Kinetic and morphologic study of the pack-aluminization of a series of refractory alloys

Abstract
Five refractory alloys, Fe-20Cr-20Ni, Fe-20Cr-36Ni, Fe-20Cr-45Ni, Fe-20Cr-60Ni and Ni-20Cr, were submitted to pack aluminization at 850 °C for different durations in a NaF-activated pack. After a short period to achieve steady state, the coating kinetics were shown to be linear. The rate of aluminization was the same for the four alloys containing iron (about 1 mg.cm^-2.h^-1) and notably higher (1.8 mg.cm^-2.h^-1) in the case of the Ni-20Cr alloy. The nature of the formed products and the morphology of the layers were shown to depend on the alloy composition. The most important feature was the presence of external FeA1₃ on the four Fe-containing alloys, whereas a mixture of Ni₂Al₃ +NiAl was observed on the Ni-20Cr alloy. Kinetic experiments and thermodynamic calculations allowed a derivation of the value of the Al-activity at the gas-substrate interface. The mechanisms involved in the aluminization were shown to depend on the Al-concentration in the pack, on gaseous diffusion for low Al concentration and on interface reaction for high Al concentration.