Hangbo Alloy Group
Shanghai · Est. 2012
Wet FGD scrubbers expose metal to hot acidic, chloride-laden slurry and abrasion. See which nickel alloys—C-276, C-22, 625—resist FGD corrosion and how Hangbo supplies them.
Flue gas desulfurization (FGD) is the workhorse of modern emissions control on coal- and oil-fired boilers, waste-to-energy plants, and many chemical process heaters. Wet limestone-gypsum scrubbers clean the flue gas exceptionally well, but they create one of the most corrosive environments found anywhere in process plant: a hot, acidic, chloride-laden slurry that is also abrasive and frequently sees oxidants such as chlorate and hypochlorite from forced-oxidation systems. Selecting the right construction material is the difference between a scrubber that runs for decades and one that needs liner repairs within a few seasons. This guide walks through where nickel alloys earn their place in FGD service and how Hangbo Alloy Group supplies them.
A wet scrubber is not a single environment but a stack of them. The absorber tower quenches hot flue gas (often 120–160°C) against a recirculating limestone slurry at pH 4–6, where dissolved SOx forms sulfite and sulfate species. Flue gas from coal combustion also carries chlorides and fluorides; chloride concentrations in the slurry routinely reach several thousand ppm and can spike far higher when the unit burns high-chlorine refuse-derived fuel. Those chloride levels, combined with abrasion from suspended gypsum crystals and alternating wet/dry zones in the upper tower and ductwork, drive both uniform corrosion and localized attack such as pitting and crevice corrosion.
Forced oxidation, added to improve gypsum quality, introduces dissolved oxygen and often supplemental oxidants that raise the redox potential—shifting the service from mildly reducing to oxidizing-acidic, which rules out the lower-alloy stainless grades that might survive in a quiescent, low-chloride pit. The lesson for specifiers is simple: design for the worst realistic chloride and oxidant level, not the average, because localized corrosion does not care about averages.
The workhorse grades for FGD liners, demisters, absorber internals, and reheater tubes are the high-molybdenum nickel-chromium-molybdenum (Ni-Cr-Mo) alloys. Hastelloy C-276 (UNS N10276, ~16Cr-16Mo) has been the long-standing default for absorber liners thanks to its balance of pitting resistance, fabrication tolerance, and weldability without post-weld heat treatment. Hastelloy C-22 (UNS N06022, ~21Cr-13Mo) offers improved resistance to both oxidizing and reducing environments and better crevice corrosion performance, which is valuable in the oxidizing zones of forced-oxidation systems. For the most aggressive, high-chloride service, Hastelloy C-59 (UNS N06059) and Alloy 686 push chromium and molybdenum even higher and are frequently specified where C-276 has shown crevice attack.
Alloy 625 (UNS N06625) is widely used for spray headers, nozzles, and demister screens where high strength and good corrosion resistance are needed together, and it is also a common cladding and overlay material on carbon-steel substrates. In less aggressive, low-chloride areas such as clean-gas ducts above the demister, super-austenitic stainless grades (e.g., 904L or 6% Mo alloys such as 254SMO) can be cost-effective, but they should never be substituted into the slurry zone without a corrosion allowance and a verified PREN margin. The fundamentals of pitting resistance that drive this grade selection are covered in our guide on PREN and pitting resistance of nickel alloys.
Material selection in an FGD system is zoned by severity. The absorber tower shell and the lower slurry zone are the most aggressive and are typically lined with Ni-Cr-Mo alloy plate (C-276 or C-22) over a carbon-steel backing, or constructed from solid alloy in smaller vessels. Demister blades and supports see both the acidic mist and mechanical loading, so 625 or C-276 are common. Spray nozzles and headers handle high-velocity slurry and are frequent failure points; solid 625 or C-276 with generous wall thickness is the norm.
Reheater tubes, if a wet stack is avoided by gas reheating, operate in a hot, moist, acidic film and are often 625 or C-22. Slurry piping and pumps are usually rubber-lined or lined with high-nickel alloys depending on velocity. The contrast with sulfuric acid plants is instructive: the same Ni-Cr-Mo family appears, but FGD adds abrasion and chloride that pure acid service does not, which is why our sulfuric acid selection guide and this one should be read together when a plant handles both streams.
Nickel alloys are forgiving to weld compared with many stainless grades, but FGD fabrication still rewards discipline. Use matched high-molybdenum filler (e.g., ERNiCrMo-4 for C-276, ERNiCrMo-10 for C-22) to avoid dilution that lowers the molybdenum at the weld and creates a preferential corrosion path. Avoid carbon-steel contamination of liners, which can introduce galvanic and crevice sites. Where liners are attached to carbon steel, design the attachment so the alloy side stays wet and the crevice is sealed, because the alloy-to-steel interface is exactly where crevice corrosion starts if ignored.
Access for inspection matters as much as the initial alloy choice. Plan for ultrasonic thickness monitoring of liner areas and demister supports so thinning is caught before penetration. Many plants also coordinate FGD work with broader corrosion programs; our notes on corrosion fatigue and on waste-to-energy incineration cover adjacent failure modes that share the same alloy toolkit.
For typical coal-fired FGD with chloride in the low-thousands of ppm, Hastelloy C-276 is usually the best balance of price and resistance and is the default liner grade. Step up to C-22 or C-59 only where chloride and oxidant levels are high enough to threaten crevice corrosion at C-276's limits—paying more upfront avoids a liner replacement that costs far more than the alloy delta.
Only in the mildest, low-chloride clean-gas zones, and even there with caution. In the slurry and lower tower, 316L will pit and crevice rapidly; its chloride threshold is far below typical FGD slurry. Where chloride stress corrosion cracking is a concern, see our guide on chloride stress corrosion cracking and alloy selection before considering any austenitic stainless.
As a dedicated nickel alloy manufacturer, Hangbo Alloy Group supplies C-276, C-22, 625, and related grades as plate, sheet, and bar for liners, demisters, and internals, with full ASTM/ASME documentation and EN 10204 3.1 mill certificates so each heat is traceable to the absorber it ends up in.
As a dedicated nickel alloy manufacturer serving power generation, chemical processing sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.