steel hull corossion & zincs

Conachair

Conachair

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Just hauled the boat out after 10 months in the water, 33´long keel steel Ebbtide. Not too happy to find some paint blistering as I had the whole lot sandblasted and resprayed in portugal last spring. Also, some anodes had quite severe corrosion while others looked brand new. Worst were 2 next to blakes seacocks, which were electrically isolated from the hull when the boat went in, haven´t check them yet to see if this is still the case.

Questions: Is abnormal zinc wear normal on a steel hull? Hull was professionaly built and I´m assuming that they knew the right amount of zinc required, but could be wrong there.
Why might the paint be blistering? blisters are very small, 3mm maybe and not uniform, some small areas of hull are quite bad large parts seem fine. The yard seemed to do a good job, sandblasted to bare metal then went over the whole lot with big blowtorch then acetone before first coat of primer. (interprotect). Could it be electrical? Only earth point is the engine with all circuits using 2 wires. Shore power is stirling batt charger which says it´s electrically isolated. Read a good guide to tracking down LX leaks somewhere but can´t find the link now. So any pointers/ideas gratefully accepted, or checks to confirm LX wiring is not the cause.
TIA
 
Abnormal anode wear is not normal on steel boats if correctly bonded.Are you connected to mains at all times? I never leave my steel boat connected when not on board.Do you moor next to steel piling?as I have found that this can cause abnormal wear on anodes.My bow thruster anode is near to steel piling when moored and wears fast.I am thinking of dangling an old anode,earthed to handrail, into the water at that point.Since fitting a galvanic isolator wear has been reduced.The prob. is that nearby boats can be creating stray currents.and a galvanic isolator should stop this.As a matter of interest some while ago my prop started to dezinc( showed a pink tinge) when I put a milliammeter from prop shaft to engine earth I got a high reading.The only way this could happen was the oil on the g/box bearing was insulating the circuit.I made a brush holder from a black and decker drill and fixed so the brush ran on the g/box coupling and a wire taken to engine earth.This stopped the prob.My first steel boat used to lose large areas of paint when lifted out at end of season,the bare areas covered in a white deposit,(magnesium),I later found that magnesium not suitable for paint finishes as too reactive, fitted aluminium and no probs.Looks like you could have a paint prob. have you opened one of the blisters? I,m sure another forum member with experience of paint finishes will be able to help.
 
By definition, abnormal anode wear cant be normal! /forums/images/graemlins/grin.gif Except in the most abnormal of circumstances of course.

The anodes to worry most about are those that "look like new". They are not doing their job of protecting things. Is there a connection between them and the hull and fittings? If not, they will not work.

The bubbles in the paint will be a paint problem and its best to talk to a paint company about the cure. Or try "passion for paint" on the web - the guy concerned is a consultant on paint and osmosis problems and is very helpful. Either way, patchy blisters sounds very much like an application problem to me. What on earth were they doing with a blowtorch?
 
Re: steel hull corossion & zincs

The blisters sound like a paint prob Paddy, my Roberts 44, (I built), never had paint probs in 6 yrs. Except, on the rudder, which was not blasted and had mill scale on it, oddly, in Tortola, I worked for a chap who had an ebbtide with paint problems, coincidence! I would just treat the localised areas (are the blisters down to the bare steel, or between coats, solvent entrapment?)

My hull annodesx8, +2 on the rudder, as per 'DUFFS' recommendation, worked evenly, welded studs and just 1 common ground welded to the hull for the 24v system.

The only concerns I experienced were from ally boat owners, who were paranoid about not berthing next to a 'tin' boat, and I don,t blame them! Bill.

(pm if I can help/suggest, I know it,s a worry)
 
It sounds like you have a lot of anodes on your boat. Paint blistering is a problem if you have too many zincs on a steel boat. If your paint job is good you should only need 2 or maybe 3 - one near the keel, one on the prop shaft and perhaps one on the rudder. Ours is a 43ft steel yacht and has just two anodes. Too much zinc will cause small blisters between 1cm and 3cm across and if you clean them off down to the steel you will see a very small brown/black dot in the centre of the bare area. If you are under zinced the blister will have more of an overall rusty bubbling look underneath.

The best way to check for the correct amount of zinc is with a multi-meter and zinc chloride anode hung over the side of the boat and connected to one terminal of the meter. You can buy these electrodes complete with cable for a few tens of pounds. The negative on the meter goes to a good earth and the electrode goes in the other socket on the meter. Check the voltage away from the marina berth and after you have been in the water for a few days (if you have been on the hard) and you should get a reading of around -0.8v (-800mV). If it is around 600mV then the zincs are not properly bonded. If it is up to 900mV or more then there is too much zinc and you will blister the paint.

A quick way to check for stray currents (on all boats - not just steel) is to connect the +ve lead of the voltmeter to the boats bonding system (or hull of a steel boat). A bonded shroud or stay will work; other wise, connect to the engine negative terminal making sure that the electrical connection is clean and sound. Connect the -ve lead to a large zinc anode , and lower the zinc into the water. Note the voltage reading at the component. Repeat the measurement near all the underwater metal masses (by moving the suspended anode around the boat). If the boats zinc are doing their job, the readings should all be less than 0.2 volt. Readings of 0.5 volt to 0.8 volt indicate a complete lack of zinc (or poor connection between anodes and hull/bonding system). Readings in excess of 0.8 volt indicate a voltage field surrounding the boat.

It sounds to me as though you have a stray current problem but it is worth checking the voltage away from the marina to make sure you are not over zinced.

Cheers
Peter
 
Blisters in 2pack paints can be caused by too heavy an aplication or too much solvent to thin it. The solvents usually used xylene and toluene are very volatile and dry fast. If the surface dries before underlying solvents have migrated through then they become trapped underneath and in a few months they form the blisters
 
Most interested in your reply PohoPetch - its the first time I've seen practical data for use on yachts. Couple of questions though, if you would be kind enough to answer.

I dont understand the mechanism by which the paint blisters. If the boat is coated with a decent epoxy there should be a non porous impervious barrier between the steel of the hull and the water. So how does anything get under the paint to cause the blistering?

Second question. I can understand your point about the voltage being low / poor anode contact but not the case in which it goes high. There is a max voltage defined by the electrochemistry. Is this the 900mv? And in any case how can you have too much of a good thing?

Genuine questions - you've posted something here that I've never seen before on the forums so its interesteing to know more of the background
 
[ QUOTE ]
It sounds like you have a lot of anodes on your boat. .... Too much zinc will cause small blisters between 1cm and 3cm across and if you clean them off down to the steel you will see a very small brown/black dot in the centre of the bare area.

[/ QUOTE ]
That's what I found! In addition the bronz heads output seacock is corroding, quite a lot of green powder on it which might explain the wear on the anodes in that area. ALthough it's showing 1megohm between the seacock and the hull, the input shows about 3megohms.
anodes.jpg

So I think course of action will be the take out the seacock and rebed it, finsh touching up and leave some anodes off then keep an eye on the others. Thanks for very informative post, Peter, I shall play around with a multimeter when i get back in the water, Though finding a zinc chloride might be tricky in the canaries. Thanks everyone.
 
[ QUOTE ]
a multi-meter and zinc chloride anode ............. You can buy these electrodes complete with cable for a few tens of pounds

[/ QUOTE ] You you mind explaining what you mean by a "zinc chloride anode" Also where can you buy them? All Google found was your own post and a description of a zinc carbon dry cell.

If you had said a silver/silver chloride electrode or a calomel electrode I would have known what you meant.
 
seacocks

Right, I'm looking at the seacocks today. Bronze blakes. Heads output is showing some greed dust on the body and a slight green tinge on other parts of the unit, like the flush handle (it's a baby blake). Heads in is showing very slight touches of green but not much. The other 2 (seawater for the galley and engine water in are prety much clean, tiny touch of green but hardly any. All show between 1.5 - 3 megohms between the mounting studs and the seacocks themselves. The worst zincs are close to the seacocks
Could stray current cause this wear on the zincs?
There was a slight seawater drip on the worst one which I didn't notice for quite a while and there a patch below the seacock where the hull has rusted a touch; would that do it?
I'm a bit lothe to take the worst one off now as it seems isolated and well bed in and will clean up easy enough. HELP!
thanks
 
Hi Vics - you are right of course - I meant silver chloride not zinc chloride - typing too fast and too many zincs on Conachairs boat! I bought my electrode from West Marine while passing through the USA. It is in their catalog at US$89.99 (approx £45). Must be a supplier in the UK.
 
We bought our steel yacht in 2004 in New Zealand - it had been launched in 2000. On the first haul-out we noticed these small blisters - typically 20 to 30 over an area of a square metre or so in 3 or 4 areas of the hull. There a loads of steel boats in NZ as they are popular for the pacific island runs as they are more reef proof than GRP, so the guys in the yards know a lot. The haul out operator who also sells zincs took one look and said "too many zincs" - his father had had a similar design and had the same problem. I ground back the blisters to bare steel, applied a very low viscosity epoxy sealer coat, epoxy primer, filler and faired, more primer and them anti foul, and removed one zinc. The next year at a different yard hauled out again and there were a few new blisters but not nearly as many. A marine electrician happened to be on the boat next door and took a look and said "too many zincs" and after I had ground the rest back and fixed them, removed another zinc and got back in the water he did the silver chloride electrode test for me and the voltage was around 850mV which he said was about right. Now 3 years later and 8 months up in the pacific islands last season and we have had no more blisters.

There are at least two bad effects from over-zincing. First, the surface of the anode gets a crust and becomes ineffective so you ultimately can end up with less protection, secondly there is apparently some sort of "reverse-cell" that gets setup and generates hydrogen at the steel surface which causes the paint blister. Quite how this happens with a good epoxy coating I don't know.

Note that the voltage to indicate over-zincing is different for GRP and wooden boats - more than 700mV is too high for GRP whereas it is 900mV for steel.

I don't pretend to understand all the details completely of how it happens (and steel boat corrosion seems to have many potential causes and be very complex) but these guys were working from experience I guess and I'm convinced they were right as it fixed my problem. I should mention that our yacht is not kept in a marina so I have no experience with stray current corrosion and this was certainly not the cause in our case.

Here is a really good detailed article on metal boat corrosion from the "Metal Boat Quarterly" http://www.kastenmarine.com/mbqCref.pdf

Here is another good link: http://www.geocities.com/bill_dietrich/BoatElectrical.html#ElectricalCorrosion

Cheers
Peter
 
Re: seacocks

Hi there conachair,
I also have a steel ebbtide 33. She has two large zinc anodes, one eiter side near the stern, and a small one fitted to the shaft also small ones fitted to the rudder pintles. They all last several years between replacement.
 
Re: steel hull corossion & zincs

Those anodes all look like what I'd expect on grp boats after 10 months in a marina - in fact I've seen a lot worse. The one at bottom right looks like it isn't connected - I'd expect some erosion.

But I don't know enough about steel boats to say anything else /forums/images/graemlins/smile.gif
 
Re: steel hull corossion & zincs

According to the Metal Boat article above (page six) your anodes are working just fine
 
Too many anodes on a steel boat can cause cathodic disbondment in the coating. As someone said hydrogen forms at the steel / coating interface causing the paint to blister. When blisters are cut away the steel is shiny underneath with no corrosion, but that changes very quickly as oxygen and moisture get to the steel.
 
When you say too many anodes do you mean in terms of total Kgs of anodes or in terms of surface ares of anodes protecting a specific area of the hull.
 
[ QUOTE ]
total Kgs of anodes or in terms of surface ares of anodes protecting a specific area of the hull

[/ QUOTE ] It'll be the ratio of surface areas that's important Roger. I have no idea where you'll find any guidance though.

Splitting a given mass of zinc up into a large number of small anodes will increase the surface area of that mass but of course anode shape is just as big a factor. For a given mass a hemispherical anode will have the smallest surface area. A big broad flat anode (if such a beast existed) would maximise the area. (and minimise its lifespan!)
 
Thanks Vic

But life depends on the mass so what i am thinking of is getting anodes with the correct surface area but the maximun mass for longest life.

I have followed the normal McDuff recommendations of 3 on hull 1 on fin keel each side and one each side of rudder and skeg and have gon for max mass for life but this may be too big as per above discussions (over protection)

So if anyone has any guide on the area of anode to area of hull that would be very nice.
 
Try the M G Duff website.

I have 14 anodes on my hull (and none on the rudder). She was built for the French Navy. When I bought her I used a specialist steel vessel surveyor (who had worked for M G Duff). He thought she "might" be overprotected.

Apart from a mains earth problem I had when moored alongside a small ship (which stripped alot of the paint off) :0( I have had no problems. I replace 3 or 4 anodes a season, and swap them round to even up the consumption. Some do develop a white crust (same location every time). It is obviously much better that they are consumed - you know that they are working; however, I wouldn't be able to judge what excessive consumption was, but overall I'm OK with 4/14=30% per annum.

It's good to have a voltage measurement process and baseline figures though (thanks), at least I can then monitor changes if any.

One bizarre thing I was told though, was that anodes should be able to 'see' each other ie any anode should have line of sight to one other anode. I can't see the logic of this.
 
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