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Dave Palmer
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Brass, bronze, and dezincification
Dave Palmer   3/29/2012 3:49:12 PM
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Brass and bronze are two different things.  Brass is an alloy of copper and zinc.  Bronze is usually an alloy of copper and tin -- although there are also aluminum bronzes, manganese bronzes, and silicon bronzes, which are alloys of copper with aluminum, manganese, or silicon, respectively.  Each of these catagories (brass, bronze, aluminum bronze, etc.) includes many different alloys with different properties.

I'm not quite sure what "shop brass" is, but cheap screw machined parts are often made out of free-machining brass (UNS C36000).  This is an alloy of copper, zinc, and lead.  The lead helps to make it readily machinable.

As the article correctly points out, copper and zinc are very far apart in the galvanic series, so when a brass part is exposed to a good electrolyte (like seawater), the zinc acts as a sacrificial anode for the copper.  Ultimately, all of the zinc dissolves out of the brass, leaving a spongy mass of copper with very little strength.  I'm willing to bet that if your uncle looked at the failed part under a microscope, he would have seen this sponge-like structure.  This process is called dezincification.

Naval brass is a type of brass (usually approximately 60% copper, 40% zinc) which also contains a small amount (0.5 - 0.8%) of tin.  Small amounts of arsenic, phosphorus, or antimony might also be added.  The presence of these elements help to inhibit dezinicification.

The absolute best copper alloys for saltwater service are copper-nickel alloys.  These alloys have excellent corrosion resistance.  However, they also tend to be fairly expensive.

This article is a good example of why proper materials selection is so important.  It pays to do your homework -- or, better still, ask a metallurgist.

Rob Spiegel
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Blogger
Re: Small component, big damage
Rob Spiegel   3/29/2012 3:43:30 PM
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Bob from Maine, it sounds like you'[re suggesting this may not be a Made by Monkeys problem but rather a simple wear-and-tear-over-many-years problem. Are there ways to check this before it causes a serious accident?

bob from maine
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Platinum
Re: Small component, big damage
bob from maine   3/29/2012 3:38:23 PM
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Don't know how long you'd had this boat, but Cheoy Lee Clippers were made in the 50's and 60's, so this incident could be due to simple age. My experience with Cheoy Lee has been their hardware is of very high quality and their hulls of that era were quite over-built. Most "Bonze" shafts, propellors, seacocks, and general fittings have some zinc, though I can't remember the exact percentage. Boats that spend time at the dock, particularly where other boats have shore-power plugged-in have a tendancy to de-zinc many of their bronze underwater fittings due to stray eddy current in the water. Also the Dolphin Striker assembly is usually not disassembled during winter storage so is never checked and unfortunately is a fairly common point of failure for this reason.

Rob Spiegel
User Rank
Blogger
Small component, big damage
Rob Spiegel   3/29/2012 9:48:46 AM
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It's amaging the damage a small weak component can do. I wojld imagine there may be liability issues with this accident.

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