Waste-to-Energy Boilers: 310S vs Alloy 625 Cladding Selection Guide

Why 310S Is Often Sufficient for Superheater Tubes

Grade 310S (UNS S31008) is an austenitic heat-resistant stainless steel with nominally 25% chromium and 20% nickel. Its high chromium content builds a dense, self-healing oxide scale that resists flue-gas corrosion up to approximately 950 °C, which makes it one of the most cost-effective solid materials for superheater, reheater and evaporator tubes in waste-to-energy (WtE) and biomass boilers. In municipal solid waste (MSW) plants, chlorine-induced wastage of correctly selected 310S is generally held below 0.5 mm/year provided steam-side temperature and gas-side flow are controlled. Because 310S is a standard, widely produced grade available as seamless tube, welded tube, plate and bar, its unit cost is a small fraction of that of nickel-base cladding alloys - the single largest factor in deciding between a solid 310S tube and a carbon steel tube with weld overlay.

When Alloy 625 Weld Overlay Is Justified

Alloy 625 (Ni-Cr-Mo, UNS N06625) is the reference overlay material for waterwall sections exposed to molten slag, reducing conditions and heavy alkali-chloride deposits. Its nickel-rich matrix, alloyed with molybdenum and niobium, suppresses both chloridation and sulfidation in the 450–600 °C metal-temperature window that governs WtE waterwalls, and it tolerates the low-oxygen, high-sulfur conditions that destroy iron-based oxide scales.

Waterwall panels facing slag impingement and falling molten ash.

Nose and throat zones where gas velocity and particle loading peak.

Overlay thickness normally 1.5–2.5 mm, applied by automatic GTAW or PTA welding in overlapping beads.

Service impact: a correctly applied overlay has extended waterwall tube replacement intervals from roughly 18 months to about 60 months in high-chlorine MSW service.

Ash Corrosion Rates and Critical Metal Temperatures

Chloride-rich ash is the controlling damage mechanism. The table below summarises the wastage rates reported for the two materials under the same high-chlorine waste conditions.

Material	Typical ash corrosion rate	Condition
310S solid tube	about 2 mm/year	High-chlorine waste, more than 1.5% Cl
Alloy 625 overlay	about 0.3 mm/year	Identical gas and deposit chemistry

Critical wastage begins once metal temperature exceeds about 400 °C in the presence of potassium chloride (KCl) deposits, because the salt becomes molten and destroys the protective oxide layer. Keeping tube-wall and steam temperatures below that threshold - by increasing steam mass flow, revising the pass arrangement or optimising soot-blowing - is frequently more effective than upgrading the alloy.

Erosion Protection Techniques

Vertical tubes: 60° spiral rifling on the tube surface reduces particle erosion by about 70%.

Horizontal tubes: alloy 625 shield caps welded over the leading edges of the first tube rows.

Refractory anchoring: alloy 601 studs and anchors rated for continuous service up to 1100 °C.

Failure Analysis and Thickness Monitoring Protocol

Metallographic cross-sections examined by SEM to locate chromium-depletion zones behind the scale.

EDS mapping of chlorine and sulfur penetration along grain boundaries to confirm active corrosion.

Deposit analysis for zinc and lead low-melting eutectics that accelerate liquid-phase attack.

Ultrasonic thickness trending on a fixed grid every 3,000 operating hours to derive a wastage rate and schedule tube replacement.

Frequently Asked Questions

Q: Is 310S good enough for waste-to-energy superheater tubes without any overlay?
In most MSW boilers 310S is sufficient for superheater and reheater tubes as long as steam-side temperatures and gas flow are controlled, so that chlorine-induced wastage stays below about 0.5 mm/year and metal temperature remains under the 400 °C KCl threshold.

Q: What decides whether alloy 625 weld overlay is worth the extra cost?
Molten slag contact, reducing gas chemistry and chloride deposits on waterwall panels. Where those conditions exist, overlay service life of about 60 months versus roughly 18 months for uncoated carbon steel usually justifies the higher material cost.

Q: Why does 310S suffer more ash corrosion than alloy 625?
310S relies on a chromium oxide scale that molten KCl dissolves, giving wastage of about 2 mm/year in waste containing more than 1.5% chlorine, whereas the nickel-chromium-molybdenum matrix of alloy 625 limits wastage to about 0.3 mm/year under the same conditions.

Q: How can erosion be reduced without changing the tube material?
Adding 60° spiral rifling to vertical tubes cuts erosion by about 70%, alloy 625 shield caps protect the leading edges of horizontal tubes, and alloy 601 studs handle refractory anchoring up to 1100 °C.

Q: Which tests confirm that chloride corrosion is the active mechanism?
SEM cross-sections reveal chromium-depletion zones beneath the scale, EDS mapping shows chlorine and sulfur penetration at grain boundaries, and deposit analysis identifies zinc and lead low-melting eutectics.

Q: How often should tube wall thickness be measured?
Ultrasonic thickness readings on a fixed grid every 3,000 operating hours give a reliable wastage trend and allow replacement to be planned before wall thickness falls below code minimum.