Metal (and glass) utensils used for food require immersion in a mikvah. What counts as a metal for this purpose? Note that there is no universally accepted scientific definition of a metal.
This article by R. Avi Zakutinsky discusses which kinds of materials need immersion in a mikvah:
When describing the obligation to purify and immerse utensils that were owned and used by non-Jews the Torah mentions only six types of metals; Gold, Silver, Copper, Iron, Tin and Lead. These items need immersion on a biblical level. The poskim discuss whether metals like aluminum or steel, which were not listed explicitly in the verse, are required to be immersed and whether the obligation is biblical or rabbinic in nature. It would seem that according to the Tiferes Yisroel (Kuntres Yevakesh Daas) the obligation is biblical. The Sefer Tevilas Keilim (page 225, footnote 113) cites the oral ruling of Rav Shmuel Wosner shlit”a, that all types of metals, including aluminum and the like, are required to be immersed m’doraysa (biblical law).
Rav Moshe Feinstein zt”l (Igros Moshe Y.D. 3:22) disagrees and he feels that there is no biblical obligation to immerse any metal which was not enumerated in the verse. He explains that G-d was obviously aware of all materials that were in use and that will be in use and if He did not list it in the posuk, clearly He felt that there is no biblical obligation to immerse them in a mikvah. That being said, there is a rabbinic obligation to immerse these utensils due to it’s similarities to the metals listed in the verse