Effects of Complex Material-Environment Interactions on Hot Corrosion Mechanisms

Low temperature hot corrosion (LTHC) is the degradation of gas turbine components by reaction with a molten salt at approximately 700 °C. LTHC is typically attributed to eutectic melts of a mixture of Na2SO4 and sulfates of alloying elements, particularly NiSO4 and CoSO4. However, characterization of NiCoCrAlY-coated field hardware shows corrosive deposits are rich in MgO and/or MgSO4 (referred to here as Mg2+) as well as Na2SO4. The literature on the effects of Mg2+ additions on LTHC is scarce and contradictory. Exposure of three different model NiCoCrAlY alloys to mixed deposits in the low velocity burner rig (LVBR) and a tube furnace, characterization of the deposits and corrosion morphology, and thermochemical and mass transport modeling resulted in the following findings:

1. Mg2+ additions to Na2SO4 accelerated corrosion in the LVBR, where exposure times were long and the temperature was thermally cycled, but not in the shorter, isothermal tube furnace exposures. Explanations for this difference were proposed.

2. In the LVBR, corrosion in pure Na2SO4 deposits resulted in localized attack around Y-rich precipitates, but corrosion in mixed Na2SO4- Mg2+ was more uniform.

3. In the LVBR, Mg2+ deposited primarily as MgO(s) and then slowly reacted with SO3(g) to form MgSO4(s). The rate of MgSO4(s) formation was faster in the presence of Na2SO4(s).

4. In the LVBR, MgO(s) deposited via the impaction of submicron particles, while Na2SO4(s) formed via nucleation at the sample surface.