Galvanic corrosion expert required to identify root cause......

[Dorset Dan]

I won't say too much of the detail on here, but I have surveyed a boat which has some galvanic corrosion issues on the starboardside in particular (affecting prop, prop nut and more importantly p bracket bolt integrity). Port side generally OK.

I have used a surveyor and engineer who both come very well recommended on this forum and they have duly identified the corrosion issue, and recommended in their reports that the root cause of the issue be found and rectified prior to purchase (noted as a possible issue with the electrical bonding). Galvanic corrosion was also noted on starboardside in a previous survey a few years ago, and starboardside prop was changed only 2.5yrs ago (now severely pitted).

Whilst it could be a marina / stray current rather than boat specific issue, the fact that it was reported in a previous survey makes me think it is the boat itself......

Any recommendations as to who to use for this to test both the systems on the boat as well as for any stray currents in the water? I am thinking that a marine electrician who is suitably qualified / experienced in galvanic corrosion issues? Or if not a marine electrician, then who? Preferably within sensible travelling distance of Poole, Dorset.

 

[burgundyben]

I lost a prop nut to galvanic action in two months. Duncan Saunders had a look and was set to refer to someone he knew that has a silver chloride electrode. So perhaps call him.

As it happens I found a failed bonding wire.

Would be worth getting this thread moved to PBO.

 

[superheat6k]

These are simple places to start a search for an issue ...

1 Is the bonding circuit intact - this means < 1 ohm between any underwater metalwork being affected, the anodes and internally to the engine block and IMHO the battery negative and AC shore power earth.

2 Check the polarity of your shore power lead Live & Neutral. Simple test plugs are available from EBAY for this, but don't leave them plugged in because if they use LEDs it can create a bypass path to the GI along the neutral line.

3 Do you have a Galvanic Isolator fitted

4 Do you have an indicator for shore power correct Live / Neutral polarity - if so is it an LED (bad) or Neon (good). Again an LED can create a GI bypass path along the shore power neutral.

5 With all your normal electrics turned off this normally means just the bilge pumps are still connected to the batteries. So do you have any 12v DC +VE leakage to the water ?

The test for this is simple. With everything that is normally off disconnect the + VE from the battery then measure the voltage between the bare terminal and lug just removed from it. If this is anything other than zero then you have a DC leakage somewhere. Find it and eliminate / remove it.

Start with any flappy bilge pump switches.

6 One side suffering more can also be caused by issues with stray current from surrounding submerged metalwork to that side of the boat.

In addition to the GI, a dangling anode can help with this. You can make one with some part worn anodes strapped to a stainless steel riser cable, in turn attached to the bonding circuit using copper wire, but do not connect the copper to the anodes or stainless riser cable beneath the water. Also don't forget to bring the dangler in when using the boat.


Be aware you may find more than one issue, so don't assume you have resolved the issue from finding one issue. Good luck - finding galvanic / electrolysis causes can be frustrating.

 

[Bandit]

I agree with superheat you may decide you want a marine electrician to do this for you but make sure they do a thorough job and not find one problem assume that is tit take their money and walk.

Boat out of the water, batteries off, shore power off check all connections every bit of under water metal to an anode.
Batteries on then check.
shore power on then check.
Normally using a silver anode in the seawater, In the water batteries off check, batteries on, shore power on. It could be the pontoon or a nearby boat or scrap metal on the seabed.
Manually check all anode wiring if green replace use tinned wire for this, check terminals for condition. check anode connection plates and check serrated washers are used, i saw major corrosion because the boat owner had used flat washers instead of serrated and corrosion caused a bad connection.

Using a good marine electrician puts a barrier between you and the vendor and it does not look like you are chipping the price.

Also check all skin fittings and valves for condition and valve action.

 

[Dorset Dan]

Thank you for the above responses, what a great forum this is to get advice.

Using a good marine electrician puts a barrier between you and the vendor and it does not look like you are chipping the price.

Agreed, I am not looking to chip the price, rather I want to get this issue (if it is indeed a boat issue rather than a marina issue) sorted prior to purchase. I have asked my local marine electrician (who I have previously used for straight forward on board electronics installs - raymarine, fusion, new plug sockets, etc) but I am awaiting his response as to whether this is something that he is comfortable to investigate and advise upon.

If in the meantime anyone has any marine electrician recommendations along South Coast it would be good to have a few options to contact.

Since my post yesterday, a prop repair place that I sent pictures to have commented that:

"Propeller nuts seem to be manufactured in HTB1 and not AB2 as the propellers are. { this is why the nuts are pink/de-zincing }".

However, I have no idea whether this is associated to the root cause of the issue or if this is just associated to the severe pitting of the prop (whereas it is the fact that there were 2 x sheared p-bracket bolts and the cause thereof that is the big worry that I need to resolve the root cause of). Galvanic corrision and electrolysis seems a very complex area with lots of possible causes and possible inter-linking issues - I just need to find someone to identify and fix these issues..........

 

[gjgm]

S.heat..surely your test only identifies a DC leak, not that the leak is into the water and causing corrosion. For example , a forgotten switch #on# would give a reading, or any minor circuit shortage...or am I misunderstanding something?

 

[Dorset Dan]

1 Is the bonding circuit intact - this means < 1 ohm between any underwater metalwork being affected, the anodes and internally to the engine block and IMHO the battery negative and AC shore power earth.

My engineer had a multimeter between prop and hull anode which read 5.8 ohms on both portside and starboardside (with the corrosion issues much more apparrant on starboardside).

 

[jrudge]

So you / he / someone needs to remove, clean, Vaseline replace each binding wire and re test.

It should be zero ohms or there abouts ( as in .1)

I have never checked mine. I don’t have a corrosion issue but next time it is out of the water it would be a good idea

 

[burgundyben]

My engineer had a multimeter between prop and hull anode which read 5.8 ohms on both portside and starboardside (with the corrosion issues much more apparrant on starboardside).

I never settle for more than 0.3ohm...and my meter reads 0.3ohm if i touch the probes...so essentially I never settle for more than my meter can measure. Essentially nothing.

Seems perfectly obvious to me that you have a bonding issue.

 

[superheat6k]

S.heat..surely your test only identifies a DC leak, not that the leak is into the water and causing corrosion. For example , a forgotten switch #on# would give a reading, or any minor circuit shortage...or am I misunderstanding something?

My list was one of suggested places to start searching, not a definitive answer to the OPs problem.

If this particular test fails i.e. there is a voltage, then the next thing is to establish why, which yes, could include eliminating a switch left on.

I learnt this one from Nigel Calder - Boatowners Mechanical & Electrical Manual and I discovered my flap switch had a DC leak into the bilge water, then via the immersed metalwork into the outside water (note I had also found - reversed polarity shorepower lead; LED being used for Earth / Neutral indicator (bridging the GI); poor bonding to one anode (and I did not discover these at one session, although I would have a better chance now as I now do not give up at first discovery).

Bonding circuits have a tough time as they live in a damp and forgotten about environment. These should be part of the annual list of checks when out of the water - as Ben says, nil resistance is almost too much.

 

[gjgm]

My list was one of suggested places to start searching, not a definitive answer to the OPs problem.

If this particular test fails i.e. there is a voltage, then the next thing is to establish why, which yes, could include eliminating a switch left on.

I learnt this one from Nigel Calder - Boatowners Mechanical & Electrical Manual and I discovered my flap switch had a DC leak into the bilge water, then via the immersed metalwork into the outside water (note I had also found - reversed polarity shorepower lead; LED being used for Earth / Neutral indicator (bridging the GI); poor bonding to one anode (and I did not discover these at one session, although I would have a better chance now as I now do not give up at first discovery).

Bonding circuits have a tough time as they live in a damp and forgotten about environment. These should be part of the annual list of checks when out of the water - as Ben says, nil resistance is almost too much.

Funnily enough, I was reading the same book, last night !