Author Topic: Internally oxidized Ag-based alloys; the passivation and the influence of chemi  (Read 2636 times)

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Author : Jožica Bezjak A. Professor
International Journal of Scientific & Engineering Research Volume 3, Issue 1, January-2012
ISSN 2229-5518
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Abstract—The aim of this work was to analyse the mechanisms of hindered internal passivation of silver based alloys which was obtained by the modification of basic chemical composition. A generalisation of the phenomenon, experimental verification and the estimated range of micro-element concentration is also introduced. The ability for inoculation of a particular alloy is determined by the differences between the formation energies of oxides, as well as their crystallographic similarity. Therefore, for the investigated Ag-Zn alloys, Mg was selected as the micro-alloying element. The influence of 0.001 up to 0.5 mass % of Mg added to the selected alloys was analysed. By changing the chemical composition,internal passivation was hindered, internal oxidation rate was increased, and considerably greater (redoubled) depths of internal oxidation were achieved.

Keywords - internal oxidation, modification, hindrance of passivation

1   INTRODUCTION
Ag, Cu and Ni based alloys with small additions of noble alloying elements like indium, tin, and antimony oxidise internally if they are exposed to oxidation atmosphere at elevated temperatures.For internal oxidation of a particular alloy, the following conditions also have to be fulfilled:

•   the base metal must have high solubility of oxygen
•   the alloying element has to be strongly electronegative
•   the diffusion rate of oxygen into the base metal has to be of some levels of magnitude higher than the diffusion rate of the alloying elements the concentration of the alloying elements has to be inside certain limits
•   sufficient partial pressure of oxygen has to be ensured in the atmosphere

If the selected alloy is isothermally annealed at an elevated temperature and all other above mentioned conditions are fulfilled,dissolution of oxygen begins on the surface of the alloy, and its diffusion into the interior, as well as its reaction with atoms of the alloying element occurs. The effect of the oxidation reaction is a precipitation of the finely dispersed oxide of the less noble component in the metal matrix (Pictures 1a and Picture 1a: the effect of oxidation reaction

Picture 1 a, b: Concentration gradient of oxygen and the alloying element in the zone of internal oxidation, as well as in the non-oxidised part of the alloy(1).

2   EXPERIMENTAL WORK
In internally oxidised Ag based alloys, where passivation normally occurs, we tried to hinder passivation with small modifications of chemical composition. The hindrance of passivation, undisturbed oxidation of the main alloying element and growth of the internal oxidation zone were obtained with small additions of micro-alloying elements (from  0.001 up to 0.5 mass %) which possess very large free energy of oxide formation.During the selection of the alloys chemical composition, the following criteria were taken into consideration:

-   the selected Ag based alloys are mono-phased
-   the concentration of the main alloying element is approximately half the critical one (N (3))
-   on the basis of the Ag-Zn binary phase diagram and relatively small free formation energy for ZnO, Zn was selected as the main alloying element
-   in the investigated binary alloying system (Ag-Zn), the added micro-alloying element has very large free oxide formation energy in comparison with the free oxide formation energy of the main alloying element
-   the concentrations of added Mg were relatively small (between 0.5 and 0.001 mass %); therefore, this element is  designated as a micro-alloying element
-   inoculation with Mg was analysed from two standpoints: differences in free formation energy of oxides and appropriate crystallographic features with regard to the silver matrix (Table 1).

On the basis of the above mentioned criteria, the selected Ag-Zn and Ag-Zn-Mg are given in Table 2 for observation of the passivation phenomenon and the conditions for its hindrance.


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