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Dealloying Corrosion
Most of the commonly used metallic material are alloys formed from mixing
two or more metals. Pure metals are usually too soft and weak to be used
structurally. In this
form of corrosion,
dealloying, corrosion selectively attacks one or more
constituent of the alloy mixture.
Definition of Dealloying.
Dealloying is the selective corrosive attack of one or more constituent of a
metallic alloy.
Mechanism of Dealloying Corrosion.
As can be seen from the
galvanic series, constituents of many common alloys
have widely separated positions on the galvanic series. In the case of brass,
the main constituents are zinc and copper. In the case of cast iron, the main
constituents are iron and graphite.
When the surface of such alloys is exposed
to an electrolyte, galvanic action proceeds with the more anodic material being
selectively attacked. In many cases, the
cathodic
material remains behind and is bound into its original shape by a residue of
remaining anodic material and corrosion products.
The strength of the remaining
material is, however, greatly reduced and will often fail during
normal handling.
Single phase material, where the alloy constituents are
well mixed, are often less susceptible to this form of attack than alloys where
phases of largely different composition are present.
In many alloys, heat
treatments have been developed specifically to make the alloy more homogeneous
and less susceptible to dealloying.
Examples of Dealloying Corrosion
The dezincification of
brass
and the graphitization of cast iron are common examples of dealloying.
Appearance of Dealloying Corrosion.
In dealloying, the size and shape of the original component is often
retained. The remaining constituent is often a different color than the original
alloy and the depth and location of attack can be easily identified by this
color change. Dealloying can either occur over the entire surface (layering) or
localized in
pits (plug type).
Significant Measurements.
Weight loss is not a significant measurement of the impact of dealloying.
The depth of attack must be measured by sectioning and microscopic examination.
The impact of dealloying on the strength of the material can be assessed through
mechanical testing. In many cases, the depth of attack is self-limiting,
particularly in the plug type of attack but the limiting depth is significant,
often in the order of 1/4 inch. The fact that there is a limiting depth is
significant only for very thick walled sections.
Source : "Corrosion Control" NAVFAC MO-307 September 1992
see also:
Corrosion Types
Galvanic Corrosion Prevention
What is Rust?
What is Corrosion? |