Increasing the 900 °C Hot Corrosion Resistance of M (Ni, Co) CrAlY Type Coatings Under Na2SO4 Deposits Through Systematically Altering Cr and Co Contents and Re-evaluating Cr and Co Effects on Long-Term Hot Corrosion Behavior

Abstract

This research was carried out to improve type-I hot corrosion resistance of MCrAlY-type coatings with modifying chromium (Cr) and cobalt (Co) contents under Na₂SO₄ salt film at 900 °C and re-evaluate the effects of Cr and Co elements on type-I hot corrosion. To evaluate hot corrosion performance, weight change versus time graphs were plotted by tracking changes in sample, total and spallation weights. The initiation stage of hot corrosion was defined by slight weight changes and local attacks where the formation of spinel oxide was observed. The propagation stage was characterized by aggressive weight change, which was directly correlated with the development of a sodium-containing hot corrosion product layer. Surface and cross-section analysis revealed that basic fluxing occurred in both the initiation and propagation stages of hot corrosion. Considering this finding, a mechanism was proposed to explain the relationship between weight change, scale morphology, key reactions and hot corrosion stages. Hot corrosion resistance was not improved by increasing Cr concentration from 22 to 30 wt% in the NiCrAlY coating alloy. Increasing α-Cr phase fraction in the structure was thought to have a detrimental effect on the formation of protective oxide in initiation stage. Conversely, a significant increase in hot corrosion resistance was achieved with the addition of Co (10 and 25 wt%). Increasing Co content led to increased Al₂O₃ scale growth rate and internal oxidation, indicating a reduction in protective scale integrity. In Co-containing alloys, initiation stage was delayed, and propagation stage was not observed until 3024h of exposure.