just wondering,

wouldnt Durgol Express Decalcifier have worked?

I work on machines in the Reno, Nevada area, where our water is very hard (even with softeners) Unless there is some urgency to put the machine back into service there are two methods I like to use that work well. The first just requires you to open the tank up and let it completely dry. The drying of the scale is much like the mud in a puddle that gets dried up and cracks. While this doesn't completely remove the scale, most of it is gone and will remove with a fingernail or a flexible narrow putty knife. If I am waiting for parts on the machine I toss the part it in a bucket of soapy water (Dawn or Joy) for a night pull it out and let it dry completely with even better results.

Remember to neutralize any acid when you use the previously posted method prior to putting the machine back into service (read that baking soda rinse after a complete cool water rinse) Any acid left over at heated pressures can have deleterious effects on other parts of you machine like flow meters and nozzles.

I've also found on extremely heavy scale deposits (where it looks like a coral reef), that light tapping with a hammer around the outside of the boiler can break it free in large pieces. I once managed to descale a Marzocco steam boiler in about 30 seconds that way. Of course, the boiler needs to have a large enough opening to get all the crap out....