What is electrolysis?
If two metals of differing electromechanical potential (i.e. alloy and steel) come into contact with each other galvanic cells are formed. The metal with the lower potential on the galvanic scale will corrode (alloy in this example). Sacrificial anodes work by offering a sample of metal which will harmlessly corrode protecting the rest of the structure.
Until about 15 years ago I’d never really taken too much notice of the need to fit anodes to boats; I felt that the fresh water on the Great Ouse was not a concern for electrolysis to occur. Then we began to notice pitting on aluminium outdrives and bright spots on the waterline of steel craft. This, I believe, is due to the conductivity of the water increasing as a result of increased usage of shore power. We now recommend fitting magnesium anodes to outdrive legs, outboards and stern gear. For brackish water, such as the tidal Norfolk Broads, alloy anodes are best suited. Remember if you take your boat out to sea magnesium anodes will dissolve very quickly, sea-faring craft will use zinc anodes. More technical information is available from the marine industry specialists in anodes MG Duff, http://www.mgduff.co.uk/.