Environmental Engineering Reference
In-Depth Information
cal model is more general and powerful in explaining the oxide scale dissolution
and reprecipitation during hot corrosion of metals/alloys.
Despite the elegance of the electrochemical model, investigations have most
often utilized the acid-base fluxing model to explaining the observations. The
mechanisms for basic and acid fluxing in the presence of Na
2
SO
4
salt deposit
are presented in the following sections.
Basic Fluxing
It is common in basic fluxing for the amount of attack to increase as the amount
of deposited salt is increased. For a fixed quantity of salt under basic conditions,
the melt is gradually consumed and becomes saturated with complex anion,
whereupon the reaction eventually subsides.
The microstructural features of the scale formed on a metal/alloy brought
about by hot corrosion damage are largely governed by the compositions of the
material and the gas phase for a particular composition of the salt deposit, apart
from other factors. Accordingly, for illustrating the basic fluxing mechanism in
hot corrosion, a simplified system, e.g., pure Ni under a thin Na
2
SO
4
deposit
exposed in pure oxygen at 1173 K, is considered here. The corresponding kinetic
curves along with the superimposed micrographs indicating the reaction sequence
are presented in Fig. 6.37. The corresponding reaction sequence is illustrated
schematically in Fig. 6.38. Here it is worth mentioning that when nickel with
Na
2
SO
4
deposits undergoes a reaction in O
2
or air (containing insignificant SO
3
),
accelerated corrosion takes place only above the melting point of Na
2
SO
4
.At
lower temperatures, Na
2
SO
4
serves as a barrier to oxidation, and the reaction rate
is reduced compared with of nickel containing no Na
2
SO
4
deposit. The kinetic
curves (Fig. 6.37) clearly demonstrate that Ni has undergone accelerated corro-
sion in the presence of a molten Na
2
SO
4
film in comparison with the bare one
but such accelerated damage lasts for a relatively brief period. Subsequently, the
rate virtually equals that for Ni without the salt deposit. The very slow rate of
reaction for Ni without salt is due to the formation of a highly tenacious, compact,
and adherent film of NiO.
Reaction Sequence.
Upon initial exposure to the oxygen atmosphere at the des-
ignated temperature, NiO forms a layer covered by molten Na
2
SO
4
. The contin-
ued formation of NiO rapidly lowers the
p
O
2
in the salt at the oxide-salt interface.
Therefore, sulfur potential increases, leading to transport of sulfur through the
thin oxide, and sulfide formation takes place at the scale-metal interface as illus-
trated in Fig. 6.38a. But there are varied opinions about the nature of sulfur trans-
port. As sulfides are detected within a very short exposure time, such transport
cannot occur by lattice diffusion of sulfur ions through NiO. A more feasible
mechanism could be transport of SO
2
molecules penetrating through the micro-
cracks of the scale. The source of SO
2
is dissociation of the sulfate ions according
to Eq. (6.35).
