dealing with a corroded aluminum plate

[Donald]

I have what they call a porpoise fin for my Mercruiser that is somewhat corroded, but still structurally fine. I think maybe it was the sacrificial metal for my outdrive.

So I have it off and would now like to clean it up and then paint it. I have wire brushes but no bead or sand blaster.

Anyone want to blast it for me if I mail it to them?

Other ideas?

 

[Brad_C]

Originally Posted By: Donald
I have what they call a porpoise fin for my Mercruiser that is somewhat corroded, but still structurally fine. I think maybe it was the sacrificial metal for my outdrive.


It is a sacrificial anode, so if you are planning on cleaning it up, painting it and putting it back don't do that. You'll knobble most of its effectiveness and probably lose expensive parts instead.

If you have replaced it and are just painting it as a trophy, then start with a slurry of lemon juice and table salt and a toothbrush to remove most of the corrosion. You can work up to stronger stuff if it's required, but it rarely is.

 

[Donald]

Originally Posted By: Brad_C
Originally Posted By: Donald
I have what they call a porpoise fin for my Mercruiser that is somewhat corroded, but still structurally fine. I think maybe it was the sacrificial metal for my outdrive.


It is a sacrificial anode, so if you are planning on cleaning it up, painting it and putting it back don't do that. You'll knobble most of its effectiveness and probably lose expensive parts instead.

If you have replaced it and are just painting it as a trophy, then start with a slurry of lemon juice and table salt and a toothbrush to remove most of the corrosion. You can work up to stronger stuff if it's required, but it rarely is.


No its a piece of aluminum you bolt on the outdrive to (hopefully) make it easier to get on a plane. They call it a porpoise fin, its sold under a few names. Its not made to be a sacrificial anode (I have those). I was joking when I commented about it being the sacrificial metal.

 

[andyd]

How cool is this? A guy in upstate NY getting advice from another guy in Perth, AUS! My simple mind marvels

 

[mahansm]

Go ahead and clean it up but do not paint or anodize. In fresh water the corrosion should be almost nonexistant. In brackish or salt water you will get a very slow overall loss of material. If the boat is kept in the water the outdrive should be left down. Raising the outdrive lifts the zinc (sacrificial anode) out of the water and also allows very high salt concentrations in crevices as the water evaporates.

If you paint/anodize the fin you will find greatly accelerated corrosion (pitting) wherever there is a break in the coating.
You already have a metal that is cathodic with respect to the aluminum (the stainless steel fasteners securing it to the existing ventilation plate).

 

[Donald]

Originally Posted By: mahansm
Go ahead and clean it up but do not paint or anodize. In fresh water the corrosion should be almost nonexistant. In brackish or salt water you will get a very slow overall loss of material. If the boat is kept in the water the outdrive should be left down. Raising the outdrive lifts the zinc (sacrificial anode) out of the water and also allows very high salt concentrations in crevices as the water evaporates.

If you paint/anodize the fin you will find greatly accelerated corrosion (pitting) wherever there is a break in the coating.
You already have a metal that is cathodic with respect to the aluminum (the stainless steel fasteners securing it to the existing ventilation plate).


The rest of the aluminum outdrive gets painted, why not the aluminum fin? I assume it was painted when new.

 

[danthaman1980]

Originally Posted By: Donald
I have what they call a porpoise fin for my Mercruiser that is somewhat corroded, but still structurally fine. I think maybe it was the sacrificial metal for my outdrive.

So I have it off and would now like to clean it up and then paint it. I have wire brushes but no bead or sand blaster.

Anyone want to blast it for me if I mail it to them?

Other ideas?


No need for the porpoise fin if you get rid of the non-needed boat ; )

http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3820476/Thoughts_on_this_safe#Post3820476

 

[Michael_P]

That fin is a sacrificial anode and a fin to counteract rotational inertia for non power steering boats. If your boat had power steering, it would be a flush fitting flat plate (no fin) bolted to the underside of the anti cavitation plate. They are zinc for saltwater and magnesium for fresh water. Don't paint it.
BTW: I have never heard of it referred to as a porpoise fin.