If you have used a metal fastener through a hole which you have drilled, or used a fastener which drilled itself. Without realising it you have started a process of corrosion. When two dissimilar metals are brought together, an electrolytic process occurs. The US aerospace industry recognised this and did something about it. They developed an anticorrosive jointing compound called DURALAC. This product is used religiously, in the marine and aircraft and other fabrication industries.

We now stock the best one - DURALAC. It comes in a 115ml tube which is all you are ever likely to need unless you are BOEING. DURALAC inhibits electrolytic corrosion between dissimilar metals.
Get in the habit of using it whenever you pick up your cordless drill.