Thanks for the info. I guess I can expect the lawnmower fuel tap to be
subject to corrosion in seawater. I misremembered that the original valve
was brass; it appears to be cast stainless, which explains why it is
expensive and why Seagull came out with a plastic replacement. If nothing
else, the Seagull manufacturers knew how to make a saltwater motor, and my
32-year-old motor has little corrosion. It hasn't had prolonged exposure,
however, since the boat and motor are rarely left in the water for long,
and I usually hose it off and spray it with WD-40 when I get home.
I haven't used the Seagull since installing the brass valve because I am
now routinely using a 70 lbf thrust MinnKota electric (saltwater model),
but I will watch for corrosion on the brass and may have to replace the new
valve with a plastic valve (also available at the lawnmower shop), if the
threads haven't fused together due to the dissimilar metals.
I should have learned by now to use only marine hardware on my boat. The
Seagull plastic valve was much shorter lived than the original metal valve,
so I was looking for something more durable, but didn't stop to think about
the saltwater compatibility. It sure looks pretty though.
Harry
>Hi Harry,
>
>Read your comments on finding a brass shutoff valve with mesh at a
>lawnmower supply store, and a couple of concerns came to mind. First, let
>me say that I am not an expert on corrosion, and would welcome comments
>from any naval architects who might wish to enlighten us further.
>Seconldy, I am not familiar with the Seagull setup, so my comments will
>encompass a couple of scenarios of brass fittings on engines.
>
>For diesel fuel engines, brass (and copper and galvanized steel) react
>with diesel fuel, although this seems to be of greater importance in tank
>material. Recommended diesel fuel lines are usually of seamless annealed
>copper, copper-nickel or stainless steel (plain steel is ok on freshwater
>boats). Solder is not recommended because of its low fatique strength and
>its low melting point (imortant in case of fire).
>
>For outboard motors, no metal should be affixed to it except stainless or
>galvanized steel. For stainless, type 316 is best; type 304 is also marine
>grade, but not as good. It goes without saying that antifouling paints
>containing mercury or copper should not be applied. Aluminum marine grade
>alloy, while resistant to corrosion when isolated, is so low on the
>galvanic table that it will be galvanically attacked (and corroded) by
>other common boat metals. Thus metals like brass, bronze, etc should be
>several feet away and have no direct electrical connection to the
>aluminum. Finally, as the experienced know and the neophyte learns,
>replace those sacrificial zinc anodes when almost eaten away.
>
>One last thought on brass and seawater. Brass is an alloy made of copper
>and zinc, in varying ratios (70/30 which is more ductal, and 60/40 which
>is stronger, are common). While they resist atmospheric corrosion,
>prolonged contact with seawater causes the zinc to dissolve out (termed
>de-zincification). The shape of the object is unchanged, but it will have
>no strength, inviting catastrophic failure. Brass with less than 15% zinc
>is usually immune, whereas 60/40 (40% zinc) is highly susceptible (adding
>1% tin helps, but it doesn't stop the process). Interestingly, 70/30, if
>0.02-0.06% arsenic is added, can be used underwater. The problem is, you
>don't know if the arsenic is there or not...certainly most folks at the
>marine stores don't seem to know what metal compositions they stock. And
>"marine grade" is too vague, as there are different levels of marine grade
>in each metal. If anyone cares, I can post some information on those
>differences (ie. stainless 304 vs 316, different!
> bronzes, different brasses, different aluminums, etc).
>
>It gets really complex, and I am still learning this stuff, so I welcome
>any and all comments.
>
>Regards,
>Leander
>leander@worldnet.att.net