preparation of interior rust on steel hull

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

  1. cgoodwin
    Joined: Mar 2011
    Posts: 47
    Likes: 1, Points: 8, Legacy Rep: 36
    Location: Seattle

    cgoodwin Junior Member

    Have a look at this demo, really interesting

    Looking into it polyurea was developed in the 1990's and its most common use is as bed liner in trucks but if you start to dig you find that garage floors in dealerships, sports stadiums, rail bridges, oil rigs, underground pipes, etc are all covered with it. It is also used commonly in fountains and pools and as a pond liner, as a matter of fact you can get it relatively cheaply from any of a number of pond liner stores. Acording to the advertised coverage rates I would be about $700 for enough to cover the area of my vessel covered with bottom paint currently, well that plus the cost of sand blasting it. Seems as though it is becomming the coating of choice for bridges and sports stadiums as well as above ground fuel and water storage tanks, sewage treatment, etc.

    Really interesting stuff and I bet we start seeing a lot more of it in the marine industry.
  2. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member

    Sure on new clean plate and welds which are accessible and easily washed off you won’t experience problems. But that’s not the application we are talking of here, we are talking of treating rust inside the hull.

    Even very low concentrations of acid activate steel surfaces. In simple terms it makes steel corrode more easily and at a much faster rate. Alkalinity has the opposite affect it passivates steel and stops it corroding.

    Inside a steel vessels hull there are frames, stringers overlapping plates paint edge voids and al the other inevitable inaccessible areas .
    Rust forms over time as not only surface rust but also in small corrosion pits with tightly packed oxide that is porous.
    Voids and remaining rust can remain acidic even after washing. That’s the real problem, inside a vessel there is a moist environment that you’ve now rendered mildly acidic.

    Consider adhering surface rust , the acid soaks in and saturates that matrix, spraying the area with water doesn’t remove the soaked in acid, it’s a sponge action. You can’t wring that sponge out to get water into the rust matrix, so it remains acidic. Scrapings of acid treated rust even after extensive washing usually show a significantly acidic state.

    Even if you dilute the acid it re-concentrates as the water evaporates, there is a mistaken idea that acid is neutralized by water, it isn’t all it does is dilute it temporarily.

    The acidic water from washing not only fails to remove all the acid but it also carries the acid it did dilute to other parts of the interior and deposits it there where it re-concentrates as the water evaporates.
    The steel corrodes rapidly now wherever it’s exposed and it’s received it’s acidic environment. Like your car battery, the acid doesn’t get used up in the oxidation process it just activates the steel and accelerates corrosion.

    Acidic conditions with steel also compromises the paint you apply over the top and the paint doesn’t bond well to the surface often causing small voids in the very areas which are still acidic.

    Overall investigation and observation over time has lead to a very strong body of evidence that has been incorporated into Industry standards for very good reason.

    Keep all your steel surfaces as alkaline as possible. If you do use acid neutralise it before you wash it into the rest of the hull but most preferably just mechanically remove as much as you can with mechanical wire brush, sanding disks, needle guns are good in my experience. There are also small sandblasters with inbuilt recovery that I’ve seen used they do an excellent first class job.

  3. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Polyurea's are a real pain to apply, they need specific and expensive spray equipment so need to be applied by specialist trained contractors. They need good primers too. A big problem is that surface wetting is very poor and they don't run into gaps and crevices.

    It's been around since circa 1950 but the practical use of the coating was limited by its application.

    There are also Polyurea polyurethane mixes. I've see Polyurea paint peeled off in large sheets when it started separating from the undercoat after 3 months. It was repainted with Epoxy !
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