My interest has been piqued by this thread, so I thought that I would run a few tests to see the effect of various beads and a steel nut.
Using a spectrum analyser and tracking generator and a little test jig which is shown with the beads and nut in the first picture:
View attachment 91166
With the analyser sweeping from zero to 100MHz, the response of the straight through connection is shown here:
View attachment 91167
The vertical scale is 2dB per division. The big dip at the left of the screen is the zero frequency marker.
Putting the steel nut on the wire shows practically no change from the straight through connection:
View attachment 91170
The small bead gave some attenuation as shown here:
View attachment 91169
The larger bead gave a bit more attenuation, as shown here:
View attachment 91168
Remember that this is taking place in a 50 ohm environment, where the circuit impedance is something else other than 50 ohm the attenuation will be different.
Going back to the original question, a steel nut does nothing, and aluminium washer is likely to do even less.
JimB