Corrosion fatigue of alloy 625 weld claddings in combustion environments

Alloy 625 weld clad boiler tubes retrieved from two supercritical pulverize d coal-fired utility boilers after less than 24 months service were extensi vely 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 chemistr y of the corrosion products was complex. The primary phase was considered a Cr-based oxide or oxide-sulfide phase mixture whereas the composition of t he secondary phase was considered a nickel-based sulfide. The cause of the cracking was attributed to corrosion-fatigue. Corrosion was attributed to t raditional oxidation and sulfidation mechanisms commonly found in combustio n environments. Sources of thermal fatigue stress and stress concentrators were identified. In the dendritic weld microstiucture, dendrite cores deple ted in Mo and Nb were susceptible to preferential oxidation-sulfidation. Cr acks initiated at these areas and propagated the farthest in the valley of weld ripples following the main axis of the columnar dendrite cores. To exp lain the corrosion morphology within the cracks, a mechanism was proposed b ased on fatigue cracking of layered multiphasic corrosion scales formed on a compositionally segregated weld microstructure.