The Inhibiting Effect of Reactive Element Oxides on the Pack Cementation Aluminide Coating Formation

Minor additions of reactive elements (Y, Ce, La, Zr, Hf, etc.) significantly affect the high-temperature oxidation behavior of protective oxide scales. Designing oxidation-resistant aluminide coatings having optimum content of reactive elements is challenging due to the observed hindering effect of these elements on the coating formation. In the current investigation, this effect has been studied by examination of a high activity aluminide coating in powder mixtures of Al, NH₄Cl, Y₂O₃, and Al₂O₃ with different Y₂O₃ contents. Oxides of Zr, Ce, and La were also examined for comparison. Cross-sectional microstructure of coatings and weight changes of the specimens were studied at different aluminizing times. The results showed that the inhibition effect applied for all reactive elements under investigation with different strengths and intensifies with more additions of Y₂O₃ up to a saturation limit. The observed inhibition effect was explained based on the equilibrium amounts of gaseous halides responsible for transfer of coating elements. It was demonstrated that the addition of Y₂O₃ can decrease the amount of aluminum carrier halide significantly. The mechanism of inhibition was attributed to the large agglomerates rich in aluminum and reactive elements detected in the pack after aluminizing treatment.