What's Below Your Waterline?

Seacocks fall into the 'out of sight, out of mind' category for many boat owners ... until they are needed.

By Steve D'Antonio
Published: April 2014

The Good

Photo of acceptable backing block materials
A proper seacock with reinforced plastic backing plate.
Photo of a Marelon seacock
Marelon seacocks are a viable alternative to traditional bronze.

The requirement for corrosion resistance limits the range of materials from which seacocks and their related components may be made. Only bronze, DZR brass, glass-reinforced plastic and, in some cases, stainless steel may be used. The terms "brass" and "bronze" encompass a wide range of copper alloys. The primary determining factor is zinc content. Zinc is an especially ignoble metal; it corrodes very easily when in the presence of other metals and an electrolyte, such as seawater, which is why it's used in sacrificial anodes. Zinc, however, also imparts strength to copper, and mixing the two, often in about a 40-60 ratio respectively, results in a brass suitable for clocks, lamps and cabin hardware, but, with few exceptions, not for seawater plumbing.

Copper-zinc alloys used below the waterline undergo a process called dezincification whereby the zinc corrodes from the alloy, leaving behind a pink, porous and very fragile structure that's almost certain to fail. For the most part, true bronze alloys are zinc-free, their primary elements being copper and tin. Other alloying elements include silicon and nickel (technically making them something other than bronze, but still acceptable for use in seawater plumbing). These are the best alloys for use below the waterline because they are not susceptible to dezincification.

Unfortunately, a huge range of alloys lies between true bronze with very little or no zinc and true brass which contains a high percentage of zinc. Two common alloys often used in marine applications are 85-5-5-5 and DZR. 85-5-5-5 contains 85 percent copper, 5 percent zinc, 5 percent lead and 5 percent silicon and can be used below the waterline. Some European manufacturers use something called DZR brass, a dezincification-resistant brass alloy. This alloy has a higher zinc composition than many other copper alloys (30 percent or more), but it also includes trace amounts of other metals meant to retard zinc corrosion or leaching. The more zinc an alloy contains, the more prone it is to dezincification, therefore, alloys with little or no zinc content are more desirable, and typically more costly. Accordingly they can be an attractive, though inferior, alternative for cost-conscious builders or do-it-yourselfers.

So what's on your boat? While there's no definitive field test for alloycomposition, there are a few things you can do to make a determination. Bronze tends to look more like copper, penny-like, darker and brownish, and it's often rough cast rather than smooth machined. Polishing a section of the metal in question as well as a known piece of bronze and holding them next to each other can prove useful, if not definitive. Brass, on the other hand, is much yellower or gold in color. Any copper alloy that looks pink, a sign of dezincification, likely contains zinc and should be replaced.

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