How to protect a battery terminal after removing corrosion?
WD40 displaces moisture but allows contacts to penetrate the thin film for a reasonable resistance ~ 100 mOhms, if there is sufficient force. 

It will eventually dry out and leave a thin residue so it is not permanent.

If you had liquid Tin, after cleaning off oxide, dip then rinse in water. That is better but you have to buy bulk (2L?) so this is not practical.  

Solder works but will oxidize rapidly in humid weather >90% RH.