Monel is the first choice then stainless, never use aluminium rivets on your rigging. To prevent galvanic reactions between dissimilar metals use Duralac compound.
www.whitworths.com.au/duralac-jointing-compound-115ml
Another product you might like to consider is LanoCote. The rigger in Port Douglas who did my mast used it. He said it was better than Duralac because it doesn't dry out after a while. Another thing he said was to use it between metals that may at first appear to be the same. An easy one is 304 & 316 S/Steel but also the aluminum mast excursion is more than likely a different alloy to the mast step and hence will be located in a different position on the galvanic table. I've noticed corrosion at this location on a few yachts. (this corrosion could also be from dodgy electrical installs on the mast) I have no indication of corrosion after 6 years.
www.crc.co.nz/LanoCote-Natural/6895-76e19afe-2d15-418a-8835-35a5762bc2c7/
www.forespar.com/products/boat-lubricant-lanocote.shtml
I think Duralac is no longer for sale in Britain for health reasons. You may need to buy some now before we catch up!