Environmental Engineering Reference
In-Depth Information
of service. Unalloyed white cast irons because of their inherent high hardness
are resistant to erosion corrosion.
Low and moderately alloyed irons contain a few tenths of copper and moderate
additions of nickel and chromium, which impart better resistance to atmospheric
corrosion and erosion corrosion. Ni-Hard is an example of this group, which
contains 4% Ni and 2% Cr.
The high-alloy irons are classified into three types:
1.
The austenitic alloys
. These irons are alloyed with nickel and chromium,
with or without copper, for improved corrosion and abrasion resistance. Me-
chanical properties and heat resistance are also improved. These are known
as Ni-Resist. Seven varieties of Ni-Resist contain 14-32% Ni and 1.75-
5.5% Cr.
2.
The high-chromium alloys
. These contain 15-30% Cr and are white irons.
They are particularly resistant to high-temperature oxidation and erosion cor-
rosion.
3.
The high-silicon irons
. The addition of more than 12% Si to irons makes
them extremely hard and corrosion-resistant because of the formation of a
passive SiO
2
surface layer. Duriron is an example of this group, which con-
tains 14.5% Si and 0.95% carbon.
Unalloyed and low-alloy cast irons perform better than steels in many atmo-
spheric exposures because the corrosion products form a protective film. Sulfur
dioxide in industrial atmospheres and chlorides in marine atmospheres tend to
increase corrosion. For domestic water supply cast iron pipes have been success-
fully used over centuries. Cast iron water pipe installed at Versailles, France in
1664 is still in service. Gray cast irons, however, suffer from severe graphitization
and pitting when exposed to seawater. Ductile iron gives a better performance
and is being increasingly used for marine appliances such as machinery founda-
tions, valve bodies, tanks, cargo pipings, etc.
Unalloyed gray iron is commonly used for stills in the manufacture of acetic
and fatty acids, but it corrodes rapidly in dilute solutions of organic acids such
as acetic, oxalic, citric, or lactic. Low-alloy cast irons perform better. Unalloyed
and low-alloy cast irons are also not resistant to corrosion in dilute to medium
concentrations of sulfuric acid and nitric acid and in hydrochloric acid of all
concentrations. However, Ni-Resist alloys are fairly resistant. High-chromium
cast irons have excellent resistance to dilute nitric acid. Duriron shows excellent
resistance to sulfuric acid (Fig. 4.2) and nitric acid of all concentrations, and the
modified grades (Durichlor) containing 2-3% Mo or Mo and Cr possess im-
proved resistance to hydrochloric acid, chlorides, and pitting. These are widely
used for drain lines, pumps, valves, and as anodes for impressed current cathodic
protection.
