Corrosion is a costant, often expensive, and sometimes dangerous problem for boaters.   Moisture, salt, electrical currents, and chemicals create a potent combination that can attack the metallic (and sometimes nonmetallic) parts of your boat.   Everett Collier, an expert in marine techenology, details all the types of corrosion - including simple galvanic, electrochemical and electrolytic - and explains how to identify, combat, and prevent them.