This is a new one. I didn't need to descale (and maintain) my machine as often, since we swapped to getting water delivery (and using a filter on top of that).

So instead of doing maintenance every six months, I've waited about a year, and to my surprise, the part immediately behind the shower head has developed a nice patina, presumably due to oxidation of the part. Previously there was only build-up of gunk that I needed to remove, but this was a new one.

So how bad is this? Do I need a new part? FWIW, I was able to scrub off the oxide (and gunk) with steel wool, but I'm not sure if I got ride of everything.

ETA: the part is called the diffusor