G. Pena

Electrochemical Impedance Spectroscopy as a Tool for Studying Steel Corrosion Inhibition in Simulated Concrete Environments—Red Mud Used as Rebar Corrosion Inhibitor

Red mud is the main by-product of the Bayer process for alumina extraction from bauxite. Rich in iron, aluminum and silicon oxides, it is believed to have corrosion protection properties. Some are studied in the present paper. The corrosion behavior of steel rebars in chlorine containing solutions and mortars is studied and compared with the same systems containing red mud in suspension (case of solutions) or added as powder to the mix (case of mortars). The results in solution show that red mud is able to maintain steel passivity for more than 90 days in highly chlorinated alkaline solutions while in NaOH and Ca(OH)2 solutions having the same pH pit formation is observable at 25 and 55 days, respectively. Additions of red mud to mortar as powder representing 2 % by weight of cement are able to block chloride depassivation. EIS data allow understanding the protecting mechanism and comparing the behavior in mortar and in solution.

Microstructural characterization of laser surface melted AISI M2 tool steel

We describe the microstructure of Nd:YAG continuous wave laser surface melted high‐speed steel, namely AISI M2, treated with different laser scanning speeds and beam diameters on its surface. Microstructural characterization of the remelted surface layer was performed using light optical and scanning electron microscopy and X‐ray diffraction. The combination of the three techniques provided new insights into the substantial changes induced by laser surface melting of the steel surface layer. The advantage of the method is that it avoids the difficult and tedious work of preparing samples of this hard material for transmission electron microscopy, which is the technique normally used to study these fine microstructures. A melted zone with a dendritic structure and a partially melted zone with a heterogeneous cellular structure were observed. M2C carbides with different morphologies were identified in the resolidified surface layer after laser melting.

Modification of AISI M2 high-speed tool steels after laser-surface melting under different operating conditions

We applied a laser-surface melting treatment to AISI M2 high-speed steel – hardened and tempered – and studied the resulting surface characteristics (microstructure) and mechanical behaviour (hardness and wear performance). The steel was treated using a Nd:YAG continuous-wave laser with different operation conditions. The influence of the laser processing parameters on the individual beads and on the characteristics of the molten surface layer obtained using the multipass system with 50% overlap was studied. The microstructure characterization for all conditions is formed of MC- and M2C-type carbides, martensite, and retained austenite; the quantities of this phase depend on the operating conditions. It has been determined that low levels of power density and high scanning speeds of the beam lead to greater homogeneity in the microstructure with high hardness values and wear resistance.

Microstructure of the Passive Layer Formed on Different Austenitic Stainless Steels Implanted with Molybdenum

Two austenitic stainless steels have been implanted at 150 keV with Mo at a fluence of 3.5x1015 ions/cm2. The effects of ion implantation in the chemical composition of the passive films was evaluated by x-ray photoelectron spectroscopy (XPS) and glancing angle x-ray diffraction (GAXRD) was used to determine the induced structural modifications. The results of the pitting corrosion studies carried out in neutral chloride medium as well as the morphology of the localized attack are discussed.

An Insight on the Influence of Ion Implantation on the Pitting Corrosion Resistance of AISI 430 Stainless Steel

Research on the effect of ion implantation on the corrosion behaviour of metals has been carried out for years, but some difficulties arise in the comparison of the obtained results due to variations in experimental conditions (alloys, surface preparation, doses, experimental techniques...). This work tries to overcome those differences, presenting the effect of several elements (Ce+, N+, Cr+ and Cr+ N+) implanted in similar conditions on the pitting corrosion resistance of AISI 430 stainless steel. Potentiodynamic measurements in 1M NaCl demonstrate the beneficial effect of all the implanted elements, showing that Ce+ is the less efficient ion, while Cr+ N+ co-implantation gives the best results in terms of localized attack resistance. Pitting morphology is explained in terms of the XPS and GIXRD data that allow chemical and structural characterization of the implanted layer. Those results help to enlighten the protection mechanisms involved in the considered implantations.

Optimization of Ageing Parameters of a Low Nickel Maraging Steel

In the present work the age hardening parameters of a 14Ni (200) maraging steel are studied in order to optimize mechanical properties of the steel. The initial characterization of the as received solution annealed steel has been carried out by optical and scanning electron microscopy (LOM and SEM), and hardness measurements. To identify the structural changes during ageing, differential scanning calorimetry (DSC) tests were performed. Different time-temperature combinations were considered for the precipitation hardening treatment of as-quenched material samples. After hardness measurements, three of these treatments were selected for an in depth study. The obtained microstructure at the maximum hardness peaks then analysed (LOM, SEM and TEM) and mechanical behaviour (strength, toughness and wear resistance) was studied for the final selection of the age hardening conditions.

The effect of the Cerium ion implantation in the passive films properties of a duplex stainless steel

Abstract The effect of Ce+ implantation on the electrochemical behaviour of SAF 2205 DSS is studied in alkaline medium. XPS studies have detected the implanted cerium as Ce3+ throughout the oxide films. The peak current of the magnetite formation peak is directly related to the passive film thickness. Nevertheless, the Cr3+ oxidation process is not affected by cerium implantation, which suggests cerium incorporation in the iron spinel.

Assisted Friction Stir Welding of Carbon Steel: Use of Induction and Laser as Preheating Techniques

The use of FSW in high melting point materials is still restricted due to limited tool life. Due to the high wear involved in the process, tool life is too short and the cost of replacing it is still too high. One of the ways that was explored to increase tool life is the use of preheating techniques to soften the material to be welded. Softening the material would cause a decrease in tool wear and therefore an increase in tool life. In this study, two preheating techniques were tested on the FSW of carbon steel: induction heating and laser heating. The data arising from the study show that the forces taking place during both hybrid processes were importantly reduced compared to those obtained in conventional FSW. Microstructure and mechanical testing of welds made in the three conditions were done showing the influence of each process on weld quality

FIB and SEM-STEM Studies of Friction-stir Processed AM60 Magnesium Alloy

In the present work, we present the microstructural study of a cast magnesium alloy AM60B processed by Friction Stir Processing (FSP) in order to achieve Ultra-Fine Grain (UFG) size (200–700nm) in the stirred zone. Focused ion beam (FIB) has been used for sample preparation, and transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and selected area electron diffraction (SAED) for the structural characterization. To explain the differences induced by the FSP, two lamellas obtained by FIB from the bottom and the upper part of the cross-section of the nugget are compared to one extracted from the parent material (unaffected zone A). The obtained results show the remarkable homogenization and grain refinement produced in the structure in the processed zone, as previous reported works indicate. The microstructure of the as cast base material reveals the presence of dendrites of solid solution of Al in Mg (α-phase), with an average grain size of ~400–500 µm, and a partially divorced eutectic structure at the interdendritic spaces, where the massive and lamellar β-Mg 17 Al 12 intermetallic phase can be observed (Figure 1). The SAED on one particle of β-phase shows a diffraction pattern that fits the crystalline structure of the cubic I -4 3 m space group (No 217). Also Mn containing phases were homogeneously distributed throughout the section. The performed analyses demonstrate that these particles are basically Al-Mn binary phases containing trace elements as Si or Fe, and the measured Mn/Al ratio allow to classify them into two types previously reported: Type I are particles of equiaxed or almost rounded shape and type II are needle like or flowerlike particles, with a lower Mn/Al ratio.

Analysis of the Passive Layer Developed on Two Stainless Steels Co-Implanted with Chromium and Nitrogen

Surface analysis techniques (XPS and grazing incidence X-ray diffraction GIXRD) and electrochemical techniques have been combined to elucidate the effect resulting from Cr+ and N+ co-implantation in the formation and evolution of passive layers generated on two different stainless steels (AISI 430 and AISI 304L) in alkaline medium. The results show that the nitrides formed on AISI 430, identified as (Cr,Fe)2N1-x, are less resistant to dissolution while the nitride phase formed in AISI 304L, NCr with nanocrystalline structure, allows the compact growth of the oxide film.