Anne Hugot-Le Goff

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.

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.

Localized corrosion processes in iron and steels studied by in situ Raman spectroscopy

The part played by molybdenum in the prevention of pitting corrosion of iron and steels in the presence of chloride ions can be explained using Raman spectroscopy. Here, molybdenum introduced as a molybdate in the electrolyte solution was particularly studied. Different polarization methods were used (potentiostatic polarization before or after the pitting potential, or voltammetric cycling), leading to different pit distributions and sizes. Around the pit, one can discriminate the different molybdate anions, in which Mo has an oxidation state of 4+ or 6+. After the pitting is generalized, there rapidly grows a thick, colloidal and unstable green rust layer (iron hydroxychlorure), which was chemically identified on pure iron using Raman spectroscopy. Beneath this layer, which represents a very agressive medium, the corrosion rate increases dramaticlly, and the sample is rapidly destroyed. On stainless steels, molybdenum (as well as chromium) can be integrated in the green rust, thereby slowing the corrosion rate. When pitting is initiated, the inner layer is formed by tetravalent molybdate, the passivity breakdown being associated with the change in oxidation state of Mo from 6+ to 4+.

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.

Raman investigation of crevice corrosion in nickel-chromium dental alloys containing beryllium

Abstract The presence of the small amounts of Be which are often added to the Inconel-type NiCrMo dental alloys to improve their castability leads to a dramatic increase of the corrosion processes in the presence of chlorides, with the formation of deep pits full of corrosion products. In this kind of alloys, crevice formation is normally unexpected and it is important to check whether Be is also able to promote crevice development. We showed that the metallurgical heterogeneities of these alloys, which are industrially used as-cast, are responsible for the tremendous increase of localized corrosion. We have proposed a simple configuration to simulate a crevice and to identify by Raman spectroscopy (RS) the corrosion products which are liable to appear, in order to characterize their differences with the products found in pits. This analysis is completed by results obtained using X-rays energy dispersive analysis (XEDS). A growth of spinels to the prejudice of the salts (molybdates, chromates) which are usually found in the pits is observed.