Hangbo Alloy Group
Shanghai · Est. 2012
Waste-to-energy and MSW incinerators attack tubes with HCl, alkali, and sulfidation. See which nickel alloys survive superheater environments, from a maker.
Waste-to-energy (WtE) plants burn municipal solid waste, industrial refuse, and biomass to generate steam and power, and they are among the most corrosive environments any alloy will ever face. Unlike a clean gas-fired boiler, a waste incinerator fires a moving feast of chlorine, alkali metals, sulfur, zinc, lead, and fluctuating oxygen — then dumps those species as molten salts onto superheater tubes. Standard boiler steels fail in months. Selecting the right nickel-based alloy is the difference between a superheater that runs for years and one that leaks before the first planned outage. This guide walks through the corrosion mechanisms, the zones that matter, and the grades that earn their cost.
The chemistry of incinerator flue gas is dominated by chlorine and alkali. PVC and other plastics release HCl; municipal waste carries sodium and potassium, which form low-melting eutectics (NaCl, KCl) that condense on cooler tube surfaces. When these molten salts meet an oxidizing-sulfidizing gas, they attack chromium-rich scales directly. Sulfur from the waste feeds sulfidation, while localized reducing pockets — common on the back side of tubes and in ash deposits — strip away protective oxide altogether. Add abrasive fly ash and thermal cycling, and the metal sees corrosion, erosion, and fatigue simultaneously.
Two variables dominate alloy life: gas temperature and the chlorine/alkali loading of the feed. Higher steam temperatures raise efficiency but push superheater metal above the salt-melting threshold, accelerating attack. That trade-off is exactly why material selection is an economic decision, not just a corrosion one.
The superheater, where steam is raised toward 400–500 °C, is the primary failure location. Here, 300-series stainless and even 253MA erode quickly under salt deposition. The reliable workhorses step up the nickel and chromium:
For the very hottest, most chloride-rich tube banks, a 625 overlay weld or solid 602CA/230 tubes are often specified despite the cost, because a single tube leak can shut the whole line.
Beyond the superheater, other components face their own regimes. The moving grate and side walls sit in lower-temperature but highly abrasive, deposit-laden zones where sulfidation and carburization dominate. Here, high-nickel alloys such as 601, 690, and 800H/HT are common, with 602CA used where chlorine is severe. Burner nozzles and secondary air lances see high heat plus impingement; 230 and 617 are typical.
Economizer and low-temperature sections are cooler and may get away with 309/310 stainless or duplex grades, but designers increasingly specify nickel alloys here too as feedstock chlorine keeps rising with richer municipal waste. The trend across the industry is clear: every year, more of the boiler is nickel-based.
Good WtE material selection is a layered decision:
When the feedstock or steam temperature changes, re-evaluate the grade rather than assuming last decade’s choice still holds. A short consultation with a metallurgist before specifying saves far more than the premium on the alloy.
For the hottest, most chloride-rich superheater banks, alloy 602CA and Haynes 230 lead on molten-salt and high-temperature corrosion resistance, with alloy 617 a strong, more widely used alternative. Alloy 625/625LCF covers moderately aggressive zones economically. The right pick depends on your steam temperature and the chlorine/alkali loading of the waste stream.
Municipal waste releases HCl and alkali chlorides that form low-melting salts on tube surfaces. In the hot, oxidizing-sulfidizing flue gas these salts attack chromium oxide scales directly, and reducing pockets under ash deposits cause sulfidation. Standard 300-series and even 253MA stainless lack enough nickel and the right scale chemistry to withstand this, so operators step up to high-nickel grades — see our notes on sulfidation of nickel alloys for the mechanism.
Often yes, for moderately aggressive zones. A 625 or 622 weld overlay on a lower-cost substrate can deliver most of the corrosion resistance at a fraction of the solid-alloy price, and many plants use overlay on economizer and lower superheater sections while reserving solid alloy 625 or 602CA for the harshest banks. The trade-off is fabrication control and overlay integrity, which must be verified by inspection.
Higher steam temperature improves plant efficiency but pushes superheater metal above the melting point of deposited alkali-chloride salts, accelerating corrosion. Each step up in temperature usually means a step up the alloy ladder — from 625 toward 617, then 230, then 602CA at the extreme. As a manufacturer we recommend mapping the actual metal temperature of each zone rather than specifying one grade for the whole boiler.
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.