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Dealloying Corrosion

 
     
                                                                                           

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

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