Hastelloy C276 for FGD Systems: Absorber and Duct Guide

Hastelloy C276 for FGD Systems - At a Glance

Hastelloy C276 is the right alloy for the small number of FGD locations where chlorides, dilute acid and repeated wetting and drying all act at the same time: the inlet duct quench zone, the absorber internals, the mist eliminator, and any duct or stack surface that can be neither kept dry nor kept fully wet.

For low-chloride, near-neutral, fully wetted duty, 904L, 254 SMO or super duplex 2507 will usually do the job at lower cost. The engineering question is therefore not whether C276 is a good alloy, but whether the chloride level, pH and thermal cycling at that specific location put the duty beyond what a 6Mo stainless steel can survive.

Flue gas desulfurization is unusual among industrial processes because it attacks metal through three separate mechanisms at once. The gas carries chlorides and fluorides that were present in the fuel; the scrubber liquor is a mildly acidic, heavily aerated, solids-laden slurry; and the hardware cycles between hot dry gas, condensing acid, and cold wet slurry every time the unit starts, stops or changes load. A material chosen only against one of those mechanisms will fail on the other two. That is the specific gap that a nickel-chromium-molybdenum alloy such as C276 fills.

What Is Hastelloy C276 for FGD Systems?

Hastelloy C276 (UNS N10276, W.Nr. 2.4819) is the broad-spectrum nickel-chromium-molybdenum-tungsten alloy used in FGD where a single grade has to survive chloride pitting, dilute acid attack, and wet-dry cycling simultaneously. It is not specified across a whole FGD system; it is specified at the locations where the three mechanisms overlap and where lower-cost stainless steels have either failed or are expected to fail.

Its chemistry explains the fit. Roughly 15.5 percent chromium provides the passive film, 16 percent molybdenum and around 4 percent tungsten provide the resistance to chloride pitting and crevice attack, and the balance of nickel keeps the alloy austenitic and effectively immune to chloride stress corrosion cracking. The carbon level is capped at 0.01 percent and silicon at 0.08 percent, which keeps the weld zone resistant to intergranular attack without a post-weld heat treatment - a decisive advantage on a site-fabricated duct or absorber. The full composition is set out in the Hastelloy C276 chemical-composition guide.

The table below sets out the mechanisms that actually drive material selection in a wet FGD system. Read it as a map of where each failure mode occurs, because that map, rather than the component list, is what determines where C276 belongs.

Corrosion Mechanisms That Drive Material Selection in FGD Systems

Why Is FGD One of the Most Corrosive Duties in a Power Plant?

FGD is exceptionally corrosive because it combines three drivers that almost never appear together in other plant systems: a chloride-rich electrolyte, a low-pH acid environment, and a metal surface that is repeatedly wetted and dried. Each one alone is manageable with a conventional stainless steel. Together they defeat 316L, and they shorten the life of even 904L and duplex grades.

• Chlorides concentrate in a closed loop. Because most wet FGD systems recirculate their scrubber liquor, chloride entering with the fuel accumulates in the slurry until a purge stream removes it. Concentrations of 10,000 to 30,000 mg/L are routine, and waste-fired or coastal units run higher.

• SO2 and SO3 form acid wherever water condenses. Absorbed sulfur dioxide becomes sulfurous acid and then sulfuric acid. In the bulk slurry the pH sits near 5 to 6, but in condensing films, under deposits, and in low-flow pockets it can fall below 2.

• Fuel-borne chlorine becomes hydrochloric acid. Chloride in coal, heavy fuel oil or waste feedstock leaves the furnace as HCl, which is highly aggressive to stainless steel at the temperatures found in the inlet duct and pre-scrubber.

• Wet-dry interfaces concentrate everything. Every time a surface dries, dissolved salts and acid concentrate in the residual film. Repeated cycles turn a mildly aggressive liquor into a severely aggressive one at exactly the same spot.

The practical consequence is that stainless steels fail in a predictable sequence. First comes crevice attack under gaskets and deposits, because that is where chloride concentrates and oxygen cannot reach. Next comes pitting on the wetted surfaces, and then chloride stress corrosion cracking at welds and restraint points.

By the time cracking appears, the component usually has to be replaced rather than repaired - which is why the cost of getting the alloy wrong in FGD is measured in outage weeks rather than in material dollars. The underlying mechanisms are covered in more depth in the Hastelloy C276 corrosion-resistance guide.

Which FGD Components Actually Need Hastelloy C276?

C276 is justified at the wet-dry interfaces, at the high-chloride internals, and at any surface that cannot be kept either fully dry or fully wet. Absorber shells and clean, dry ductwork rarely need solid C276 plate, though they often need a C276 lining or overlay where the gas condenses.

The inlet duct quench zone is the single most reliable candidate. Hot flue gas at 120 to 160 degrees Celsius meets the first spray of slurry, the temperature collapses, and the metal surface moves from dry to wet and back again on every start-stop cycle. Concentrated sulfuric acid and chloride salts form in that transition, and it is the location where C276 or a 625 overlay has effectively become standard practice.

FGD Components and Where Hastelloy C276 Earns Its Place

Note that the table describes where C276 is justified, not where it is mandatory. On a low-chloride, base-loaded unit with stable operation, several of these components are served perfectly well by 904L or 254 SMO. The C276 case strengthens as the chloride level rises, as the unit cycles, and as the fuel mix moves toward waste-derived or high-chloride feedstock.

How Does C276 Perform in Each FGD Process Type?

C276 relevance scales directly with the chloride and HCl load of the process. Waste-to-energy and high-chloride coal units are the strongest cases, seawater FGD is a split decision, and the dry and semi-dry processes rarely need nickel alloy at all.

The reason is that different FGD processes generate very different electrolytes. A dry sorbent injection system has almost no liquid phase and therefore almost no chloride-driven corrosion. A wet limestone absorber has a continuously recirculating chloride-rich liquor. A seawater system has sea-strength chloride but also a high dissolved oxygen content, which keeps the chemistry oxidizing and allows titanium to perform well where it would fail badly in a reducing, low-pH acid stream.

Hastelloy C276 Relevance by FGD Process Type

What Is the Role of Chlorides, Fluorides and Acid Dew Point in FGD?

Chloride is the single most decisive variable in FGD material selection. It sets the pitting and crevice resistance that the alloy must have, and it is the reason duplex and 904L reach their limits well before C276 does. Fluoride and acid dew point matter because they define where and how the attack begins.

Chloride attacks in two ways. The first is localised breakdown of the passive film, which produces pitting and crevice corrosion under gaskets, deposits and scale. The second is chloride stress corrosion cracking, which needs tensile stress, a susceptible alloy, and chloride at temperature - all three of which are present in a welded, restrained, warm duct. C276 addresses both: its molybdenum and tungsten content raises the pitting resistance well beyond the 6Mo stainless steels, and its high nickel content makes it practically immune to chloride cracking.

Fluoride behaves similarly but arrives from a different source. Fluorides in coal and in some waste streams leave the furnace as hydrogen fluoride, which dissolves readily into condensing water and attacks the same low-pH locations as HCl. Where fluorides are present, the case for a nickel alloy in the inlet duct and stack strengthens considerably.

Acid dew point is the third variable, and the one most often under-designed. Flue gas containing SO3 has a dew point typically in the range of 120 to 150 degrees Celsius. Any metal surface colder than that will collect concentrated sulfuric acid. In wet FGD the absorber and the saturated outlet duct are below the dew point by design, so they are wet and must be built for wet acid service. The risk sits in the components that are supposed to be dry: bypass ducts, dampers, and stack sections during start-up, when the metal is cold and the gas is not. Insulation, reheat, and - in the worst cases - a C276 or 625 lining are the responses. For chloride-driven acid attack specifically, see the C276 hydrochloric acid service selection guide.

Hastelloy C276 vs 316L, 904L, Alloy 20, 254 SMO, Duplex and Titanium

C276 is not the cheapest alloy that can work in FGD - it is the alloy that keeps working where the others have already failed. The comparison below is a screen for deciding when that premium is genuinely required, and when a 6Mo stainless steel or a super duplex grade will deliver the same service life for less money.

Hastelloy C276 vs 316L, 904L, Alloy 20, Duplex and Titanium for FGD Service

Two cautions apply when reading the table. First, the PREN values are indicative and calculated as chromium plus 3.3 times molybdenum, which understates C276 and C22 because it ignores their tungsten content and therefore their real pitting resistance. Use PREN as a ranking aid, not as a design limit. Second, cost rankings move with nickel and molybdenum markets, so treat them as relative positions rather than as fixed multipliers.

The practical hierarchy for FGD is straightforward. 316L survives only in low-chloride, near-neutral, continuously wetted service. 904L and Alloy 20 extend that range into moderate chloride and sulfuric acid duty respectively. 254 SMO, Alloy 31 and 2507 duplex cover most of the remaining wet internals. Above roughly 10,000 mg/L chloride, in low-pH pockets, or at any surface that cycles between wet and dry, C276 becomes the defensible choice. Direct comparisons for the neighbouring grades are set out in the C22 vs C276 comparison and the Inconel 625 vs C276 comparison.

How Do You Choose a Material by Chloride Level?

Use a chloride band as a screening tool to narrow the alloy choice, then confirm it with corrosion testing in the actual liquor. Chloride alone is never the whole answer, because pH and temperature move the limits by a wide margin - the same alloy may be comfortable at pH 5 and unacceptable at pH 2.

Indicative Chloride Bands and Alloy Choice for Wet FGD

The bands above are deliberately broad and should be treated as an initial screen only. A system running at 30 degrees Celsius with 15,000 mg/L chloride and a pH of 5 is a materially easier duty than the same chloride level at 70 degrees Celsius and a pH of 2, and a coupon test in the real liquor will separate the two cases far more reliably than any published table.

Quick-start: Treat the chloride band as a shortlist generator, not a verdict. Confirm the final alloy with an ASTM G48 pitting and crevice test and, where possible, an ASTM G31 immersion test in liquor drawn from the actual system, and check the C276 temperature limits for the hot, dry end of the duty.

What Are the Temperature Limits of C276 in FGD and Duct Service?

C276 is not chosen for its high-temperature strength in FGD, because process temperatures sit far below its limits. The temperature question in FGD is really a dew-point question: whether a given surface stays above the acid dew point, and therefore stays dry, or drops below it and collects concentrated acid.

Four temperature zones dominate design decisions. The wet absorber runs at roughly 50 to 60 degrees Celsius, fully wetted, with chloride-rich slurry - a corrosion problem rather than a temperature problem. The inlet quench zone sees gas that may enter at 120 to 160 degrees Celsius and is cooled to saturation within a short distance, so metal temperature, gas temperature and dew point cross each other inside one component.

The saturated outlet duct and stack run at 50 to 55 degrees Celsius in a fully wet state. The bypass duct, the damper, and the stack during start-up are the components that are meant to be dry, and they are the ones at risk whenever metal temperature falls below the dew point.

• Wet, saturated zones. Corrosion is chloride- and acid-driven. C276, C22 and the 6Mo stainless steels all compete here, and chloride level decides between them.

• Wet-dry cycling zones. The most damaging duty, because the surface concentrates acid and salts as it dries. C276 is the default answer.

• Dry zones above the dew point. Ordinary carbon or low-alloy steel with insulation is often sufficient, and stainless is used mainly for erosion or for cold start-up.

• Dry zones below the dew point. Acid condensation at 120 to 150 degrees Celsius. Insulate, reheat, or line with C276 or 625 - and do not rely on a stainless steel that cannot tolerate the resulting acid concentration.

How Should You Specify C276: Solid, Clad, or Lined?

For large surface areas, specify C276 as ASTM A265 clad steel plate or as a two-layer weld overlay on carbon steel. Reserve solid C276 for internals, headers, small components and anywhere that a thin lining would be damaged by the duty. This split typically removes 70 to 90 percent of the alloy content from the project while leaving the corrosion performance where it is actually needed.

Solid C276 sheet is the simplest option to fabricate and the easiest to repair, but it is also the most expensive per square metre of surface protected, and on a large absorber or duct wall it is difficult to justify economically. Clad plate solves that by bonding a 3 to 4 mm C276 layer to a carbon or low-alloy steel backing, so the structural thickness is paid for at carbon steel prices. Weld overlay goes further still and deposits the alloy only where it is needed, at the cost of a more demanding procedure and a requirement for at least two layers to guarantee that the exposed surface is undiluted.

A third route, common on absorbers and ductwork that are already built, is the wallpaper lining: thin C276 sheet, typically 1.6 to 2 mm, attached by resistance spot welding or plug welds and sealed at the seams with matching filler. It is economical and repairable, but the attachment points and seam quality become the critical inspection items, because a lining that lifts or a seam that leaks creates a crevice that is worse than the bare metal would have been.

Whichever route you take, specify the product forms by their ASTM designation so that the mill, the fabricator and the inspector are all working to the same document.

Product Forms and ASTM Specifications for FGD Construction

Product availability for these forms is summarised on the Hastelloy C276 product page, with pipe, plate and fittings covered separately under Hastelloy steel pipe, Hastelloy sheet and plate and C276 pipe fittings.

How Do You Weld and Fabricate C276 in FGD Construction?

Weld C276 with a matching nickel-base filler, keep heat input low, and treat iron contamination as a defect rather than as an inconvenience. The alloy has a low thermal expansion and a sluggish, viscous weld pool, so techniques that work well on stainless steel need to be adjusted rather than copied.

Welding C276 for FGD Construction: Process and Filler Selection

• Use dedicated tools. Grinding wheels, wire brushes and cutting discs that have touched carbon steel will embed iron in the C276 surface and produce rust spots that look like corrosion failures but are contamination. Keep a separate set for nickel alloy work and check the finished surface with a ferroxyl or copper sulfate test.

• Control dilution on overlay work. On a carbon steel substrate, dilution from the first pass can pull enough iron into the deposit to reduce its corrosion resistance. A qualified procedure with two or more layers is the control, and PMI of the finished overlay surface is the proof.

• Avoid chloride-bearing consumables and cleaning agents. Chlorinated solvents, hydrochloric-acid descalers and contaminated wash water can leave residues that initiate pitting. Use clean water and chloride-free cleaning materials.

• Do not rely on post-weld heat treatment. The low carbon and silicon content means C276 welds do not need solution annealing to remain resistant to intergranular attack, which is why the alloy is practical on large site-erected structures.

• Prepare for distortion. Thin lining sheet and low-thermal-expansion nickel alloy both move differently from carbon steel, so sequencing, tacking and fit-up need to be planned rather than improvised.

The welding variables specific to the alloy, including filler selection, heat input and interpass control, are set out in the Hastelloy C276 welding guide. Machining and forming operations, which matter mainly for internals and nozzles, are covered in the C276 machining tips.

What Standards and Codes Govern FGD Materials?

Three families of documents govern an FGD material specification: ASTM product specifications that define what the metal is, ASME codes that define how it is designed and joined, and environmental regulations that determine why the FGD system exists in the first place. A complete specification references all three.

• ASTM product specifications. B575 for plate, sheet and strip; B574 for bar and rod; B622 for seamless pipe and tube; B619 for welded pipe; B626 for welded tube; B366 for wrought fittings; B564 for forgings; B462 for forged flanges, fittings and valves; A265 for nickel alloy clad steel plate.

• ASME design and construction codes. Section II for material properties, Section VIII Division 1 for pressure vessels such as the absorber, B31.1 for power piping and B31.3 for process piping, Section IX for welding procedure and welder qualification, Section V for non-destructive examination, and STS-1 for steel stacks and chimneys.

• Fitting and flange standards. B16.5 and B16.47 for flanges, B16.9 and B16.11 for fittings, and B36.10 and B36.19 for pipe dimensions. See the C276 flange dimensions and the ASME B36.19 pipe dimension charts.

• Filler metal specifications. AWS A5.14 for ERNiCrMo-4 bare wire and AWS A5.11 for ENiCrMo-4 covered electrodes.

• Environmental drivers. In the United States, the Mercury and Air Toxics Standards and the New Source Performance Standards for utility units; in the European Union, the Industrial Emissions Directive and its BAT conclusions for large combustion plants; in the United Kingdom, environmental permitting for combustion installations. These rules set the SO2 and HCl removal duty that the materials then have to survive.

How Do You Test and Qualify C276 Before It Goes Into Service?

Positive material identification of every heat and ASTM G48 and G28 testing of the critical heats and weld zones are the non-negotiable checks. Everything else in a quality plan supports those three, because the two ways a C276 fabrication fails are the wrong material being installed and the right material being welded or heat-treated into a susceptible condition.

Mix-ups are a genuine risk rather than a theoretical one. C276, 625, 904L and 316L are visually indistinguishable, and a single substituted flange or header run in a high-chloride location can fail within months. PMI on receipt, on cut pieces and on finished welds closes that gap at trivial cost relative to the consequence.

Inspection and Testing That Protects C276 FGD Fabrication

Where the duty is genuinely marginal - a chloride level close to the limit of the grade, or a temperature that sits near a known threshold - a field coupon programme in the actual liquor is worth the effort. A coupon rack in the absorber or the outlet duct, evaluated to ASTM G1 and G46 and weighed to ASTM G31, turns a specification argument into a measured corrosion rate.

What Does C276 Cost in an FGD Project, and Is It Justified?

C276 typically costs roughly 6 to 10 times as much as 316L per kilogram and about 1.5 to 2 times as much as 904L, so the premium is real. It is justified when the alternative is a shorter replacement cycle on a component that is expensive to access - which, in an absorber, a duct or a stack, is almost always the case.

Construction Options for Corrosive FGD Zones and Their Cost Logic

The economic argument rests on where the cost falls. Alloy cost is paid once, at fabrication. The cost of premature failure is paid in scaffolding, confined-space entry, disposal, re-lining and lost generation, and on a stack or an absorber internals package that figure routinely exceeds the entire alloy premium by a wide margin. In that framing, the relevant comparison is not C276 against 904L; it is C276 against the probability of replacing 904L a second time inside the plant design life.

That said, the premium is only defensible where the mechanism actually demands it. Applying C276 across an entire FGD system because it is the most corrosion-resistant option wastes capital on dry ductwork and low-chloride wetted surfaces that would never have failed. The disciplined approach is to spend the alloy budget where the three drivers - chloride, low pH and wet-dry cycling - genuinely overlap.

When Should You NOT Use Hastelloy C276 in FGD?

Do not use C276 in low-chloride oxidizing duty, in abrasion-dominated locations, or in oxidizing seawater service where titanium performs better for similar money. C276 is a broad-spectrum alloy, which means it is rarely the cheapest answer for a narrow problem.

• Low-chloride, near-neutral, fully wetted duty. Below roughly 500 mg/L chloride with a pH above 5, 316L or a duplex grade will give the same service life. C276 adds cost without adding life.

• Strongly oxidizing streams. Where free chlorine, hypochlorite or other strong oxidizers dominate, C22 or C2000 resist better than C276, and titanium may be cheaper still.

• Oxidizing seawater ductwork. Seawater FGD with high dissolved oxygen is a titanium duty in many designs. C276 remains the right answer for the reducing, low-pH zones but should not be assumed for the whole gas path - see the 625 vs C276 seawater comparison.

• Abrasion-dominated zones. C276 is a relatively soft alloy. Where high-velocity angular gypsum solids cause metal loss by erosion rather than by corrosion, a hardened or ceramic-lined solution, or a change in geometry and velocity, will outlast an alloy upgrade.

• Dry ductwork above the acid dew point. Insulated carbon steel is sufficient and costs a fraction of the price.

• Where the real problem is design, not material. Stagnant dead legs, poor drainage, uncontrolled chloride build-up and excessive slurry velocity will eventually defeat any alloy. Fixing the design is cheaper than upgrading the metal.

What Are the Most Common Mistakes When Specifying C276 for FGD?

The most common mistakes are applying C276 uniformly across a system instead of targeting the severe zones, and treating it as an alloy problem when the underlying cause is a design or operating problem. A third cluster of failures comes from fabrication discipline rather than from alloy choice.

Quick-start: Before releasing a C276 specification, confirm four things: the chloride level is a measured design value rather than an assumption, every wet-dry interface has been identified, the welding and overlay procedures are qualified for the dissimilar joint, and PMI is required on all material and finished overlay surfaces.

• Assuming a chloride level instead of designing to one. Chloride in the recirculating liquor is a design parameter that must be controlled by purge rate. If it is not written down, it will drift upward and undermine the alloy selection.

• Over-specifying the alloy everywhere. Lining a dry bypass duct with C276 while leaving a high-chloride quench zone in 316L is a predictable way to spend a large budget and still suffer a failure.

• Ignoring the wet-dry interface. It is not a component, so it does not appear on a component list, and it is consistently the first place to fail.

• Unqualified overlay procedures. A single-layer overlay on carbon steel can pass visual inspection and still have iron levels at the surface high enough to pit in service.

• No iron contamination control. Shared grinding and brushing tools are the most common source of rust staining on new C276 surfaces, and the staining is then misdiagnosed as a material defect.

• Treating a lining as maintenance-free. Wallpaper linings need scheduled inspection of seams and attachment welds, particularly in the quench zone and at the slurry line.

• Using the wrong filler for convenience. Substituting a stainless filler to save a spool of wire creates a weld that is less corrosion resistant than the parent metal and will fail first.

How Does This Article Fit the Hastelloy C276 Content Cluster?

This article is the FGD industry spoke inside the Hastelloy C276 hub-and-spoke cluster. The Hastelloy C276 ultimate guide is the hub: it covers composition, properties and the full application map, and it links back to this page from its industry section.

Alongside this page, the cluster covers the alloy fundamentals in the what is Hastelloy C276 primer, the mechanical properties guide and the corrosion resistance guide; sibling industry spokes for chemical plants and acid service; and the nickel alloy pipe selection guide by acid type. Where the FGD duty involves a chemical process stream rather than flue gas, the chemical equipment overview is the better starting point, and where the corrosion driver is a reducing acid rather than a chloride-rich slurry, start from the C276 acid service guide.

For a worked example of the same selection discipline applied to a different environment, the Inconel 625 LNG pipe spool case study and the 316H sulfuric acid service analysis show how alloy choice follows from the measured chemistry rather than from habit.

Key Takeaways: Hastelloy C276 for FGD Systems

• C276 is a targeted solution in FGD, not a system-wide default. Spend it where chlorides, low pH and wet-dry cycling overlap.

• The inlet duct quench zone, absorber internals, mist eliminator and wet-dry interfaces are the components that most reliably justify the alloy.

• Chloride concentration is the decisive design variable, and it should be a controlled, measured value rather than an assumption.

• Use clad plate or two-layer weld overlay for large surfaces, and solid C276 for internals and small parts.

• Weld with ERNiCrMo-4 or ENiCrMo-4 filler at low heat input, and treat iron contamination as a defect.

• Require PMI on every heat and ASTM G48 and G28 testing on critical heats and weld zones.

• Where chlorides are low and the chemistry is oxidizing, titanium or a 6Mo stainless steel may be the better engineering answer - C276 is the broad-spectrum choice, not the universal one.

Related Products and Further Reading

• Hastelloy C276 product range - plate, sheet, pipe, bar, forgings, flanges and fittings

• Hastelloy steel pipe for absorber slurry, wash-water and quench circuits

• Hastelloy sheet and plate for duct, absorber and stack lining

• A complete guide to C276 flanges

• Everything you need to know about C276 plate

• C276 pipe price per kg 2026 for budget screening

• JN Alloy FAQ list for general material and ordering questions

Frequently Asked Questions

What is Hastelloy C276 used for in FGD systems?

Hastelloy C276 is used in the specific FGD locations where chlorides, dilute acid and repeated wetting and drying occur together: the inlet duct quench zone, absorber internals such as spray headers and oxidation air lances, the mist eliminator and its wash system, and any duct or stack surface that can be neither kept fully dry nor kept fully wet. It is not specified across an entire FGD system.

Why is Hastelloy C276 better than 316L for FGD?

Because 316L cannot survive the combination of high chloride and low pH that wet FGD produces. 316L depends on a passive chromium oxide film that chlorides break down locally, causing pitting, crevice attack and chloride stress corrosion cracking. C276 uses molybdenum and tungsten to resist that breakdown and a high nickel content to resist cracking, so it holds up in the locations where 316L fails within a few years.

Is 904L good enough for an FGD absorber?

Often yes, in moderate-chloride duty. 904L is widely and successfully used for absorber internals and spray headers at chloride levels up to a few thousand mg/L with a pH above roughly 4. It becomes marginal above about 10,000 mg/L chloride, in low-pH pockets, and at wet-dry interfaces, which is where C276 takes over.

What chloride level requires Hastelloy C276 in FGD?

As a screening guide, C276 becomes the defensible choice above roughly 10,000 mg/L chloride in the recirculating slurry, and at any chloride level where the surface cycles between wet and dry or sits in a low-pH pocket. Chloride alone is not the whole answer, because pH, temperature and oxidizing potential move the practical limit by a wide margin.

Where does corrosion start first in an FGD system?

At the wet-dry interfaces, and above all in the inlet duct quench zone where hot gas meets the first slurry spray. As the surface dries, dissolved chlorides and acid concentrate in the residual film, so a liquor that is only mildly aggressive in bulk becomes severely aggressive at that one spot. The absorber splash zone and any slurry line that drains between cycles behave the same way.

Can Hastelloy C276 be used for FGD duct lining?

Yes, and it is one of the most common applications. Large duct surfaces are usually protected with ASTM A265 clad steel plate or a two-layer C276 weld overlay rather than solid plate, and existing ductwork can be wallpaper-lined with 1.6 to 2 mm C276 sheet attached by plug welds and sealed with matching filler.

What is the difference between C276 and C22 for FGD?

C22 has more chromium and less molybdenum and tungsten, which makes it stronger in strongly oxidizing conditions and slightly less strong in reducing acid. In FGD, C276 is generally preferred for the reducing, low-pH and chloride-rich zones, while C22 is the better answer where free chlorine or other strong oxidizers dominate. The C22 vs C276 comparison sets out the detail.

Is Inconel 625 a substitute for C276 in FGD ductwork?

It is a common and often economical substitute for duct and stack overlay work, because 625 resists chlorides well and is easier to weld. C276 remains stronger in reducing acid and low-pH service, so 625 is usually chosen for large overlayed areas and C276 for internals, nozzles and the most aggressive low-pH locations.

What filler metal do you weld C276 with in FGD construction?

Use a matching nickel-base filler: AWS A5.14 ERNiCrMo-4 for GTAW and GMAW, or AWS A5.11 ENiCrMo-4 for SMAW. Never substitute a stainless filler to save time, because the resulting weld is less corrosion resistant than the parent metal and will be the first place to fail in service.

Does C276 need post-weld heat treatment?

No. C276 is capped at 0.01 percent carbon and 0.08 percent silicon, which keeps the weld and heat-affected zone resistant to intergranular attack without solution annealing. That is what makes the alloy practical for large site-erected duct, absorber and stack structures that cannot be heat treated after fabrication.

What temperature can C276 handle in an FGD duct?

The alloy's high-temperature limits are far above any FGD process temperature, so temperature itself is not the constraint. The real limit is the acid dew point of the gas, typically 120 to 150 degrees Celsius. Any surface colder than that collects concentrated sulfuric acid and must be lined, insulated or reheated.

What is acid dew point corrosion and why does it matter in FGD?

It is the condensation of sulfuric acid onto metal that is colder than the gas dew point. Because SO3 raises the dew point to 120 to 150 degrees Celsius, bypass ducts, dampers and cold stacks during start-up can collect concentrated acid even though they are supposed to be dry. Managing it with insulation, reheat or a C276 or 625 lining is usually much cheaper than upgrading the whole duct.

How much does C276 cost compared with 904L for FGD?

C276 typically costs about 1.5 to 2 times as much as 904L per kilogram, and roughly 6 to 10 times 316L. Cladding or overlaying removes 70 to 90 percent of the alloy content, so the premium applied across a whole project is usually far smaller than the per-kilogram figure suggests.

Is solid C276 necessary, or is cladding enough?

Cladding or a two-layer weld overlay is enough for large surfaces such as absorber shells, duct walls and stacks, and it is the normal choice. Solid C276 is reserved for internals, spray headers, nozzles, fasteners and small components where a thin lining would be damaged or where the geometry cannot be clad reliably.

What ASTM specifications cover C276 for FGD?

B575 for plate, sheet and strip; B574 for bar and rod; B622 for seamless pipe and tube; B619 for welded pipe; B626 for welded tube; B366 for wrought fittings; B564 for forgings; B462 for forged flanges, fittings and valves; and A265 for nickel alloy clad steel plate. Filler metal is covered by AWS A5.14 and AWS A5.11.

Can C276 be used in seawater FGD systems?

Yes, but selectively. Seawater FGD has sea-strength chloride plus high dissolved oxygen, which keeps the chemistry oxidizing and often makes titanium the better and cheaper choice for the wet duct and stack. C276 remains the right answer for the reducing and low-pH zones, and it is not attacked by the chlorides that limit stainless steels. See the 625 vs C276 seawater comparison for the detailed split.

How do you inspect a C276 lining in an absorber?

Check the seams and attachment welds, not only the sheet surface. Use a vacuum box or bubble test on lining seams, dye penetrant on weld toes, a ferroxyl or copper sulfate test for iron contamination, and positive material identification to confirm the lining material. The quench zone, the slurry line and any restrained area are the priority inspection points.

What causes a C276 weld to rust in FGD service?

Almost always iron contamination rather than a material defect: carbon steel particles embedded by a shared grinding wheel, wire brush or handling equipment. The fix is dedicated nickel-alloy tooling, clean working practice and a ferroxyl test on finished surfaces. Genuine corrosion of a correctly made C276 weld in FGD service is rare.

Does C276 resist hydrochloric acid in flue gas?

Yes, and that is one of its strongest qualifications for FGD. Chloride in coal, oil or waste feedstock leaves the furnace as HCl, which is highly aggressive to stainless steel in the inlet duct and pre-scrubber. C276 is resistant across a wide HCl concentration range, which is why it is close to a default choice in waste-to-energy FGD.

When should you not use C276 in an FGD system?

When chlorides are low and the chemistry is oxidizing, when the failure mode is abrasion rather than corrosion, or when the real problem is a design defect such as a stagnant dead leg or poor drainage. In those cases a duplex or 6Mo stainless steel, titanium, a hardened liner, or a design change delivers a better result for less money.