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Two different metals in a salt water corrode very quickly. The concept behind a zinc anode is that because zinc is more reactive to the seawater than aluminium or steel and so the corrosive action will first dissolves the zinc instead of the aluminium or steel engine parts.
What was happening in Aceh, was the zincs would last one year, the second year the seawater was corroding the engine, the third year the corrosion reached a point where the engines were starting to fail.
All we had to do was install a couple of inexpensive zinc anodes on a couple of outboards and the steel corrosion stopped; everyone was convinced.
We also showed them that they could use zinc from other sources; zinc is three dollars a pound and they could protect ten boats with that amount. As opposed to paying the twenty dollars for the brand name outboard replacements. Seventeen dollars may not sound like a lot, but when you are only making five dollars a day, it is.