Magnesium anodes in salt water

pault1216

I checked the archives but did not see a spot-on answer. I have a freshwater runabout that I assume has magnesium anodes (installed by previous owner). They are in good shape. I will be in the Florida Keys next month for 2 weeks (hurricanes not withstanding) and the boat will be in the water the entire time. I have done this several times before and not given much thought to mags in salt water. I don't care if they deteriorate faster than in fresh water circumstances. Looking for opinions on whether I should switch to zinc for 2 weeks or just accept that they will wear out sooner. Anyone else see fault with this approach? Thanks!  -Paul

LaRea

I've done it in the low-salinity waters of the Chesapeake Bay, but not the ocean.  I found this quote:

"Additionally, the high rate of electrons served up by magnesium can create too much current — i.e., “overprotect” — when used to protect a metal that itself is already very active, like aluminum, especially one used in waters that are more conductive of electricity. This means salt water and polluted fresh water. In fact, if you take a freshwater boat protected by magnesium anodes to the coast and put it in a slip for a week, you’ll have a catastrophe on your hands."

http://www.boatingmag.com/how-to/choosing-right-sacrificial-anode

Looks like you should change the anodes.

pault1216

Very interesting. I would never have guessed that it would be that much of an issue. Thank you!!   -Paul

MDboater

.....or at least verify that you indeed have magnesium.  

pault1216

The boat has been in the same body of water with zero corrosion issues since new. I can't imagine it being anything but magnesium. Not sure how to verify as I cannot find any visible markings in their current state (about 80%).

Handymans342

I wouldn't stress out about it too much

LaRea

From what I've read, the magnesium anodes can make the paint on the aluminum bubble and flake off.  "Catastrophe" is not the word I'd choose to describe it, but if it happens, you'll need to repaint the drives.  It's maintenance work you could maybe avoid or postpone by changing the anodes.  

MDboater

Put vinegar on it and if it doesn't bubble it's aluminum.  If it bubbles then file off some shavings and try to light them on fire (away from the boat and anything flammable as magnesium burns hot).  If it burns fairly easily then it's magnesium,  if not it's zinc.  

Do you keep your boat on a trailer or in the water?  

pault1216

Thanks MD. Great test and good to know. Boat is kept on a trailer except for vacation (14 continuous days in salt water). I will be testing it this morning.  -Paul

MDboater

@pault1216: Did you ever determine your anode material?  My money was on zinc.

pault1216

Reasonably sure it is magnesium. Sorry. I got the zincs off of Amazon cheap. Not worth the risk even though I have had mag anodes in salt for 4 weeks and noticed no damage. Better safe than sorry. Maybe I will find a home in the keys this month and get my $$ worth.  -Paul

Michael T

Normally magnesium in salt water is quick death to your boat's metals. A short period of time is a guess. But, even if it only abrades your drives' paint that is not good. How fast and bad will depend on nearby boats' shore power and cathode systems and galvanic isolators. It may not be an issue but it is a bit risky.