Dan, I agree with most of what you said and that was great information about the aluminum protection of the carbon steel docks. I'm not very knowledgeable about the protection other then all the stuff we did back in the 80's and 90's with stern drive and outboard lower unit corrosion. First change was Mercruiser and Outboard Marine Corporation changed from using Zinc anodes to Aluminum. Magnesium anodes have a electrical potential of -1.6V, Zinc -1.05V and Aluminum slightly higher -1.1 then the zinc.This adds more protection to the drive. I'm not sure if this is still true but in the 90's if zinc anodes were used on Mercruisers it would Void the corrosion warranty . (aluminum housings are a different alloy then the Aluminum anode) Aluminum anodes have aluminum, zinc and indium, this is called Navalloy. The issue with Magnesium use is it can over protect the lower unit. The only time Magnesium should be used is in clean fresh water. Aluminum anode is your best choice for Lower Units.
Many use zinc on their inboard boats protecting the bronze and stainless steel hardware under the water, Aluminum is very close to the same voltage potential and both are effective an can not over protect the hardware. Both material should have close to the same life span used in the same environment. Some manufactures using true Navalloy claim longer relative life with Al. over the zinc. I agree with you on this statement " The degree to which an anode deteriorates is not necessarily indicative of how well it is protecting. If it is not deteriorating that may simply mean that the environment is passive."
My issue with zinc especially in fresh water is not that the Zinc will not protect hardware under water because it will. The issue with zinc ( especially in fresh /polluted/and brackish water) anodes can become inactive after only a few months due to the build up of an insulating film of zinc hydroxide. Aluminum will not do this and remain active. A boater can be using the Zinc anode and pull the boat and think that the fact that the zinc looks good he is in good shape but in reality he has no protection at all. After our first season with zinc I found no change in the zinc other then it was a very dull grey. I used magnesium on Lake Michigan which is clean water and find a considerable amount of wear or erosion of material. Because our boat is trailerable and I periodically pull it out of the water. I believe the aluminum anode is the good choice for protection in all waters. This agrees with the standards set by the ABYC. Another factor is the thruster anodes are recommended to be aluminum I like having the same anode materials in the water. Good information Dan I enjoy reading your posts.
