Environmental Engineering Reference
In-Depth Information
hinder deposition of copper. However, since the dezincification of
brass is not
prevented by such alloying, the question remains regarding the mechanism.
More recent studies indicate that both processes occur in separate but overlap-
ping potential regimes.
β
Remedial Measures
1.
Use of a more resistant alloy is the most practical approach to minimize the
occurrence of dezincification as the control of environment is not often feasi-
ble or economical. Red brass with less than 15% zinc is almost immune.
Improvement in the performance of yellow brass (70-30) has been achieved
with the addition of 1% tin (admiralty brass) and small amounts of arsenic,
antimony, or phosphorus. Cupronickels provide better substitute in severely
corrosive environments.
2.
Removal of stagnation of corrosive, particularly if acidic.
3.
Periodic removal of scales and deposits from the inside surface of pipelines.
4.
Use of cathodic protection.
3.7.2 Dealloying of Other Copper Alloys
Selective leaching of aluminum takes places in copper-aluminum alloys (alumi-
num bronze) when exposed to hydrofluoric acid or acids containing chloride ions.
The alloys containing more than 8% Al are particularly susceptible; these are
two-phase alloys containing a copper-rich
α
phase and an eutectoid of
α
phase
and an aluminum-rich
and is preferentially
attacked. Dealloying of nickel in copper-nickel alloys has been observed in refin-
ery condenser tubes under the condition of high heat flux and low water velocity
at temperatures above 100
γ
phase. The
γ
phase is anodic to
α
C. Selective leaching of tin in tin bronzes in hot brine
or steam and desiliconization of silicon bronzes in high-temperature steam con-
taining acidic species have been reported.
°
3.7.3 Graphitic Corrosion
Selective leaching of iron in gray iron is termed
graphitic corrosion
. This is
observed in gray iron pipes buried in soil or manhole covers exposed to mildly
corrosive waters and in similar appliances. Iron leaches out selectively, leaving
the cathodic interconnected graphite flakes as a porous mass on the metal surface
(Fig. 3.25). There is little change in metal thickness, but the entire structure is
rendered weak and tends to give way to fluid pressure from within or to impact
load from outside. Graphitic corrosion does not occur in ductile iron or malleable
iron because in these varieties graphite does not form a network to hold the metal
residue.
The term ''graphitization'' is sometimes used to describe this phenomenon,
