Alumina–spinel castables are critical refractory linings in ladle furnaces, significantly impacting refining efficiency and steel cleanliness. During secondary refining, slag composition fluctuates considerably due to varying steel grades, ranging from CaO‐rich basic slags to SiO 2 ‐rich acidic slags. Extensive research exists on basic slag corrosion with established mitigation strategies. In contrast, acidic (low‐basicity) slag corrosion remains a significant challenge, leading to drastically reduced service life and compromised steel purity. This study systematically investigates the corrosion behavior of alumina–spinel castables against slags of varying basicity, complemented by thermodynamic simulations using FactSage to elucidate the underlying mechanisms. To address the critical issue of acidic slag attack, La 2 O 3 was employed to enhance the refractory. Based on comprehensive slag corrosion test, detailed microstructural analysis, and structural characterization, a novel slag corrosion resistance mechanism of La 2 O 3 ‐doped castables was revealed. La 2 O 3 primarily incorporates into the calcium hexaluminate (CA 6 ) phase within the matrix. Upon contact with SiO 2 ‐rich acidic slag, La 2 O 3 promotes the preferential formation of a high‐melting‐point, high‐viscosity lanthanum‐calcium–silicate (La‐Ca‐Si‐O) phase. This newly formed silicate network with high structural stability and more bridging oxygen bonds reduces the rapid dissolution of alumina and CA 6 in the matrix, thereby enhancing the acidic slag corrosion resistance. Consequently, a dense, protective interfacial barrier analogous to the CA 2 /CA 6 layer formed under basic slag conditions is established.
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