Did not mention color of scale?
[QUOTE=Larry Horwitz]Also, quit using metasilicate based cleaners (pbw) in your kegs. The only thing I've seen that can remove it is hydrofloric acid...very nasty stuff. BTW this is NOT beerstone. also +1 on acid cleaning cycle. [QUOTE]
I agree w/ Larry, with the following additions:
PBW- Leaves a white scale behind, this could be your problem itself. It took me several acid cycles to remove pbw residue from the tanks when I first started here. A blended caustic product with occasional acid cycles is the way to go.
Hydrofluoric Acid- Dont think of using, this shit will kill you. Really. If you are using it use the same hazmat as if playing w/ Ebola....
Not Beerstone- If it is black this is true, its caused by Co2 meeting Caustic and creating Calcium oxalate as mentioned before. Make sure you have a good rinse cycle and purge kegs w/ compressed air not Co2, prior to washing.
Acid Cycle- use a good nitric/phos blend at WASHING concentration not RINSING concentration. For the acid I use that means 10oz/gal vs. 2oz/gal.