Steel hull pitting

[Mollymawk]

We have a fairly major problem with pitting of the steel hull plating below the waterline (see photos). It's a good few years since we've been able to find anywhere to haul out. This problem started to become obvious about 2 years ago, after some 6 years since our last haulout. (At that time we dried out against a quay on several sequential tides and cleaned and filled all the pits with epoxy, and though the pitting has increased since, most of the pits we repaired then haven't got worse.)

We can't really work out what is causing our problem. It certainly seems to be electrolysis, judging by the nature of the pits. They seem to start with a bubble under the paint – but I'm not sure if it's all the layers of paint separating from the steel, or just the top layers of Coppercoat (epoxy) separating from the layers of epoxy paint below, since this is certainly happening over some areas.
Then the pit seems to gradually get deeper and deeper. However, not all of the pits have a rusty surface. Many of them are shiny metal. Not steel colour though (and bare steel should rust), so I'm wondering if this is a coating of zinc built up by the electrolytic reaction, either coming from the anodes or from our zinc epoxy base coating.

Some areas are much worse than others. Basically the problem gets worse as you go aft – at the bow there is almost nothing – and it's also worse under the lower chines and near the surface than in between. In particular, though, the rudder is far worse than the rest of the hull, and actually ended up with several holes (it has thinner plating, too). It's also worse near the toilet outlet (which is a stainless pipe welded into the hull, with a stainless seacock).

Several people here have told us we don't have enough anodes (we have 8, plus the shaft anode - none on the rudder, which may explain why it is worse than the rest of the hull). And we have been getting through anodes quite fast (they are half gone within a year).

So we seem to have an electrolysis problem, but I'm not sure what is likely to be causing it. I'd appreciate any advice from any experts or people with similar experience.

Our paint system consists of a coat of zinc-rich epoxy, two epoxy build coats, and then Coppercoat (epoxy resin impregnated with copper powder).
Coppercoat insist that the copper causes no electrolysis problems since each particle is surrounded by epoxy and thus electrically isolated. The zinc has worked very well (above the water at least), you can get a small chip in the paint and it doesn't rust for a long time due to being protected by the surrounding zinc, just like galvanising. All the paint is now 17 years old, except the top layer of Coppercoat (which is on top of the older, original layer).

We never go into marinas, so no stray current issues from other boats/docks. Any problems must be our own…

We also have some quite bad inside rust in a few small areas, under the water tanks. Not pitting, just areas of 'normal' surface rust. Interestingly, at least one bad area of pitting outside exactly coincides with a bad area of inside rust. Is this just coincidence, or could the rust on one side of the plate cause or accelerate the electrolysis on the other side?

[tkeithlu]

Hey. I've got a 44' steel trawler. I use 14 large aluminum plates distributed about the hull, plus a zinc "baseball" around the prop shaft because I can't find aluminum shaft anodes. My prop shaft is bonded to the rest of the electrical system with a brush (I have a spare if you need one). I have no corrosion, period, below the waterline, even in places that all the paint has been scrapped off and bare metal is against seawater. You don't have enough anodes, and you may be using zinc where aluminum is best for steel hulls. When you upgrade your anodes purchase a silver corrosion reference electrode (the anode dealers sell them for $100-$125) and keep your hull more than one volt from the reference. That will tell you when you need to be replacing anodes. I keep two sets, wire brushing the surfaces of the set that comes off for use the next time. Some are on their third year of use. Note here that the goal is maximum aluminum surface area bonded to steel. The irony is that while nothing under water rusts, I'm forever chasing rust above the water line. Good luck with it.

[micah719]

Have you looked at your electrical system?

[tkeithlu]

Micah has a good point. If your hull is being used as the negative return to your battery like a car's frame is used, all bets are off. While the hull must be electrically bonded to the batteries (zero ohms between battery negative and hull), it must not be used as an electrical route - it must not carry any current at all. A fine but damned important distinction. If your ammeter shows anything between your disconnected battery ground and your hull......

[Mollymawk]

Thanks for the replies tkeithlu and micah.

tkeithlu, your reply is very interesting; I have never come across aluminium anodes – only zinc for salt water and magnesium for fresh (I must add that prior to first noticing the problems we had spent at least 6 months in fresh water, still with our zinc anodes. I wonder if we should have considered some temporary magnesium).
What exactly are the advantages of aluminium rather than zinc? I would have expected it to provide less protection, as a less noble metal. The prop shaft is not bonded. Interesting idea to do so with a brush. I fear that it may be hard to get yours to us in Chile! :-)

As for the electrical system – we haven't done much in the way of checking anything yet; I thought I'd fish for some ideas first. No, we certainly don't use the hull as a negative return. The batteries are not grounded to the hull directly; the only connection would be via the engine – which itself I guess is only connected by the ball bearings on the shaft (the same applies to the prop shaft – which is not connected directly to the gearbox; we have a belt drive). I know for a fact that there is an electrical connection there though, as in the past I have touched a live positive wire to bare metal and it sparks.

My understanding is that if you can achieve full isolation of the electrical system from the hull that's better than bonding it, but that otherwise it should be bonded. So would we be better trying to isolate it fully (perhaps with 2-pole switches on the alternator and starter), rather than bonding it? Or is my understanding of this wrong?

I'm curious about the reference electrode you refer to. How does this work – where are we measuring our >1V?

[Mollymawk]

Oh, someone here told us that our stainless water tanks should be isolated from the hull. At the moment they are bolted to (painted) steel tangs. He said he'd been told that the movement of the water inside the tanks creates an electric charge which can cause electrolysis. Is there any truth to this? I don't think our problem areas coincide with the tangs really.

[Pauls]

Movement of water in tanks does not create a charge.

A common problem with anodes mounted to metal hulls is that if they are only bolted to the hull (as is common) it is possible for the joint to corrode and have enough electrical resistance that the anodes no longer protect the hull.

On a steel hull you should use the zincs which are cast around a steel bar. When you bolt them on, use a welder to make a small spot weld between the bar and the hull mount. This guarantees good electrical contact. The spot weld can be easily broken by hand when you next need to change the anode.

[Jim Cate]

You might contact CF member "BoatPoker" on this question. He is well versed in such issues, and has been helpful to others in the past.

Jim

[hpeer]

First I would suggest you get a copy of Scott Fratchers online book. Metal Boat Maintenance or something like that. So folks don’t like his approach. It I think it’s a decent introduction.

Now some of my experience, I’m no expert, I’m just sharing observations as a guy with 2 Steel sailboats.

I note that whenever I have stainless NEAR but not welded to mild steel I will have moderate to sever rusting. It may not even be stainless just two different alloys of Steel. Our bowsprit ate out just where the anchor laid near it and no where else. Why? Damned if I know. I see this fairly regularly. I never see this kind of problem where I have a welded Ss to mild steel connection. I’ve now got a fair bit of Ss welded to mild steel.

I’ve got tons of conflicting, and heated, advice about using a zinc rich primer under the waterline. I’ve done it but don’t think it is mandatory.

Our big boat has 4 big zincs on the hull, and 2 smaller ones on the rudder. They last a few years. As near as I can tell she has the origional bottom paint and it seems to be holding up well. Our small boat has only 2 zincs. You sound to have a lot more zinc than I do.

The small boat had some similar pitting not near as bad as yours. I believe the 30+year old paint job was starting to fail. I blasted and recoated the hull starting with zinc rich epoxy only to have it develop osmotic blisters BEFORE launch. Long story short, and many hours of sanding later, I’m convienced the blistering was due to refinery smokestack contamination getting onto the bare metal. I’ve got it licked, I think, I hope.

As to boat electrical grounding..... our small boat zincs lasted forever. Then I changed the engine mounts, the old ones were VERY odd, and grounded the motor to the hull. The new mounts removed the ground, and the zincs were disappearing fast. I reestablished a hard ground to hull connection and that seems to have resolved the problem. I do have one additional ground, the antenna at the top of the mast. I’ll tolerate that.

I think your pictures indicate an electrolysis problem. Ive heard of problems from having too much zinc, but I don’t understand it. What I see on at least some of your pics looks like electrolysis, especially active where you have shinny metal. I think of that like looking at a grinder scar, where the metal has been recently gnawed out, that’s an active hot spot...I THINK.

My GUESS is you have at least one and maybe a number of interior wires and/or devices (light fixtures?) that are grounding out. Maybe someone installed lights with a single wire and relied on a hull connection for the ground?? Sounds silly and likely is. Maybe you have some wires that have cuts in the insulation, especially on the ground/return wire.

My most sincere condolences for you problem. What you face would eat at me like cancer. Best of luck.

[hpeer]

Let me offer some additional arguments I have heard.

That whole thing about the copper being covered in epoxy so it’s isolated? Well I’ve heard paint “experts” use that same argument against zinc rich epoxies.... the zinc is in a epoxy bath and thus isolated so it does no good. Maybe, but if that is true then there is no basis for your zinc primer.

And if that’s true for zinc it’s true for your coppercoat. It if it’s true for the Cooper it’s then how is the Cooper providing any antifouling protection? If it’s electrically isolated it’s probably isolated from the wildlife.

So let’s say it is NOT true. In that case you require a PERFECT barrier between the zinc and the copper If the barrier is in anyway compromised then you will have a battery between the zinc and the copper. The zinc will get eaten away. It at some point the Steel under the zinc will be exposed. And when it is the IT will be eaten away.

Now you say you filled old holes with some kind of epoxy, but new ones formed. Makes sense, you isolated the zinc/Steel at the active sites but then the copper attacked the next most vulnerable sites and created new pits.

I’m not positive that is what is occurring but IMHO it’s a good chance that is the problem.

BTW, fresh water is more likely to create an osmotic reaction (blister) than salt. Counterintuitive but true. You had the boat is fresh water for some years, that may have created some small blisters that went unnoticed but compromised the coatings. Then going into salt water it provided the electrolyte needed to start the battery action.

Now this is a pretty tale I tell. I’ve no way to know for sure what the problem is. I may be full of it, I’m just offering this theory as a potential to consider.

Again good luck and best wishes.

[Jim Cate]

More inexpert and anecdotal evidence:

Our timber boat has a steel shell fin keel. At time or purchase, the keel was sandblasted back to bare steel and two coats of Jotun zinc-epoxy mastic were sprayed on, followed by eoxy hi-build, fairing and bottom paint. All done by an "expert".

For the next 12 years, at annual slipping, we found small blisters in the paint on the keel, much as described by the OP. No pitting noted, but the blisters were clear down to bare metal. No rusting occurred until the boat sat dry for a few days, and then slight surface rust would appear in some of the blister areas. (These were small, from 1/8 inch to 1/2 inch in diameter). We would do various palliative things, including spot repair of the Zn, spot priming with epoxy, spot priming with Vinyguard (like Primocon) and similar approaches. None worked noticeably, and year by year the surface grew more and more moon-like, with craters galore. Yuck! Not happy were we!

A couple of years ago a chap who worked as a painter in a proper shipyard wandered by, took one look and said "you've got zinc epoxy paint under there, don't you?". When I said "yep", he gave a discourse which I can't quite reproduce, but the gist of it was that such mastics were very good above the waterline, but not so good below, and often exhibited blistering such as ours. No specific reason was presented, just historical observations.

This past August we slipped the boat, took all the coatings off the keel, down to bare again, and, on Minaret's advice, coated with Interprotect 2000 followed by hibuild and fairing followed by a vinyl base tie coat followed by antifouling. So far, underwater inspection has shown no blistering, but the time frame is short, and the water here in TAsmania is too damn cold for me to do much checking in the immediate future. Fingers are crossed!

All of the above are just one boat's experiences, but when the OP mentioned that he had zinc epoxy coatings, well, I hadda butt in with our tale.

Jim

[boatpoker]

First of all .... Do not take any advice from anyone who uses the term "electrolysis" as they clearly do not understand the issue. I suggest you read this first "Electrolysis Mythology".

You have either a galvanic issue or a stray current issue but I would not hazard a guess without seeing this one myself.

Now invest a few dollars in a silver silver chorlide half cell like this one which comes with a pretty good booklet that explains how to use it. If you already have a multimeter you are all set to track down your issue.... it ain't rocket surgery.

[Lepke]

On ships, tugs and commercial fishing steel boats, several zinc plates are fastened to the hull. A tug might have 10-20 large plate zincs, a battleship might have a hundred. Ships and fishing boats use zincs in ballast tanks and fish holds, too.
If they waste away fast you have a ground problem. If they can't find the problem, some people add more zincs.
The compact pitting may be from the steel alloy not fully mixed before the plate was formed. Finished steel has many other ingredients added like carbon, nickel, chrome, molybdenum that can be half of the steel by weight.
The spots where the pits occurred may have been a different alloy than the surrounding area. I sand blast and weld up the pits.
I've always used zinc, but mostly in salt water. I never noticed a problem from from sitting over the winter in fresh water. But if you're always in fresh water use aluminum or magnesium for a steel hull. Don't forget the engine zincs if you change over.

[boatpoker]

Forgot about this one ..... Corrosion Control Workbook.

I have a few minor quibbles with it (it's "silicon" not "silicone" bronze) but overall it's not bad and shows you how to plan a corrosion survey with that new silver/silver chloride half cell that you just ordered on-line

[hpeer]

Jim,

I’ve heard the same argument about zinc under water. And they can be compelling. Then again I had an Ameron rep come out and look at the boat and he told me that the USCG uses zinc epoxy underwater and that he recommended it. He had his own reasons. I find the divergence or opinion very frustrating.

ALL agree the top coatings must be maintained perfectly if you have zinc underneath.