Help w/ Outdrive Corrosion and Improving Anode Performance

[Horigan]

My '95 OMC Cobra SX (Volvo SX-M) outdrive has many layers of black Trilux 33 anti-fouling paint that is chipping and flaking off due to corrosion. I had installed a new Volvo aluminum anode kit during winterization and it's working fine on the transom mount, but I'm having difficulty getting a good electrical bond to the outdrive. When I noticed the increased corrosion on the drive I re-installed the anode with new stainless star washers under the bolts and sanded the area under the bolt heads. I also applied some dielectric grease on the sanded surface under the star washer to help mitigate corrosion at this interface. Is this really the only electrical bond area between the outdrive and the anode, at the bolt heads? Should I also be sanding the perimeter of the outdrive where the rounded surface of the anode sits on the outdrive? Overall it seems like a small electrical bond area. I'm seeng more corrosion on the anode since the rework.

I boat 50/50 fresh and salt water, with 75% of the outings in fresh for a couple of hours, and the remaining time in salt for one or two days at a time. I rinse at a fresh water ramp after every salt outing. My research says aluminum anodes are what to use in combined fresh and salt water applications. Should I consider a zinc or magnesium anode?

My plan to address the corrosion and flaking is to use a small Dremel sanding wheel to sand off all the loose paint and corrosion, prime with green zinc primer, and put a couple of coats of Trilux 33 on sanded areas the drive. I did all this last fall, but the corrosion and old flaking paint has exposed more areas than last fall due to the poor anode performance during the first half of the season.

Any other thoughts on how to improve the anode performance on my drive? Should I be evaluating how well the bonding straps are attached to the various components on the drive? They seem pretty good with a cursory look.

Thanks in advance.

 

[GA_Boater]

Dielectric grease is not conductive, so using it is counter productive.

With the multiple coats of paint, it might be worth using a tap to chase the threads in the outdrive for the anode bolts. And do check all bonding wiring and connections, not just the outdrive. The anodes aren't painted over are they?

 

[Horigan]

No paint on the anode. I'll check the bonding straps.

Unfortunately, the bolts thread into the anode, not the drive. So the only electrical bond contact to the drive is between the bolt head/star washer and the drive. I'm referring to the round anode below. Are all you guys with SX-M drives just bolting these on and getting good anode performance?

Dielectric grease won't interfere with mechanical metal to metal contact between the bolt/star washer and outdrive. It should help prevent corrosion at the bolt interface though to help maintain the electrical bond. I could apply a different corrosion inhibiting compound to help maintain the bond.

 

[RCSConstruction]

Is the boat docked and/or around shore powered boats when in salt?
My buddies outdrive was trashed (well not trashed, but corroded badly) at a local marina over a period of 9 months.
Stray currents in the water did it, that's what he was told anyway.

 

[Thalasso]

Aluminum anodes is what you want