What Does Dealloying Mean?
Dealloying is an electrochemical reaction in which one element constituting an alloy is selectively separated and removed from the alloy, causing deterioration of its essential properties.
The study of dealloying is significant because:
- It causes widespread damage and failures in specific alloys.
- Damage can be monitored and failures can be prevented.
- Heat treatment methods are being developed to avoid and minimize this type of corrosion.
Dealloying may also be known as selective leaching.
Corrosionpedia Explains Dealloying
Dealloying is an unusual type of corrosion, occuring mainly in certain alloy metals such as copper alloys as well as in gray cast iron. When the dealloying takes place, the alloy metal loses its reactive element and retains the more stable corrosion-resistant element in a porous state.
The mechanism of dealloying corrosion can be explained with the help of the difference of materials in the galvanic series, wherein one of the alloying elements acts as an anode with respect to the other element, in the presence of any electrolyte, which can initiate an electrochemical reaction.
For example, for brass alloy, the reactive component, zinc, becomes the anode and is selectively removed due to galvanic corrosion, while copper remains in a spongy, porous surface.
In the case of cast iron, the main constituents are iron and graphite, and graphite becomes the anode to be removed by galvanic action. The cathodic material, such as copper, remaining behind is structurally bound to the original shape by the remaining anodic matter. The location of corrosion attack is distinguished by a change of color. Depth and severity of attack can be thus easily recognized by this change. Dealloying can be found as sporadic localized pits or over the entire surface, depending upon electrolyte and the other factors. The properties, such as mechanical strength, of the remaining material are significantly reduced and can fail during normal loading.
Weight loss due to dealloying corrosion is not an indicator of any impact of dealloying. The depth of attack must be measured. The impact on the mechanical strength can be evaluated through mechanical testing. In some cases, the depth of attack stops at a certain stage, in a self-limiting phenomenon. This phenomenon is significant, as in the case of thick wall components, the depth of attack has been observed to be around 1/4 th of an inch.
If the alloy components are well mixed and they are in the same phase, such alloys are generally less vulnerable to this type of attack compared to components of alloys, where phases are largely different. Particular heat treatment methods have been found and developed for making the alloys homogeneous and less vulnerable to dealloying corrosion attacks.