pcrussell50 wrote:For the record, I'm not sure it's hard plastic. I think it's aluminum. It's light like plastic, but aluminum is light. And it's hard and "rings" like aluminum. When I scraped off some of the "bubbles" on mine, it looked like freshly exposed (i.e. not yet oxide layered) aluminum. Do you know for sure that the removable disc is not aluminum?
You're idea that it's galvanic corrosion is a good one. And the stainless screw passes through the disc on it's way to threading into the group head. Don't quote me on this, but if I remember my strength of materials classwork from engineering in college (25 years ago), certain stainless alloys are more reactive with common Al alloys than come carbon steel alloys.
My dispersion disk is definitely plastic, but that isn't to say they didn't change the material at some point.
I'll have to see if the replacement I ordered from Breville is the same as my old one.
This site selling a replacement part seems to indicate they did change it at some point due to the issues you've experienced. Maybe the inner shower screen has acted as a sacrificial piece and saved your actual shower head from any issues.