Environmental Engineering Reference
In-Depth Information
Table 3.2
Combinations of alloys and environments for selective leaching
Alloy
Environment
Element removed
Aluminum
Hydrofluoric acid, acid chloride solutions
Aluminum
Bronzes
Brasses
Many waters
Zinc
Cupronickels
High heat flux and low water velocity (in
Nickel
refinery condenser tubes)
Gray iron
Soils, many waters
Iron
Gold alloys
Nitric, chromic and sulfuric acids, human
Copper or silver
saliva
High-nickel alloys
Molten salts
Chromium, iron,
molybdenum,
tungsten
Iron-chromium
High-temperature oxidizing atmospheres
Chromium
alloys
Medium- and
Oxidizing atmospheres, hydrogen at high
Carbon
high-carbon
temperatures
steels
Monel
Hydrogen and other acids
Copper in some
acids, nickel in
others
Nickel-
Oxygen at high temperatures
Molybdenum
molybdenum
alloys
Silicon bronzes
High-temperature steam, acidic solution
Silicon
Tin bronzes
Hot brine, steam
Tin
low in salt content and at room temperature. Plug-type attack is favored in neutral
and alkaline water, high in salt content and above room temperature.
Dezincification gets accelerated under the conditions of high temperatures,
stagnant solutions, and porous inorganic scale formation. Crevice conditions un-
der a deposit or scale tend to aggravate the situation.
Mechanism
Two different mechanisms have been suggested for dezincification: (1) simulta-
neous dissolution of the components and redeposition of copper as a porous layer,
and (2) selective dissolution of zinc from the lattice leaving behind a porous
copper-rich structure.
The selective dissolution mechanism envisages the following steps:
1.
Anodic dissolution of zinc, which may proceed in pure water even in the
absence of oxygen with the cathodic reduction of water to hydrogen and
