Im not a big fan of just crimping the radials. While this works fine in other environments, these radials are on the ground and over time they will tend to collect dust, dirt, and water. I dont see that working out well at RF especially long term. If you use stranded wire, crimping will also encourage a bit more wicking of water due to capillary action.
I agree with K3GM. I originally used SnPb 63/37 solder and it chalked badly. While it wasnt rotting away at the time-it was well on its way. Im now trying SAC305 to see how that works.
After four months, no problems, but a longer time is needed to evaluate. I do not attach copper directly to stainless steel as I found it tended to create a corrosion point (dissimilar metals).
I have always believed in a good electrical as well as a good mechanical connection.
That is an excellent point that sums it up.
Whatever you decide to do, good luck.