POWER February 2014 - 26

Corrosion Protection for FGD Vessels
Roughly five years ago, the power industry readily embraced the
new Alloy 2205 metal as a more lightweight and cost-effective
substrate for the construction of flue gas desulfurization (FGD)
absorbers and vessels than traditional heavy-duty stainless steel
and carbon steel substrates, including the more common Alloys
304 and 316. Assuming that the surfaces of the Alloy 2205 FGD
absorbers and vessels were properly prepared and lined, early test
reports indicated that the substrate would hold up well, except
perhaps under conditions such as crevice corrosion or corrosion
under film buildup (Figure 7).
New Substrate Gives Way to Old Corrosion
Challenges
Prior to lining, Alloy 2205 requires a clean surface with a sharp
profile and blast profile (depth). For most high-build linings or
coatings, a 3-mil profile is needed. However, for most alloys, adjacent
unlined surfaces also need to be protected during abrasive
blasting to ensure that the surfaces are not contaminated
by iron.
Because most slag abrasives contain iron, garnet or aluminum
oxide abrasives are preferred choices, provided that they are able
to produce the required sharp profile. As with carbon steel, the
surface must be tested for salt contamination, and cleaned if
necessary, to the lining manufacturer's requirements. One different
feature with alloys is that the surface must be coated or
primed soon after blasting, unless dehumidification control holds
the relative humidity to less than 40%, as the surface will not
show the typical discoloration, as carbon steel does, due to surface
oxidation.
Despite acceptable surface preparation and application of linings,
many Alloy 2205 vessels located at more than 40 power
plants across the U.S. are now showing evidence of severe pitting
and corrosion under scale buildup, including " worm holing, " after
just one year in service. Apparently, much of the pre-testing of
Alloy 2205 did not take into consideration the effects of a high
concentration of chlorides/fluorides under the scale buildup, or
that it creates the same corrosive effects of crevice corrosion.
Not All Solutions Are Created Equal
Increased demand for corrosion-related repairs to Alloy 2205 vessels
has given rise to some new corrosion solutions with varying
degrees of efficacy.
One repair alternative used by plant owners is to clad the interior
of the FGD vessel with an alloy metal that is known to
perform well in these service conditions and costs less than other
alloys, such as Alloy C276. Nevertheless, common drawbacks to
this option include material availability. Alloy metal can often be
in short supply, which forces owners to wait for extended periods
until the material becomes available.
The biggest drawback to this repair solution, however, is cost.
Between the high price of the material and the skilled welders
needed for installation, some estimates put the cost of cladding
up to $75 per square foot. Furthermore, localized leaks can still
seep under the cladding and attack the alloy underneath virtually
unnoticed. Worse, this repair process is fairly time-intensive,
resulting in significant downtime, which some plants estimate at
up to $1 million per day in lost revenues.
Over the past 10 years, vinyl ester linings have been the most
common method of corrosion repair for both carbon steel vessels
and many types of alloys. These linings can be very effective
and are increasingly being utilized in Alloy 2205 corrosion re26
www.powermag.com
POWER
| February 2014
8. An installer spraying Ceilcote lining onto tank
walls. Courtesy: International Paint
7. Typical flue gas desulfurization vessel that will be
lined to protect the substrate. Courtesy: International Paint
9. An installer rolling fiberglass reinforcement into
wet basecoat. Courtesy: International Paint
http://www.powermag.com
POWER February 2014

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