RE: Re: Seagull

happy life skills foundation (leander@worldnet.att.net)
Wed, 28 Jul 1999 05:06:41 -0700


- - - - - - - - - - - - - - - - - - - - - - - - - - -
West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
- - - - - - - - - - - - - - - - - - - - - - - - - - -
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