Modelling the corrosion behaviour of Al2CuMg coarse particles in copper-rich aluminium alloys

The corrosion behaviour of 2024 aluminium alloy in sulphate solutions was studied; attention was focused on the influence of coarse intermetallic Al2CuMg particles on the corrosion resistance of the alloy. Model alloys representative of the aluminium matrix and of Al2CuMg coarse intermetallics were synthesized by magnetron sputtering. Open-circuit potential measurements, current–potential curve plotting and galvanic coupling tests were performed in sulphate solutions with or without chlorides. Further explanations were deduced from the study of the passive films grown on model alloys in sulphate solutions. The results showed that model alloys are a powerful tool to study the corrosion behaviour of aluminium alloys

Identification of intermetallic precipitates formed during re-solidification of brazed aluminium alloys

This study shows that Fe and Mn bearing phases in Al–Si alloys containing a low level of Fe and Mn are essentially cubic greek small letter alpha-Al(Fe, Mn)Si with Im3 space group and δ-AlFeSi which is observed with both tetragonal and orthorhombic structures. As this latter phase is not expected to form according to the ternary Al–Fe–Si phase diagram, the present results suggest that it is stabilized in the quaternary Al–Fe–Mn–Si system with respect to other phases such as β-Al4FeSi

Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water

The corrosion behavior of 304L grade stainless steel (SS) in high-temperature steam and in a simulated Pressurized Water Reactor (PWR) is studied. The goal was to characterize the nature of the oxide coating generated during 500 h exposure of samples in a 400 °C steam (200 bar) or a 340 °C simulated PWR. Accelerating the effect of the steam environment as well as the influence of surface preparation have been studied. Two initial sample surfaces were used: mechanical polishing and finishing grinding. Oxide coatings were investigated using TEM imaging coupled with EELS spectroscopy and R – SIMS (Secondary Ion Mass Spectroscopy)

Electrochemical Behavior of Magnetron-Sputtered Al–Cu Alloy Films in Sulfate Solutions

Model alloys, generated by magnetron sputtering, have been employed to understand the role of copper on the corrosion behavior of aluminum alloys. Binary Al–Cu alloys, with copper contents between 0 and 100 atom %, were synthesized with well-controlled compositions, embracing single-phase alpha and theta alloys together with multiphase alloys. Electrochemical measurements confirmed the stability of the thin alloy films and revealed that the corrosion behavior of the alpha, theta, and eta2 phases differed strongly in the cathodic region. Further, in the anodic region, phases of high copper content suffered pitting in sulfate solutions, while the alpha phase remained passive

Effect of modification by Pt and manufacturing processes on the microstructure of two NiCoCrAlYTa bond coatings intended for thermal barrier system applications

Few studies have already shown that Pt influences the diffusion of aluminium and therefore the microstructure of β-NiAl or γ-Ni/γ′-Ni3Al materials. Besides, several works have revealed that the addition of Pt to MCrAlY (M = Ni and/or Co) improves the oxidation/corrosion behavior of the material. Nevertheless, very few data have been published on the microstructure of such modified MCrAlYs. Then, the present work deals with the addition of Pt to two NiCoCrAlYTa coatings that differ by their manufacturing process. Characterization is carried out in order to understand the influence of Pt diffusion but also the effect of the manufacturing process on the final microstructure. The collected data from XRD, SEM, EDS and TEM analyses reveal that an Al uphill diffusion occurs during heat treatment due to the presence of the Pt layer. The Al diffusion from the NiCoCrAlYTa bulk to the Pt-rich surface is so extensive that no more β-phase remains within the core of the coating. Pt may also dissolve TaC, precipitates largely present in the non-modified NiCoCrAlYTa coatings. In addition to Pt, the microstructure of the NiCoCrAlYTa prior to Pt deposition and heat treatment, dependent on the NiCoCrAlYTa manufacturing process, greatly influences the final microstructure

Short Term Oxidation of a Ti-46Al-8Nb Alloy at 700°C in Air

Thermogravimetry was used to study the oxidation behaviour of a lamellar Ti46Al8Nb alloy during holding at 700°C in synthetic air. A parabolic plot of the oxidation kinetics shows three different regimes over the total duration (50 h) of the tests corresponding to decreasing values of the parabolic rate constant. The oxide scale was characterized by glancing-angle X-Ray diffraction and transmission electron microscopy. The scale was found to be bi-layered with an outer part that consists of amorphous aluminium rich oxide whilst the inner layer is made of very small cristalites of titania distributed in the same amorphous oxide

Structural characterization of the scale formed on a Ti–46Al–8Nb alloy oxidised in air at 700 C

The structures of the oxide scale formed on a lamellar (gþ a2) Ti46Al8Nb alloy after heat-treatment in air at 700 C for 50 and 1000 h have been compared. Though the outer scale is thicker after 1000 h, both oxide scales are similarly layered with an outer aluminium-rich layer. The main difference is that the aluminium-rich oxide is amorphous after 50 h while it becomes cubic gamma alumina after 1000 h. After 50 h, the presence of TiN was detected at the interface between the scale and the base metal. After 1000 h of oxidation, the subscale region is divided into two clearly separated zones differing by their composition, with a thick TiN layer close to the oxide scale. From these observations, it appears that the development of the oxide scale at 700 C is similar to the scheme proposed in the literature for short times at higher temperatures, with the initial formation of a transient aluminium-rich oxide layer, which here is amorphous, and precipitates of TiN at the scale/metal interface that oxidise afterwards to rutil

Characterisation and understanding of the corrosion behaviour of the nugget in a 2050 aluminium alloy Friction Stir Welding joint

The corrosion behaviour of the nugget of a Friction Stir Welding joint employing a 2050 Al–Cu–Li alloy was investigated. The results showed that the nugget was susceptible to both intergranular and intragranular corrosion. Such corrosion behaviour was related to microstructural heterogeneities observed on a microscopic scale. Furthermore, heterogeneities in the corrosion behaviour of the nugget observed on a macroscopic scale were evidenced by a different corrosion behaviour from the top to the bottom of the nugget and by a localisation of the corrosion damage related to the ‘‘Onion ring structure’’. Critical microstructural parameters were identified to explain the results

Synthesis, characterization and photocatalytic activity of TiO2 supported natural palygorskite microfibers

This study deals with the synthesis of TiO2 supported Moroccan palygorskite fibers and their use as photocatalyst for the removal of Orange G pollutant from wastewater. The TiO2-palygorskite nanocomposite synthesis was accomplished according to a colloidal route involving a cationic surfactant as template (hexadecyltrimethylammonium bromide) assuring hence organophilic environment for the formation of TiO2 nanoparticles. The clay minerals samples were characterized before and after functionalization with TiO2. Anatase crystallizes above ca. 450 °C and remarkably remains stable up to 900 °C. In contrast, pure TiO2 xerogel obtained from titanium tetraisopropoxide (TTIP) showed before calcination a nanocrystalline structure of anatase. By increasing the temperature, anatase readily transforms into rutile beyond 600 °C. The remarkable stability at high temperature of anatase particles immobilized onto palygorskite microfibers was due to the hindrance of particles growth by sintering. Homogeneous monodisperse distribution of anatase particles with an average size of 8 nm was found by TEM and XRD onto palygorskite fibers. This anatase particle size remains below the nucleus critical size (ca. 11 nm) required for anatase–rutile transition. The TiO2 supported palygorskite sample annealed in air at 600 °C for 1 h exhibits the highest photocatalytic activity towards the degradation of Orange G compared to nanocomposite samples prepared under different conditions as well as pure TiO2 powders obtained from the xerogel route or commercially available as Degussa P25

On the Understanding of TGO Growth and Spallation in Nickel Aluminides

This paper describes various parameters influencing oxidation kinetics, oxide’s morphology and spallation phenomena encountered while studying nickel aluminides in high temperature oxidation. Questions are raised about the possibility of making precise lifetime or failure predictions of coatings and TBCs systems. Changes in growth mechanism due to the presence of elements other than Ni and Al in nickel-aluminide base coatings, the effect of atmosphere, particularly water vapor, and the effect of surface preparation and of crystallographic orientation are the main parameters discussed in this work