Rapid corrosion on motor

profisher said:
Easy way to include metal thru hull fittings it to strip back an inch to 2 inches of insulation off that end of your bonding wire and then use a stainless hose clamp around the threaded stem of the fitting.....simply tighten down the clamp around the stem with the bare wire in between.
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A yup, that’s how mine are conected
 
Here are some articles:

Marine Grounding Systems | West Marine

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Pay attention to the illustration in the first article where it shows all the grounds connected together with seawater at ONE & only ONE point. If you connect to the neg bus which is connected to the engine, which is in the water, that will work as the negative cable from the battery to the engine is on large size & therefore well below the 1 ohm of resistance specified in both articles.
Resistance of seawater seems to be around .2 ohms.

IMO this aligns perfectly with what profisher states.
 
Jon said:
www.yachtsurvey.com

Corrosion on Boats: How to Identify & Correct Them - by David Pascoe

This essay is intended to give you a fundamental understanding of the causes and effects of corrosion, as well as how to identify problems and correct them before they become severely damaging.
www.yachtsurvey.com
Some good reads on this site too
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Awesome read, thanks. After reading this there a few things I need to look into on my boat now, but for most part I think I got it right
 
Still fighting this battle . Brought my prop into maple marine to show the guys as the pitting on the leading edge and in the exhaust port are concerning . After chatting we think it could be a cavitation problem and also the heat from the exhaust could be an issue. Previous owner installed a keel guard and this could be causing this to happen. I’m gonna remove that and gonna get the shop to go through the motor fuel system and such to see if it’s burning lean and producing some in wanted heat.
Guess I’ll be pulling the lower unit off again and rinse and repeat the sand down and fill and paint Job. Sure hope I can figure this thing out . Been chasing my tail for years thinking it’s electrical. Practically replaced all electrical components and wiring besides my 2 main +\- going up to the front.
 
Sea Chaser said:
Honda BF 90 outboard lower unit starting to rapidly corrode. Not sure what I’m doing wrong or what is going on? Rinsed , flushed cleaned after every use. Zincs all look good . Little bit corroded I think I’ll be changing them just to make sure. Any suggestions gladly accepted
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Are your sacrificial anodes(zincs) aluminum or zinc? My mercury dealer told me to use the aluminum anodes as the will work better, not last longer but work better
 
Once the corrosion is that far into the aluminum there is no stopping it, looks like poor quality when cast. I would start hunting for another case while it still holds gear oil.
 
turbomack said:
Once the corrosion is that far into the aluminum there is no stopping it, looks like poor quality when cast. I would start hunting for another case while it still holds gear oil.
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Been thinking that too. But really want some assurance that it won’t happen again before forking out thousands for a lower unit .
 
I may be wrong or misinformed, but when I bought an aluminum boat I did a ton of research on causes of aluminum corrosion because my boat was going to be in the chuck year round.

With regard to your corrosion issue, and please correct me if I'm wrong, given that your boat lives on a trailer and out of the water, whatever sacrificial anodes you have installed will be of use only when submerged. In other words, if the corrosion is advancing while the boat is on the trailer, it is not likely to be electrolysis causing your corrosion. Air is a poor conductor of electricity, while salt water is a pretty good conductor of electricity, hence the need for the sacrificial anodes.

Having said that, I would bet 2012 CAD Dollars to Honeys Doughnuts that your issue is caused by contamination of the aluminum at some point.
If a person uses a steel wire brush when
cleaning aluminum, iron molecules will be deposited into the aluminum causing an aggressive reaction between the two. Sanding disks and other surface preparation tools can be a common cause as well. When welding aluminum, the only suitable brush to be used is a stainless brush that is new or has never been used on anything but clean aluminum. Same goes for sanding disks, etc... The issue can also be caused by the composition of the filler or paint that was used to compete the repair, which may contain elements No Bueno with aluminum.


Some rules of thumb I consider accurate.

1) Electrolysis really only happens under water

2) Atmospheric corrosion (oxidation) can be eliminated by removal of oxygen, like applying a thin coat of fluid film when not in use. Also is normal on bare aluminum and stops once the surface is fully oxidized

3) Galvanic Corrosion (dissimilar metals) is the most common cause of aluminum corrosion, and the easiest form to create (and most difficult to stop) when repairing aluminum.

If I was to guess at the root cause (just a guess), I would guess the following happened:

At some point in the distant past the leg was repainted, perhaps because the paint had worn off in a few areas. The preparation before painting involved some sanding or wire brushing which introduced some foreign metal into the surface of the aluminum. Since then, you have been fighting a battle to stop the ongoing corrosion, without much success.

If this is in fact the cause of your corrosion problem, you will need a chemical solution to solve it. The leg will need to be media blasted, then acid dipped, then epoxy primed, then painted with paint specifically for aluminum outdrives. Aluminum begins oxidizing rapidly (like 1 hour) , so once the acid dip is complete, it will need to be primed quickly.

Note: Given the state of your leg, you can't really make it worse, so have at it and try to have fun learning the process.

Hope this is helpful.
 
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Sea Chaser said:
Been thinking that too. But really want some assurance that it won’t happen again before forking out thousands for a lower unit .
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Of the many brands of outboards I've owned only 2 have corroded like this
and they were both Honda's.
You see lot's of good ones, but sadly once corrosion starts it just keeps getting worse.
 
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