The deposition is calcium carbonate and other carbonates; the very
same scale formation as when sea water's temperature is above 143-145
deg. F.

Urine when mixed to seawater lets the carbonates 'drop
out' (precipitate) out of seawater at a lower temeperature .... its
the very same scale thats inside your raw water circuit of your
engine, especially inside the exhaust manifold.

I use a commercial (environmentally degradable) boiler descaling on my
ENGINE, waste plumbing AND holding tank to remove the scale ... all at
the same time.... the engine is more important so I save the engine
cleaning solution then simply soak the head, plumbing AND holding
tank, etc, before dumping.

Rydlyme or Marsolve is what I use. Vinegar is for salads, its too
weak and takes too long to put calcium carbonate back into solution.
www.marsolve.com/ www.rydlyme.com/ www.rydlyme.net