Wierd corrosion on 9.9 leg.

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catalacmarc
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Wierd corrosion on 9.9 leg.

Postby catalacmarc » Wed Aug 31, 2016 10:13 am

Friends, one of 9.9s on newly purchased 36 has a divot corroded out of the back of the leg on the sloped area just below tthe base for the cover. It appears to have been partially filled with compound and that compound mayde exacerbating the corrosion. It actually foams some. Could it be some sort of acidic polyester resin filler, would the"liquid aluminum" fillers do this in salt water? Would like to save the motor which seems to run well.

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Re: Wierd corrosion on 9.9 leg.

Postby duetto » Wed Aug 31, 2016 1:18 pm

years ago we had a 9.9 long shaft with the exact problem you describe. i took the head off and found serious corrosion in the upper part of the leg. we chose to replace with a new part.

if you choose to fix i would recommend grinding away all corrosion and then filling with marinetex, assuming holes aren't huge. i've had very good luck repairing outboard castings with it.
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Re: Wierd corrosion on 9.9 leg.

Postby catalacmarc » Thu Sep 01, 2016 3:09 am

Here is a photo of the divot out of the back of the motor leg. You can see the aluminum foaming up? Never seen corrosion like this before. I am going to knock out the filler and clean up the hole to see if that decreases the corrosion.
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divot.jpg

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Re: Wierd corrosion on 9.9 leg.

Postby amytom » Thu Sep 01, 2016 7:03 pm

We had one with corrosion similar to that but didn't catch it until a big chunk of metal was completely missing. Exhaust noise was our first clue.
My advice is to start setting money aside for a new engine. You might be able to milk out a few more months but the days are probably numbered.
Before the funeral make sure the organ donor card is honored. Still lots of good parts left.

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Re: Wierd corrosion on 9.9 leg.

Postby thinwater » Sun Sep 11, 2016 1:27 pm

I would strongly suspect stray current corrosion.

* Are the outboards often left down while plugged in? Do they lift 100% (at least 3") clear of the water?
* Good anodes on the engines? However, if there is stray current, this is not enough.
* Could the phases be reversed?
* Good galvanic isolator?
* Are these engines all from the same source? Please list source (I wonder if there is a wiring harness fault), since these appear to be replacement engines.

This is probably NOT just the action of saltwater. Check voltage (AC and DC) from engine shell to water when fully raised out of the water.
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Re: Wierd corrosion on 9.9 leg.

Postby catalacmarc » Sun Sep 25, 2016 6:37 pm

Friends, excavated the filler, cleaned it up, the space behind it was packed with salt which I dug out as best I could. Flushed it with fresh water. Then ground the opening and indented it. Also drilled holes like mouse ears to lock the new filler. Used a piece of thin fiberglass as a backing for devcon aluminum putty. Also have been debugging the electrical system. Last step is installing a galvanic isolator. We'll see how long it lasts. And it was salt not calcium cloride packing the space.

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Re: Wierd corrosion on 9.9 leg.

Postby amytom » Fri Sep 30, 2016 3:37 pm

How often do you freshwater rinse the engines?

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Re: Wierd corrosion on 9.9 leg.

Postby catalacmarc » Fri Sep 30, 2016 11:23 pm

Live in Seattle and moor inside the Government Locks in fresh water. Everytime we come home we run in fresh. We only bought her about a month ago so until recently 9she has been kept in salt. Locking is time consuming but the fresh water is great.

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Re: Wierd corrosion on 9.9 leg.

Postby amytom » Sat Oct 01, 2016 9:43 pm

You might consider looking at the other engine too. Maybe cycle a vinager/water mix for a while to clear the buildups. The extra heat will kill those engines.

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Re: Wierd corrosion on 9.9 leg.

Postby catalacmarc » Sun Oct 02, 2016 2:09 am

Overheating is an interesting issue here. When I had he boat surveyed neither motor was pumping water through the telltale. Both according to the surveyor's infrared thermometer were operating within range. Only a fraction of the cooling water leaves the telltale.First thing I did was get a compressor (Thanks Goodwill) and blow out the cooling system. That got the telltale working. The material in the repaired engine appeared to be salt. When dug out, it was solid, is dissolved freely in water so I don't think that vinegar will help that much. The problem in the "good engine" is that if it is as packed with salt, the dissolving will be slow since it will have limited surface contact with the fresh water. Without the divot, the location of the build up is unreachable.

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Re: Wierd corrosion on 9.9 leg.

Postby thinwater » Sat Oct 08, 2016 3:52 pm

The "salt" is almost certainly a mix of calcium carbonate and magnesium carbonate, with a little calcium silicate and traces of borate and barium. This is called scale for clarity, not salt, although they are salts. Chloride salts of sodium and calcium are very water soluble and dissolve very quickly, as you know. (NaCl is table salt, and CaCl2 is used for drying, and absorbs water so fast it can dissolve in the water it absorbs).

Ca and MG are nearly saturated in seawater and crystallize easily if the temperature rises above a certain threshold. Fortunately, the solubility of Ca as carbonate is very pH dependent. Seawater is alkaline (pH about 8) already, so even small pH adjustments with weak acid can dissolve it without degrading the aluminum, so long as the pH stays above 4-5. Vinegar is relatively safe when diluted, with a pH of ~ 3.5-4 (2.6 undiluted), while CLR (1.4 undiluted) and muratic acid (very low) are more risky.

The best bet is to mechanically remove as much as you can, use a very weak vinegar solution to loosen the rest, and then pick some more. Repeat. Otherwise, the acid sits and eats the good Al.

For what it is worth, freshwater flushing is really not the solution, since the scale is not formed then. This is very nearly an old wives tale, but it is the only thing we can do. But it doesn't do much, I know from industrial experience. The actual cause, well known by the HD diesel generator community, is coolant temperatures over about 130F with seawater. Much different from AF cooled engines. This will make the scale form fast, and then as flow drops and temperatures climb, it accelerates. The actual cause is usually an obstruction, a weak water impeller, or over-warm Florida water. That is the REAL cause.

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Re: Wierd corrosion on 9.9 leg.

Postby catalacmarc » Mon Oct 17, 2016 5:43 pm

A question, would scale disolve in fresh water when disrupted? The stuff inside the exhaust housing appeared to disolve when I dug it out and put water on it. At the Yamaha dealership, their explanation for this was design flaw in the hollow exhaust housing. A large space did not drain when the engine stopped. The salt water would evaporate, this allows for the slow accumulation of salt. This imay not be the same as the material that can accumulate in water runs in the powerhead.

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Re: Wierd corrosion on 9.9 leg.

Postby thinwater » Tue Oct 18, 2016 11:25 pm

Possible, if some scale first blocked flow to the area.

I'd bet it's a bit of both.
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