High-temperature oxidation evaluation usingcrystal microbalance

High-temperature oxidising environments are frequently encountered but the limited number of in situ techniques that can be implemented has hindered the monitoring possibilities and a better comprehension of the oxidation phenomenon. In this paper, the high-temperature oxidation behaviours of three alloys (AISI 316L, AISI 310 and HAYNES\uae HR-120\uae) were studied by using crystal microbalances. Two types of crystal were tested: quartz or gallium orthophosphate crystals. First the behaviour of thin sputtered deposited alloys on quartz slides was studied at 400 and 700\ub0C under air oxidising conditions and compared to bulk samples. Kinetics measurements were performed on the three alloy films deposited on the resonators at 400 or 700\ub0C: it was possible to measure very small mass variations associated with thin oxide formation between 5 and 180 nm of thickness. The crystal microbalance technique gives promising perspectives in understanding the high-temperature corrosion and scaling mechanisms and also for in situ monitoring

Poly(phenylene methylene)-based coatings for corrosion protection : replacement of additives by use of copolymers

Poly(phenylene methylene) (PPM) is a thermally stable, hydrophobic, fluorescent hydrocarbon polymer. Recently, blended PPM has been proposed as a valuable anti-corrosion coating material, and, in particular, rheological additives such as external plasticizers resulted crucial to prevent crack formation. Accordingly, to avoid common problems related to the use of external plasticizers, the development of PPM-related copolymer-based coatings containing n-octyloxy side chains and their anti-corrosion behavior were explored in this study. The aluminum alloy AA2024, widely employed for corrosion studies, was selected as a substrate, covered with a thin layer of a polybenzylsiloxane in order to improve adhesion between the underlying hydrophilic substrate and the top hydrophobic coating. Gratifyingly, coatings with those copolymers were free of bubbles and cracks. The n-octyloxy side-chains may be regarded to adopt the role of a bound plasticizer, as the glass transition temperature of the copolymers decreases with increasing content of alkoxy side-chains. Electrochemical corrosion tests on PPM-substituted coatings exhibited good corrosion protection of the metal surface towards a naturally aerated near-neutrally 3.5% wt.% NaCl neutral solution, providing comparable results to blended PPM formulations, previously reported. Hence, the application of rheological additives can be avoided by use of proper design copolymers

Study of Al alloy corrosion in neutral NaCl by the pitting scan technique

The corrosion behavior of commercial Al alloys was studied in neutral 0.6 mol L(-1) NaCl by using single-cycle polarization. Qualitative interpretation of pitting scans in both deaerated and naturally aerated NaCl solution, with the aid of corrosion morphology characterization, allowed for inference of the features of localized corrosion as a function of Al substrate nature. Electrochemical characteristic parameters such as pitting (E(pit)), repassivation or protection (E(prot)) potentials and pit transition potential (E(ptp)) were determined. The validity of the difference Delta E = vertical bar E(pit)-E(prot)vertical bar as criterion for susceptibility to localized corrosion of aluminium alloys is discussed

\u3b3 -alumina as a support for catalysts : A review of fundamental aspects

A review; the present review discusses the most important aspects to take into consideration to improve the properties of \uf067-Al2O3 as a support for catalytic applications. We show that the synthetic route to \uf067-Al2O3 is the starting point that dets. the micro- and macrostructure of the oxide and, consequently, allows control of the support characteristics. The relevance of the adequate structural characterization of the oxide as well as of its surface sorption behavior through the proton-affinity distributions, are considered. The usefulness of the latter during supported catalyst prepn., for catalytic active sites characterization, and for stability evaluation after calcination are discussed for the Co/\uf067-Al2O3 system in particular

Pyrrole-based silane primer for corrosion protection of commercial AA 2024 T3

An alternative approach was developed for surface treatment of as-received commercial AA 2024 T3 by using a pyrrole-based silane (SiPy). For film deposition, just one immersion step is enough, followed by curing. SiPy layer structure contains both polysiloxane bonds and pyrrole oligomers, with some degree of doping, giving a highly coherent layer. The superior film quality with respect to simple polysiloxane, is probably the main reason for the better corrosion performance obtained for SiPy on 2024

Relation of anodic and cathodic performance to pH variations in membraneless microbial fuel cells

One-compartment (membraneless) microbial fuel cells (MFCs) are effective tools to test new bio-technology at a laboratory level. More efforts in MFC design and materials are necessary to move from laboratory tests to real applications. In such a context, this paper presents the experimental results that investigate pH variations of three single chamber and membraneless MFCs having positive and negative electrodes made of graphite-based materials without any addition of chemical catalysts. MFCs were built and operated with raw wastewater (inoculum) and sodium acetate as substrate. The progression of the power in the MFC and the relationship between performances and induced pH variation (from pH 6.7 to 10.2) will be discussed. A general connection between SEM images, chemical analyses, pH trends and reactions in the MFCs will be attempted, by connecting all processes with thermodynamic and chemical equilibria considerations

Anodic films containing zirconia nanoparticles for corrosion protection of AA1050 aluminum alloy

In this paper, production of anodic aluminum oxide (AAO) is based on a parameter-optimized literature scheme. Highly ordered tubular structures are achieved as a starting point for subsequent modification steps. These steps include sealing of AAO pores. Sealing is here attempted via the spontaneous adsorption of ZrO2 nanoparticles on preformed AAO. Nanoparticles are synthetized by several different methods, in particular, the innovative water-free microwave-assisted synthesis. These novel water-free nanoparticles are, here for the first time, used for AAO sealing and corrosion protection. Increase in corrosion resistance is tested by electrochemical methods

Detection of Sigma Phase in 22% Cr Duplex Stainless Steel by Electrochemical Methods

Duplex stainless steels (DSS) may undergo various structural transformations as a function of temperature, leading to formation of secondary phases when exposed to temperatures in the range 600-1000 degrees C for prolonged periods of time. Among these phases, apart from carbides and nitrides, sigma is the most prominent intermetallic compound, since it is known to adversely affect mechanical properties and corrosion resistance of DSS as a consequence of Cr and Mo depletion in the boundary zones of ferrite grains. As a result of sigma precipitation, DSS become susceptible to localized corrosion via a mechanism resembling sensitization in austenitic stainless steels. Electrochemical methods, known to successfully detect sensitization in austenitic SS, have been used to identify reliable test conditions with appropriate sensitivity also for DSS. A modified electrochemical potentiokinetic reactivation method was developed to investigate 22% Cr DSS sensitivity under different thermal cycles. Modifications were designed by closely analysing DSS anodic behaviour changes with respect to the operating testing conditions. All findings were confirmed by SEM, optical microscopy and XRD

Polypyrrole films on Al alloys: The role of structural changes on protection performance

Polypyrrole (PPy) films were electrochemically synthesized on AA6082, AA5083 and AA2024 Al alloys by a two-step galvanostatic procedure: (1) activation in nitric acid and (2) deposition in oxalic acid. PPy films were characterized by elemental analysis, cyclic voltammetry, SEM/EDX. RAIR, solid-state (13)C NMR, and EPR. The corrosion behaviour of PPy-coated alloys was evaluated in near neutral NaCl solution by single-cycle anodic polarization and open circuit potential monitoring. Film adhesion and polymer structure are principally determined by the nature of the Al substrate. Good quality films are obtained on the most reactive AA5083 and AA2024, while highly degraded, partially saturated polymer is electrogenerated on AA6082. The performance of PPy films was observed to depend strongly on the electrochemical test used for corrosion protection evaluation. Accordingly, the rate at which structural changes are driven within the polymeric matrix is of paramount importance to understand the corrosion inhibition properties of conducting polymers