Alloy 625 Weld Overlay

Alloy 625 weld overlay of boiler pressure parts, although widespread in Waste to Energy and Biomass-fired boilers, has a limited life depending on the operating conditions of the unit: its aging starts with dealloying, followed by pitting and after some time boiler operators understand that the remaining sponge-like structure can’t protect the tubes and can’t be repaired. Replacing these pressure parts becomes the only choice.

Can weld overlay ageing be stopped at an earlier stage to avoid a costly replacement?

• The answer is Yes! IGS does it and can provide excellent references.

In the steam boiler environment, Alloy 625 weld overlay can degrade quickly depending on the metal skin temperature. Below 400°C deterioration is rather slow: it can take over 10 years to see signs of degradation; above 430°C it degrades much faster: it can reach degradation rates of over 0,5-0,6 mm/year in cases of superheater tubes or high-steam-pressure boilers.

The centers of wide membranes are also the hottest areas on the water walls where degradation initiates more rapidly, especially on the junctions between panels, since the quality of this field operation is typically lower than the shop applied 625 WOL.

The quality of the 625 overlay also impacts the lifetime: the more homogeneous, smooth, and consistent the thickness the better. That is why shop weld overlay performs better than field or on-site welding.

Alloy 625 forms a Chromium oxide layer to resist corrosion but this layer can be eroded or degraded; consequently, another layer forms which consumes the chromium from the 625 alloy layer. Since the concentration of Cr will vary (due to a lack of homogeneity) the consumption can be faster in Cr-depleted areas creating pitting and developing into a “spongy” looking texture on the 625 WOL.

Signs of Weld Overlay Degradation

When the operator starts seeing this “spongy’ texture the corrosion rate of the Inconel is going to increase. Iron dilution of the weldment can be a significant reason for this acceleration, therefore, the corrosion rate of a welded alloy is never linear: it is slower at the surface and accelerates with increasing proximity to the base (a combination of Cr consumption and higher Iron (Fe) content when closer to the substrate).

Typically, the first sign of ageing of a welded alloy is the disappearance of the welding ripples on the surface; then, we can start to see small pits on the surface, these pits grow wider and deeper with time.

When measuring the remaining weld overlay layer thickness, the surface should be ground smooth to remove the pitted appearance (to reach the same level as the deepest pit or the minimum thickness) or the measurement will be overestimated (as you will not measure the minimum thickness) leading to later unwanted surprises.