Emmanuel Rocca

The tin effect in lead-calcium alloys

Abstract The effect of tin in the depassivation of lead-calcium alloys under conditions of deep discharge (0.7 V versus Hg/Hg2SO4 in 0.5 M H2SO4) is investigated by measurements of the polarization resistance, the oxidation rate of ferrous ions added to the electrolyte, the potential decay with/without applied cathodic current, and surface analysis and visual inspection. The results may help to choose the optimal level of tin in the alloys.

Evaluation of new non-toxic corrosion inhibitors for conservation of iron artefacts

Abstract The anticorrosion performances of new non-toxic inhibitors based on carboxylic acids extracted from vegetable oil have been evaluated for the protection of iron artefacts. Electrochemical measurements and natural aging tests have demonstrated the efficiency of those inhibitors in the context of temporary treatments. Surface analysis coupled with in situ X-ray absorption near edge structure experiments has revealed that their anticorrosion properties are correlated to the precipitation at the metal surface of a protective layer made of iron carboxylate.

Mangrove (Rhizophora apiculata) tannins: an eco-friendly rust converter

Abstract Rust converters are chemical formulations that can be applied to corroded surfaces to convert residual rust on iron/steel surfaces into harmless and adherent chemical compounds. The known hazardous effects of most synthetic rust converters have led to a search for safer, more environmentally friendly natural products. Tannins, a class of natural, non-toxic and biodegradable polyphenolic compounds, extracted from plant sources have been proposed as an alternative. The transformation of rust in the presence of 5 g L−1 tannins extracted from mangrove (Rhizophora apiculata) barks was studied. Three different methods of pre-rusted sample preparations were tested. The rust composition of the steel was dependent of sample preparation. The pre-rusted samples prepared formed predominantly lepidocrocite and magnetite. Transformation of rust components to ferric-tannates, particularly lepidocrocite, in the presence of mangrove tannins that could be observed via Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy analysis indicated their potential as rust converters.

Development of Burner Rig Corrosion Tests for Gas Turbine Alloys: Study of Protective Metallic Coatings and Inhibitors for Hot Salt Corrosion

The efficiency of all types of gas turbine engines (aircraft, terrestrial and marine) is proportional to firing or turbine inlet temperature. In most cases, the use of heavy or contaminated fuels (by Na, S, V) in such combustion equipment is the life-limiting factor for the metallic parts constituting the hot gas path. Since several years, a burner rig was developed in the laboratory to evaluate the corrosiveness of the ash deposit formed on the gas turbine alloys in dynamic conditions. It has been designed to easily adapt the ash deposit rate and the level of the contaminants. The first part of the present paper is devoted to the evaluation of the resistances of several types of metallic coatings against sodium-induced hot corrosion. These coatings were deposited on IN738 alloy and have been treated during 1000h of test. For this, a NaCl solution was injected in the hot gas flux to obtain a deposit rate of 1 mg/cm 2 /100h at 850°C. Different types of aluminide coatings obtained by pack cementation were tested and compared to a NiCrAlY coating produced by LPPS process. Among all these coatings, the Pt-modified alumides show the best anti-corrosion behaviour against the Na-induced hot corrosion. In the second part, the burner rig has been used to test additives introduced into crude oil burned in gas turbines. Indeed, the usual and low-cost defence against (Na,V)-induced hot corrosion is the addition to the fuel of some metals as corrosion inhibitors. To simulate this corrosion and its inhibition in the burner rig, an aqueous solution containing adjusted concentrations of sodium, vanadium with or without inhibitor was injected into the combustion gas just after the burner. A Mg/V ratio equal to 3 was tested to evaluate the efficiencies of the inhibition and the deposition rate in the presence of magnesium.

Synchrotron radiation contribution to the study of aluminium corrosion layers of air and space museum aircrafts for their preservation

Materials made of aluminium alloys are nowadays encountered in museum collections, especially in air and space museums. Due to their size, these materials have been exposed for decades to climatic changes, allowing the development of thick corrosion layers. To improve the conservation of such collections, curators need information on the physico-chemical characteristics of the thick corrosion layers. To achieve this, samples collected on aircrafts were characterized. μXANES investigations (Al-K edge) have allowed us to determine aluminium local chemical environment in amorphous corrosion products. Electrochemical measurements were realised to establish correlation between the composition and the electrochemical behaviour of such Al alloys.

Rust Phase Transformation in the Presence of Mangrove Tannins

The transformation of rust in the presence of 5 g L � 1 tannins extracted from mangrove barks was studied. Pre-rusted samples prepared resulted in the formation of predominantly lepidocrocite and magnetite. Partial transformation of rust components, particularly lepidocrocite in the presence of mangrove tannins was found to occur when observed via infrared spectroscopy, X-ray diffraction and SEM analysis. The anti-corrosive properties of the mangrove bark extracts seem to be related to the tannins which are capable of chelating with Fe 3+ giving a highly stable, and insoluble ferric-tannate complexes. A molecular modeling study of the reaction of catechin, a flavanoid of mangrove tannins with Fe 3+ to form a ferric- tannate complex was performed to explain the adsorption mechanism of tannins on iron. The correlation between the electronic density of the catechin molecule and the inhibiting properties is reported.

Revisiting the Inhibition of Vanadium-Induced Hot Corrosion in Gas Turbines

Since the 70’s, nothing substantially new has been published in the Gas Turbine Community about the hot corrosion by vanadium and its inhibition, after the “inhibition orthodoxy” based on the formation of magnesium vanadate, was established. However, the experience acquired since the late 80’s with Heavy Duty Gas Turbines burning ash-forming fuels in Southern China, shows that the combustion of very contaminated fuels does not entail corrosion nor abundant ash-deposit on gas turbines buckets. Analyses of deposits collected from gas turbines fired with these crude oils showed that the ash-deposit contains a large amount of nickel.These new facts led to revisit the role played by nickel and envisage its possible inhibiting action against the vanadium-induced hot corrosion. A thorough review of the literature on the vanadium-induced corrosion have been carried out, and the study of the nickel effects with respect to magnesium effects on the ash deposit have been performedResults show that nickel presents an interesting way to substitute magnesium for the inhibition of vanadium-induced hot corrosion. The advantages of nickel with respect to magnesium are to be efficient at a low Ni/V ratio, to produce less abundant, less adherent ash and to act, to some extent, as a self-cleaning agent for the blades of the turbine.© 2001 ASME