Environmental Engineering Reference
In-Depth Information
of mixed oxide/metal layer, is composed of finer, light gray sulfide particles in
the metal matrix. Since the sulfides are usually rich in chromium, the underlying
metal consists of chromium-depleted discontinuous fragments of the base alloy.
Under exceptionally severe service conditions, liquid Ni-rich or Co-rich sulfides
may also develop in the inner layer, leading to very rapid degradation of the
alloy [55].
6.7.4 Mechanisms
The mechanism of hot corrosion has been the subject of a number of investiga-
tions, which have been extensively reviewed by Stringer [55]. Some of the first
proposals that protective scales could be removed from the surfaces of metals/
alloys by molten deposits were generated in studies related to fireside corrosion
of boilers [58]. It was also mentioned earlier (Sec. 6.7.1) that at relatively elevated
temperatures, comparatively high SO
3
pressures are required to form sulfates of
such elements as Ni, Co, and Al. Furthermore, for the same amount of sulfur in
the gas stream SO
3
pressures are lower at higher temperatures. Therefore, SO
3
can be considered to have a progressively less dominant role as the operating
temperature of the metallic component is increased. Accordingly, investigators
started developing mechanisms of hot corrosion attack without involving SO
3
in
the gas phase at temperatures above about 1023 K.
Two different models are reported in the literature to explain the mechanism
for breakdown of the protective oxide scale formed on metal/alloy surface during
the incubation period, leading to propagation stage of hot corrosion attack. These
are (1) the acid-base fluxing model and (2) the electrochemical model.
Perhaps the most popular is the acid-base fluxing model, which has been used
extensively since it was first proposed. Bornstein and Decrescente [59] were the
pioneers who proposed that hot corrosion of alloys involves a basic fluxing pro-
cess, as opposed to acidic fluxing process involving SO
3
. According to their
model, the protective oxide scales are destroyed as a result of reactions with
oxide ions in the salt where the oxide ions are produced by removal of sulfur from
Na
2
SO
4
. Goebel et al. [60] subsequently extended the high-temperature fluxing
reactions to acidic processes where the component to make the salt acidic was
proposed to be oxides of the elements in the alloys (e.g., MoO
3
,WO
3
). The
proposal embodies the fact that porous oxide scales are formed during basic or
acidic fluxing by precipitation from the molten salts in which these oxide scales
had initially dissolved. The dissolution and precipitation processes are eventually
controlled by the oxide ion activity of the melts, which in turn is governed by
the removal of sulfur from the salt (Na
2
SO
4
) or by addition of oxides of certain
elements (e.g., MoO
3
,WO
3
) to the salt.
An idealized model for fluxing a protective oxide scale in a molten layer of
salt deposit [61] is shown schematically in Fig. 6.36. In this model, it is assumed
that the metal is covered with a thin protective oxide scale that is continuously
