Page 51 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

METALLURGICALLY INFLUENCED CORROSION 29

Figure 1.12 Uniform dealloying of admiralty brass. (Reproduced by permission, John Wiley
and Sons (8).)

1.5.1.11 Dealuminification Recent studies have shown the importance of the
dealloying of S-phase (Al CuMg) particles on the corrosion of aluminum aircraft
2
alloys such as 2024-T3. In this alloy, the S-phase particles represent nearly 60% of
the particle population. These particles are of the order of 1 mm in diameter with
a separation of the order of 5 mm amounting to a surface area fraction of 3%. The
selective removal of aluminum and magnesium from these alloy particles leaves
behind porous copper particles that become the preferential site for oxygen reduction
(45, 46).
Dealloying has also been observed in the case of Ag–Au, Cu–Au, Cu–Pt, Al–Pt,
Al–Cu, Cu–Zn–Al, Cu–Ni, and Mn–Cu alloys (46).
Evidence for dealloying has been reported in austenitic stainless steel and
iron–nickel alloys in acidified chloride solutions, reduction of titanium dioxide in
molten calcium chloride, Cu–Zn–Al alloy in NaOH solutions giving rise to Raney
metal particles (46).

1.5.1.12 Mechanism of Dealloying In the dealloying process, the mechanism
involves alloy dissolution and replating of the cathodic element or selective dis-
solution of an anodic alloy constituent. In both types of mechanism, the residual
product left behind is spongy and porous and loses most of its strength, hardness,
and ductility (9). In the case of brass, the attack involves dissolution of both zinc and
copper and subsequent deposition of copper. In the case of gold and silver, selective
removal of silver is observed in 0.1 M HClO .
4
Two models have been advanced to explain the dissolution and rapid supply of
more active or less noble metal. According to one model, the less noble metal dis-
solves. The remaining more noble element is now in a highly disordered state and
begins to reorder by surface diffusion and nucleation of islands of almost pure metal.
The coalescence of these islands continues to expose fresh alloy surface where further
dissolution will occur, leading to the formation of tunnels and pits (4).