G. Béranger

A statistical study of the role of molybdenum in the pitting resistance of stainless steels

Abstract The local breakdown of passive films grown on stainless steels in the presence of chloride ions generally leads to pitting. To study this phenomenon, the pitting potential of several stainless steels in neutral chloride media has been determined, by a statistical technique. This parameter is obtained when the statistical pit generation rate corresponds to a preset critical value. Two cases were considered: (a) stainless steels without alloyed Mo (AISI 430 or 304) in NaCl or NaCl + Na 2 MoO 4 , aerated or de-aerated, solutions, and (b) Mo alloyed stainless steels (AISI 434 or 316) in NaCl aerated or de-aerated solutions. The experimental results are compared, and discussed in terms of the influence of alloyed or dissolved molybdenum. It appears that the role of this element is different in each case, and also that dissolved oxygen in the solution plays a significant role in the improvement of the passivity due to molybdate ions action.

Mechanical properties and microstructural evolution of a cobalt base alloy coating

Abstract Several isothermal mechanical tests have been performed on a cobalt superalloy to determine the parameters of a viscoplastic behavior law in the range (20–700 °C). These parameters have been identified by means of a systematic numerical procedure (a computer program for simulation and identification). The microstructural evolution during the mechanical tests have been examined in relation to the evolution of the internal variables of the viscoplastic model. It was found that the material softens at room temperature, mainly as a result of an isotropic component decrease. However, at high temperatures (500–700 °C), the material hardens, mostly owing to a kinematic component increase. The deformation mode, i.e. the martensitic transformation, has been identified for all the temperatures studied. At high temperatures carbide precipitation on stacking faults and deformation bands has been observed. Complementary tests have shown that, at high temperatures, the further addition of carbide precipitates dissolves by dislocation glide and a diffusion mechanism. At room temperature, after heat treatment at 700 °C, carbides precipitated on deformation bands induced the formation of “hard” zones. These microstructural observations have been correlated with the important part of the kinematic term in the stress increase.

Role of oxyanions in the improvement of the stainless steels passivity

Abstract The stainless steels passivity in aqueous media can be locally destroyed by halide ions. By using a potentiokinetic technique, the pitting probability ω E was measured on aisi 304, 430 or 434 stainless steels, and the pit generation rate g E was calculated. This parameter has permitted to study the variation of the inhibitor efficiency ζ with the contents of aggressive and inhibitive species in the media, and to discuss the process occuring in different cases : competitive adsorption of ions on the active sites, displacement of chemical equilibria in the medium, diffusion of species through the passive film … A comparison of respective role of chromate or molybdate ions was made, and the role of intrinsic or extrinsic molybdenum for the alloy was also discussed.

The mutual interactions of oxygen and hydrogen in α-titanium during heat treatment

Oxygen and hydrogen dissolve in α-phase titanium at different concentrations up to 34 at.% and 0.14 at.% respectively. Oxygen is an alphagene element while hydrogen is betagene. Firstly, the oxygen diffusion coefficient in hydrided titanium TiH1.46 has been measured at 750 °C and additional tests have been carried out on TiH0.2. Secondly, the hydrogen diffusion coefficient (in fact for tritium) has been determined at 400 °C in solid solutions of oxygen in titanium at 1 at.% and 2 at.%. For the case of oxygen diffusion in Ti-H, the chief effect is the disappearance of hydrides when the oxygen concentration is over 5 at.% (for TiH0.2 by analogy with titanium) or at 2 at.% (for TiH1.46 by direct measurement). The diffusion conditions of tritium depend greatly on the oxygen content of the metal. A value of 1–2 at.% seems to be critical; for higher oxygen concentrations, the quantity of tritium dissolved diminishes markedly. Diffusion coefficients have been measured.