Galvanism/Electrolysis

[tonybarley]

Can anyone give me some help/advice on discovering the source of 12 vdc affecting the sterndrives on my Sealine 328?
The boat is currently on the hard having a number of replacement parts fitted to the drives. The legs, over the last seven months have been corroded by this internal current leak from the boat and the steel pile on the mooring.
There is no mains connected to the pontoon and there are no boats nearby.
I am proficient with an Avometer.

 

[Eygthene]

For the steel pile to be involved in the corrosion circuit for your outdrives, there would need to be an electrical connection (a wire?) joining the steel pile to a metal component on the boat. Instead, look for another immersed metal part of the boat which could be responsible (bronze seacock, stainless rudder shaft, stainless trimtabs?) The circuit would then be from the outdrive, through the water to this immersed metal and back to the outdrive via some internal metal path (anode bonding wire, control cables etc). If there is no internal metal path, the circuit will not be completed and the metal item can be eliminated. You can use the AVO on resistance range to measure the resistance of any suspected path, by placing one probe on a scraped place on the outdrive and the other probe on a scraped place on the suspected fitting. When you find the culprit, we will take it from there.

Otherwise, fit a pair of zinc anodes, one close to each sterndrive, and connect them together and to the sterndrives by internal wires.

 

[tonybarley]

Thanks John. Presumably, I can do the resistance test between the sterndrives and trim tabs etc whilst the boat is on the hardstanding and the elec\trics switched on. I will try this.

However, it was my understanding that if the pile is grounded(and it is, literally), a 12V or less supply could create an anodic effect between the sterngear (aluminium) and the pile (iron). Do I need to stand corrected?

 

[alan006]

Good evening,
If there is no conductive link between your boat and the pile then the only corrosion between the two is galvanic. By looking up the galvanic series we can see that between steel (pile) and your outdrives ( aluminum alloy) the potential difference is about 0.3 volts.
To understand what is happening in your case more info is needed.
a) What state are the leg anodes in?
b) How does this compare to past experience?
c) Can you check to see if their is a high resistance between any part of your outdrives and your skinfittings and trim tabs?
If you can check that I may be able to help.
d) What makes you think that the problem is caused by 12v DC supply?
I look forward to hearing more.
Alan

 

[tonybarley]

[ QUOTE ]
Good evening,
If there is no conductive link between your boat and the pile then the only corrosion between the two is galvanic. By looking up the galvanic series we can see that between steel (pile) and your outdrives ( aluminum alloy) the potential difference is about 0.3 volts.
To understand what is happening in your case more info is needed.
a) What state are the leg anodes in?
b) How does this compare to past experience?
c) Can you check to see if their is a high resistance between any part of your outdrives and your skinfittings and trim tabs?
If you can check that I may be able to help.
d) What makes you think that the problem is caused by 12v DC supply?
I look forward to hearing more.
Alan

[/ QUOTE ]

Hi Alan

Thanks for responding.

a) & b) The leg anodes are 80% eroded after 10 months in the water; I would normally expect about 50%

c) The boat is on the hardstanding at the moment awaiting the legs being repaired. Excuse my ignorance but can I perform this resistance test with the boat being 'dry'?

d) The marina has no electricity supply and there are no boats close by. I assume that the problem results from my boat's battery system. Three sets of batteries - one each to port and starboard engines and a double set (in parallel) to power the on board electrics.

Hope this helps.

Kind regards

Tony

 

[Eygthene]

You asked how you could use the AVOmeter to investigate whether the batteries were causing excessive corrosion of you sterndrive legs, or if not what was.

Using the 1 volt DC range of the AVO, with the boat ashore and the boat's batteries turned on, you can measure the voltage from one (scraped to bare metal) part to another. Measure from one leg to the other, measure from each leg to any underwater metal fitting (scraped to bare metal) - seacock, trimtab, rudder shaft ... etc. what have you. The voltage should be close to zero. If there is a voltage between any two items, write it down. This way we can probably eliminate the batteries from the list of suspects.

Now using the AVOmeter on the resistance range, with the boat batteries turned off, to avoid any possible stray currents confusing the resistance readings, measure the resistance between each leg and all the other items as above. Write down each reading and which items were involved.

If there is no internal connection you will get a high resistance- more than 100 ohms say. Even if there are two dissimilar metals, inherently capable of generating a galvanic voltage when both are immersed in seawater, there can be no significant current - to cause corrosion, unless there is a low resistance return path between them to conduct the current round the circuit. This is why I said before that the steel piles were unlikely to be involved.

Now we can study the results to decide where the source of the trouble is located.

If you get one or more reasonably high voltage readings in the first set of measurements, we can look to see what connections to the batteries are causing it.

If you get a low resistance in the second set of readings, we can see where the current's return path goes and put forward a solution.

Happy hunting.

I would not expect the problem to be due to batteries, or to other metal parts, but to the legs themselves and their anodes. So if these measurements are all OK, we should look at the legs themselves and their anodes. The large surface area of the leg needs to be painted with a good underwater paint system, to keep out the water and reduce the effective area for the corrosion currents, so that the small anodes can last longer. These anodes need to be free from paint and mounted with good metal to metal contact (perhaps through the fixing bolts) onto the leg. This way the zinc gets to form a cell with the surface of the leg, when immersed, but the current (and rate of corrosion of the zinc) is reduced because most of the surface of the leg is masked by the paint. The leg itself does not corrode, because in the cell formed, the zinc is sacrificed to keep the leg protected. As long as zinc remains, in electrical contact (ie low resistance path) with the leg, the zinc will be dissolved preferentially.

 

[alan006]

I think John has covered this very well and at this stage there is little more I can add. As he says as long as the legs are electrically bonded to the leg anodes and the leg anodes are not eaten away they should save the legs.
If you had a 12volt problem you would get very severe corrosion. This ( as John agrees) is not in my opinion likely to be the problem .
Good luck.

 

[tonybarley]

Thanks John - seems very logical and I will follow your advice over the next day or so down at the marina. I will post the results. Thanks again

 

[tonybarley]

Hi John

Sorry for the delay in responding; been away.

I have done the checks that you recommended.

Voltage whilst boat is ashore and batteries switched on:
Sterndrive to sterndrive - zero volts
left trimtab to sterndrives - 0.44 volts
right trimtab to sterndrives - 0.34 volts

Resistance measurements
sterndrive to sterndrive - 0 ohms
sterndrives to port trimtab - 5.3 megohms
sterndrives to starboard trimtab - 2.5 megohms

Does this set of readings provide a clue?

Regards

Tony

 

[jimmy_the_builder]

Is your bonding good between the upper and lower leg casings, and from the casings to the transom shields? I had a corrosion problem on one of my outdrives on the T37, and we traced it to a bonding issue between the upper casing and the transom shield. We added some simple external bonding between the two, and after a year in the water it was clear that this had solved the problem. (Both transom shields were internally bonded to a big transom anode).

Cheers
Jimmy

 

[Eygthene]

Hi Tony,


The measurements that you made show that the resistance is so high and the voltage so low, that there is no problem associated with nearby metal fittings (trimtabs), just the saildrives themselves. As you didn't report any measurements from seacocks or the rudder stock, I presume that these are not metallic underwater objects on your boat.

That leaves just the sterndrives as I said before. I expect they are made of aluminium alloy castings and probably have bronze propellors. The bronze/aluminium pair of metals sets up a significant voltage and the resistance from prop to sterndrive leg will almost certainly be quite low (near zero ohms). This drives a quite significant current which causes the aluminium to dissolve in the seawater.

The usual way to protect the aluminium leg is to connect it to an even more anodic metal like zinc. Then the zinc dissolves instead and protects the aluminium. Of course, the small zinc anodes on the legs may dissolve quite fast, so to help reduce the rate of disappearence of the zinc, we can reduce the effective surface area of the bronze and the aluminium, by covering these parts with a good layer of paint.

Alternatively, as Jimmy suggests, you can attach a much large anode to the hull close to each leg and connect the anodes together and to the legs with stout wires. Usually these cables connections are made inside the hull, from the anode mounting bolts to the saildrives and to each other. It would still be worth replacing the smaller anodes on the legs and using paint (not on the anodes) to slow things down as much as possible.

John

 

[halcyon]

[ QUOTE ]
left trimtab to sterndrives - 0.44 volts
right trimtab to sterndrives - 0.34 volts


[/ QUOTE ]

Which one had the negative probe on, i.e. leg or trim tab, out of interest.

Brian