Selective leaching
In metallurgy, selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the initially homogenous material.
The less noble metal is removed from the alloy by a microscopic-scale galvanic corrosion mechanism.
The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, e.g. copper and zinc in brass.
The elements most typically undergoing selective removal are zinc, aluminium, iron, cobalt, chromium, and others.
The material remaining is a copper-rich sponge with poor mechanical properties, and a color changed from yellow to red.
Affected surfaces develop a layer of graphite, rust, and metallurgical impurities that may inhibit further leaching.
Similar effects for different metals are decarburization (removal of carbon from the surface of alloy), decobaltification, denickelification, etc.
The way that porosity develops during the dealloying process has been studied computationally to understand the diffusional pathways on an atomistic level.
Thus, the slowest step, and that which is most important for determining rate of porosity evolution is the dissolution of these higher coordinated less noble atoms.
Therefore, as dissolution proceeds, any more noble atoms will move to more stable positions, like steps, where its coordination is higher.
[3] Due to the relatively small sample size achievable with dealloying, the mechanical properties of these materials are often probed using the following techniques:[5] A common concept in materials science is that, at ambient conditions, smaller features (like grain size or absolute size) generally lead to stronger materials (see Hall-Petch strengthening, Weibull statistics).
However, due to the high-level of porosity in the dealloyed materials, their strengths and stiffnesses are relatively low compared to the bulk counterparts.
The GA relations can be used to estimate the strength and stiffness of a given dealloyed, porous material, but more extensive study has revealed an additional factor: ligament size.
[7] However, when the ligament size is under 100 nm, which is very common in many dealloying processes, there is an addition to the GA strength that looks similar to Hall-Petch strengthening of bulk polycrystalline metals (i.e., the yield stress increases with the inverse square root of grain size).
Combining this relationship with the GA relation from before, an expression for the yield stress of dealloyed materials with ligaments smaller than 100 nm can be determined:[3]
Either way, there would be significant surface and small volume effects in the ligaments <100 nm, which lead to this increase in yield stress.
[7] A relationship between ligament size and Young's modulus has not been studied past the GA relation.
[3] Occasionally, the metastable nature of these materials means that ligaments in the structure may "pinch off" due to surface diffusion, which decreases the connectivity of the structure, and reduces the strength of the dealloyed material past what would be expected from simply porosity (as predicted by the Gibson-Ashby relations).
[3] Dislocation behavior is extensive within the ligaments (just as would be expected in a metal): a high density.
of partial dislocations, stacking faults and twins have been observed both in simulation and in TEM.
[3] However, the morphology of the ligaments makes bulk dislocation motion very difficult; the limited size of each ligament and complex connectivity within the nano-porous structure means that a dislocation cannot freely travel long distances and thus induce large-scale plasticity.
[3] Countermeasures involve using alloys not susceptible to grain boundary depletion, using a suitable heat treatment, altering the environment (e.g. lowering oxygen content), and/or use cathodic protection.
Selective leaching can be used to produce powdered materials with extremely high surface area, such as Raney nickel and other heterogeneous catalysts.
