Desulfurization towers are critical in power plants and industrial facilities—they remove sulfur dioxide (SO₂) from exhaust gases to reduce air pollution. At the heart of these towers are spray pipes: they distribute a corrosive limestone slurry (the “desulfurizing agent”) evenly across the tower, where the slurry reacts with SO₂. This job is brutal for pipes: the slurry is acidic, temperatures reach 60–80°C, and constant vibration from pumps wears on the material.
2205 duplex steel has become the go-to for these spray pipes. It blends 22% chromium (for corrosion resistance) and 5% nickel (for strength) with a mix of austenitic and ferritic microstructures—making it twice as corrosion-resistant as standard stainless steel (like 304) and stronger than carbon steel. But here’s the catch: even the best material fails if installed poorly. A wrong weld, a scratched surface, or a mismatched bracket can turn a durable 2205 pipe into a leaking, corroded mess in months.
We’re breaking down the critical installation and construction points for 2205 duplex steel spray pipes—from pre-installation checks to post-installation care. These steps don’t just ensure the pipes work; they keep them working for 15–20 years, saving plants from costly downtime and repairs.
1. Pre-Installation: Inspect and Protect the 2205 Steel—Don’t Skip This Step
Before lifting a single pipe, you need to make sure the 2205 duplex steel is in perfect condition. Even small flaws from shipping or storage can turn into big problems in the tower’s harsh environment.
What to Check
Surface Condition: Look for scratches, dents, or oil/grease stains. 2205’s corrosion resistance comes from its thin, protective oxide layer—scratching this layer creates a “weak spot” where slurry can attack the steel. Use a wire brush (stainless steel, not carbon steel!) to remove light rust, and wipe oil stains with isopropyl alcohol. For deep scratches (deeper than 0.1mm), grind the area smooth with a stainless steel grinding wheel—then check with a magnifying glass to ensure no grooves remain.
Material Certification: Verify that the pipes have a “mill test report” (MTR) proving they’re genuine 2205 duplex steel. Counterfeit or mislabeled steel (like 316 passed off as 2205) will corrode in months. Check the MTR for chromium (21–23%), nickel (4.5–6.5%), and molybdenum (2.5–3.5%)—these are the markers of true 2205.
Storage: If pipes sit unused for weeks, store them indoors or under a waterproof cover. Avoid placing them directly on concrete (moisture seeps up) or next to carbon steel (Contact of dissimilar metals causes corrosion). Use wooden or rubber supports to keep pipes off the ground.
Real-World Mistake: A power plant once skipped surface checks and installed 2205 pipes with shipping-related scratches. Within 6 months, the scratches turned into rust pits, and the pipes leaked. They had to shut down the desulfurization tower for a week to replace the pipes—costing $100.000 in lost production.
2. Cutting and Beveling: Keep Heat Low to Protect 2205’s Microstructure
2205 duplex steel’s strength and corrosion resistance depend on its balanced austenitic-ferritic microstructure. Too much heat during cutting can disrupt this balance—turning parts of the steel into brittle ferrite or weak austenite, which corrodes easily.
Best Practices
Use Cold or Low-Heat Cutting: Avoid oxy-acetylene cutting (high heat!)—instead, use plasma cutting (with a water-cooled torch) or a bandsaw with a carbide blade. Plasma cutting is faster, but keep the torch moving at 10–15 cm/min to limit heat exposure. For thin-walled pipes (less than 5mm thick), a bandsaw is safer—it creates no heat-related damage.
Bevel for Welding: Spray pipes need beveled ends to ensure full weld penetration (partial penetration leads to leaks). Use a mechanical beveler (not a grinder) for smooth, uniform bevels—aim for a 30–35° angle. After beveling, wipe the edge with acetone to remove metal shavings; these shavings can cause “inclusion defects” in the weld.
Pro Tip: If you must use a grinder (for on-site adjustments), use a low-speed setting (1.500 RPM or less) and keep the grinder moving—don’t let it sit in one spot. After grinding, polish the bevel with a 120-grit stainless steel sanding disc to remove heat marks.
3. Welding: The Most Critical Step—Get the Microstructure Right
Welding is where most 2205 spray pipe failures happen. If the weld doesn’t maintain the steel’s austenitic-ferritic balance, it will corrode faster than the pipe itself. Here’s how to get it right:
Choose the Right Filler Metal
Use ER2209 welding wire (for TIG/MIG welding) or E2209 electrodes (for stick welding)—these are designed to match 2205’s composition. Never use 308 or 316 filler metal: they have less chromium and molybdenum, so the weld will corrode first.
Control Welding Parameters
Heat is the enemy here—keep parameters tight:
TIG Welding: Current = 80–120 A, Voltage = 10–14 V, Travel Speed = 5–8 cm/min. Use argon as a shielding gas (99.99% pure) to prevent oxidation.
MIG Welding: Current = 120–160 A, Voltage = 18–22 V, Wire Feed Speed = 3–5 m/min. Add 2–5% nitrogen to the argon shielding gas—it helps maintain the austenitic phase.
Preheat/Post-Heat: Don’t preheat 2205 unless the pipe is thicker than 12mm (then preheat to 100–150°C). After welding, avoid post-heating above 300°C—high temperatures destroy the balanced microstructure. Instead, let the weld cool naturally in air.
Inspect Welds Thoroughly
After welding, use dye penetrant testing (DPT) to check for cracks or pores. DPT works by spraying a colored dye on the weld, letting it seep into defects, then wiping it off and applying a developer that pulls the dye out—revealing even tiny cracks. For critical joints (like pipe-to-nozzle connections), use radiographic testing (RT) to check for internal defects (like Incomplete fusion).
Success Story: A waste incineration plant used ER2209 wire and strict TIG parameters for their 2205 spray pipes. After 5 years, inspections showed the welds were as corrosion-free as the pipe itself—no leaks, no rust.
4. Installation: Secure Pipes to Avoid Vibration and Slurry Erosion
Desulfurization tower spray pipes face two mechanical threats: vibration from slurry pumps and erosion from fast-moving slurry. Poorly secured pipes will shift, leading to cracked welds or worn pipe walls.
Support and Bracing Tips
Use 2205 Stainless Steel Supports: Don’t mix 2205 pipes with carbon steel brackets— galvanic corrosion will eat away at the 2205. Make brackets from the same 2205 material, or use plastic-coated carbon steel brackets (the plastic acts as a barrier).
Space Supports Correctly: For horizontal pipes (common in spray headers), place supports every 1.5–2 meters. For vertical pipes, add guide brackets every 3 meters to prevent swaying. Too few supports cause pipe sagging; too many restrict thermal expansion (2205 expands slightly when heated, and rigid supports can crack welds).
Protect Against Slurry Erosion: The areas around spray nozzles get the most slurry scour. Add a 2205 steel “wear ring” around nozzle connections—this extra layer takes the brunt of the erosion, extending pipe life by 5–7 years.
5. Post-Installation: Clean and Passivate to Boost Corrosion Resistance
Even with perfect welding and installation, welds and cut edges have thin oxide layers (or no oxide layer at all). Post-installation cleaning and passivation fix this, creating a strong, uniform oxide layer that repels slurry.
Step-by-Step Passivation
Wash the Pipes: Use a high-pressure water jet (3000 psi) to remove weld slag, grinding dust, and any slurry residue from installation.
Acid Pickling: Apply a pickling solution (10–15% nitric acid + 2–3% hydrofluoric acid) to welds and cut edges. Let it sit for 5–10 minutes—this dissolves heat-affected zones and creates a fresh surface.
Rinse Thoroughly: Wash the pipes with clean water until the pH of the runoff is neutral (6–7). Residue acid will corrode the steel over time.
Passivate: Apply a 20–25% nitric acid solution to the entire pipe surface. Let it sit for 20 minutes, then rinse again. This step builds up the oxide layer—tests show passivated 2205 has 40% better corrosion resistance than unpassivated steel.
Note: Always wear chemical-resistant gloves and goggles when handling acids. Work in a well-ventilated area to avoid fume exposure.
Conclusion
2205 duplex steel is an excellent choice for desulfurization tower spray pipes—but its performance lives or dies by installation. From pre-installation surface checks to post-installation passivation, every step matters. By following these key points—protecting the steel from scratches, controlling heat during cutting, using the right weld filler, securing pipes properly, and passivating thoroughly—you’ll create a spray pipe system that resists corrosion, handles vibration, and lasts for decades.
For power plants and industrial facilities, this isn’t just about installation—it’s about reliability. A well-installed 2205 spray pipe system means fewer shutdowns, lower maintenance costs, and consistent compliance with environmental regulations. In the tough world of desulfurization, getting installation right the first time is the best investment you can make.
