I've been careful to avoid identifying the "victim" (Ouch! I haven't killed it yet!) because I don't want product-specific answers here, I am seeking general shop practices guidelines to apply to all the machines I work on.
I've been using a liquid phosphoric acid descaler manufactured by a local company (it's not sold in the US) which has mixing directions for use in coffee urns. I've been using a higher concentration because it does nothing to the heavy scale I've been finding at the recommended concentration.
I've been using the liquid because at the time I bought it, I was told that citric acid powder is best for maintenance-level descaling where no disassembly is required, and for heavy scale I needed something more aggressive. Since then I've seen Doug at OE using citric acid exclusively to tackle in some cases some pretty heavy scale in the many videos he and Barb have posted, so I'm starting to question my decision. If "bronze disease" or any other ill effects are caused by using phosphoric acid, I'll definitely stop using it, but I'd like to know more about it if any of you care to humour me.
In any case I want to avoid immersing chrome or plated parts to avoid damaging them. I'm just not sure what else to do with them. I upended the group head casting and filled the warming passages with diluted acid, and immersed that in a small bucket of hot water, changing the water when it got cold, and that seemed to go okay. I'm now descaling a vertical passage near the back, resting the grouphead upright and running hot water through the warming passage with a hose to keep it warm, but it's pretty tedious having to babysit one part for a whole evening. There has to be a better way!
Thanks again for reading and any help you can offer. Cheers