84 research outputs found
Study of cathodic reactions in defects of thermal spray aluminium coatings on steel in artificial seawater
Electrochemical and surface characterization techniques were used to study the corrosion protection provided by Thermal Spray Aluminium (TSA) coatings on carbon steel substrates with and without defects in artificial seawater. Results showed that the presence of defects accelerates the formation of a protective corrosion product layer on the TSA coating. Voltammetry and Electrochemical Impedance Spectroscopy (EIS) revealed that both calcareous deposits and aluminium corrosion products hindered diffusion of dissolved O2. Analysis of corrosion products by SEM-EDX, μ-Raman spectroscopy and XRD revealed Al oxides/hydroxides and hydrotalcite on top of TSA coating and a bilayer of calcareous deposits on the defect area
The reduction of chromate ions by Fe(II) layered hydroxides
International audienceThe reduction of chromate ions by Fe(OH)2 and the iron (II)-iron (III) hydroxysulphate green rust, GR(SO42-), was studied to evaluate whether such synthetic layered hydroxides and the corresponding natural green rust mineral could be involved in the natural attenuation of contaminated environments. The resulting Cr (III) bearing phases, which would govern the subsequent behaviour of chromium, were clearly characterised. Both compounds proved to be very reactive and oxidised instantaneously while chromate ions were reduced to Cr (III) as evidenced by X-ray photoelectron spectroscopy. Mass balance (ICP-AES) demonstrated that the Fe/Cr ratio inside the solid end product was equal to the initial Fe/Cr ratio. The solid phases, analysed by X-ray diffraction, Raman and Mossbauer spectroscopies were identified as Cr-substituted poorly crystallised iron (III) oxyhydroxides in both cases, more precisely d-FeOOH when starting with Fe(OH)2 and ferrihydrite when starting with GR(SO42-)
Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle
Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C), borosilicate septum bottles (70°C) and HDPE tubes (22°C) in the presence of Fe and Ni metal, awaruite (Ni80Fe20) and tetrataenite (Ni50Fe50). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm Ntot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts
Effets des ruptures et reprises de protection cathodique sur la couche de produits de corrosion des aciers en zone de marnage
Des coupons en acier au carbone ont été exposés en zone de marnage dans deux sites
portuaires (Le Havre et Nantes-Saint Nazaire) pendant 7 ans en étant soumis à une
protection cathodique permanente, à une protection limitée aux 6 premières années ou à une
protection appliquée seulement la dernière année. La couche de produits recouvrant les
coupons a été analysée en détail par diffraction des rayons X et
μ-spectroscopie Raman et les vitesses de corrosion ont été estimées par
des mesures de perte d’épaisseur. Les coupons ayant été protégés en permanence se
caractérisent par des vitesses de corrosion environ 10 fois plus faibles que celles
observées en l’absence de protection. Les produits de la corrosion résiduelle sous
protection cathodique, similaires à ceux observés en corrosion libre, forment une fine
couche au contact du métal sous le dépôt calcomagnésien. L’arrêt de la protection au bout
de 6 ans conduit à une ré-accélération de la dégradation qui se traduit par l’accumulation
rapide des produits de la corrosion libre. Ceci confirme que la protection cathodique a
essentiellement un effet modérateur sur la cinétique d’un processus qui reste globalement
le même. Enfin, l’application différée de la protection après 6 ans induit la
transformation de certains constituants de la rouille, notamment celle de la rouille verte
sulfatée en rouille verte carbonatée. D’autres constituants prédominants, comme la
magnétite, ne sont pas affectés. La couche de rouille pré-existante subit donc des
modifications plus ou moins importantes selon sa composition
Corrosion processes of carbon steel in argillite: Galvanic effects associated with the heterogeneity of the corrosion product layer
cited By 7International audienceS235 carbon steel electrodes covered by a compacted layer of natural argillite were immersed for 15-20 days in a 0.01 mol L-1 NaHCO3 + 0.01 mol L-1 NaCl solution at 80 °C and the resulting corrosion product layers were characterized by μ-Raman spectroscopy. In some cases, a heterogeneous layer could be obtained, the center of the electrode being covered by a (FeS + Fe3O4)/Fe2(OH)2CO3 bilayer and the rest of the surface by a (FeS + Fe3O4)/FeCO3 bilayer. Heterogeneous corrosion product layers could also be generated reproducibly by coupling an initially bare electrode (anode) with an electrode covered by a Fe3O4/FeCO3 bilayer (cathode). Three zones were generated on the anode, the nature of the corrosion product layer varying with the distance from the cathode. The galvanic couplings induced by heterogeneous corrosion product layers could be evidenced using the scanning vibrating electrode technique. © 2015 Elsevier Ltd
Mécanismes de la corrosion des aciers au carbone en zone de marnage
Des coupons en aciers au carbone ont été exposés en zone de marnage dans deux sites
portuaires (Grands Ports Maritimes du Havre et de Nantes-Saint Nazaire). La couche de
produits de corrosion recouvrant les coupons après 6 mois, 12 mois et 7 ans d’exposition a
été analysée en détail par diffraction des rayons X et μ-spectroscopie
Raman. Les résultats obtenus aux temps courts (6 et 12 mois) mettent en évidence un
processus général qui conduit principalement à la formation de magnétite
Fe3O4, produit typique de la corrosion atmosphérique favorisépar
les cycles humide/sec, et d’oxyhydroxydes ferriques FeOOH. La rouille verte sulfatée RV(SO42−), produit caractéristique de la corrosion marine des
aciers, est néanmoins observée en fine couche à la surface du métal sous la magnétite.
Pour des temps plus longs d’exposition (7 ans), deux types de comportement sont observés,
l’un s’apparentant à la corrosion atmosphérique, l’autre s’apparentant à la corrosion en
immersion continue dans l’eau de mer. Cette différenciation est probablement liée aux
conditions de séchage des couches de produits de corrosion lors de la période d’émersion
de l’acier. A ces deux comportements sont associées des vitesses de corrosion différentes
(~0,09 mm/an dans le premier cas et ~0,05 mm/an dans le second cas)
Nickel incorporation in Fe(II, III) hydroxysulfate Green Rust: effect on crystal lattice spacing and oxidation products Incorporação de nÃquel em Fe (II-III) Grenn Rust hidroxisulfato: efeito sobre a estrutura cristalina e produtos de oxidação
Ni(II)-Fe(II)-Fe(III) layered double hydroxides (LDH) or Ni-containing sulfate green rust (GR2) samples were prepared from Ni(II), Fe(II) and Fe(III) sulfate salts and analyzed with X ray diffraction. Nickel is readily incorporated in the GR2 structure and forms a solid solution between GR2 and a Ni(II)-Fe(III) LDH. There is a correlation between the unit cell a-value and the fraction of Ni(II) incorporated into the Ni(II)-GR2 structure. Since there is strong evidence that the divalent/trivalent cation ratio in GR2 is fixed at 2, it is possible in principle to determine the extent of divalent cation substitution for Fe(II) in GR2 from the unit cell a-value. Oxidation forms a mixture of minerals but the LDH structure is retained if at least 20 % of the divalent cations in the initial solution are Ni(II). It appears that Ni(II) is incorporated in a stable LDH structure. This may be important for two reasons, first for understanding the formation of LDHs, which are anion exchangers, in the natural environment. Secondly, this is important for understanding the fate of transition metals in the environment, particularly in the presence of reduced Fe compounds.<br>Amostras de hidróxidos de dupla camada (HDC), ou "sulfate green rust" (GR2), contendo Ni foram preparadas utilizando-se sulfatos de Ni(II), Fe(II) e Fe(III) e analisadas por difração de raios X. O Ni está incorporado na estrutura do GR2 e forma um sólido entre GR2 e um HDC contendo Ni(II)-Fe(III). Há correlação entre os valores de "a" da célula unitária e os da fração de Ni(II) incorporado na estrutura do Ni(II)-GR2. Desde que haja forte evidência de que a razão entre os cátions divalente/trivalente no GR2 seja igual a 2, é possÃvel, a princÃpio, determinar a extensão da substituição do cátion divalente por Fe(II) no GR2 a partir dos valores de "a" da célula unitária do cristal. Sob o efeito da oxidação, é formada uma mistura de minerais, porém a estrutura do HDC não é alterada se pelo menos 20 % dos cátions divalentes na solução inicial forem de Ni(II). Isso parece indicar que o Ni(II) está incorporado à estrutura estável do HDC, o que é importante por duas razões: para entender a formação dos HDC, os quais são trocadores aniônicos em condições naturais no meio ambiente; e para entender o destino dos metais de transição no ambiente, particularmente na presença de compostos de Fe na forma reduzida
Structure and stability of the Fe(II)-Fe(III) green rust "fougerite" mineral and its potential for reducing pollutants in soil solutions
International audienc
Mossbauer and XAS study of a green rust mineral, the partial substitution of Fe2+ by Mg2+
International audienc
- …