties towards catechol

Summary Two different methods were used to obtain polypyrrole/AuNP (Ppy/AuNP) composites. One through the electrooxidation of the pyrrole monomer in the presence of colloidal gold nanoparticles, referred to as trapping method (T), and the second one by electrodeposition of both components from one solution containing the monomer and a gold salt, referred to as cogeneration method (C). In both cases, electrodeposition was carried out through galvanostatic and potentiostatic methods and using platinum (Pt) or stainless steel (SS) as substrates. Scanning electron microscopy (SEM) demonstrated that in all cases gold nanoparticles of similar size were uniformly dispersed in the Ppy matrix. The amount of AuNPs incorporated in the Ppy films was higher when electropolymerization was carried out by chronopotentiometry (CP). Besides, cogeneration method allowed for the incorporation of a higher number of AuNPs than trapping. Impedance experiments demonstrated that the insertion of AuNPs increased the conductivity. As an electrochemical sensor, the Ppy/AuNp deposited on platinum exhibited a strong electrocatalytic activity towards the oxidation of catechol. The effect was higher in films obtained by CP than in films obtained by chronoamperometry (CA). The influence of the method used to introduce the AuNPs (trapping or cogeneration) was not so important. The limits of detection (LOD) were in the range from 10−5 to 10−6 mol/L. LODs attained using films deposited on platinum were lower due to a synergy between AuNPs and platinum that facilitates the electron transfer, improving the electrocatalytic properties. Such synergistic effects are not so pronounced on stainless steel, but acceptable LOD are attained with lower price sensors.

Application of minielectrochemical cell to corrosion studies of welded joints of austenitic stainless steel

Abstract A specific electrochemical cell for small scale electrochemical testing has been built. The electrochemical cell has been made bearing in mind the microstructural features of the stainless steel weldments and the performance of the design has been verified. Two polarisation methods (potentiodynamic anodic polarisation measurements and cyclic potentiodynamic polarisation measurements) and two reactivation electrochemical techniques (the electrochemical potentio-kinetic reactivation test, EPR, and electrochemical potentio-kinetic reactivation double loop test, EPRDL) have been applied to two austenitic stainless steels: AISI 304 and AISI 316L. The results are reliable and have been compared to large scale experiments. Scanning of the welding joints has been performed and the results have been correlated with their microstructural features. It, thus, makes it possible to study the intrinsic heterogeneous microstructures such as the heat affected zone of a welded joint.

Monitoring the Phenolic Ripening of Red Grapes Using a Multisensor System Based on Metal-Oxide Nanoparticles

The maturity of grapes is usually monitored by means of the sugar concentration. However, the assessment of other parameters such as the phenolic content is also important because the phenolic maturity has an important impact on the organoleptic characteristics of wines. In this work, voltammetric sensors able to detect phenols in red grapes have been developed. They are based on metal oxide nanoparticles (CeO2, NiO, and TiO2,) whose excellent electrocatalytic properties toward phenols allows obtaining sensors with detection limits in the range of 10−8 M and coefficients of variation lower than 7%. An electronic tongue constructed using a combination of the nanoparticle-based sensors is capable to monitor the phenolic maturity of red grapes from véraison to maturity. Principal Component Analysis (PCA) can be successfully used to discriminate samples according to the ripeness. Regression models performed using Partial Least Squares (PLS-1) have established good correlations between voltammetric data obtained with the electrochemical sensors and the Total Polyphenolic Index, the Brix degree and the Total Acidity, with correlation coefficients close to 1 and low number of latent variables. An advantage of this system is that the electronic tongue can be used for the simultaneous assessment of these three parameters which are the main factors used to monitor the maturity of grapes. Thus the electronic tongue based on metal oxide nanoparticles can be a valuable tool to monitor ripeness. These results demonstrate the exciting possible applications of metal oxide nanoparticles in the field of electronic tongues.

A tandem allylsilane–vinylsilane difunctionalization by silylcupration of allene followed by reaction with α,β-unsaturated nitriles

Silylcupration of allene using phenyldimethylsilylcopper 1 followed by BF3-mediated reaction with α,β-unsaturated nitriles at −40 °C affords allylsilane–vinylsilane-containing ketones resulting from consecutive addition (1,2 and 1,4) of the intermediate allyl- and vinylcopper species formed in the silylcupration of allene.

Influence of sinter-cooling rate on intergranular corrosion of powder metallurgy superaustenitic stainless steel

Abstract Powder metallurgy superaustenitic stainless steel sintered in N2–H2 atmosphere (95%–5%) was obtained. Three different processes were applied by controlling the sinter-cooling rate (furnace, gas and water). Intergranular corrosion resistance was evaluated. Furthermore, microstructure, density, porosity, macro- and microhardness were investigated. Microstructures were characterised by optical microscopy and scanning electron microscopy with energy dispersive analysis of X-rays. Intergranular corrosion behaviour was evaluated by immersion and by single and double loop electrochemical potentio-kinetic reactivation tests. The superaustenitic steel after fast cooling by water quenching showed the best intergranular corrosion behaviour, the highest microhardness values and the most homogeneous microstructure.

Improvement in Mechanical Properties and Corrosion Resistance of Powder Metallurgy Type 430L Stainless Steel Sintered in Nitrogen-Hydrogen Atmosphere

Abstract Powder metallurgy (PM) AISI 430L (UNS S43000) sintered in a nitrogen-hydrogen atmosphere was investigated to obtain an acceptable balance of mechanical properties and corrosion resistance. High-temperature solution annealing (1,150°C, 60 min) and some post-sintered aging heat treatments (100, 200, 300, 375, 675, and 850°C) for different times (90 min, 4, 6, 8, and 48 h) were performed. The effects of nitrogen and temperature/time aging on localized corrosion were analyzed by using an exposure method (ferric chloride test, ASTM G48) and anodic polarization measurements (ASTM G5). Experimental results showed that high-temperature solution annealing provides even better corrosion resistance than the material sintered in vacuum. Also, aging at low temperatures shows a positive effect on mechanical reinforcement without losing significant levels of corrosion resistance.