Effects of chromium in steel substrates on interfacial IMC growth and thermal durability of hot-dip Al-9 wt% Si coatings

초록

This study investigates the influence of chromium in steel substrates on the interfacial reactions and thermal durability of hot-dip Al-9 wt% Si coatings during isothermal exposure at 450-550 degrees C. Low-carbon steel (LC) and two ferritic stainless steels (STS409L and STS439) with different Cr levels were examined using complementary microstructural identification techniques. In LC, i-Fe2Al5 rapidly became the dominant interfacial phase and exhibited nearly linear growth kinetics, controlled by Al flux through the t5(H)-Al8Fe2Si layer. In contrast, the Crcontaining steels stabilized t5(C)-Al8(Fe,Cr)2Si with minor 0-FeAl3, with t5(C) following parabolic growth. The higher activation energy in STS439 reflected greater Cr incorporation and slower Al-Fe interdiffusion. Cr also significantly delayed i-Fe2Al5 nucleation, suppressed Kirkendall voiding, and improved coating integrity. Surface discoloration correlated with the growth of interfacial intermetallic compound (IMC) layers. LC blackened early due to rapid topcoat depletion and Si enrichment, whereas STS409L and STS439 exhibited an intermediate yellowish discoloration prior to blackening, attributed to thicker alumina scales formed by reduced Si enrichment at the surface. Overall, increasing Cr content stabilized t5(C), suppressed i-Fe2Al5 formation, and raised the diffusion barrier for Al-Fe interdiffusion. These effects progressively retarded IMC growth and enhanced the high-temperature durability of Al-Si coated steels.

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