Starsan has its corrosive abilities and in the wrong concentration could definitely cause some corrosion. Mix it with the recommended dose and hammer down. What I usually do Is just keep one keg full of starsan (and the others just clean and pressurized) and when im ready to keg something move it to the next clean keg and fill the sanatized one with my beer. Starsan, esp if kept under pressure, will last a long time if you are just sanatizing kegs with it. I have friends that have done 20+ on one batch of starsan with no problems, I like to refresh mine a little more often tho like every 5-10 batches.