Rusty nuts sir?
Stainless steel I say sir.
Under the car where it can stay wet if used on rainy days, caution should be given to dis-similar metal use like this.
(this is not criticism of your choice, as I don't understand the detail of the bolt/nut/bracket make up in the picture shown).
By fitting stainless nuts on e.g. low alloy steel bolts, if the components get wet (and noting that the screw thread is a crevice that can retain water) then any exposed bare low alloy steel of the bolt will sacrificial corrode and 'protect' the stainless steel even further. This is basically a tiny electrochemical battery. In these dis-similar metal situations, the more reactive metal can be consumed fairly quickly vs a similar metal joint.
If the bolts are replaceable and the ability to protect the nut and undo the joint is the highest priority, and the bolt can be replaced, this is a good solution.
If the bolt is welded into position and is difficult to replace, then aluminium nuts could be a good option, as they will corrode preferentially to the low alloy steel bolt, thereby protecting the bolt
The only trouble being that you would need to replace the nuts every year or two.
The use of low alloy steel nut and bolt has even/equal and generally slower rate of corrosion between the two, so liberally application of corrosion inhibiting spray after installation and at regular interval would help slow the corrosion down even further.
So, armed with this information, you can choose which metal combinations might be suitable.
. Have you used any wheel sealant?
