Fake Silicon Bronze that's Really Brass
Too many merchants around the web will lure the unsuspecting customer in with "bargain" prices for silicon bronze fasteners and fittings, only for the customer to later find out that the "bronze" they bought is a cheap imitation and won't have the resistance to galvanic corrosion that is needed on a boat.
Cheap "bronze" fasteners are probably brass!
True silicon bronze is copper alloyed with silicon, while brasses are copper (and much less copper at that) primarily with zinc. Shady manufacturers will try to save money on the alloy by selling fake silicon bronze with less copper in it, and replacing the silicon content with zinc. Copper is of course a precious metal and the less of that metal the unscrupulous manufacturer has to use, the higher their profit margin. That "silicon bronze" that was a bargain but didn't hold up? It's probably actually a brass.
Brass is often used on some elements of a yacht, but should not be used for structural purposes. The zinc in the fake metal is so far apart from copper on the galvanic scale that the brass will suffer from severe corrosion called dezincification. The zinc literally dissolves away, leaving a brittle honey-combed structure of a fastener with no strength at all. While brass does make fine interior hardware and low-strength deck fittings, it's not corrosion resistant enough for anything below the waterline.
Maybe they're bronze, but not the right bronze for a fastener.
The four most common of these fakers are manganese bronze, Tobin bronze, commercial bronze, and naval brass. Even worse could be a mystery alloy that is bronze colored. Often made from leftover scraps at the manufacturer, this new tactic is being employed more and more often at foundries overseas.
Manganese bronze is a complete misnomer (though it is the correct name for some unknown reason). In fact, manganese bronze is 58% copper and 39% zinc, with only 0.8% manganese! With all of that zinc content it's really and truly a brass. Tobin bronze is 60% copper, 39.2% zinc, and 0.7% tin - again, I don't know how they got away with calling that a bronze with over 39% zinc content. Commercial bronze is 90% copper and 10% zinc, too much zinc to be a true bronze, but at under 15% zinc it's moderately resistant to dezincification. Naval brass is at least properly named as a brass. It is 60% copper and 37.5% zinc. Because it has the word “naval” in it, however, there’s a tendency to think it can be used for marine applications underwater. It shouldn’t be. It should be limited to use on deck and in the interior.
Manganese bronze, tobin bronze, and commercial bronze are widely employed for not only fittings like cleats, chocks, and portlights, but even for propellers and struts. They're used because they are initially strong, but more importantly easy to cast, and easy to machine. In fact, they give adequate service on deck and excellent service in the interior, but are potential problems below the waterline.
Silicon Bronze is the standard for below the waterline.
When used for propellers or propeller struts, which is surprisingly common, care must be taken to protect these manganese, Tobin, or commercial bronze (really brass) fittings with zinc anodes. Proper silicon bronze is much better and far more resistant to corrosion.
There is a reason that silicon bronze has been the standard fastener material on wooden boats for over 150 years - it's simply the right thing to use.
Don't get fooled. Buy your silicon bronze fasteners from a supplier that names and guarantees the alloy that is used.