preparation of interior rust on steel hull

Discussion in 'Metal Boat Building' started by owensp, Oct 26, 2010.

  1. pdwiley
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    pdwiley Senior Member

    Ah. That explains why Richard has a reputation score of 1300+ and yours is negative.

    PDW
     
  2. NZ_Shipwright
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    NZ_Shipwright Junior Member

    NZ_Shipwright
     
  3. michael pierzga
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    michael pierzga Senior Member

    The water and powerful soap wash is the very first step...before surface prep. Not a good idea to be using heavy grinders or abrasives on thin plate yachts. After ten of 15 years you will be left with an egg shell. Even sandblasting is erosive. The thing to get over is that you dont need "white" steel surfaces for decent paint protection. Besides, Sandblasted "White" steel is unabtainble in something like a steel yachts bilge. Clean brown steel with no oily contaninates...no scale and use paint which tolerates brown steel, applied to the correct film thickness. . I can show you the interior of yacht steel watertanks...always very difficult to surface prepare...that are in great shape after 5 years and the system was soap and water wash, needlegun, wire brush and surface tolerant paint. As mentioned earlier a Die Grinder is a very effective tool for tight inside welded corners. And always..as was mentioned earlier..Follow up every year with touch up painting. Its part of the maintenance cycle for steel boats.
     
  4. MikeJohns
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    MikeJohns Senior Member


    When you use the quote button it's better not to write your opinions amongst theirs within the quote without some clear demarcation ( colours good). Otherwise It gets very confusing. I had to extract your questions from my quote!

    As for the acids: Read Johns (Landlubbers) post #9 on the first page of this thread. That's my view too of these products.

    Every effort should be in getting as much of the rust off as possible and all the scale with no exception. Small pits the needle gun gan't get into can be hit with a centre punch that shatters the packed rust infill. At some point you have to call it a day, then make sure its dry and get it well covered with paint.preferably with a zinc rich coating on the steel first. That should completely passivate the surface.

    I mentioned the CR zinc because it’s an interesting case, The CR is the small amount of binder for a high build zinc coating ( but you wouldn’t know it was CR) it’s suitable for overcoating with epoxy. Sometimes they can work better than the epoxy zinc rich paints on pitted surfaces because they can contain a lot of thinners and still work so they carry the zinc into areas that an epoxy zinc paint couldn’t. They are also a quick drying one pot.
    The result is quite satisfactory for protected surfaces certainly under cabin soles and behind ceilings. I’ve performed standard adhesion tests (on all the paints we specified) and they were good. You can overcoat with another CR of course and there are some very good tough paints available in that formulation now (from Dulux here) and they are cheap good quality industrial paints.


    Rust itself is quite inert, it’s a very stable oxide. The only reason it carries on rusting the parent steel is that there’s moisture and often chloride ions within the rust itself. Because it absorbs moisture even airborne, it keeps the surface supplied with just enough moisture to slowly carry on corroding often digging a pit. Drive ALL the moisture out and it will be inert. But I was only thinking of the tiny spots or cavities with powder surface rust after the scale has been knocked out that it’s not worth grinding out. Make sure it’s dry and paint it. You should never paint over anything other than light superficial surface rust.

    No grinding or sanding? I don't agree at all. Abrasive cleaning of a surface removes a little of the surface regardless of whether you sand it grind it or sand blast it. Providing you are sensible you are not going to remove enough to cause a problem. Unless you are particularly heavy handed. If it is so thin as to be an invalid option then you'd better replace the plate . Also grinding or sanding smooths the sharp edges lumps and bumps that could otherwise compromise the paint film or initiate a stress riser.


    Most of this is just common sense.



     
  5. michael pierzga
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    michael pierzga Senior Member

    I keep seeing this MgDuff product sold in marine stores. Its not cheap. Zinc spray. Is it of use for small difficult to prepare and paint surfaces ? Hydraulic hose fitting , small angular mounting frames and the like. As for heavy gringing, abrasives. Works fine, assuming the operator of the machine is competent. Be aware. Most times you deploy the full range of tools to attack the job
    http://www.mgduff.co.uk/zingard.html
     
  6. castaway
    Joined: Nov 2010
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    castaway New Member

    I also am new to steel boats and have developed a healthy interest in the 'R' word.

    My Mauritius 43 has 11 coats of 2 pack on its topsides and below the waterline, and I'm pretty happy about that, however the interior of the boat ( which was built in a shipyard on the Clyde in the 90's) still has its original 'commercial' heavy paint coating, which appears to be in good condition.

    The interior is fully lined with thinsulate down as far is the cabin sole and that also appears to do an excellent job of preventing condensation.

    The hull interior below the cabin sole is pretty accessable and all the longitudinal stringers have been faired with cement/concrete to prevent any water trapment, the paint all appears to be on good condition.

    Any input or comments on the above would be appreciated....

    But..does anyone use wax ( in the UK we have a product called Waxoyl), on non exposed surfaces on steel boats, as is used in the auto industry and DIY to seal cavities and areas that flex .

    To my ignorant eyes it would appear to have good qualities for bilge surfaces but I'm prepared to be corrected.. !

    Many thanks

    Nick
     
  7. michael pierzga
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    michael pierzga Senior Member

    Sounds like you have a well built steel boat. Perhaps the best thing to do...and the easist, cheapest, is to maintain her as she was built. The phrase heavy commercial coating sounds good to my ears. The secret with steel boats is that when you see a defect, a paint chip, a rust bleed, fix it...even the simplest...soap and water wash then a dab of paint is effective and preventitive.

    The wax product you speak about is often used on the classic Narrow, canal boats. I have no experience with it on salt water yachts.
     
  8. apex1

    apex1 Guest

    Utter nonsense as usual.

    Of course one must blast to SA 2,5 or needle / grind to same level, and of course it is difficult in finicky areas, to get a surface which is NOT maintained every season twice. Once done to bare, white metal you must not blast or sand it for the next 25 - 30 or more years, when touched up after minor scratches.
    When done the sloppy way YOU recommend, sure one will be through the hull soon.

    If SA 2,5 is not to achieve, the way Mike recommends is a valid, and proven one.

    Your soap wash is a house wife recipe.

    castaway

    leave the original coating intact and touch it up when need be. Do not use these oily or waxy coatings, they do not stand salty air, and worse, they contain and collect acids and water. Not what you need. If it has to be dirt cheap, smeer any paint on, but not oil.

    Regards
    Richard
     
  9. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Hi, Sorry about the quote mess up.
    Please explain how acids complicate things? Acids are widely used in our industry.

    Also post #9 is a contradiction.

    We will have to agree to disagree on the grinding.

    Regards NZ_Shipwright
     
  10. MikeJohns
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    MikeJohns Senior Member


    My view is the standards view that John Posted. Don't acid clean in this situation if you can help it. steel doesn't like acid it corrodes significantly faster in even a mildly acidic environment. Small amounts of trapped acid will cause more problems than small amounts of trapped Iron oxide. Acid 'activates' the surface of steel.

    The higher the PH the better in fact if you can get the PH above 10 steel doesn't corrode at all. Builders hydrated lime has a PH around 12 which is why a simple lime wash can protect bright steel so surprisingly well.
     
  11. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Hi,You are forgeting the removal of the thin black Magnetite rust
    film found below Iron Oxide film on rusted steel. Iron Oxide is a higher
    oxidation state and occurs at the rust/air interface on steel and adheres to the Magnetite layer.
    Magnetite is a lower oxidation state and occurs on rusted steel as a thin film adhering directly to the metal. When the lower Magnetite film is removed, the Iron Oxide has nothing to adhere to and is also removed.
    The acid should be only strong enough and capable of penetrating the rust layers and preferentially removing the lower Magnetite layer.
    Every Car manufacturer pre cleans before painting new cars with Phos acid.
    Foundries worldwide pickle plate with a lot stronger acids than I suggest.
    NZ_Shipwright
     
  12. MikeJohns
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    MikeJohns Senior Member

    You are simply not expressing a professionally accepted view for the preparation of plate on a steel hulled vessel.

    Experience and an understanding of the pros and cons of this method have even driven our standards view, and I have never seen acid recommended for a ships hull nor would I recommend it. It’s a poor alternative to mechanical preparation and it’s cons are significant enough for it to be specifically professionally discouraged.

    If you don’t hold with the standards. I don’t think you’ll have much luck changing them
     
  13. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Here you go ,
    National Association of Corrosion Engineers (NACE) Standard RP0178.

    4.3.4 CORROSION REMOVAL. Abrasive blasting is the preferred method of removing corrosion; other
    mechanical methods (SSPC SP-2 or SSPC SP-3) that may be used are grinding, chipping, sanding, or wire
    brushing. Chemical corrosion removal may be used when there is no danger of the chemical becoming entrapped.
    The chemical method should be used on complex shapes and machined surfaces. Chemical rust removers are of
    two types: acid or alkaline. The acid type can be used in removing rust and black oxide by immersion or brush
    application. This phosphoric-acid-type remover must not be used on high-strength steel heat treated above 1.24
    gigapascals (GPa) [180,000 pounds per square inch (psi)] tensile strength because of possible stress corrosion or
    hydrogen embrittlement problems. The alkaline type (sodium hydroxide base) is suitable for use by immersion
    only. It is preferred for use on machined surfaces where a dimensional change would be objectionable.
    Above is copied from another website.
    NZ_Shipwright
     
  14. MikeJohns
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    MikeJohns Senior Member

    Inside a steel boat or ship hull that's one big reason for it to be discouraged.

    How do you ensure that there is no acid residue under paint edges, left in pits, or trapped between frames and the plating ?

    When you wash it where does the waste water go (it's acidic) dose any of than get trapped anywhere?
    Remember too that you liberate acid when you wash it, that acid doesn't get neutralised by water only diluted. It reconcentrates as the water evaporates.

    You'd really need a high pressure fresh water wash. Of the treated area and the bilge afterwords. Then after you waterblasted the surface you've got the added complication of drying the now wet interior fast and that means under all the framing nooks crannies crevices that you just introduced water to.

    It's a process with issues and it's discouraged on many steel structures and expressly forbidden on some. It's certainly not encouraged inside boat hulls.

    Done with care, diligence and common sense it will remove rust you could have removed using mechanical means. But I'd avoid it for this application unless it was the bitter end of options.
     

  15. Landlubber
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    Landlubber Senior Member

    NZ Shippy....yes post #9 is a contradiction......clearly the use of acids (phosphoric acid generally) is NOT recommended by the major paint companies NOR is it recommended by the standards for protective coatings on metals (steel).....sometimes we do what we have to do to justify our means.....sometimes a product is used to hold the current situation from gettimg any worse, at least till the proper techniques can be used, and the ONLY way to treat steel is to sandblast to SA2 1/2 (white metal)...AS1627....all other products and processes are substitutes.....and as stated above, they are often used to hold the situation.....not expected to be the be all and end all....do it once and do it right...sandblast white.

    ....as a qualified NZ shippy, surely you are aware of the standards for steel treatment...they do not waddle on about doing anything but sandblasting to white metal

    again, another pdf link that is very helpful....
    http://www.duluxprotectivecoatings.com.au/technotespdf/1.1.2 Mild Steel - Surface Preparation.pdf
     
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