What Does Dealuminization Mean?
Dealuminization is a type of corrosion that consists of the selective loss of aluminum in aluminum bronzes and nickel-aluminum bronzes. Massive effects have been observed in crevices on aluminum bronze where the solution contains some chloride ions.
The primary concerns with dealuminization are:
- Adverse effects on the ultimate tensile strength of components
- Potential for non-ductile failure
- Loss of leak tightness
It is a concern for nuclear power generation facilities that utilize aluminum-bronze alloy piping and piping components for the transport of brackish water or seawater.
Some aluminum alloys are susceptible to this form of corrosion, particularly in seawater, and specify suitable measures (either temper anneal or compliance with chemistry formula) for these castings. This approach eliminates premature failures due to dealuminization.
Corrosionpedia Explains Dealuminization
Dealuminification is the specific process of the leaching of aluminum in aluminum-bronze (or other Cu-Al) alloys. Aluminum bronze is commonly used in seawater or brackish water applications because of its good resistance to corrosion and erosion.
Such attacks are exceedingly rare, if not unknown on the single-phase alloys, and especially those containing less than 8.5% aluminum. Dealuminization is generally confined to the higher aluminum, multi-phase alloys, and then only to attack on the gamma-2 phase, whose formation results from the slow cooling of such materials.
Dealuminization causes a loss of strength from the loss of the aluminum and the less-than-100% dense copper that is left behind. In seawater, rapid selective corrosion can occur in aluminum bronzes and nickel-aluminum bronzes, particularly under conditions where hydrogen sulfide is formed. One example of this is found in a water jet engine made of nickel-aluminum bronze, used in a high-speed vessel.
When dealuminization is significant, it can be detected visually, identified by the presence of a deep copper color. Methods for preventing dealuminization in seawater include:
- Giving the casting a special heat treatment known as a temper anneal
- Increasing nickel content suppresses the formation of the gamma-2 phase. If gamma-2 forms on slow cooling, the material will not suffer dealuminization in service.
It can be used to produce powdered materials with extremely high surface area, such as Raney nickel. Raney nickel catalyst can be prepared by selectively removing aluminum from Al-Ni alloy by action of a caustic.