Louis wrote:Commercial machines like the Vivaldi are expected to be fed with appropriate water in order to prevent scaling (water softener). In a commercial environment, descaling is just not practical: with the amount of water going through, scaling would happen too quickly and down time to descale is lost sales.

The question I would ask myself after seing those pictures is rather: "Where is that nickel now?" Did it went with the descaler (not really an issue) or did it go, slowly dissolved, in the brew water?

In the later case, and if it is a health concern, is it worse to drink nickel rather than copper or lead?

I would be more worried about copper and lead than I am about nickel. While scaling is bad for boiler/heaters, it can be considered good for health, as it lowers the contact surface between metal and water, reducing metal dissolution in water. On the contrary, water softening (along with heat) can augment metal dissolution, as in this example from the WHO: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2560737/

I have not done much research into this but HB threads on the subject are many and some patient Google Scholar research could help understand if this is a real issue or not.

Personally I would say that water coming from a HX tube or a small boiler (like one in a Vivaldi), drank in very small amounts (60ml at a time), is probably not an issue. One could ask questions though on frequently drinking tea mugs out of huge brass steam boilers on espresso machines.

But then I digress and we are far from your original flaking question.