The importance of the plastic liners in soda cans

I've recently started reading the CR4 discussions. One of their latest experiments was where they removed the plastic liner from Al cans before filling them with muriatic acid; it is a good example of focused curiosity. The discussion and youtube video outline their results. They wanted to see if the contents of the soda would eat through an unprotected can. They found that it did, but mold growth brought their experiment to an end. So to push the limits and get quicker results, they filled the cans with muriatic acid. The result; in 5 min, the three unprotected cans are completely eaten away by the acid while the unaltered can remains.