Hot zinc spray?

Discussion in 'Metal Boat Building' started by Wynand N, Mar 27, 2005.

  1. Wynand N
    Joined: Oct 2004
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    Wynand N Retired Steelboatbuilder

    To prevent corrosion (electrolyses) on boats in seawater we uses sacrificial anodes made from zinc. This raises a question.

    Although I have never hot zinc sprayed a steel hull or any experiences of it, how good is this.
    Assuming you have a bad case of unnoticed electrolyses present, and/or your hull sit in a area with stray currents - and the paintwork underwater damaged to the zinc coat, what will happens?

    The way I see it the whole hull becomes an anode. Perhaps better to stay with epoxy paints :?:
     
  2. lazeyjack

    lazeyjack Guest

    not really On alloy boats now we use copper antifoul, as do all the major superyacht builders this we achieve by using ZECA primer first over the bare alloy, they won't say whats in it but buy the feel its zinc rich, then 300 microns epoxy then the copper,
    will be same for steel , call ZECA OR ALTEX PAINTS OR DEVOE
     
  3. jehardiman
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    jehardiman Senior Member

    The military (US) has used flame/plasma sprayed aluminum as a topside coating . It works well (25+ years coverage) but color choices are limited, it needs to be sealed and it is a large initial expense. I know of no cases of sprayed zinc being used below the waterline.
     
  4. lazeyjack

    lazeyjack Guest

    all steel boats are sprayed 75microns dfm below waterline, in this neck of the woods, enough to cover blast profile before high build epoxys are laid over, the painr is cold sprayed
     
  5. sailrjim
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    sailrjim Junior Member

    My cutter is mild steel with two coats flame sprayed zinc all over, .004 inches (.1mm) /coat . Fourteen years in service and good performance. Know that the total coating system is important consideration. Below WL, the zinc layer is coated with a barrier coat: coal tar vinyl paint that is filled with aluminium powder (probably no longer available as it is fifteen years old). Over this is a 60% CuO ablative antifouling paint. The only problem over many years seems to be some localized reactions between the copper in the antifoul and the aluminium powder in the barrier coat. This produces small holes in the antifoul film and some attack of the aluminium powder, but does not seem to attack or to breach the zinc. I repair it at haul out by adding some barrier coat (I still have some).

    I have had some minor spots on the topsides where there was likely some contamination of the zinc coat before painting, causing the paint to bubble; very localized. The only place for serious concern is on the leading edge of the rudder where the prop wash has beaten the coating off to steel. So, when I haul out every third year, I must repair that area using a premium urethane primer with aluminium powder for anodic protection.

    A local company is offering hot zinc spray service for props, but I have my own scheem so cannot comment on their experience.

    A flame sprayed surface may have a very slight washboard appearance due to a narrow spray pattern.

    Because of the unavailability of the vinyl paints, I may eventually need to strip the paints to zinc and recoat, probably with epoxies and acrylics or urethanes.

    When doing the original zinc coating job, we blasted no more than could be coated within four hours of blasting. then move to new area and overlap the coating edges.

    J
     
  6. MikeJohns
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    MikeJohns Senior Member

    It's galvanic corrosion Wynand (Electrolytic needs an external current source). Zn or Al spray on topsides deck and interior is a good option although a bit expensive. Cold Galv (zinc rich) paints work very well too.
    Don't spray below the waterline, the zinc will deplete where you get any abrasion or small holes in the paint and you will suffer from paint disbondment. Far better to keep your anodes in good order.
    The waterline is a bit trickier and often the hardest area to protect well, a good high build epoxy and a decent boot top to the antifouling paint will protect this area, but it could also be zinced. I usually specify Zn rich primer then several coats of epoxy above WL, below just the epoxy and anti-foul.

    Cheers
     
  7. Wynand N
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    Wynand N Retired Steelboatbuilder

    Mike,

    Thanks for explaining the galvanic corrosion.
    My question, what if you have a stray current in or around the boat and the Zn is exposed to seawater?
     
  8. sailrjim
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    sailrjim Junior Member

    Electrolysis and hot zinc spray coatings

    Wynand,

    I suggest you check the library for a basic text on the subjects of electrolysis and galvanic cells. The explanations are available in many texts, including several available through chandlers and are intended specifically for the boating public. Check West Marine as an example.

    The so called stray currents around a boat is a bit of misstatement. There is no such thing. In order for a current to flow, there must be a complete circuit. If you are not connected to shore power, you will be immune to electrolysis, unless it is created by you and the electrical system in your vessel or you are connected in a complete electrical circuit with another vessel; for example, if there was a conductor connecting the elctrical systems aboard the two vessels, or the hulls, and there was a different potential and the hull/zinc(s) was in the circuit too. I think the term comes from the concept that current is flowing through an unintended (and unknown) circuit; ie. has gone astray. If that was also your understanding, disregard first sentence of paragraph. But the fact remains, no circuit, no current flowing, no electrolysis. Electrolysis is an impressed phenomenon. Galvanic cells are a natural phenomenon in that no external voltage is applied.

    One way to avoid electrolysis is by never leaving the vessel connected to shore power without using an isolation transformer in the supply side of the line. You will find info on this subject in the same places as I suggested above.

    I avoid galvanic corrosion by having a floating two wire DC system within the vessel. I use an electrical isolator in the prop shaft. This causes problems with some equipment that requires a large ground plane, but my steel hull can provide that. And, of course then I would need to monitor for "stray currents". But, if my equipment is in good condition, there should be none.

    Several times, I have tried zincs on my vessel; Mil-spec, small Campbell disc style, etc. The results were either no better, or actually worse galvanic problems. I do use a tear drop zinc on the rudder and a shaft zinc on the prop shaft. Both get wasted, but mostly the prop shaft zinc because of the bronze prop and fitting nuts.

    Probably, your greatest concern should be from galvanic corrosion, especially if you are tied near a vessel that has a large mass of low nobility metal immersed and may not have adequate zincs (think large commercial barge).

    Specific to your concern regarding the zinc coating: If the paint film is breeched to the coating, the zinc becomes an anode.

    An electrical engineer on the forum could explain in greater detail, but I think you will get the basic idea.

    Regarding the use of epoxy systems below the WL, as opposed to zinc coatings: That is common in my area, south Chesapeake Bay, and I think it is a good idea too. If I were to build another steel boat, I would consider the all epoxy systems because of simplicity, ease of application without special equipment, availability, and low VOC content. Of course, then individual anodes are required.

    Jim
     
  9. mareng
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    mareng Junior Member

    Wynand,
    There is a book that might be helpful:

    Metal Corrosion in Boats, (second edition) Sheridan House
    by Nigel Warren

    Have a look, it might be helpful.
     
  10. D'ARTOIS
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    D'ARTOIS Senior Member

    Hi Wynand, Hoe gaat'ie?

    You better keep with epoxy: for a few reasons: hot zinc spraying is a well known
    method of laying up an anti-corrosion skin on steel, practised in Holland for a long time.
    Environmental law forbids such methods now and also the use of tar-epoxies here are illegal. In fact, yachbuilding in Holland becomes each year more difficult and the governmental officials know how to drive up costs so that over some years yachtbuilding might be extinguished as it already happenend to our commercial ship building.

    I would prefer a zinc-sprayed hull above an epoxy treated one, provided the zinc-spraying can be done professionally. The majority of Dutch shipbuilders will agree with me, as I do believe.

    According to the corrosion issue, there are about that as many opinions as are men. I do not believe in galvanic corrosion on steel boats that much. Provided of course that you are not provoking galvanic corrosion by connecting electrical wires straight on the steel hull as many did and are still doing. (Using the hull as earth or negative). But you know that.

    Holland is a steel building nation, or used to be so, although at present the zinc spraying has been replaced by the application of epoxy-based paints.

    If you have no experience with zinc spraying, let it be! Keep yourself at what you are used to do and you can't go wrong on that.

    Nevertheless, it could be a good idea - as Mike says (and Mike is like you and me an afficionado of steel boats) to study something about Electrolytic Galvanysis. Prevention is better than repair!

    (PS: lost your mail address - you have still mine? Mail me yours)

    Regs
    Brien
     
  11. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hi Brien,

    Nice to hear from you again. Check your email.

    I have no experience in hot zinc spray and in my part of the woods there is no pro doing it. Epoxy seems to be the answer.
    In the past I sprayed all my hull inside and under the waterline outside with coaltar epoxy over the epoxy primer. Approx thickness about 140 microns on average.

    What would the reason be for Holland to ban coaltar epoxies? It is a wonderfull rust preventer.
     
  12. pengreg
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    pengreg Junior Member

    Anode?

    Nee man - I am no expert but I think the hull remains the Cathode, its the coating that becomes the Anode. This from Nigel Calders Boatowners Mechanical and Electrical Manual - "any time the coating is damaged, allowing moisture access to the metal and so establishing a localized galvanic cell, the zinc will corrode protecting the steel and in fact plating out the steel to "heal" the scratch"

    Contingent Sea was "flame sprayed" in Durban in the early eighties according to her records

    Regards
     
  13. D'ARTOIS
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    D'ARTOIS Senior Member

    Wynand - to ban something in Holland, you don't need to have a specific reason.
    At least the government doesn't. There is a leegloop of industries right now that go to the eastern countries that are presently in the EU and that don't have yet the strict ruling. The Government in Holland is the largest destroyer of jobs of all time. Due to their strangulating rulins, the one after the other industrie disappears. Therefore most of the heavy shipbuilding has disappeared to countries like Poland and Rumania - Ukraine. A reasonable ferryboat for the services between the Dutch isles was build in the Phillipines!
    People are disinterested to carry out heir job. Save for the few entouhsiasts working in our field. Be glad that you are in S.A. where you can build a boat just to your liking, where you can blast a boat without receiving a killing fine.
    Can you imagine that you are forced to make the floors of your shed impermeable to fluids?
    In about 5 years time, all non-water solvable paints are illegal and forbiden to use or applied on whatever!
    bye bye yacht industry.
     
  14. jerryniff
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    jerryniff Junior Member

    I would like to share some experience that I have with flame spray.
    I have friend with a 36 foot steel sailboat that was built in Holland more than 80 years ago. On the inside the boat was flame sprayed with zinc at the time it was manufactured. Insulation in the form of rock wool was installed and over the years got wet. There is not a sign of rust, on the unpainted zinc, in any area that we inspected on the inside of the boat. I can't speak for what was applied on the outside of the boat because there looks to be 80 years worth of paint.
    I have flame sprayed the inside of my 39 foot steel sailboat with zinc so I have at least a minimal amount of experience with sand blasting and zinc flame spray. I bought a type Y Metco gun and a box full of spare parts on ebay for 140 US$. The gun works perfectly. The pure zinc wire costs around 7US$ per kilo and covers around 5 square meters per kilo. It takes a minute or two to spray a square meter, on a straight run. Add in the cost of gas and oxygen and the cost would come in below 1 US$ per square foot. Compared to epoxy coating systems, primer ... primer. paint. I believe the zinc is the cheaper of the two, or at least in this neck of the woods zinc is cheaper? There are issues about welding over zinc etc. etc. but since this is already longer than most will want to read, I wont go into them.
    Gerald
     

  15. Wellydeckhand

    Wellydeckhand Previous Member

    Trust Spray than epoxy even developing country is doin it ........ not much advertisment is goin around promote its advantage n only big firm would wan to try it....... wata pity...
     
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