AL-6XN® Case Study

January 13, 2026
2:13 pm

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AL-6XN® Resisted corrosive environments at indianapolis power and light for over 5 years

Specifications

UNS: N08367

ASTM: B 688, A 240, B 675, A 312, B 676, A 249, B 804, B 691, A 479, B 462, A 182, B 564, B 366, B 472

ASME: SB-688, SA-240, SB-6 75, SA-312, SB-276, SA-249, SB-691, SA-479, SB-462, SA-182, SB-564, SB-366 Code Case N-438-3, B-31.1 Case 155-1.

Chemical Compositon (%)

	Ni	Cr	Mo	Mn	Cu	Si	C	N	S	P	Fe
MIN	23.5	20	6	--	--	--	--	0.18	--	--	--
MAX	25.5	22.00	7.00	2.00	0.75	1.00	0.03	0.25	0.03	0.04	Balance

Case Study

Coal-fired power plants generate flue gas as a byproduct of combustion. In many modern plants, especially those burning coal with high sulfur and chloride content, this flue gas can become highly corrosive to carbon steel and even conventional stainless steels.

The risk increases dramatically when flue gas cools below its acid dew point, allowing condensation to form. That condensate often contains sulfurous acid, sulfuric acid (H₂SO₄), and hydrochloric acid (HCl), a combination that aggressively attacks ductwork, absorber internals, and downstream equipment.

The Corrosion Challenge in Flue Gas Ductwork

Ductwork carrying flue gas from the boiler, electrostatic precipitator (ESP), or scrubber to the exhaust stack is particularly vulnerable. Corrosion frequently concentrates in:

Duct floors and lower sidewalls
Corners and crevices
Areas exposed to cold spots caused by external supports or structural steel

These locations are prime sites for acid condensation, leading to rapid wall loss, unplanned outages, and costly repairs.

Real-World Case Study: Indianapolis Power & Light (IP&L)

In the late 1980s, Indianapolis Power & Light Company (IP&L) experienced severe, localized corrosion in a rectangular steel flue gas duct.

Operating Conditions

Flue gas temperature: ~260°F
Duct length: 150 ft
Configuration: Rectangular duct from ESP outlet to induced draft fan
Fuel characteristics: High sulfur and chloride content

Corrosion appeared randomly along the floor and sidewalls, particularly near duct corners supported externally by steel braces.

Root Cause

IP&L engineers determined that:

Cold spots lowered metal temperature below the flue gas dew point
Acidic condensate formed at wall/floor interfaces
Condensate chemistry included approximately:
- 33 lb/hr of chlorides (Cl⁻)
- 29 lb/hr of sulfuric acid (H₂SO₄)

This environment proved too aggressive for carbon steel and standard stainless options.

Material Selection: Why AL-6XN® Alloy

After consulting with Rolled Alloys®, IP&L evaluated several high-alloy stainless steels, including 904L and 317L. Based on corrosion resistance data and service history, AL-6XN® alloy was selected.

Key Reasons for Selection

Proven durability in flue gas and FGD-related applications

Installation Approach

The solution was to install AL-6XN alloy as a liner inside the existing carbon steel duct.

Project Details

Duct size: 12 ft wide × 24 ft high
Material supplied: 35,000 lb of AL-6XN sheet
Sheet size: 4 ft × 12 ft × 14 gauge
Fabricator: Sterling Boiler (Evansville, IN)

Lining Method

Sheets installed using a “wallpapering” technique
90° bends formed at floor-to-wall interfacesSheets overlapped to prevent leakage paths
Plug welded to carbon steel on 2-ft centers
Overlaps covered with AL-6XN diamond patches
All seams and interfaces seal welded using GMAW with Alloy 625 filler

This approach created a continuous, corrosion-resistant barrier while utilizing the existing duct structure.

Performance Results

Five years after installation:

The duct was removed due to construction of a new scrubber
AL-6XN liner remained in excellent condition
In some areas, original mill markings were still visible

The liner showed no significant signs of pitting, crevice corrosion, or acid attack.

Extended use in FGD Scrubber Service

Based on the success of the duct lining project, IP&L selected AL-6XN alloy to be installed downstream of the quench section of the No. 3 scrubber absorber tower at the same plant.

After five years of service, the AL-6XN alloy components were reported to be in good condition and remained in operation for years to come.

Why AL-6XN® for FGD & Flue Gas Applications

For coal-fired power plants operating FGD systems, AL-6XN alloy offers:

Exceptional resistance to chloride-rich condensates
A significant degree of protection against acid corrosion
Reduced risk of under-deposit corrosion
Long service life with lower total lifecycle cost

Looking for a proven solution?

If your plant is experiencing corrosion in:

Flue gas ductwork
FGD absorber towers
Quench sections or downstream components

AL-6XN alloy has a proven track record in real-world, high-sulfur, high-chloride environments.

Contact us to discuss your operating conditions and explore whether AL-6XN alloy is the right solution for your application.

Alloy	Weight Loss, g/cm2, 75°F	Weight Loss, g/cm2, 112°F	Weight Loss, g/cm2, 158°F
316L	0.0006	0.0343	0.039
317L	0.0007	0.0377	0.05
317LMN	0.0000	0.0129	0.0462
904L	0.0000	0.0221	0.0419
AL-6XN®	0.0000	0.0000	0.0266
625	0.0000	0.0000	0.0149
C-276	0.0000	0.0000	0.0004

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