Suzanne Joiret

Polyaniline Layer for Iron Protection in Sulfate Medium

Polyaniline (PANI) has been electrodeposited on iron in oxalic medium in order to evaluate the protective character of this polymer. PANI has been found to be efficient for corrosion protection during at least 10 h in a pH 4.5 sulfate medium. Interaction between the passive layer and polymer has been studied using spectroelectrochemical techniques such as Raman spectroscopy and reflectance measurements.

Anodic behaviour of manganese in alkaline medium

Voltammetry, Electrochemical Impedance Spectroscopy (EIS), Rotating Ring-Disk Electrode techniques (RRDE), Electrochemical Quartz Crystal Microbalance (EQCM) measurements, and in-situ Raman Spectroscopy were applied to investigate the anodic behaviour of Mn in 1 M NaOH solution over a wide potential range. Prior to these experiments, for EQCM, an improved plating bath was designed for coating the thin gold electrode of the quartz sensor with Mn. The results obtained revealed clearly that various oxides, depending on the electrode potential, cover the electrode surface. The oxidation–reduction processes between these different oxides and the associated exchange of species with the solution constitute the main characteristic of this electrode. When the Mn electrode is left in 1 M NaOH solution, it becomes spontaneously passive through two consecutive steps. In-situ Raman spectroscopy indicated that the electrode surface is covered by Mn3O4, Mn2O3, and MnO2 as the potential is shifted towards more anodic values. The polarisation curves showed two anodic current peaks, in agreement with the two-step passivation process. EIS spectra exhibited the typical shape of passivation reactions with a large capacitive loop in the low frequency range. The double layer capacitance and the faradaic capacitance determined from EIS data indicate the increase in expanded surface area and bulk volume of the surface oxide with anodic potential. From RRDE measurements, the dissolution of Mn through Mn2+ and Mn3+ species were evaluated. EQCM measurements corroborated the growth of surface oxide species with the potential, and gave valuable information on the nature of the chemical species involved in the oxidation–reduction processes. A reaction mechanism of the Mn electrode in 1 M NaOH in a wide potential range is proposed.

Protection of Iron Against Corrosion Using a Polyaniline Layer: I. Polyaniline Electrodeposit

Polyaniline (PANI) has been studied for some years in order to protect mild steels against corrosion in the absence of inorganic conversion layers. During the electropolymerization of PANI film, iron is passivated; the subsequent protection is dependent on the nature of this passive layer, and PANI acts as a stabilizer of this layer. The best conditions for PANI electrodeposition are discussed here, using a simple electrochemical test to characterize the passivity breakdown.

Protection of Iron Against Corrosion Using a Polyaniline Layer: II. Spectroscopic Analysis of the Layer Grown in Phosphoric/Metanilic Solution

Electropolymerized polyaniline (PANI) is able to protect iron against corrosion for several weeks, even in the absence of inorganic conversion layers or topcoats. During the PANI deposition process, iron is passivated and the subsequent protection is dependent on the nature of the passive layer, which is stabilized by PANI. The composition of the solution in which PANI is polymerized can modify the passive layer as well as the polymer and Raman and optical spectroscopies are used to characterize the properties of PANI potentiostatically electrodeposited from phosphoric/metanilic solution (electrodeposition solution). The presence of phosphoric acid strengthens the passive layer, while metanilic acid is inserted in the polymer backbone to form a copolymer which presents higher protective properties.

Influence of Ni, Mo, and Cr on pitting corrosion of steels studied by raman spectroscopy

Abstract Corrosion layers grown on stainless steels (AISI 302, 316), polarized at the pitting potential in alkaline solutions containing chloride, were identified by Raman spectroscopy. Several binary alloys such as Fe-10Ni, Fe-6Mo, and Fe-18Cr, were studied to separate the influence of the different alloy elements. A comparison was also made with the layers formed on these alloys by voltametric cycling (which allows the thickening of the passive layer), the anodic reverse potential being in this case below the pitting potential. The parts played by the two main elements, Cr and Mo, are described. After the passivity breakdown, i.e., once the chromia layer has broken down, the inner layer is formed by tetravalent molybdate, the pitting initiation being related to the passage of Mo from valency6+ to valency4+. In the outer green rust layer, bichromate ions fill the sites available for Cl− and then slow down the ingress of chloride.

Oxide scale stress determination by Raman spectroscopy application to the NiCr/Cr2O3 system and influence of yttrium

The performance of nickel-based alloys as heat resistant materials is strongly dependent on their oxidation resistance. A strong resistance to oxidation of Ni-Cr alloys is expected due to the formation of a protective Cr{sub 2}O{sub 3} layer which blocks the diffusion of nickel ions to the oxide scale/gas interface and prevents from fast growth of a NiO scale. Raman spectroscopy was used here in order to determine the stresses in chromia scales formed at 900 C on Ni-30Cr alloys which develops only Cr{sub 2}O{sub 3}. Chromia has the same structure as corundum, and therefore it should have seven Raman active bands (2A{sub 1g} + 5E{sub g}). The most intense mode is the 552 cm{sup {minus}1} A{sub 1g} vibration and this one was used for monitoring the stress. It has long been known that the addition of so-called reactive elements, such as yttrium, to chromia former alloys improves the protective character of such scales by reducing the scale growth rate and, in particular by spalling during thermal cycling. Thus, Ni-30Cr samples doped with yttrium were tested and the influence of this reactive element on the oxidation kinetics and on the stress level in the oxide scale was determined.

Combined impedance and raman analysis in the study of corrosion protection of iron by polyaniline

Chemically prepared Polyaniline coatings on iron electrodes were tested in chloride or sulphate solutions at neutral or slightly acidic pH for which only the emeraldine base moiety exists. However, the Raman results indicate the presence of polarons and the impedance diagrams were explained by the presence of a faradaic reaction evolving on an electronically conducting porous medium.

Polyaniline layer for iron protection in sulfate medium

Abstract Polyaniline has been potentiostatically electrodeposited on iron in acidic medium to protect the electrode from corrosion. Two acids have been tested : oxalic and phosphoric acids to look for the influence of the passive layer on the system behavior. Influence of additives in the deposition have been examined together with temperature. Careful control of temperature deposition is necessary due to the high activation energy of the process. Metanilic and camphorsulfonic acids act as plasticisers for the polymer layer but also have beneficial effects on electrodeposition by lowering the passive potential and active currents.

Characterization of passive films on Ni and Ni alloys by ReflEXAFS and raman spectroscopy

Abstract Local order and composition of passive films formed on nickel and nickel molybdenum alloys have been studied by combining two in-situ techniques, ReflEXAFS and Raman Spectroscopy. Passive films can be described by a model consisting of NiO entities perpendicular to the electrode surface and separated by channels containing OH − , SO 4 = or water molecules. X-Ray reflection measurements show that the density of the passive films is four times lower than those of the bulk fcc NiO. It is deduced from the number of oxygen first neighbours that Mo enhances disorder in the passive film. Ex-situ ReflEXAFS measurements and RHEED observations show a recrystallization phenomenon which is accompanied by a thinning of the passive layer and a better ordering. It results that the density of the film is about fifty percent of NiO fcc. ReflEXAFS studies at the Mo k edge reveal an increase of the Mo concentration at the metal-film interface.

Protection of iron against corrosion using a polyaniline layer. III. Spectroscopic analysis of the mechanisms accompanying the breakdown

The protection of iron against corrosion brought by polyaniline (PANI) potentiostatically polymerized in phosphoric/metanilic solution is due to the combination of the passive layer and the polymer. The passive layer is strengthened by phosphate incorporation, and sulfonated aniline is inserted in the chain to yield a copolymer (SPAN). The potentiostatic polarization is partly responsible for the heterogeneity of polymer (block-polymer). Raman and optical spectroscopies are used to characterize the modifications in the PANI composition preceding and accompanying the passivity breakdown. The breakdown is associated with the loss of PANI reoxidability, but this step is preceded by slow modifications in the polaron distributions.