You're fine for short periods, but don't do it over an extended period.
I had a bucket I used just for Star San, and over the course of about 9 months the bottom of the bucket developed a slow leak.
No. It makes foam because it contains a small amount of surfactant. It sanitizes because it's phosphoric acid based. As others said, the pH is the only indicator. A graduated food grade bucket is like $5 at Home Depot, the bucket isn't something worth worrying about.