Hmm, sailorchic34
Your instincts would be correct if all alloys were just a mixture of discrete elemental metals, dissolved in each other, but retaining their separate properties at the macro scale.
For various reasons, this does not usually happen. One simple example is the formation of 'intermetallic' compounds.
This is where the atoms of different metals share electrons, the most commonly known being the various compounds of Fe and C, like austenite, martensite, pearlite, cementite, etc. - which give different grades of
steel their peculiar properties.
Some of these compounds of iron and carbon have properties (strength, resistance to attack) which are unrecognisably different from either constituent, and from each other.
Whereas, in cast iron, which has roughly an order of magnitude more carbon than ordinary "carbon steel" (which we usually just call steel), most of the carbon is present as clumps of graphite. The graphite is
not chemically compounded with the Fe of iron.
And zinc, in brass, is (to a crude approximation) like graphite in iron: it retains its separate identity.
Because of this, zinc exhibits much the same vulnerability to attack in seawater, when present as an alloying constituent
(perhaps more accurately as a solid in solution) in brass, as it would if you studded the brass with little zinc rivets.
Aluminium bronze, far from being unsuitable for seawater
service, is one of the most suitable bronzes of all.
Nickel aluminium bronze is even better, and is widely used when
money is no object, or when small amounts are involved
To make sure you were not onto something I didn't know about, I checked with one of the "bibles" on the topic, "Corrosion Metal Environment Reactions", Vol I of which runs to over 1400 pages.
Here's an excerpt which is relevant to this discussion, particularly the first paragraph:
<< Many of the alloys of copper are more resistant to corrosion than is copper
itself, owing to the incorporation either of relatively corrosion-resistant
metals such as nickel or tin, or of metals such as aluminium or beryllium that
would be expected to assist in the formation of protective oxide films.
Several of the copper alloys are liable to undergo a selective type of corrosion in certain circumstances, the most notable example being the dezincification of
brasses.>>
I noted a couple of instance of 'dealuminisation' in the book, but they were not in the body text; they appeared in the title line of a couple of obscure fifty-year old scientific papers. Dealuminification did not appear.
In comparison, the word "dezincification"appeared roughly a hundred times.
But this pricked my interest, as I have never encountered selective dealloying (aka 'parting') of aluminium from bronze, either in the real world or in the literature, so I searched further.
And came across this, in
"Aluminium Bronze Alloys - Corrosion Resistance Guide"
when I hunted for "dealuminification", I found this:
<<It can be very largely prevented under most conditions of
service by ensuring that the alloy used is free from gamma 2 phase.>>
Gamma 2 phase is a particular structure of an intermetallic compound of copper and aluminium, which forms only on
cooling, and only in certain conditions.
Which probably explains why it is not a well-known problem: the metallurgists who
work for bronze founders would surely know all about it, though; it's their job to make sure nobody else needs to know about it.
For anyone interested in the detail, and the implications for Nickel Alu Bronze:
<<The danger of selective phase attack occurring on the gamma 2 phase in aluminium bronzes has already been discussed in Section 1, where it was also explained that the formation of this phase can be avoided by suitable control of composition and/or cooling rate.
Under free exposure conditions in fresh waters or sea water, aluminium bronzes free from gamma 2 phase do not show selective phase corrosion but, under crevice conditions, beneath deposits or marine growths or under the influence of galvanic corrosion or of electrical leakage corrosion, selective phase attack can occur. In the alphabeta alloys this takes the form of slightly preferential attack on the beta phase.
In the nickel aluminium bronzes, selective phase attack may affect small amounts of residual beta phase if any is present, but is more likely to affect the narrow band of alpha phase immediately adjoining the lamellar kappa and to spread from that into the kappa phase itself. This selective phase attack in aluminium bronzes is not usually of great significance and occurs only when they are subjected to particular severe service conditions. For such conditions of service it can be beneficial to apply to nickel aluminium bronze castings the heat treatment required in DGS Specification 348 (six hours at 675°C + 15°C followed by cooling in still air). This is, however, only necessary if the rate of cooling of the casting from about 900°C has been too rapid for formation of the normal alpha-plus-kappa structure.>>
In short: the problem is negligible for aluminium bronzes, and minuscule for nickel aluminium bronzes. Which helps explain the popularity of the latter for propellors, where the amount of metal is not large, and even shallow corrosion is potentially very consequential.
The problems they mention for alu bronzes, even in the worst cases, tend to be self limiting. They are not in any way comparable with the crevice corrosion vulnerability of, say, 316 stainless.
Stainless is almost unique in having the capacity to attack itself under certain (regrettably, commonly encountered) conditions.
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