Environmental Engineering Reference
In-Depth Information
Figure 6.31
Stability diagram to illustrate the phases of nickel (- - - - -), aluminum
(⋅⋅⋅⋅⋅⋅⋅⋅), and chromium (xxxxx) that can exist in an Na
2
SO
4
layer on a Ni-Cr-Al alloy at
1200 K [58].
alloy at 1200 K [58]. This diagram has been constructed by superimposing the
Ni-O-S, Al-O-S, and Cr-O-S stability diagrams on the Na-O-S phase diagram as
a function of log
p
O
2
versus log
p
SO
3
. In this diagram, the Na
2
SO
4
region is
bounded by Na
2
O and Na
2
S and is indicated by solid straight lines. It has already
been mentioned that basic fluxing is dependent on the ability of the salt to main-
tain a high concentration of oxide ions (O
2
) for reaction with a protective layer.
This diagram suggests that NiO will be dissolved in Na
2
SO
4
in preference to
Al
2
O
3
and Cr
2
O
3
at the composition of the melt designated by the symbol
due to generation of oxide ions, thus making the melt more basic. On the other
hand, for the composition of the melt designated by the symbol
,Al
2
O
3
will
be dissolved as sodium aluminate (Na
2
Al
2
O
4
) in preference to Cr
2
O
3
. Thus, the
melt is made more acidic due to depletion of oxide ion or release of SO
3
within the
melt. These concepts are useful in understanding the mechanisms of hot corrosion
processes.
