Steel corrosion and stray currents

I recently inspected a potential boat purchase, an appx. 20 year old mild steel hull. Overall, the coatings seem to have been applied properly and have held up reasonably well, inside and out. The topcoat and possibly the barrier coat seems to be letting go in places, particularly along the chine and waterline in some spots. I think a sandblast and new coatings would be appropriate, but I am concerned about the cause of corrosion and whether stray currents, galvanism, or other electrical corrosion problems might be contributing and should be addressed first.

In the past, I have experimented with electro etching-steel knives using DC current in salt water. The process creates hydrogen and oxygen gas bubbles and a rusty color in the water.



I couldn't help but think of this when I saw these brownish bubbles along the chine where the paint is peeling/failing.





The bubbles were spotted on a day of still water. The next day, after a rain and some wind, no bubbles were present.

I have some knowledge of corrosion issues, but my practical experience is limited. I may be imagining a problem where there isn't one. From what I can tell, the white epoxy coating has some pitting, though maybe this is simply fairing compound that has degraded. The boat in question has a mixture of best practice and somewhat egregious electrical installations, including "zip wire"secured with washers and electrical installations (motors, batteries, wiring) in areas of high moisture and condensation. There seems to be a single negative bus for the DC system, but I am not certain how the electrical systems are bonded.

Has anyone here seen this before? Any advice appreciated.

 

Was the hull connected to shore power, f.e via a charger when you saw it?

 

Shore power being attached would be my question too. I'm thinking bubbles could be organic gases rising from the mud below?

I would imagine checking the difference in electrical potential between the hull and shorepower earth would give you an answer, as if there is enough power available to create that much gas I would imagine crew would be getting zaps from the boat.

Never having seen this before I'm guessing.

 

Yes, the hull was connected to shore power. According to the owner, the shore power system was professionally installed and included an isolation transformer, but unfortunately I did not see this present when I inspected the hull. It may have been tucked into the back of the panel behind some cabinets. Or it may not exist, but I will trust the owner's explanation until I learn otherwise.

From what I understand, AC isn't usually the cause of corrosion, but DC shared through the shore power ground between boats on the dock.

 

Usually DC is the culprit but from your description I doubt that is the problem. Has it been in saltwater or fresh? It makes a difference. I think the brownish water is just rust, It's a steel boat after all and steel rusts. If it is 20 years old and hasn't been repainted in all that time, it's time for a complete sandblast and recoating. When was the last time it was hauled and painted? If you still are concerned about a stray current corrosion, measure the current in the water around the boat. A surveyor should have the equipment to do that. Or you can do it yourself if you have a multimeter and a silver-silver-chloride Probe.

BTW some studies have shown that AC can cause stray current corrosion but it is rare. The real danger from AC in the water is shock, usually resulting in drowning.

 

Yes, AC corrosion is much less insidious than corrosion from DC, but it does exist.
At the microscopic level, tiny pits in the steel form their own little diode structures.
This causes a situation in which the AC cycle does not have EXACTLY EQUAL areas on each side of the sine wave.
A billion electrons flow one way, but a billion minus 1 flow the other way, (just for an example).
The waterline area? Much of that is the constant wet/dry cycle, (and heat/cool).
You must do whatever is necessary though to prevent any DC from getting into the steel.
I know, it's a frustrating job.
Stray electrons are devious Machiavellian creatures, they can't be allowed to roam around outside of the proscribed path.

 

@rangebowdrie you are saying that smal pits can form diodes, which necessarily raises the question can these semiconductors create bridge rectifiers and cause the steel to constantly conduct DC if stray AC is present? It takes little power to create electrolytic corrosion with direct current.

 

The difference between the areas of the two halves of the sine wave is the DC component.
It is miniscule compared to directly applied DC.
The DC applied to the steel from an outside DC source is the Goliath.
The DC applied from the difference in the sine wave areas of AC is but the ant on one of his sandals, (in a manner of speaking).
As has been said a million times "The first line of defense is an un-broken layer of paint".
What we've been talking about is corrosion that is caused by "outside" electricity, (stray current,) being connected to steel, (any metal really,) in an electrolyte.
If we lay a bronze bar against the steel shell, or bolt on a bronze fitting, and there is an electrolyte present, that is "internally" produced electron flow, we call that "galvanic corrosion".

 

I'd be looking for a big old DC charger like the old tombstone welder style ones Nampa used to sell. I've seen that pronounced electrolysis a couple times, both caused by bad chargers.

 

Those are about the worst kind of charger you could ever put on a boat.
Miserable devices, atrocious voltage regulation, no ability to match voltage to the charging curve of the batteries.
No ability to maintain a proper float voltage.
They're basically just "farm chargers" to get the tractor started.
Most of them have an internal circuit of what is called an "auto transformer", where there is a tap connecting the two windings.
Useless heavy junk on a boat.
What's going on in the bucket in the first picture is "electrolysis".
What's going on with that boat is "stray current corrosion".
They are NOT the same thing.
As a side note, a friend of mine is a court certified expert witness on maritime electrical corrosion and has taught classes on the subject at the CG academy.
Some of the worst cases he'd ever seen were on steel fish boats in Alaska.

 

I confirmed that there is an isolation transformer installed by professionals, along with the rest of the shore power system. Of course it may not be functioning correctly, but everything is speculation at the moment. My main question was whether the bubbles near the failing paint were evidence of some kind of increased corrosion activity or more benign.

The boat has been in cold salt water. Ike has suggested that it may be just normal rusting. Based on some of the wiring practices and the amount of moisture present in the hull, my suspicion is that it is being enhanced by onboard electrics, but I don't have real evidence to support that yet.

These were the instructions I received from a friend to check for stray currents:

"Use a DVM 0-2Volt scale. Take a small zinc anode – connect about 20ft of 18 gauge wire and place a banana plug on the other end (reference electrode). Connect to the common. Use about another 20’ of 18 gauge with an alligator clip which you will use to ground to the boat and connect to the positive.

Disconnect all electrics and put the zinc in the water at about 2ft depth. Starting at the bow measure the potential between the zinc and the boat along both the port and starboard sides. The potential difference should never be more than 200mV. You may also wish to do it at different depths."​

 

Stray current corrosion.

I’m not a steel boat guy, but can’t you simply test to see if the boat commons are continuous to the hull?

Both the DC and the AC systems are free floating which means their grounds cannot be grounded to the hull.

If you suspect current leakage; disconnect the boat shore power, and check the hull for continuity to the AC and then the DC ground. If either one is continuous; it is wrong. You only need to make sure your not insulated by paint. So check your probe continuity by verifying two points on the hull are both continuous and use one of those points for one side to the bus tests. This is like a 5 minute test max.

The next moves are much harder as you need to determine how grounding is occuring, but bilge pumps are always suspect, as well as a boat owner who skips wiring a common by using the hull which does happen. Checking mechanical gear like shafts and rudders probably also wise.

corrections are welcomed

The other test I believe is anode testing. A good next step..

I suppose you could check the hot side for continuity to the hull as well; odd as it would be…but use caution to disconnect all power sources first, including batteries (it should not be the case), so reluctant to even mention.. And, of course, if the commons already fail, why bother as that would be a dead short..and blow fuses, but on the off chance the first test has no failure..

 

Thanks Fallguy. From my understanding everything should be connected to a negative bus, which should be returned to the battery negative, then have one connection to ground at the engine or nearby frame. I did not have time to investigate this to the full extent, but I did find a negative bus that was connected back to the battery.

 

the grounding system needs to be free floating, not grounded to hull … free floating means grounds to itself, not through the vessel, cars ground thru frame, not boats

I welcome corrections.

@gonzo may have experience with older steel hulls as well as @bajansailor , but if you are grounding to the hull; then that is stray current corrosion and the system is wrong

Another good contributor is @Ike , who has authored papers on this subject.

I will post a link to an article as well, if I can find a good one.

 

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