Pewter (Solder)

Tin and Lead bond (Fusion), a variety of Pewter that's called Solder, generally needed for construction in the Metals society to put things together (like a glue). So that's mainly the reason these two would ever get together like this. Pewter's not exactly the fusion you want for him to stay together for long. At first Tin's pretty good and in control, then over time Lead kind of overtakes the reigns. As poor Zinc is witnessing right now. Zirconium knew not to keep these two together for long, he just was a bit too slow to catch it. Zinc's also saying this in a nice manner compared to what Zircon told him to say. 

Considering it's Lead, they probably told Tin that he needs them, that he's weak without this. 

(Note: Not all pewter contains lead, especially nowadays. A good sign that it does contain lead is if it has a blue sheen to it, as that's lead's part, which is older pewter since the other components or options for pewter were more expensive. Pewter itself is an alloy mainly consisting of tin, usually with antimony, copper, bismuth, and occasionally lead. Solder on the other hand, is tin + lead. It's generally used to bind two sheets/pieces of metal together, especially in electric wiring. It has a low melting point.)