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I am going to try aluminum anodes on my DP - SMTD outdrive this up coming season. I had the drive prepped and painted for the 2012 season and used magnesium anodes. I noticed bubbling patches under the paint in about in about 5 places . I boat in out of Midland in so called fresh water but I have doubts about how fresh it is in the marina and at the mouth of some of the tannen rich rivers in the area. Some on-line anode experts advise that sometimes hydrogen bubbles will form under paint because of magnesium anodes overdoing their job in fresh but polluted /acidic waters. Has anyone tried these aluminum anodes ?

http://lmgtfy.com/?q=why+is+the+paint+o ... e+bubbling

Two things for sure will cause the bubbling. 1. over protection which i doubt you have. 2. not good adhesion to the primer. Hope you used zinc chromate or Alodine for your primer.
Recommended etching for aluminum is Alumiprep. (It shouldn't be used on copper or silicon bearing aluminum alloys)
This is where you should be using Metal Prep 79
◦Alloy is manufactured to meet or exceed US Military Specification (MIL-A-18001K)
•Aluminum Alloy Anodes = Salt or Brackish water
◦Not recommended for use in fresh water
◦Proven to last longer than zinc due to increased capacity

◦Alloy is manufactured to meet or exceed US Military Specification (MIL-A-24779(SH))
•Magnesium Alloy Anodes = Fresh water only
◦Not recommended for use in salt or brackish water
◦The only alloy proven to protect your boat in fresh water

Thanks for the info THalaso ! I think you are the same person that helped me out on my trim tab problem a while back. As you suggested I acquired a trim tab o ring kit from Bennet free of charge and it arrived 10 days after calling them. I will install the kit in 3 to 4 wks from now. AS for as the bubbling under the paint on my outdrive you are probably correct about the primer , all I can do is trust that the company that did the repaint did a good job as thats all they do as well as prop work. Anyway the article on-line that led me to the aluminum anodes is called "Whats the truth about Aluminum sacrificial anodes " written by Martin Wigg ( President of performance metals, inc and Paul Fleury who founded Marine services a marine electrical engineering and corrosion consulting co. ) Who knows ? These guys say that the major sterndrive manufactures have accepted Aluminum anodes ( an alloy of aluminum,zinc and indium) as the best all round anode material to use. I think I will clean up the bubble patches first before launching install the AL anodes and the haul out in about 3 weeks to check to see what is going on. Change back to mag if need be.

I think Merc has gone to using aluminum on all there drives. Glad you saved yourself some big bucks on the rams.Just make sure you use some zinc chromate primer.
It might be hard to find locally other then West Marine. You can also get it online. (Moeller)
Check all your bonding wires on the drives. Every thing should have one.