The primary alloying metal is tin. Other metals are introduced in trace amounts to affect the properties, like zinc, aluminum, etc., the same way you'd add tiny amounts of arsenic or other elements to harden lead. Copper and zinc make brass.Depends on the particular alloy. Bronze used for prop shafts and struts contains zinc.
Bronze - Wikipedia
en.wikipedia.org
Right, and when the zinc leaches out of bronze, it changes the alloy which means the alloy no longer has the engineered properties. In some applications this might be completely inconsequential, while in other applications this could lead to failure of a critical piece of equipment.
This is archaic information. It's the common lay-persons distinction between a brass and a bronze, where brass is a copper-zinc alloy and bronze is a copper-tin alloy.
From the looks of your images, I would tend to agree with the yard that your strut appears to be in good enough shape to not require replacement, at least at this point in time. The pink areas are a concern, but clean them up well. From here it's not possible to really know but it looks superfical at this point. I'd certainly defer to the advice of someone that can actually look at it.
No, it's not archaic information, it is fundamentally correct. While there certainly are alloys referred to as bronzes that have no tin, like aluminum-bronze, the fundamental alloy called bronze has as its primary alloying metals copper and tin. Certainly, in marine applications, in struts, shafts, through-hulls, motor boat rudders, propellers, and so on, are copper-tin bronzes. Note that deviations from this are named with qualifiers like phosphor-bronze, aluminum-bronze, and so forth.
@jviss Modern bronzes are more complex than the historic use of the bronze definition of copper-tin alloys. I attach the current definition from the CDA, the Copper Development Association. If you'd care to read it, it states that a bronze is a copper based alloy that does NOT contain as the major alloying element of zinc or nickel. But, it does not state that a bronze is a copper-tin alloy. It also goes on to say that "Originally "bronze" described alloys with tin as the only or principal alloying element. Today, the term is generally used not by itself but with a modifying adjective."
The cause is lack of protection, especially if the strut is made from manganese bronze. Dezincification is the "normal" degradation of this kind of alloy in seawater. The OP doesn't say the age of the boat, the Newport 30 Mark iii's were made between 1971 and 1996. So the boat is somehwere between 51 and 26 years old. 26 years is a bit short for the amount of dezincification observed, but 51 years, and the obvious lack of protection, and I'd say that's darned good.
