I have a keg dedicated with unscented oxiclean that I flush/fill the beer lines with after each keg blows. I let it sit in the line until the next keg is ready to tap. I have another keg dedicated with star-san that I fill/flush the line with just prior to tapping the next beer. The star-san sits in the line no more than 5 minutes before I run beer thru it. It has served me well over the years... Cheers!!!
I left a keg filled with a weak Oxyclean solution for a few days, the stone that formed was a mess. It took a few baths in vinegar, and eventually Star San, to remove it.
You leave that solution in the keg all the time? What does the inside of the keg look like?