Marie-Christine Lafont

A corrosion inhibition study of a carbon steel in neutral chloride solutions by zinc salt/phosphonic acid association

Electrochemical measurements (steady-state current-voltage curves and AC impedance) were coupled with analytical techniques (reflection-adsorption spectroscopy at grazing incidence and XPS) to investigate the inhibition of corrosion of a carbon steel by a mixture of a zinc salt and a phosphonic acid. From the polarization curves and impedance measurements it was shown that the electrochemical reactions occurring at the metal interface were not controlled by mass transport. The inhibitor film acts as a protective layer impermeable to ionic or molecular diffusion. Corrosion proceeds only through small defects of the inhibitor film. XPS analysis shows that the film is very thin and homogeneous in composition. It is essentially composed of Zn, P and O. The formation of an iron oxide under the inhibitor layer was observed. The FTIR spectrum of the film formed on steel indicates a reaction of the phosphonic acid with the zinc hydroxide and the iron oxide to produce metal salts.

A synergistic effect between zinc salt and phosphonic acid for corrosion inhibition of a carbon steel

This work is devoted to the corrosion inhibition of a carbon steel by a zinc salt/phosphonic acid association. Steady-state current-voltage curves and electrochemical impedance measurements carried out in the presence of each compound and for the mixture show a synergistic effect between the two molecules. The concentrations of the compounds in the mixture were lower than the concentrations used for each compound separately. Phosphonic acid was observed to act as an anodic inhibitor whereas cathodic action was shown for the zinc chloride. Electrochemical measurements and surface analysis (XPS and reflection-adsorption spectroscopy at grazing incidence) showed that the synergistic effect afforded by the mixture was attributable to the reaction of the phosphonic acid with the zinc salt. The inhibitor film acts as a protective layer impermeable to ionic or molecular diffusion. The film is very thin and homogeneous in composition. A chemical structure of the film is proposed.

Study of the corrosion and inhibition processes of a carbon steel in a low conductivity medium by electrochemical methods

Abstract Ohmic drop corrected steady-state current—voltage curves and electrochemical impedance diagrams have been obtained for carbon steel rotating disc electrodes in 200 mg l−1 NaCl solutions with and without addition of 1 g l−1 of the association of oleylamino propylene and amino-tri[methyl-phosphonic] acid (FA + TMPA). Corrosion rates determined from both steady state and impedance data are in fairly good agreement with those determined from gravimetric measurements carried out on a semi-pilot scale circuit. Because impedance measurements are non-destructive and not affected by low electrolyte conductivity, they allow us to follow the evolution of the corrosion rate as a function of time: in the absence of inhibitor the corrosion rate increases during the early stages of immersion whereas it decreases in the presence of 1 g l−1 (FA + TMPA). Impedance diagrams obtained in inhibited solutions reveal a high frequency loop unobserved in the absence of the inhibitor; the very low value (1–2 μF cm−2) of the capacity associated to this loop suggests that the inhibitor (FA + TMPA) acts in forming a relatively thick and compact protective film on the metal surface.

Microstructural Characterization of Graphite Spheroids in Ductile Iron

The present work brings new insights by transmission electron microscopy allowing disregarding or supporting some of the models proposed for spheroidal growth of graphite in cast irons. Nodules consist of sectors made of graphite plates elongated along a 〈a〉 direction and stack on each other with their c axis aligned with the radial direction. These plates are the elementary units for spheroidal growth and a calculation supports the idea that new units continuously nucleate at the ledge between sectors.

Corrosion inhibition of pure iron in neutral solutions by electrochemical techniques

Abstract Steady-state current-voltage curves for various disk rotation rates were combined with transient measurements (frequency analysis of the electrochemical and electrohydrodynamical impedance) in order to investigate the corrosion inhibition by an organic surfactant of pure iron in an aerated 0.5 M NaCl solution. In the cathodic range, physical adsorption of the compound with the interfacial potential was observed leading to the thickening or strengthening of the film. Thus, for short times of polarization, due to the adsorption relaxation, it was necessary to compensate the ohmic drop in the circuit to obtain the electrohydrodynamical (EHD) curves. The inhibitor acts by forming a porous layer covering the surface. From the analysis of the EHD diagrams the thickness and the porosity of the film were estimated. Nevertheless, a non-classical behaviour in the HF range of the EHD diagrams was observed and interpreted by a mechanical action of the fluctuating flow on the film. At the corrosion potential, the electrochemical impedance diagrams were essentially representative of the charge transfer process. Diffusion across the inhibitor film is very slow and requires extremely low modulation frequencies (below 1 mHz).

MINERALOGICAL AND PHYSICO-CHEMICAL CHARACTERIZATIONS OF FERRUGINOUS BEIDELLITE-RICH CLAY FROM AGADIR BASIN (MOROCCO)

The mechanism of formation of detrital, beidellite-rich clay occurring in the Agadir basin (Morocco) is well documented, but its detailed characterization is incomplete which limits its application. The aim of the present study was to provide further details of the mineralogical and physico-chemical characteristics of this clay. Bulk raw clay and its Na+-saturated, <2 μm fraction were characterized using chemical, structural, and thermal techniques. Measurements of induced streaming potential (e.g. particle charge) and of specific surface area and porous volume are reported. The raw clay contained carbonate and quartz as associated minerals along with phyllosilicates (<2 μm particle size). X-ray diffraction and scanning electron microscopy analyses showed that the <2 μm fraction was dominated by a dioctahedral smectite. Because dehydroxylation of this mineral occurred at 510°C, and because it re-expanded in ethylene glycol after Li+-saturation followed by heating at 240°C for 24 h, the mineral was shown to be a beidellite rather than montmorillonite. This assertion was further supported by 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectra showing predominantly negative charges in the tetrahedral sheets due to notable Al-for-Si substitutions. The chemical composition of the <2 μm fraction showed an Fe2O3 content which was ~7.52 wt.% greater than those of other beidellite occurrences but not so much that it would be identified as a nontronite. The absence of stretching and bending absorption bands corresponding to characteristic (Fe2OH) units in mid-infrared spectra and their corresponding fundamental overtones or combination bands in near-infrared spectra supported this notion. The structural formula of the beidellite in the present study was determined to be (Si7.51Al0.49)(Al2.99Fe0.68Mg0.33) (Ca0.03Na0.54Mg0.11)O20(OH)4, having dioctahedral ferruginous characteristics with almost 60% of the negative charge found in tetrahedral sheets. The cation exchange capacity determined from the structural formula was ~108 meq/100 g. The specific surface area and total pore volume were ~82.2 m2/g and 0.136 cm3/g, respectively. Interestingly, a detrital rather than a hydrothermal-alteration origin, as reported for other beidellite occurrences, explains its natural abundance and emphasizes the great interest in it.

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.

Photocatalytic activity of TiO2/stevensite nanocomposites for the removal of Orange G from aqueous solutions

Abstract TiO2/stevensite nanocomposite photocatalysts were synthesized by a solvothermal method using TiCl3/HCl as reactants and the stevensite clay mineral extract as support. The prepared photocatalyst samples were then characterized using various techniques such as X-ray diffraction (XRD), Infrared spectroscopy (IR) and Transmission Electron Microscopy (TEM). The Points of Zero Charge (PZC) of the various samples were evaluated by titration of the non-modified and the Ti-modified clay aqueous dispersions, with cationic surfactant solutions. The photocatalytic activity of the resulting nanocomposites samples were evaluated for the removal of Orange G (OG) from aqueous solution as a model dye pollutant. The data indicate that the formation of Na+-stevensite by the TiO2 particles leads to TiO2/stevensite nanocomposites having higher specific surface areas and mesopore volumes, and lower PZC values. Further, the photocatalytic activity was greater for the TiO2/stevensite nanocomposites having the greatest Ti amount, as compared to a pure TiO2 sample, and increased with the increase of the TiO2 content in the TiO2/stevensite nanocomposites.

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.

Influence of Localized Plasticity on IASCC Sensitivity of Austenitic Stainless Steels under PWR Primary Water

The sensibility of precipitation-strengthened A286 austenitic stainless steel to Stress Corrosion Cracking (SCC) is studied by means of Slow Strain Rate Tests (SSRT). First, alloy cold working by Low Cycle Fatigue (LCF) is investigated. Fatigue tests under plastic strain control are performed at different strain levels (Δ ep/2=0.2%, 0.5% and 0.8%) in order to establish correlation between stress softening and deformation microstructure resulting from LCF tests. Deformed microstructures have been identified through TEM investigations. Three states of cyclic behaviour for precipitation-strengthened A286 have been identified: hardening, cyclic softening and finally saturation of softening. It is shown that the A286 alloy cyclic softening is due to microstructural features such as defects — free deformation bands resulting from dislocations motion along family plans , that swept defects or γ’ precipitates and lead to deformation localization. In order to quantify effects of plastic localized deformation on intergranular stress corrosion cracking (IGSCC) of the A286 alloy in PWR primary water, slow strain rate tests are conducted. For each cycling conditions, two specimens at a similar stress level are tested: the first containing free precipitate deformation bands, the other not significant of a localized deformation state. SSRT tests are still in progress.