You are better off with citric or sulphamic acid, both are less likely to eat away the cast iron. they are a bit slower than hcl, but attack the metal itself much much slower.
DS40 (mostly citric acid) run round for a week, measure the ph daily to see how it is going, initial ph is about 3, then part way through it drifts up to a ph of 6.3, after that it is spent. It is a three acid tails on the molecule, so ph varies through its use.
Fernox recommend up to a week each time, so some DS40, volume up to you, when spent or up to a week, then drain and flush to remove loose crud, and repeat. If the ph stays at 3, then it is clean and you can stop.