Author Topic: The formation of Passivation in internally oxidized Ag-based alloys  (Read 2829 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 research was to analyse the inhibition of internal passivation by changing the chemical composition of silver alloys and to estimate the concentration boundaries of the selected microalloying element at which the passivation is still inhibited. Since, the ability of inocculation or modification is mostly based on the large free energy of formation of oxides of microalloying elements and their crystallographic similarity, Mg in the quantities of 0.001 to 0.5 mass% was chosen as a microalloying element for Ag-based alloys in addition to the main alloying element (Zn). For the Ag-5.8%Zn alloy, the boundary conditions of the inhibition of passivation were established. For concentrations of MgO below 0.005 vol.% not enough nuclei are formed. For too high concentration of MgO (above 1.2 vol.%) defect microstructures are formed which are characteristic for MgO precipitation. This produces a local or general passivation. By changing the composition of microalloying the internal passivation of Ag-5.8%Zn alloy could not only be inhibited but increased. Thus under some microalloying conditions much bigger depths of internal oxidation were reached.

Keywords – Ag-based alloys, Passivation, oxidation kinetics, alloys, microalloying, internal oxidation.

1   INTRODUCTION
The passivation is a phenomenon where at certain depth from the surface of the investigated material an oxide barrier is formed. The formed barrier is thick enough that further progress of oxidation through the material is not possible any more. In internally oxidised alloys the passivation has been observed for certain conditions of concentration of main element, partial pressure and temperature of oxidation[1 to 8]. We attempt to study the inhibition of passivation by microalloying. With the addition of microalloying elements (0.001 to 0.5 mass%) with very high free formation energy of oxides, regarding the oxide of the main alloying element, we tried to inhibit this phenomenon and to enable an undisturbed process of oxidation, as well as a growth of internal oxidation zone.
Our previous investigations[6 to 8] established that Mg, Si, Ti, Al, Zr, Be are important microalloying elements for Ag-Zn, Ag-In and Ag-Sn alloys. These microalloying elements have large affinity to oxygen and therefore oxidise at lower partial pressures (concentrations) of oxygen in silver than the main alloying elements (Zn, In, Sn etc.). Their oxides form the nuclei for the growth of oxides of the main alloying element. Therefore, by their distribution in the metal matrix, the distribution of the main alloying element oxide is also determined (Fig. 1)[1,2].

 
Fig. 1 
The scheme of  the concentration profile of oxygen and alloying elements at the beginning of the internal oxidation zone.

This heterogeneous nucleation is very effective at the defined concentration interval of a microalloying element.
The aim of our research was to analyse the inhibition mechanism of the formation of passivation in the Ag-Zn alloys by changing the fraction of main alloying element, as well as the fraction of microalloying element (Mg) and to estimate the range of concentrations at which the microalloying element Mg is still effective as a modificator.

2   EXPERIMENTAL
The different alloys (Table 1) were prepared in evacuated (up to 10-2 bar) quartz tubes in an induction furnace. After 30 hours of homogenisation and annealing at 800 oC the alloys were hot forged and recrystallization annealed. Finally, they were cold rolled into 3 mm thick stripes, which were then ground and etched in 10 % solution of HNO3.


Main alloying element
Zn    Type of oxide
ZnO   Additional  alloying
element   Types of oxides
MgO   Alloy designation
      mass%        at%   mass%          vol.%   mass%      at%  of Mg   mass%    vol.%   
6.3            (9.97)   7.74            (13.73)         2S1
0.045       (0.20)   0.08        0.217  (MgO)         2S0
5.8            (9.18)   7.12            (12.64     0.51       (2.29)     0.85         2.46   3S1
5.8            (9.18)   7.12            (12.64)      0.29       (1.31)   0.48         1.39   3S2
5.0            (7.91)   6.14            (10.89)     0.25       (1.18)   0.43         1.20   1S4
5.8             (9.18)   7.12            (12.64)      0.21       (0.95)   0.35         1.01   2S2
5.8             (9.18)   7.12            (12.64)     0.065     (0.29)   0.108       0.31   2S3
5.8            (9.18)   7.12            (12.64)      0.012     (0.05)   0.02         0.06   3S3
5.0            (7.91)   6.14            (10.89)     0.001     (0.005)   0.002       0.005   1S6
5.8            (9.18)   7.12            (12.64)   <0.002     (0.009)   0.003       0.009   2S4
5.8            (9.18)   7.12            (12.64)     0.001     (0.005)   0.002       0.005   3S4

Table 1 Chemical composition of the investigated Ag-based alloys

The tests of internal oxidation kinetics were made at different temperatures (in the temperature range between 750 oC and 850 oC), and at different time (from 5 to 140 hours) in a tube furnace, where air flow circulation was assured.
In the most cases, the oxidising atmosphere was air. However, some tests, in the oxidising atmosphere of oxygen (with partial pressure of 1.01105 Pa) were also performed.
Microstructural changes in the zone of internal oxidation were observed by optical (Leitz Wetzlar, Germany and Nikon Microphot–FXA, Japan) and scanning electron microscopy (SEM/WDX analyser Jeol JSM 840 A). Volume and energy changes, as well as the process of internal oxidation were measured by thermogravimetric analysis (TGA, Mettler TG 50, Germany). Applying the Wagner’s theory[1 to 3], the changes of system kinetics were calculated.

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