Corrosion and Corrosion Fatigue Behavior of Nickel Based Alloy Weld Overlay and Coextruded Coatings

Ni-based weld overlay claddings are often used to provide corrosion resistance for waterwall tubes in low NOx boilers of coal fired power plants.  However, they are susceptible to circumferential cracking, which is attributed in large part to the welding process.  High residual stresses, reduction in alloy content due to dilution, and localized depletion of alloying elements due to microsegregation all contribute to circumferential cracking. Coextruded coatings are fabricated under solid state conditions and therefore offer the potential to eliminate the factors that promote corrosion fatigue.  As such, a study is being conducted to understand the corrosion and corrosion fatigue of Ni based alloy coextruded coatings as compared to weld overlays coatings.  The corrosion behavior of Alloy 622 coextruded and weld overlay coatings were evaluated using thermogravimetric techniques under simulated low NOx combustion conditions.  Coextruded and weld overlay coatings of Alloy 622 are also currently being tested under representative gaseous corrosion conditions in a Gleeble thermal and mechanical testing system.  The results of the corrosion fatigue tests will be discussed.  The corrosion results demonstrate that the coextruded coatings provide corrosion resistance that is significantly better than the weld overlay coating.  The improved corrosion resistance is attributed to elimination of dilution and microsegregation in the coextruded coating.  Dilution of the weld overlay causes an increase in the iron content and a decrease in the chromium content, which is detrimental to the corrosion resistance.  In addition, microsegregation depletes the dendrite cores in critical alloying elements (such as molybdenum) making the dendrite cores susceptible to preferential corrosive attack.  Understanding the corrosion and corrosion fatigue mechanisms in coextruded and weld overlay coatings will allow for the optimization of the corrosion resistance of Ni-based alloy coatings on waterwall tubes.