Suitable sealants for cracked aluminized austenitic steels and their oxidation behaviour

Abstract

Aluminizing of steels may result in the appearance of tensile cracks from the coating surface to the substrate due to the difference in thermal expansion coefficients. The cracks often need to be sealed to make the coatings impervious to chemical attack and to limit their mechanical degradation. In this work, dipping of the cracked aluminide coatings elaborated on IN-800HT in 0.5, 1 and 3.75mol·L−1 concentrations of H3PO4 in acetone and increasing the dipping time are investigated. The three concentrations resulted in the formation of P-O compounds and of Al2O3. However, the 0.5mol·L−1 allowed better wetting of the surface. Further reduction of the concentration from 0.5mol·L−1 to 0.2mol·L−1 brought about the complete sealing of both large and fine cracks. The efficiency of the sealing process with 0.2mol·L−1 and 0.5mol·L−1 was thereafter evaluated by cyclic oxidation in air at 650°C for >1000cycles. The overall microstructure and composition of the aluminide coatings remained identical after sealing and the oxidation behaviour comparable to the untreated counterparts. The oxide scale formed on the untreated samples appeared thicker compared to the sealed ones. The latter were covered by a thin layer of AlPO4 which evolved towards a thin protective alumina scale. However, for the sealed samples, some minor cracks were detected after oxidation but they appeared completely healed with Al2O3 and no propagation of them beyond the additive layer/interdiffusion zone was noticed.