Kinetics of low-temperature pack aluminide coating formation on alloy steels

The kinetics of pack aluminide coating formation and growth on alloy steels was studied in the temperature range of 550 DGC to 700 DGC. The pack Al content was varied from 1 to 10 wt pct and aluminizing time from 1 to 16 hours. The halide salts AlCl3 and NH4Cl were studied as activators. In the AlCl3 activated packs, it was observed that all the coatings consisted of a single Fe2Al5 layer with an abrupt coating/substrate interface and varying the pack chemical compositions and processing conditions affected the growth rate of the layer thickness but not the microstructure of the coating. In these packs, it was revealed that the growth kinetics of the layer thickness (12 in 1.i.m) that accounts for the effects of temperature (T in K), time (t in hours), and pack Al content (W in wt pct) can be described by h = 75,141.8 exp (-8820.8/T)W1/2t1/2. In the NH4CI activated packs, it was shown that coating formation and dissolution took place simultaneously at 650 DGC. However, increasing Al content in this type of packs will increase the coating formation rate, making it possible to form a sufficient coating layer.