KINETIC AND MORPHOLOGIC STUDY OF THE PACK-ALUMINIZATION OF A SERIES OF REFRACTORY ALLOYS

Five refractory alloys, Fe-20Cr-20Ni, Fe-20Cr-36Ni, Fe-20Cr-45Ni, Fe-2 0Cr-60Ni and Ni-20Cr, were submitted to pack aluminization at 850-degr ees-C for different durations in a NaF-activated pack. After a short p eriod to achieve steady state, the coating kinetics were shown to be l inear. The rate of aluminization was the same for the four alloys cont aining 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 a nd the morphology of the layers were shown to depend on the alloy comp osition. The most important feature was the presence of external FeAl3 on the four Fe-containing alloys, whereas a mixture of Ni2Al3 + NiAl was observed on the Ni-20Cr alloy. Kinetic experiments and thermodynam ic calculations allowed a derivation of the value of the Al-activity a t the gas-substrate interface. The mechanisms involved in the aluminiz ation were shown to depend on the Al-concentration in the pack, on gas eous diffusion for low Al concentration and on interface reaction for high Al concentration.