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Old 08-08-2003, 18:05   #1
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Corroding Engine mounts

The engine mounts for my Yanmar engine are corroding. The mounts, bolts, and nuts appear to be made of steel (they are rusting). The engine compartment is realtively dry (I say relatively, because I've found traces of water but not enough to ever pump out). The mounts, and bolts are bolted to aluminum stringers on my boat (an aluminum hulled boat). Does anyone have an idea as to why the mounts, bolts, and nuts are corroding, how to remove the current corrosion, and finally how to prevent further the corrosion? Any help would be greatly appreciated.

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Old 09-08-2003, 12:28   #2
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Electrically Induced Corrosion


There are two general types of electrically induced corrosion - Galvanic & Electrolytic.

Galvanic (Cathodic)Corrosion is self-generating, and occurs when two dissimilar metals are electrically connected, and immersed in an electrolyte (water).
In this instance, an electrochemical ncell is formed, and a voltage is developed between the two metals (or alloys), with water acting as the connecting electrolyte.
The more active metal (anodic) will have a higher electrical potential than the less active (cathodic) metal, and will deteriorate. Most Steel is generally higher on the Galvanic Series than most Aluminum Alloys.

Electrolytic or Stray Current corrosion refers to the erosion resulting from an “unintended” current flow, and may be DC induced, AC induced, or Telluric (a natural phenomenon caused by geomagnetic disturbances).
This unintended current flow typically occurs when an AC or DC electrical source is connected through immersed metals to ground. Damaged wiring & terminations, and improper grounding & bonding techniques are among the commonest causes of electrolytic errosion.

It appears that you are on the right track, in asking “how to remove the current corrosion”.
I agree with your implied assessment that you may be suffering from electrically induced electrolytic corrosion (given the “dry” conditions).

Nonetheless, I suggest that you first eliminate galvanic or cathodic corrosion, as you do have dissimilar metals (steel mounts & aluminum stringer/hull).

1. Generally speaking, Steel fittings should be electrically & mechanically isolated from Aluminum hulls. Are your mounts in electrical contact with the aluminum stringers?
A general “rule of thumb” for galvanically compatible metals (USE /W CAUTION)
1. Magnesium & it’s alloys.
2. Cadmium, Zinc, Aluminum and their alloys.
3. Iron & Steel, Lead, Tin, and their alloys (NOT Stainless Steel).
4. Copper, Chromium, Nickle, Titanium and their alloys (including Stainless & Graphite).

2. Are there salt deposits bridging the steel to aluminum interface? This would indicate previously wet conditions - and would in itself form an electrical “couple”.

3. Is wood used as an isolator (twixt St. & Al.), and (if so) is the wood “white”, “punky”, or “foamy”?

4. Is the steel mount (& engine block) effectively “bonded” to the hull?

5. Is the steel mount painted? Metalic base paints can cause galvanic corrosion.

And more ...

OK - so none of the above apply, and the corrosion is electrically induced.

6. The Aluminum hull is the main grounding conductor and ground plate (electrode). Any metal part NOT connected to the hull (isolated as above) must have a bonding conductor attached to the hull (see 1 & 4 above).

7. Do you have a “Reverse Polarity” indicator or alarm? I recommend that reverse polarity indicators be either: VERY high resistance circuits, or TEMPORARILY connected via a momentary switch.

8. Are you connected to shore power? If so, install a galvanic isolator, and check the marina’s wiring for AC polarity, and ground continuity - and/or disconnect from shore power except as absolutely necessary.

9. Examine your wiring (AC & DC) for frayed insulation, chaffe, thermal injury (scorching etc), and/or poor terminations. All wiring should be run high & dry, supported, and protected against mechanical or thermal damage. All terminations should be tight, isolated (insulated) from each other & ground, and sealed against moisture.

CAUTION: DISCONNECT AC SHORE POWER, and INVERTERS (if installed) prior to performing the following tests!

10. You can check for DC POTENTIAL with a Voltmeter as follows:
-a- Turn OFF all AC & DC Loads, at their local switches.
-b- Turn ON all DC Breakers, except where the breaker directly controls a device .
DO NOT PROCEED unless you fully understand the differences between a & b above!
-c- Set the meter to 25VDC range.
-d- Connect the Positive Voltmeter test lead to the Positive Battery Terminal, and the
Negative lead to the hull.
-f- Connect the Negative Lead to the Negative Battery Terminal, and the Positive Lead to the Hull.
If no voltage either test (d & f) OK - If Voltage present proceed.
-g- Set the meter to “OHMS”
-g- Disconnect & Isolate the Positive & Negative Battery cables from the Battery
-h- Connect Ohmmeter between Positive Cable & Hull, then between Negative Cable & Hull.
If infinite resistance (open circuit) OK - If continuity present (lower resistance) proceed.
-i- Test each circuit individually.

11. You can check for stray DC CURRENT with an Ammeter (20A - 10mA) as follows:
-a- Turn off all AC &DC loads.
-b- Remove the Positive Battery Cable from the Battery post.
-c- Connect Positive Lead of Ammeter to the Positive Battery Post
-d- Connect Negative Lead to the (removed) Pos. Battery Cable.
-e- Read current (starting @ highest Amps scale - preferably 20A, then switch to lower scales as practical)
-f- No current should flow, even at the lowest setting! (OK - perhaps as much as 1 milliamp = 0.001 A).
-g- If more than 1 mA, each load circuit must be individually tested.

I hesitate describing the similar testing procedures for AC systems, as they could be very dangerous! I recommend that anyone not EXPERT in electrical testing seek professional assistance.

Note re: Copper Wire & Aluminum Hulls
EXTREME caution should be taken to insure that NO copper strands, strippings or other remnants are allowed to come in contact with Aluminum!

You’ve raised a very interesting and complex subject, about which many books and learned papers have been written. I hope the forgoing helps in some measure, but caution that a forum , such as this, cannot purport to provide comprehensive advice.

E. & O. E.
Gord May

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Old 09-08-2003, 14:37   #3
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Thanks for the info


Thank you for the lengthy reply. I didn't mean to imply that the mounts were suffering from from electrically induced electrolytic corrosion. As I'm not on the boat right now, I have few answers to the questions you posed. I have printed your response and will check each item, and report my findings later. I can say that I'm not connected to shore power in a marina. I may very well have developed a some stray current, when last on the boat I did notice 0.1A draw with all AC/DC loads off. I don't remember noticing it before so it must be new. I double check upon my next visit to the boat (next weekend). Testing each individual load circuit sounds like a daunting task.

Any ideas as to how to remove the corrosion? The working space is extremely limited and it may be difficult to attack with a wire brush, also I fear a wire brush will spray iron oxide (rust) all over the engine bilge, thus creating more problems. Aside from isolating the mounts, bolts, and nuts from the Al stringers should I cover them with vaseline, or some other product to keep the moist air away?

Thanks again Gordon

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Old 10-08-2003, 09:40   #4
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Now I understand, you were referring to “existing” corrosion, not necessarily electrically (current) induced - oops.

Let me continue by cautioning - I am NOT an Aluminum boat expert!

It’s good that your AC shore power is not connected.
How about neighboring boats - are they connected? Often, nearby (100's of feet away) boats can cause severe problems, especially to Aluminum boats (& Outdrives).

You’ve noticed a 100mA (o.1 A) draw /w all loads ‘off’. This is at or beyond the maximum I’d normally consider permissible - and I’d recommend (at some time) you undertake the ‘daunting’ task of testing each circuit individually.

You didn’t indicate how severe the corrosion was. If the rust has compromised the engine mounts, nut or bolts, or the ‘rubber’ vibration isolation is worn - you’ll want to replace them. This would be the time to ‘isolate’ the steel mounts from the aluminum stringers (Zinc Chromate, Polysulfide, Nylon, Teflon, HDPE, etc).

Vaseline is not very useful as a permanent sealant. Commercial/Industrial Electricians use anti-oxidant products on Copper-Aluminum cable joints:
Widely available as: Ideal “No-Alox”, Burndy “Penetrox, GB “Ox-Guard”, etc.

Your fear of spreading contamination around the hull is valid (wire brushing). A Phosphoric Acid product , such as ‘Ospho’, can stabilize rust - but (I think) you’d want to be absolutely assured that you don’t get any on the aluminum (I’m on ‘thin’ ice here). Do some independent research on ‘compatibility’, prior to painting the steel (or aluminum) with anything.

Have you noticed any pitting, or Aluminum Oxide on the stringers or proximate hull etc?
The determination of which of two coupled dissimilar metals erodes is determined by their position on the galvanic scale - and their relative mass or surface area (area ratio effect). The mounts being much smaller, I’d expect them to erode (*) - and be VERY concerned at any damage to the Aluminum.
(*) It’s difficult to determine your galvanic couple, without knowing the exact composition of the Aluminum alloy, and Steel - and even the, perhaps, beyond my competence.

Look forward to your further report.

PS: There is LOTS of information on the web. Here's one site /w some good basic info':
Seaguard Ltd-Corrosion Basics Tutorial

Regards and good luck!
Gord May
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Old 27-09-2015, 22:07   #5
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Re: Corroding Engine mounts

I had the same rusty motor mounts on my survey. I purchased rust bullet on Amazon for $12 for 1/4 pint. It neutralizes the rust. That was five years ago and still no rust returning.
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Old 28-09-2015, 09:52   #6
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Re: Corroding Engine mounts

Steel rusts on boats unless it's covered with thick layers of paint. The mounts for the Yanmar need more paint to stop them from rusting and their convoluted design makes it near impossible to hit all the nooks and crannies. Mine were a mess when I changed them out for aftermarket alloy mounts.

Big problem I now have is leaking at the exhaust manifold connection to exhaust pipe. Normal rocking of the engine when running seems to make leakage over the stbd rear mount inevitable. It also leaks salt water onto the steel external oil line on the GM series of engines. Yanmar now makes that line in a copper alloy so it doesn't rust out. The steel one was almost rusted through when changed it on my 3GM30F.
Peter O.
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Old 28-09-2015, 23:13   #7
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Re: Corroding Engine mounts

Mine rusted within a couple of years of being installed in the boat and eventually the rust worked it's way under the elastomers. For the very slight difference in costs Yanmar could make the damned things out of stainless or at least hot dip galvanize them before they glued the elastomers on.

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