Corrosion fatigue of alloy 625 weld claddings in combustion environments

Alloy 625 weld clad boiler tubes retrieved from two supercritical pulverized coal-fired utility boilers after less than 24 months service were extensively cracked. Most of the cracks tapered towards a single crack tip whereas some cracks branched at the crack front. Corrosion products in the cracks consisted of a primary phase and a secondary phase or “spine”. The chemistry of the corrosion products was complex. The primary phase was considered a Cr-based oxide or oxide–sulfide phase mixture whereas the composition of the secondary phase was considered a nickel-based sulfide. The cause of the cracking was attributed to corrosion-fatigue. Corrosion was attributed to traditional oxidation and sulfidation mechanisms commonly found in combustion environments. Sources of thermal fatigue stress and stress concentrators were identified. In the dendritic weld microstiucture, dendrite cores depleted in Mo and Nb were susceptible to preferential oxidation–sulfidation. Cracks initiated at these areas and propagated the farthest in the valley of weld ripples following the main axis of the columnar dendrite cores. To explain the corrosion morphology within the cracks, a mechanism was proposed based on fatigue cracking of layered multiphasic corrosion scales formed on a compositionally segregated weld microstructure.