<p>The resistance to acid corrosion of oxygen-free copper (OFC) heat tubes is a significant concern during long-term operation. The addition of the rare-earth element lanthanum (La) can refine grains and strengthen grain boundaries, thereby enhancing the corrosion resistance of the material. However, the mechanism underlying microstructural evolution during the high-temperature sintering preparation process of OFC heat tubes, as well as the effect of La addition on the microstructure and corrosion resistance, remains inadequately understood. Therefore, this study employs in situ high-temperature metallographic microscopy to observe the microstructural evolution during the sintering process. The results indicate that the addition of La promotes the precipitation of Cu₆La nanoparticles during grain growth. The Cu₆La nanoparticles are pinned at grain boundaries, leading to boundary roughening and impeding grain growth and boundary migration. Consequently, the final grains show a small-sized ellipsoidal shape. This microstructural configuration enhances the stability of the OFC heat tubes and mitigates localized pitting corrosion. Moreover, the addition of La facilitates the formation of a dense and uniform oxide film, thereby suppressing both the formation of corrosion pits and the tendency towards ant-nest corrosion.</p>