What is "dry corrosion"?

Dry corrosion occurs in areas not in direct contact with water - exhaust systems, for example.  The outside of most exhaust system components is cooled by raw water to prevent overheating.  When today's ethanol-enhanced fuel is burned, it creates by-products known as sulfate salts.  These salts are highly corrosive, especially when exposed to very hot temperatures.  If the outboard's cooling water passages are not kept clean by regular flushing (the exhaust area in this example), hot spots can form on the interior of exhaust components, concentrating the sulfate salt's corrosive effects.  In effect, it's corrosion from the inside out.  That's why flushing your engine with fresh, clean water for 15 minutes after each trip is a vital part of preventing even dry corrosion.  It helps the cooling system run at maximum efficiency by keeping the cooling water passages clean and clear, which helps minimize the heat inside the engine, making it less suceptible to dry corrosion.