Oxidation-reduction equilibrium of Cu 2+ /Cu + in binary alkaline sulfate melts
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I.
INTRODUCTION
THE disposal of garbage is normally based upon combustion in refuse bumers. In such incineration systems, alloys are inevitably brought into contact with oxidizing atmosphere. In oxidizing gases, metals and alloys can be protected from oxidation by the formation of an adherent, slow growing oxide film, generally comprised of Cr203, A1203, and SiO2. The combustion product gas in refuse incinerators, however, contains inorganic impurities such as sulfur and chlorine. Because of such impurities, when the gases are cooled, fused salt film may condense on the hardware to generate a highly corrosive condition, corresponding to accelerated degradation known as "hot corrosion." As pointed out by Rapp,t,,2.3] an important initial aspect of hot corrosion is the consideration of thermodynamic stability of protective oxides and fused salts. Under the presence of fused salt, such oxide exhibits limited stability. Figure 1, cited from the review paper of Rapp, indicates the phase stability of the Na + Cr + S + O system at 1200 K. Fused Na2SO4 is frequently a major constituent of corrosive fused salt because of its high thermodynamic stability coupled with the general presence of Na and S in refuse burners. Within the Na2SO4 stability field, Cr203 may react and dissolve into Na2SO4 to form corrosion products, depending upon the partial pressure of 02 and the activity of 02-. The dissolution of Cr203 (s) into N~SO4 (l) can be expressed as
Cr203 (s) + (3/2)02 (g) + 202- = 2CrO,:-
[11
Cr203 (s) = 2Cr 3+ + 302-
[2]
Reactions [1] and [2] correspond to the formation of and Cr2(SO4) 3 (Cr3+), respectively. The equilibrium constants for Reactions [1] and [2], respectively, are given as N a 2 C r O 4 (CrO24 - )
K (1) = a(CrO42-)2/a(02-)2 P o f 2
[3]
K (2) = a(Cr3+) -' a(O2-) 3
[4]
T. YAMAMOTO, Graduate Student, and M. IWASE, Professor, are with the Department of Energy Science and Engineering, Kyoto University, Kyoto 606, Japan. N. YAMANO-UCHI, K. MASAMURA, and M. TAMURA, Senior Research Engineers, are with the Materials and Processing Research Center, NKK, Kawasaki 210, Japan. Manuscript submitted February 23, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
where a(i) denotes the activity of i. It follows, from Eqs. [3] and [4], respectively, that at a fixed oxygen partial pressure, d log {a(CrO42-)}/d log {a(O2-)} = +1
[5]
d log {a(Cr3+)}/d log {a(O2-)} = - 3 / 2
[6]
In principle, the absolute values of a(O 2-) are not measurable. For pure sulfate, however, oxygen anion activities would be related to the N%O activities via Temkin's model; a(Na20) = a0Na+)z a(O 2-) = a(O 2-)
[7]
Attention, hence, can be focused on measurable quantities of a(N~O) rather than a(O2-). For the experimental determinations of the NaEO activities, pure NazSQ would be brought into equilibrium with gas mixtures of SO2 + 02: Na20 (salt) + SO2 (g) + (1/2)O2 (g) = NazSO4
5
[8]
1200 K
3r2(SO4)3 o_ O. ~'(~
(salt)
N~
0
Cr203
o
......................
No2C
m_ .2s_o_4_ _
Na2S -25
I
I,
I
-20
-15
-10
log
N
I
I
-5
0
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