Friday, July 19, 2013

The Bronze Rage

Here's what we already knew before today:

Copper  +  Tin  =  Bronze
&
Copper  +  Zinc  =  Brass

But here's what we learned today (!) from one of our customers who uses bronze on boats:
Until I joined this industry, from the heavy equipment industry where everything is steel, the only bronze I knew was the type used in statues.

The fasteners are "silicon" bronze, zinc making up about 1.5% and silicon about twice that much. In a salt water environment the zinc is the sacrificial metal and it can be leached out of the bronze (by galvanic corrosion) leaving the base copper resulting in a very "pink" fastener. The composition of the bronze is much like grains of sand, lots of copper grains and in between some silicon or zinc grains. When the zinc is leached out the metal becomes porous like osteoporosis in bones - and just as weak and brittle. When you see pink in a bronze fitting think green - it's going to cost to get it fixed because if it's pink it has to be replaced.

Silicon bronze fasteners in wooden boats can last 50+ years but with the advent of electricity on boats and docks supplying shore power and poor power maintenance on docks or boats in a "hot" dock, any or all of theses problems can "cook" the fasteners in months - the last thing you want in a wooden boat is to have a fasteners breaking and planks coming loose - we call that a "boat sinker" incident.

The worse case of galvanic corrosion I've seen was a boat that was "refastened" (new screws put into the wood hull) with galvanized screws without the original bronze screws being removed. Not only did the new screws rot out but the resulting brew of decomposing chemicals rotted out the wood. Entire sections of planks had to be replaced - see what the local lumber store gets for a single piece of  2" by 6" x 120" white oak. This boat was eventually cut up - too expensive to repair.

And stainless underwater is another can of worms. Embedded in wood where oxygen cannot get to the fastener results in crevice corrosion - another "boat sinker" incident waiting to happen.

My first eye-openers in this business were: (1) what time do you need me here in the morning? 3am? because of the tide? You've got to be kidding? No? and (2) If you want to keep your boat afloat why do you put so many holes in the boat under the waterline? (engine raw water intakes and discharges, head raw water intakes and overboard discharge, air conditioning/refrigeration raw water intakes and discharges, grey water discharges, bait tank intakes,...)*

GREAT INFORMATION!!!

*He did want us to mention that he's no expert on bronze fasteners but what he's learned about them is from people who have owned boats or built boats, or both, for most of their lives.  

1 comment:

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