I finally found some information on calcium citrate, hardly soluble at all, not worth trying in my view, you need to make a soluble salt.
Acetic acid definitely worth trying, as calcium acetate is quite soluble. Vinegar is not very strong, so it will take a large quantity applied by many rinses. Possibly gentle rocking the boiler for agitation.
Be careful when you handle that brick cleaning acid. Check the strength on the label as it is sold here it is really concentrated and fumes when you open the lid. Gloves, goggles, even a face mask, and definitely out doors, remember it dissolves concrete.
Then remember the chemistry, with the molecular weights of hydrochloric acid and calcium carbonate, it will take a lot of acid if you have a significant weight of scale, it does not just “disappear” with a quick rinse. As the limestone is dissolved, the acid is neutralised and has to be replaced. I am guessing many times to get it all.
So as not to dissolve the boiler, you need weak acid, add acid to the water to make only about 10% strength or less. But it will always be a balance between time and copper loss. Think of what you see when pickling for silver soldering. Then it will take a lot of rinsing and emptying. Scale is probably mostly in the bottom half and around the tubes.
As Willy says an iron chloride is used to dissolve the background for pcb manufacture. I don’t know the chemistry there, but you clearly don’t want it that aggressive. Perhaps the navy uses a very weak form to keep boilers clean.
It’s not a quick or easy process as Derek has found with his iron scale. There are many reports of the problem, not so many reports of a solution in my observation. We all want you to win, but if all else fails, look up Ramon’s amazing Wide a Wake build log for his boiler restoration.
MJM460