I have been contacted by our old friend David T who has a Lavac sea toilet. These use a Henderson type bilge pump for emptying. David has found that hydrochloric acid attacks a small piece of aluminium in the pump. (see his reply) What David does is to strip the pump and use a domestic loo cleaner to clean the parts.
I did not realise that there were aluminium parts
in these pumps, I do have one somewhere and will check but think that the fairly recent version I have is all plastic.
Worth checking if you have a Lavac!!
Another method of keeping the system clear of calcium is to use vinegar. This is considerably more expensive and not as aggressive as hydrochloric acid so may well be OK where there are aluminium parts in the system. Used in a similar fashion to the acid it should help to stop the calcium build up and the need need to regularly strip the system.
As far as I am aware all the normal heads such as the almost universal Jabsco plastic pump type only have stainless parts.
Some years ago I tested all these stainless parts from an old loo along with some 304 and 316 bolts by leaving them in 50% strength acid. After 3 days I examined them under a powerful magnifier and found only a slight dulling but no corrosion.
Our current Jabsco is around 6 years old and has not needed the valves or pipes cleaning yet.