Why did copper sheeting corrode the iron nails on HMS Alarm?

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Anyways, can anyone help me with this question..

In 1763, the British Admirality covered wooden ship HMS Alarm with copper sheeting to protect it from marine worms. This was successful but, they reported: '... we were surprised to perceive the effect of the copper had upon iron where the two metals touched... the nails and staples... were found dissolved into a rusty paste.' Briefly explain this occurrence and how it might have been prevented.

This should be easy for you guys...

I know that both the metals undergo wet corrosion, but I don't know anything about the effect on having the two metals in contact.
 
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This phenomenon is known as galvanic corrosion. It occurs when two different metals are in contact with each other in an electrolyte, such as saltwater. In this case, the copper sheeting and iron nails and staples created a "battery" which caused an electric current to flow between the metals. This caused the iron to corrode much faster than it would have otherwise. To prevent this, they should have used some form of insulation, such as a rubber pad or coating, between the copper and the iron. This would have prevented the flow of electricity and the accelerated corrosion.