What Does Parting Limit Mean?
The parting limit is the minimum concentration of a noble element in an alloy above which simultaneous dissolution of both components occurs rather than selective dissolution of the less noble component. Selective dissolution of less noble components leaves behind a porous structure of mostly noble metal. This causes corrosion damage due to porous layer formation, resulting in reduction of mechanical resistance of an alloy.
Parting limits have been suggested in many alloy systems, particularly in binary systems. Parting limit is the threshold where selective dissolution is possible. Copper-zinc and copper-aluminum alloys exhibit well-defined parting limits.
Parting limit is also known as de-alloying threshold.
Corrosionpedia Explains Parting Limit
Parting is the selective corrosion of one or more components of a solid-solution alloy. Parting limit refers to the minimum concentration of a more noble component in the alloy, above which parting does not occur within a specific environment.
De-alloying is a significant factor, especially in stress corrosion cracking (SCC) and corrosion fatigue, and parting limit is an important parameter associated with de-alloying. Parting limit has significant importance in corrosion studies and noble metal technology.
De-alloying displays sharp parting limits, expressed as critical atom percentages of the more reactive component above which that component can be removed from the alloy by electrochemical dissolution in an oxidizing environment such as nitric acid.
Parting limits range from about 20 to 60 atomic percent (at. %). This concept is still used in noble metal technology to separate noble from base metals. For example, an alloy of 55 at. % gold and 45 at. % silver does not de-alloy, but if it is re-melted with additional silver so that the atom fraction of Ag is greater than 60 percent, the gold can be separated almost completely by nitric acid immersion.