How long can stages be left underwater?

[Tracy]

Why would the aluminum corrode before the aluminum?

It is a different grade and alloy.
Zinc isn't useful on aluminum in fresh water, it is a cheaper option in salt water as it corrodes slower.
Aluminum is the most common alloy used for aluminum. It is a decent option in fresh water and works great in brackish. For somebody paying a diver to changes their anodes, it is the most common option with aluminum boats.
Mariloy or Mil-Spec A-24779(SH) aluminum alloy offers the greatest protection for the aluminum as it is the softest and corrodes the fastest. It was specifically made to protect 5000 and 6000 series aluminum boat hulls in fresh water. I normally get about a year out of them.
In the context of a scuba tank left in fresh water and installed with a hose clamp, the Mariloy will offer the greatest protection for the tank and typically last years.
Magnesium is another good option for aluminum in fresh water. It would be eaten monthly in saltwater.

For a bit of context. My wife is a metallurgical engineer and we own multiple aluminum hulled boats kept in fresh water. A lot of research was done on how to best protect them as they exceed the cost of our home many many times over.

***edited to correct a couple items. My wife read it and pointed out that I am an idiot.

 

[fsardone]

It is a different grade and alloy.
Zinc isn't useful on aluminum in fresh water, it is a cheaper option in salt water as it corrodes slower.
Aluminum is the most common alloy used for aluminum. It is a decent option in fresh water and works great in brackish. For somebody paying a diver to changes their anodes, it is the most common option with aluminum boats.
Mariloy or Mil-Spec A-24779(SH) aluminum alloy offers the greatest protection for the aluminum as it is the softest and corrodes the fastest. It was specifically made to protect 5000 and 6000 series aluminum boat hulls in fresh water. I normally get about a year out of them.
In the context of a scuba tank left in fresh water and installed with a hose clamp, the Mariloy will offer the greatest protection for the tank and typically last years.
Magnesium is another good option for aluminum in fresh water. It would be eaten monthly in saltwater.

For a bit of context. My wife is a metallurgical engineer and we own multiple aluminum hulled boats kept in fresh water. A lot of research was done on how to best protect them as they exceed the cost of our home many many times over.

***edited to correct a couple items. My wife read it and pointed out that I am an idiot.

Any good reading about the issue available on the net?
I’d like to understand better … I mostly have experience in protecting outboard engine in salt water or stainless pipes undergroud (cathodic electrical protection).
Thanks

 

[Tracy]

Any good reading about the issue available on the net?
I’d like to understand better … I mostly have experience in protecting outboard engine in salt water or stainless pipes undergroud (cathodic electrical protection).
Thanks

Possibly, I don't honestly know. I'm not the smart one in the relationship. I just fix things when they break. My outboards have magnesium anodes. Again, because that is what she told me to run.
I will ask her if she knows of any.