protecting contact between aluminium and copper wires
This may depend on your location and local electrical codes.  I have the same issue in my house.  In the USA, there are NEC (National Electrical Code) compliant wire nuts rated for Al to Cu connections.  These come pre-packed with a protective goop. The goop can also be purchased separately in a tube for use with ordinary wire nuts. You may have to ask at your local electrical supply store.  In my area the pre-packed wire nuts are a distinctive purple color not found on regular wire nuts (for example: Home Depot Al/Cu nuts).

For code compliance, use approved solutions only, even if you think you know more chemistry than the electricians. Having your house burn down from a wiring issue is no fun.