Void space corrosion inhibitor

Discussion in 'Metal Boat Building' started by sailrjim, Nov 28, 2005.

  1. sailrjim
    Joined: Mar 2005
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    sailrjim Junior Member

    Seeking info on a coating for void spaces. Generic term is "float coat". It is used for void spaces that are inaccessible by usual means. Need source in U.S.A. I searched this forum, but found no posts on the subject.

    My www search finds two methods and several products.
    The two methods:
    1. Introduce dry crystaline posder to the void space. Seal the space. Powder out gasses a corrosion inhibiting product that fills the space. Rated for two years protection max. Generic term VPCI, vapor phase corrosion inhibitor.

    2. Introduce liquid product to void space and allow to form a layer on bottom of space. Introduce water at bottom of space. Product rises on surface of water, coating the surfaces of the space, and again coats when water is withdrawn at bottom. Two makers of this type, both are in europe. Not avail in USA as best I can determine.

    I prefer the liquid product as I perceive it offering longer term protection. Also, best to be nonflamable.

    Intended use is the keel of a steel sailing boat. The space is the aft portion of the keel and is not used for any purpose. Boat has been in service many years. Access to space is to be through pipe plug fittings, one high, one low in external surface of the keel.

    Your advice, info, experience greatly appereciated.

    TIA

    Jim
     
  2. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    Jim
    No need for expensive solutions and fancy chemicals.
    Seal off the void and low pressure test to ensure the seal then fill with a cheap available inert gas such as Nitrogen. With no available oxygen the space will be as you left it in many decades time even with bare untreated steel.
     
  3. sailrjim
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    sailrjim Junior Member

    Mike,

    Thank you for the response and suggestion. Seems to me that this will require installation of a permanent valve through which to admit the gas. If so, I will need to weld in a threaded collar (pipe coupling) to attach the valve. Is there a valve style that is best for this purpose?

    Jim
     
  4. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    Jim
    You do not need to be too fussy , you need an inlet and a vent hole valves are not required beyond simple sealing. (A lump of putty would do ). Remember it is not a critical application, its a matter of reducing the oxygen content to a relatively low level. Air is mainly nitrogen anyway with a bit of carbon dioxide and Oxygen.

    If you are using a gas cylinder Warm nitrogen is slightly lighter than air so a vent pipe from the bottom of the cavity and a gentle in feed at the top will result in good expulsion.
    You can simply vent the gas near a flame lamp until it extinguishes but introduce the gas slowly so as not to get turbulent mixing. The alternative is to let rip and vent away for a while allowing the gasses to stir and mix together achieving a dilution which will also work.

    The alternative is a flask of liquid nitrogen simply poured into the void and left to boil off, The cold vapor is denser than air but you would have to allow a simple flap valve to allow for the vapor expansion or just another lump of putty !.

    The holes can be simply welded up or plugged with some sealant after filling or capped but no permanent valve needs installing. Large voids on commercial vessels pressurize the void and attach a pressure gauge but you do not need to go to those extremes.

    If you pressure test your tanks then 2-3 psi should be more than adequate.
     
  5. sailrjim
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    sailrjim Junior Member

    Thank You, Mike. Understood and easily done.

    BTW, removed the old shaft tube today and had a look at the surfaces in the void area. Only a hint of surface rust after fifteen years service in salt water. I expect most of that is from the recently discovered hole in the shaft tube. drained approx. ten gallons (twenty litres) water from the space. Capacity of the space is estimated to be 120 gallons. Except for the single hole in the shaft tube (1.3 m long), the tube was nearly as new.

    Jim
     

  6. saeble
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    saeble Junior Member

    whilst you may think its a slight waste, if you are using a MIG, you have a ready supply of inert oxygen displacing gas at your disposal... your MIG cylinder is full of a nice argon mix. :) not to mention it comes with a nice easy to use dispensing system. Pull the wire from your gun and pump away. :)
     
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