Galvanic Corrosion

[kodacachers]

Hi All,
Now to start, I know I should have addressed this issue when I first spotted it but I didn't. My outdrive is suffering from rather sudden corrosion. I notices a spot last fall, but didn't think anything of it (see first sentence). But this year its pretty bad. The drive is 4 years old. Had new anodes at that time and I replaced them last year. This is a trailered boat, rarely in the water overnight. Used for both fresh and saltwater (about 50/50). I checked for wires dangling in the bilge--none. I had a second battery installed don't know if the wiring there could cause something.
I'll wind up taking it to the shop, but curious what your thoughts are on 1) why its happening and 2) once I identify the problem, how to repair the drive?
Thanks.

[4n6fisher]

I would guess that stray current, not galvanic corrosion (which is typically a connection between dissimilar metals) is the root cause, but it is hard to tell exactly from the picture. I could not get a good look from the photo.
The only way to test for that would be with the boat in the water and a marine electrician. If you have a good multimeter and some know how you might be able to check yourself, although there are a few tricks to do it correctly. I only know since I work with a few people that specialize in boat corrosion and impressed current protection systems, but do not know exactly myself. I may be able to get a quick procedure if you want to try yourself.
Could have also been initiated from stray current if it was in a marina (only takes a short amount of time depending on currents), then just progressing once the bare aluminum is exposed. Anodes are best locally and do not fully protect if there are coating breaches away from them as they need a direct conductive path. The white stuff is the aluminum oxides.
And just because there were no wires hanging in the bilge does not mean something is not grounded or properly connected.

How did the anodes look, normal wastage or heavily pitted?

For repair you will want to remove all corrosion product, probably wire brush first, then a mild chemical treatment (naval jelly) rinsed well. After that you should be able to prime and repaint.

[4n6fisher]

Add a couple more close up image or send me a PM for further details.

[kodacachers]

Thank you for the information! I'll take some better pics. Any suggestions on someone who I can take this to for a full eval? I'm wondering if my current boat guy did something wrong.

[Larry3215]

Its easy to screw something up when installing new gear or re-wiring. Its also easy for a connection - usually a ground - to get loose or corroded and cause this type of problem. Bilge pumps and especially float switches are notorious for causing this type of issue.

Like was said above, even a few hours near anther boat that has issues in a marina or bad wring in the marina itself can screw you up big time.

You will need to replace all your zincs/anodes for sure, but I agree - you need a pro to track down the source of the problem. They use something called a Reference Electrode to test for stray currents, bad grounding, bonding etc.

You need to find a pro to do the testing - not just some random electrician.

As far as repairing the outdrive, you will need to take it to a shop and have it checked. The visible damage may not be all there is. Internals like bearings etc may also be damaged. Hopefully not.....

[Sideburns]

The second battery being recently added is the most likely root cause in my opinion. Grounds are complicated, and easy to cause problems like this when introducing another battery. (doesn't mean the mechanic is at fault, as larry mentioned - a ground could have been simply disturbed) But I don't know why the corrosion would be so isolated to one side of that corroded outdrive flange, unless there are some electronics that mount outboard of that flange. If the outdrive was separated there recently, I would guess that an inappropriate sealant could have been used on the flange and or fasteners that might interfere with good electrical grounding between the parts. Just my ideas.... good luck.

[kodacachers]

Thanks all. This is an aftermarket drive so I suppose it could have been an install issue. Here's some more pics of what I'm dealing with. Now trying to find a decent pro as Larry suggests.

[Larry3215]

It looks like there is a removable zinc or anode on the hydrolic cylinder, but it doesnt look worn much at all. Makes me wonder if its installed/bonded properly or maybe not the correct one or correct type. There should also be another one on the small fin above the prop or near there plus maybe some others. How do they look?

Its looking to me like that one at least isnt doing the job its supposed to be doing. You may have more than one issue - stray currents from some source AND an outdrive that isnt properly protected.

Id check the manual and find all the zincs/anodes that are on that outdrive and check them all. Then, if they look good, start asking why they are not doing the job.

[4n6fisher]

Make sure the surface is clean underneath the anodes or they will not make proper contact, limiting effectiveness. Also if there is any white scale build up wire brush it off to expose clean metal, the oxides are not very conductive. Probably not an issue since you replaced the anodes a year ago, but some I have seen some that are painted over, that will not work.

Some of this corrosion could just be from environmental exposure after the coating was breached, as the anodes only work when the boat is submerged. Aluminum will corrode just exposed to the atmosphere. Maybe a just bad paint job that just started peeling exposing the aluminum, that may explain the corrosion around the seams.
One other thought is that once the coating was breached salts may have become trapped under the coating and were not removed while rinsing, they would reactivate and cause corrosion anytime the surface gets wet (rain).

[Bodofish]

Looking at where it started ....... galvanic or poor grounds or what ever, I'm going to suggest that the finish on the parts has lost it's integrity for some reason, little nick or scratch when it was being assembled or not prepped properly at the factory. I'd surely get all the parts separated and it taken down to bare metal, sand all the oxide off and have it powder coated or shot with zinc chromate primer with in 8 hours to start with. You then have enough time to think about what type of epoxy you're going to shoot on it. For any marine environment equipment, an extremely hard undercoat and a somewhat softer top coat. 8 hours seems to be the magic number for any alu alloys getting too much oxidation for any sort of paint to stick properly. If you can't do it within 8 hours start over when you can.

Grounds, grounds, grounds, make sure you have a hell stout bus running from one end of the boat to the other to ground everything too. Boat looks glass, yes? If it's not, for any metal hull boats make sure the bus is isolated from hull and nothing, I mean nothing is grounded to the hull. And as already has been mentioned good anodes with the proper goop in between.

[zen leecher aka Bill W]

you didn't say if your boat was fiberglass or aluminum. I had a "hot" boat, aluminum, and really had a problem with galvanic corrosion. It was two unterminated ends up by the drivers side that were less than an inch apart. That "juice" was enough to cause corrosion. Until we figured out the problem that was a fish catching boat in saltwater.

[kodacachers]

Its a fiberglass boat. I appreciate all the feedback. I'm going to replace the anodes, check the wiring and take it to the shop and will report back what the issue is. I'm just surprised it came on so quickly for a trailered boat that's in the water maybe 30 days a year.

[Bodofish]

Actually a boat that's not stored in the water can corrode faster than one stored in the water. The corrosion starts in the water and goes to town when it gets out in the air.

[kodacachers]

Hi All,
I appreciate all the input and thought I'd give you a report. I took the boat in and the diagnosis was paint failure on the drive rather that corrosion causing paint to bubble. There is the one spot of clear corrosion, but their thought is that it started once the paint came off. A sand/prime/paint and it should be good as new. After hearing this I looked and others have had similar problems with SEI drives (should have googled that first).
Thanks again.

[Bodofish]

Hi All,
I appreciate all the input and thought I'd give you a report. I took the boat in and the diagnosis was paint failure on the drive rather that corrosion causing paint to bubble. There is the one spot of clear corrosion, but their thought is that it started once the paint came off. A sand/prime/paint and it should be good as new. After hearing this I looked and others have had similar problems with SEI drives (should have googled that first).
Thanks again.

Clean to raw metal., zinc chromate primer, then epoxy primer (hard) finish with epoxy paint (soft).
It'll be better than new.

[4n6fisher]

Kodacacher,
Thanks for the update, just heard back from one of my ABYC contacts (was on vacation) and he confirmed that those outdrives have notoriously bad paint jobs. They just do not take all the care and necessary steps to coat aluminum. Follows Bodofish's advice and should be better than new. Glad that is an easier repair than some odd stray current that could have affected internal components as well.