What should i do with my iphone 4?

ok, so i accidentally went into a hotub with my iphone 4 and then i put it in rice for 2 days and it worked, then recently it started getting really hot and dying fast. So i went to apple and they said it was corrosion or something, so for christmas i just got a new iphone 4, beecause they took my old one and asked for 170$. anyways i turned the new phone on and it was already at 50% battery, so i went to charge it. When it was on the charger it was normal temperature but when i take it off it gets hot! just like my old phone! and the battery dies fast! my mom said i was supposed to let it die and then charge it to 100% battery but i didnt know that. What should i do its a day old can i get a new one if i take it to apple with the reciept?