Description
Most metals used in engineering are alloys; mixtures of more than one metal or element. Dealloying is a corrosion process that may selectively attack, and leach out one of the elements, normally the more reactive, leaving a porous and weakened structure behind which is rich in the less reactive element. One of the most common dealloying corrosion mechanisms is the dezincification of brass.
Mechanism
The mechanism of dealloying is caused by the differences in electrical potential of different elements and hence phases within the alloy, and this is a form of galvanic corrosion. For example, brass is an alloy of copper and zinc with zinc being more reactive than copper as they are anodic and cathodic respectively in this combination. In the presence of an electrolyte, a corrosion cell is set up and the zinc, or zinc-rich phase, is selectively attacked leaving a copper-rich and porous material behind. Often, the zinc is reprecipitated out elsewhere on the component as a white powdery deposit.
Some brasses exhibit a duplex microstructure consisting of alpha and beta phases with the beta phase being richer in zinc than the alpha phase. In this case the beta phase is more prone to attack.
Copper-aluminium and copper-nickel alloys can also suffer from dealloying, or dealuminification and denickelification respectively which are the same processes as dezincification. Some cobalt-based alloys may also be prone to decobaltification.
Appearance and Examples
The surface of an alloy may exhibit corrosion deposits or changes in colour. The image below shows the flaking of paint from the surface of a brass water fitting, revealing a pink-coloured area; this is the copper-rich phase left over from dezincification.
Microexamination of a section through this location revealed the early stages of dezincification and specifically the preferential attack of the beta phase in addition to deposition of copper-rich phases on the surface as shown below.
Avoidance
The main methods of avoiding dealloying relate to either changing the environment, or modification of the alloy to reduce the more reactive elements. For example, the use of copper alloys with lower levels of zinc would be preferable such as those that contain <15%Zn. These alloys tend to have a microstructure consisting of a single phase, rather than the dual or duplex phase structure of high zinc copper alloys (e.g. >40%Zn). Elements such as antimony, arsenic, and phosphorus may also be added to reduce the potential (i.e. inhibited) for dezincification and some grades are specified as being resistant to dezincification.
The dealuminification of copper-aluminium alloys may be caused by secondary phases within the microstructure due to incorrect heat treatments and so tight controls of these treatments is crucial.
Since dealloying is a corrosion process, measures taken to reduce the overall corrosion potential should be considered. This may include prevention of the build-up of scale or other deposits that can alter the local environment, but also by the use of cathodic protection e.g. sacrificial anode or impressed current protection.
