Christophe Rapin

Corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D alloys

The microstructure and the corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D magnesium alloys have been investigated. Semi-solid processing leads to a structure with large rounded grains of a solid solution of magnesium (α phase) whereas die-cast alloys are more homogeneous. Electrochemical measurements, particularly with impedance spectroscopy, have shown that the semi-solid cast alloy possesses a corrosion rate at least 35% below that of the die-cast alloy. This can be explained with considering the differences between the composition of the primary α phase and the volume fraction of the β phase, in the two types of alloy.

Electrochemical study of chromium corrosion in Na2O-xSiO2 melts

The systematic study of the chromium corrosion in Na 2 O-xSiO 2 melts is performed with varying glass basicity using electrochemical techniques. The metal can exhibit several typical behaviors (active or passive state) depending on the melt composition. Each possibility is described in terms of corrosion layer morphologies and thicknesses. The effects of the melt basicity, preoxidation treatment of the metal, and temperature are studied. The conclusions are given by putting face-to-face the Na 2 O activities in the melt with quantitative data extracted from this work.

Development of chromia forming Mo-W-Cr alloys: synthesis and characterization

The sintering process and microstructural characteristics of tungsten-based and molybdenum-based alloys containing chromium and group VIII metals as sintering agents have been investigated. The influences of the alloy composition, the nature of the sintering agent and the synthesis process on the microstructure and microhardness of these materials have been studied. Homogeneous alloys can be obtained with palladium or nickel as the sintering agent. The mechanisms are totally different with these two metals. In the case of nickel, diffusion of the refractory metals through a nickel layer is responsible for the densification of the alloys, whereas with palladium, a CrPd liquid phase at the grain boundaries leads to homogeneous sintering. Formation of the CrPd phase is directly dependent on the chromium content wich influences the solubility of palladium in the MoW matrix. Consequently, a low chromium content leads to a high CrPd content, and to an increase in the grain size. On the contrary, with a nickel sintering agent, ahigh Ni content leads to an increase in thickness of the interdiffusion layer and thus a decrease in the grain size. Sresses generated by uniaxial sintering and mechanical alloying are not released during the annealing sequence and contribute to increase the microhardness of the alloys. Microhardness is also a strong function of the tungsten content.

Relationship between Chromia Solubility and Superalloy Corrosion in Silicate Melts – A First Attempt

Most of equipments used in glass industry are superalloys containing up to 30 wt% chromium. The ability of these alloys to resist against silicate melt corrosion is directly linked to the formation of a chromia (Cr2O3) layer at the alloy/melt interface which can be protective under particular conditions. These conditions have been previously identified and are the temperature, melt composition and redox conditions. The aim of the present study is to establish the relationship between the protective (or non protective) behaviour of the chromia layer with the solubility of chromia in silicate melts under given conditions. The combination between results from the electrochemical study of pure chromium corrosion and total Cr solubility in Na2O-xSiO2 melts clearly indicates that both parameters are linked: the lower the total Cr solubility, the better the Cr is corrosion resistant.

Kinetics and Mechanisms of Tantalum Corrosion in Glass Melts

The kinetics and mechanisms of corrosion of pure tantalum in three molten glasses are investigated in the 1200-1400°C temperature range by thickness loss measurements. The corrosion layers formed at the metal/glass interface, the elemental concentration profiles in the glass, and the corrosion rates are determined. Corrosion rates are reported on an Arrhenius representation for each glass. They yield to the determination of the activation energies of corrosion processes that depend on the glass composition. The limiting process is probably the diffusion of oxygen through one of the corrosion layers. The corrosion mechanisms are described in the form of successive redox reactions between the glass and the substrate. The Ta V /Ta 0 standard potential is estimated to be below -1.200 V. The formation of an intermetallic compound containing boron (TaB) as a corrosion layer is evidenced for the first time.

Three Examples of High-Temperature Corrosion of Metals by Molten Glasses

The effects of glass redox, glass basicity and glass temperature on the metallic material corrosion in molten glass are illustrated with regard to three examples. a) The corrosion of a platinum alloy immersed in a Te-bearing glass under reducing conditions is due to the formation of low melting temperature PtxTe1-x compounds. b) The corrosion of a Ni-base superalloy immersed in a Mo-bearing glass in a reducing atmosphere is due to the dissolution of the chromia protective scale. Indeed the cathodic reaction (Mo◊ Mo) generates O ions which can promote the basic dissolution of Cr2O3. c) The corrosion of a Co-base superalloy in molten C-glass is investigated as a function of temperature, yielding to the demonstration of the existence of a maximum using temperature, characterized by a passive-to-active state transition.

State of the Sub-surface Microstructure of Carbides strengthened cast Superalloys after High Temperature Oxidation - Use of Thermodynamic Modelling for a better Understanding

Several cast strengthened superalloys, Ni base and Co base, were exposed to high temperature oxidation for long times and metallographically examined. Different phenomena occurred in the sub-surface microstructure, depending on both alloy and temperature. Thermodynamic modelling was used to know what it happened for carbon during oxidation, then to explain the observed microstructural changes. It appears that carbon atoms either quit the alloy probably after its oxidation into gases, or on the contrary go deeper into the bulk where they promote the precipitation of new carbides by solid state transformation. Thereafter, thermodynamic modelling allowed to know the new local refractoriness of the zones affected by oxidation, then to appreciate the new mechanical properties in the sub-surface.

Oxidation of chromia forming molybdenum-tungsten based alloys

The oxidation behaviour of tungsten and molybdenum based, chromia-forming alloys prepared by powder sintering activated with group VIII metals has been investigated. The influences of the alloy composition, nature of the sintering agent and oxidation temperature have been studied. A good oxidation resistance is observed with palladium as sintering agent. This metal is rejected at the grain boundaries and allows a fast diffusion of chromium to the metal-oxide interface. Contrary to palladium, nickel leads to a catastrophic oxidation of the sample. The formation of a two-phase interface enriched in nickel leads to a non-protective oxide layer constituted with Cr2O3 and NiWO4.Catastrophic oxidation is observed when the refractory metals are oxidised into volatile oxides, i.e. in the case of the alloys with a high molybdenum content. Contrary to molybdenum, a high tungsten level leads to high oxidation resistance, even at temperature as high as 1300°C. In this latter case, alloys are two-phase: this result has led to the investigation of the ternary section of the Cr-Mo-W system at 1300°C.

Development of a Laboratory-Scale Pilot for Studying Corrosion on MSWI Heat Exchangers

The efficiency of Waste-to-Energy (W-t-E) boilers is affected by fireside corrosion of the heat exchangers that involve unexpected shutdown of facilities for repairs and limit the increase of steam conditions used to produce electricity. The parameters governing fireside corrosion are various and mechanisms are very complex, nevertheless, they are relatively well documented in the literature. In this paper, a laboratory-scale corrosion pilot, which reproduces MSWI boilers conditions, is described. The specificity of our approach includes simultaneous simulation of the temperature gradient at flue-gas/tube interface, the velocity of flue-gas and ashes. Corrosion rates obtained on Tu37C carbon steel at a metal temperature equal to 400°C and a flue gas temperatures of 650°C and 850°C (1100 ppm HCl, 110 ppm SO2 and synthetic ashes free of heavy metals) are respectively around 1.6 2m/hour and 5.6 2m/hour. Preferential metal loss, attributed to erosion-corrosion phenomena, is also observed at low flue-gas temperature (T=650°C) on the face exposed at 90° to the flue-gas. The analysis of corrosion scales demonstrates the reproducibility of results and the reliability of corrosion mechanisms determined from experiments, with degradation observed similar to superheater tubes from EfW facilities. Thus, the corrosion pilot developed can be used as an accurate simulator of the environment encountered in MSWI.

Influence of the Microstructural Texture of Cast Superalloys on their High-Temperature Oxidation Behaviour

The high-temperature oxidation behaviours of Ni base and Co base cast superalloys were studied to determine the effect of different grain sizes and different surface dendritic orientations. These microstructural characteristics were obtained by varying solidification rate and cutting orientation with regard to the external surface. Thermogravimetry tests were run at 1000 and 1100°C and parabolic oxidation constants were considered. It appears that oxidation is faster for fine microstructures than for coarser ones for Ni or Co alloys including tungsten in their chemical composition, while it is the contrary when W is not present. When the sample surface is mainly parallel to the dendritic network, the oxidation rate is greater than for a surface perpendicular to dendrites, for the studied Ni alloy. The same phenomenon is observed for the studied Co base alloy at 1100°C but the order is inverted at 1000°C. These different behaviours can be explained by the grain boundary densities and orientations obtained on surface, since they can influence the diffusion of species involved in the oxidation phenomena. It is also possible that the characteristics of the chromia scale, such as grain size and general quality, depend on the microstructural texture of the alloy.