HRSGs and boilers |


reactive, often leading to unplanned repairs or premature component replacement.


Protection strategies for waterwalls and other components


Panel replacement


Full panel replacement remains the most invasive option. It involves removing sections of waterwall tubing and installing new prefabricated panels, often with shop-applied overlay, followed by additional on-site welding. While this approach effectively resets the asset’s condition, it introduces significant logistical complexity, costs, and outage duration.


On-site weld overlay


Weld overlay has become the industry standard for protecting carbon steel components in WtE boilers. In this process, a corrosion-resistant nickel-based alloy, most commonly Alloy 625, is deposited onto the tube surface. The overlay acts as a sacrificial barrier, protecting the base material from aggressive combustion environments. On-site application offers flexibility,


allowing operators to: ● extend the life of existing components; and ● target specific high-risk areas.


Weld ultrasonic thickness UT mapping. Photo: IGS


● economisers; and ● gas flow turning zones.


In these regions, fly ash particles remove protective oxide layers from tube surfaces. The freshly exposed metal is then subjected to accelerated corrosion, creating a self- reinforcing degradation mechanism. The severity of erosion–corrosion depends on gas velocity, ash loading, and local flow geometry, and is often intensified in modern plants operating at higher efficiencies.


Inspection strategies and corrosion monitoring


Given the variability of both fuel composition and operating conditions, regular inspection is essential. Effective inspection programmes


typically include: ● visual assessment of exposed surfaces; ● ultrasonic thickness (UT) mapping; ● evaluation of protection system (weld


overlay, thermal spray) condition and integrity; and


● identification of pitting and localised attack.


Establishing corrosion trends is critical. In many cases, operators observe measurable wear rates in protective overlays, while base material degradation can accelerate rapidly once exposure occurs.


Without structured inspection, maintenance strategies tend to become


Boiler weld overlay. Photo: IGS 20 | October 2026 | www.modernpowersystems.com


Re-overlay – extending asset life Increasingly, operators are adopting re- overlay strategies to extend the life of previously protected areas.


Rather than waiting for full degradation, controlled re-overlay enables the restoration of protection before base material exposure occurs.


This approach can significantly delay the need for panel replacement and reduce lifecycle costs. However, re-overlay is not


simply a repeat of the original process. It


requires: ● thorough surface preparation and removal of fatigued material;


● precise control of heat input and dilution; and ● understanding of existing metallurgical condition.


Good execution quality is essential. Poorly executed re-overlay can lead to defects such as lack of fusion, porosity, and irregular surfaces that compromise both performance and inspectability.


Common operational and maintenance pitfalls


Field experience across European waste-to- energy plants highlights several recurring


challenges: ● Delayed inspection and intervention Operators often delay inspection or continue operating beyond safe parameters, accelerating degradation. In some cases, remaining wall thickness becomes insufficient for weld overlay, leaving replacement as the only option.


● Operating beyond design conditions Running boilers above intended production rates or burning more aggressive waste streams increases both corrosion and erosion rates, reducing the effectiveness of protective systems. ● Poor execution quality


Common technical issues include: ● inadequate surface preparation; ● lack of bead overlap and exposed base material;


● incorrect welding parameters; and ● excessive or uneven overlay thickness.


Such deficiencies not only reduce protection performance but also create inspection challenges and localised corrosion initiation points.


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