Conference vicki::boats

    
      I have some excellent reference material on this. I'll give a
    thorough reply after I can get home to consult it.
    
    				Kenny

    CAN WAIT...
    
    THANKS..
    
    

    Electrolsis is caused by current flowing from one metal to another.
    The current will flow from the metal that is less noble to the more
    noble on the galvanic scale. That is why you use zinc which is less
    noble so it will lose electrons instead of your shafts, props,etc.
    
    You can measure if you have a problem by taking a sensitive meter
    and measuring the potential difference between different parts of
    your boat. For example measure the difference in potential between
    your engine ground and your shafts.
    
    The best protection is zinc of course but a bonding system will
    help. A bonding system is nothing but a wire you run around inside
    your hull and complete a circuit between all your underwater metals
    like through-hull fittings, struts, rudders, etc. It is easy to
    do. Use #10 wire.
    
    You didn't mention what kind of boat, whether or not is has shore
    power or a generator. All kinds of sources can get you a problem.
    I once had a new boat start boiling the zincs off in about 4 months.
    After measuring the potentials, I found the manufacturer had the
    12V ground tied to the 120V ground tied to AC neutral. Once I separated
    all these onto their own ground bar, the problem disappeared.
    
    Good luck, you have an investigation ahead of you.
    

    The boat is a:
    
    1988 36 ft. Sea Ray, twin 454's. no gen. but shore power and
    
    battery charger...
    
    
    Jim.

    I am sure that a 1988 36 Searay has a bonding system. What kind
    of problem do you have? You should have zincs on your shafts and
    rudders and tabs. I would assume the Searay also has a transom zinc
    tied into the bonding system. Is your shore power of proper polarity?
    You probably have a polarity indicator built in, don't you?
    

  A chemical reaction occurs whenever 2 different types of metal
are joined by an electrical conductor and immersed in an
electrolyte. Electrons move through the electrolyte and return
through the conductor.

  The hardware on a boat is made of many different metals. The
boat hull acts as a conductor between the metals; even if the
hull is made of wood or fiberglass. The slightest film of water
within the hull provides conductivity. The salt water makes a
VERY good electrolyte. This combination creates a good
environment for electron flow. This elctron flow results in
galvanic corrosion. That is, one of the metals is erroded, or
eaten away by the process. The amount of electron flow, and
therefor the amount of corrosion depends on 3 factors:

     1. The types of metal involved.
     2. The efficiency of the conductor. (is it just a film of
          water, or are the metals tied together by a wire)
     3. The strength of the electrolyte.

  The type of metal involved has a very large effect on the
amount and speed of galvanic corrosion. Certain metals are more
resistant than others.

  Some metals act as the negative side of this electric
production and some act as the positive. The negative metals are
called "noble" metals and are the cathode in the reaction. Metals
which are subject *TO* the corrosion are electrically positive,
and are called the anodes.

  The most IMPORTANT thing to remember is that there is no *one*
noble metal! What matters, is which metal is MORE noble. The
metal which is MORE noble than the other becomes the negative
pole in the reaction, the other metal becomes the positive one.
Zinc and magnesium are two of the *least* noble metals. Titanium,
nickel, and stainless steel are some of the more noble metals.
Thus if a piece of nickel and a piece of magnesium are in the
water, the magnesium will be corroded buy the current flow
between the two metals, the nickel will not. The fact that zinc
is cheap and is very far down on the scale of which metal is the
most noble, is why it is used as a "sacrificial" ANODE. Since it
acts as the anode in the reaction, the ZINC loses metal, and not
the piece you wish to protect.

  Even though the hull of the boat can act as a conductor between
the different types of metal, large pieces of equipment,
outdrives, propellers, shafts and other fittings can also act as
conductors. When these pieces are firmly attached together, they
are even better conductors of electricity. Cool fresh water is
not as good a conductor as warm water, warm salt water is a much
better conductor.

If current flow is added by an external source, as in a previous reply, 
the corrosion rate will increase.


Things to watch out for:

1.  Use fasteners of a more noble metal that what is being
fastened. That way, if there is corrosion, the fastener which is
the smallest part, will be protected, the idea being that the
larger piece will be more able to tolerate some corrosion.

2.  Keep all painted surfaces in good condition. If bare metal is
exposed, corrosion will increase rapidly. Vinyl or plastic based
paints will act as insulators.

3.  Metal based anti-fouling paints should NOT be applied to
metal parts unless the paint is *specifically* made to be used
over THAT type of metal. Otherwise, the paint can actually
*cause* the other metal to corrode.

4.  Whenever a zinc or other anode is used, it must be immersed
in the water with the parts it is supposed to protect AND must be
electrically connected to the parts it is supposed to protect.
For example, if you have anodes on an I/O outdrive that are out
of the water when it is tilted, you must have some others which
are IN the water or protection is lost whenever the outdrive is
tilted up.

5.  NEVER paint over a zinc anode because that would insulate it
from the corrosion process AND the zinc anode *MUST* have a good
electrical connection to what ever you are trying to protect.

  This is a list of metals arranged in descending order with the
MOST noble metal at the top. When any of these metals are used
together, the one which is *lowest* on the list will be the one
to corrode(for example, if you use Tin and 316 Stainless together 
the Stainless will corrode and the Tin will not!). The
metals with a '*' are metals which can be used together without
any galvanic corrosion.

     Platinum                                M O S T   N O B L E
     Titanium
     Gold
     Graphite
     Silver
     Monel*
     Copper Nickel Alloys*                        
     Bronze*
     Copper*
     Brass*
     Nickel
     Naval Brass (whatever that is)
     Tin
     Lead
     Lead/tin alloy
     316 Stainless Steel
     304 Stainless Steel
     Chromium Iron
     Iron or Steel
     Aluminum ST
     Cadmium
     Aluminum 25
     Galvanized Steel
     B605 Aluminum Anode Alloy
     Zinc
     CB75 Aluminum Anode Alloy
     Magnesium and it's alloys            L E A S T   N O B L E


               Kenny

    There is an easy answer if you weren't inboards, I have a 30 foot
    formula and have merc cathodes installed, they do help but I found that
    stainless steel props cause more of a problem than if I could run 
    aliminum.   But you probably have bronze props and shafts??? or
    stainless???  
    
    Any way you have a nicea boat.....
    
    mimi

    Just as an FYI -  A MAJOR cause of electrolysis is the Bilge Pump
                      Float Switch.
    
    Typical failure mode is the Positive lead gets immersed in the bilge
    water either due to a poor installation or broke insulation.
    
    The current path is out of the ground (i.e. your bonding system)
    via the bilge water to the positive lead of the float switch.
    A very good way to eat up through hulls.
    
    I saw a boat loose it's props due to a similar failure of a Bait
    well pump.
    
    Also in in setting up a bonding system be sure it in no way ties
    into any form (deliberate or not) of lightning protection -- blows
    out the thru hulls.
    
    Another major failure is the marina wiring -- Nice new Searay follows
    all the bonding rules etc.. and it's thru hulls props etc are tied
    to the shore power ground via the Green (not the white) wire ---
    everything is really at ground.   On the next dock your neighbors
    older boat has a wiring error the puts his boat at some potential
    or the marina wiring is a slightly different and voltage drop causing
    his boat to be at some potential etc....   The result is You Loose.
    
    It's a good idea NOT to leave your shore power pluged in from week
    to week just because of this; however, the food in the fridge spoils!
    
    SO if you are going to remain hooked to shore power Check to be
    sure all the neighboring boats indeed have the same AC feeders or
    if not that the grounds (white & green) are indeed wired correctly
    all the way back to the source (in fact the green should be grounded
    to earth at the outlet boxes - like 1:1000 really does it)
    
    I'm one who does not believe in the concept of a bonding system
    for some of these reasons.
    
    				Take care,
                                           Lou    

    
    What's the difference between electrolysis and galvanic corrosion?
    Or are they one and the same?
    
    Rick W.

    The book I have at home describes electrolysis as the result of
    dissimilar metals having different electrical potnetials, while
    electrolytic corrosion is due to externally applied electric current
    (improper wiring / grounding). The latter can cause much more 
    rapid and severe corrosion.
    
    The book makes a big point about the two processes being not the same.
    
    Bill
    

    
    Re.10 - Right, they're not them same, BUT they have the same 
    	effect - the metal corrodes.
    
    Re.9 - the two terms are about the same.
    
    	Electrolysis is decomposition of a material in an electrolyte
    by an electric current.
    
    	Galvanic Corrosion is corrosion cause by a small electric current.
    
    				Kenny

    The Fundamentals of Corrosion
    J.C. Scully
    Pergamon Press
    TA462.S39 1990 620.1'623-dc20 89-72143

    
    Regarding the original problem...
    
    You have a *big* problem somewhere.  With a boat only two years old
    there could be a few "etches" from small things wrong in the 
    grounding, etc., but to have a prop eaten through...
    
    I'd suggest first checking active leaks - is the shore power system
    grounded right - is there a battery charger that isn't grounded
    properly - check both the leads to al these devices *and* the screws
    that attach the boxes to the hull.
    
    Next check the zincs... Are they connected to the grounding system...
    
    Finally, use the list of most noble metals and least noble metals a few
    notes back and find out what metal the props are corroding to and
    check the grounding on it...
    
    Sounds like the boat next to you has 110AC dragging in the water...
    
    Regards,
    
    ted
    

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Note 776.0                          Corrosion                            1 reply
EXPRES::GILMAN                                       10 lines  18-DEC-1990 10:58
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    I did a dir/title and did not find anything related to corrosion or
    appropriate fittings for saltwater boats.  Anyway, this string is 
    to discussion various materials (brass/bronze/stainless/iron/copper)
    etc. and their problems and appropriateness for use in boats.
    
    To start this off. NEVER use brass underwater in a saltwater boat.
    It will dezincdefy and leave you with a spongy fitting which will
    fail.
    
    Jeff

Moved by moderator...

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Note 776.1                          Corrosion                             1 of 1
ULTRA::BURGESS "Mad man across the water"            28 lines  18-DEC-1990 11:25
                      -< Find it under what its called. >-
--------------------------------------------------------------------------------
re                       <<< Note 776.0 by EXPRES::GILMAN >>>
>                                 -< Corrosion >-

>    I did a dir/title and did not find anything related to corrosion or
>    appropriate fittings for saltwater boats.  Anyway, this string is 
>    to discussion various materials (brass/bronze/stainless/iron/copper)
>    etc. and their problems and appropriateness for use in boats.

	Try    dir/title = electrolysis

	"Electrolysis"  is technischerspeak for what you seem to be 
wanting to discuss/read about.
    
>    To start this off. NEVER use brass underwater in a saltwater boat.
>    It will dezincdefy and leave you with a spongy fitting which will
>    fail.

	Conflict between "in" a saltwater boat and "underwater";  
does it matter if the boat has sunk ?

	"dezincdefy" ?    right.    

	See high school chemi texts re:  "electrolysis", etc.



	R

    Quite right, I should have said UNDERWATER in a saltwater boat.
    Actually brass should not be used in an interzone either, where the brass
    is being lapped by waves rather than completely underwater.
    
    Another misnomer. Manganese bronze is actually a brass because brass is
    an alloy of copper and zink. Mag 'bronze' is copper, zinc, and
    manganese.  
    
    Reference:  Corrosion in Boats, can't remember the author.
    
    Jeff

    As I continue to read the book: Metal Corrosion in Boats I realize how 
    involved picking the correct materials for different marine
    applications really is. For example a S.S. prop shaft might seem ideal
    on a saltwater boat.... the cathes are: S.S. NEEDS highly oxygenated
    water to maintain its protective surface film, in oxygen deprived water
    it is subject to rapid pitting.  How could that happen? How about under
    the rubber bearing pads in a cutless bearing? Oxygen deprived area =
    pitting. THEN you start the boat up and rip the cutless bearing by the
    sharp pit edges on the shaft!  Another trouble area is in prop to shaft
    mounting area if enough water can't get under there = oxygen
    deprivation and pitting again! Bonding and zincs help with this
    situation but are not a cure all. 
    
    
    Jeff