Corrosion Basics
Understanding the basic principles and causes of corrosion
Corrosion Behavior of Cast Irons
ast iron is a generic able service life of gray cast iron piping,
C
many environments. The notable excep-
sometimes well beyond 100 years. tion is hydrofluoric acid (HF). In fact,
term that applies to
these high-silicon irons are the most uni-
high-carbon/iron al- White Cast Irons versally resistant of the commercial (non-
loys containing sili- White cast irons have practically all of precious) metals and alloys. Their inher-
their carbon in the form of iron carbide. ent hardness makes them resistant to
con. The common These are extremely hard and brittle. erosion-corrosion. A straight high-silicon
ones are designated as gray Silicon content is low because this ele- iron contains about 14.5% silicon and
ment promotes graphitization. Graphite 0.95% carbon. This composition must be
cast iron, white cast iron, mal- formation is related to the rate of cooling closely controlled within narrow limits to
leable cast iron, and ductile or from the melt, so chilling can produce a provide the best combination of corro-
nodular cast iron. white iron from one that would normally sion resistance and mechanical strength.
be gray. The excellent corrosion resistance of
Gray Cast Irons high-silicon irons is due to the formation
Malleable Cast Irons of an inert silicon dioxide (SiO2) surface
Gray cast irons contain about 2 to 4%
Malleable cast irons are produced by layer, which forms during exposure to the
carbon and 1 to 3% silicon. These are the
high-temperature heat treatment of environment.
least expensive of the engineering metals.
white irons of suitable composition. The High-silicon cast irons are available
The dull or grayish fracture is due to the
graphite forms as rosettes or clusters
free graphite flakes in the microstructure. only in cast form for drain lines, pumps,
instead of flakes, and the material shows valves, and other process equipment.
Gray cast irons can be readily cast into
good workability (hence the name mal- Because of their low consumption rates,
intricate shapes because of their excellent
leable).
fluidity and relatively low melting points. high-silicon cast irons have also found
They can also be alloyed for improved extensive use as anodes for impressed
Ductile Cast Irons
corrosion resistance and strength. current cathodic protection.
These materials, also known as nodu-
In most cases, gray cast iron is about lar cast irons, exhibit ductility in the as-
twice as resistant to natural waters as steel Other Alloy Cast Irons
cast form. The graphite is present as nod-
because of the bonding of rust products In addition to silicon, molybdenum,
ules or spheroids as a result of special
by the graphite flakes. In modern engi- nickel, chromium, and copper are added
ladle additions to the molten metal. The to cast irons for improved corrosion,
neering practice, most cast-iron piping mechanical properties of ductile cast
receives an internal cementitious lining abrasion, and heat resistance, and
irons can be altered by heat treatment enhanced mechanical properties. In par-
for improved water-side corrosion resis- similar to that used for ordinary steels.
tance. Soft or low pH waters may cause ticular, copper additions impart better
The superior mechanical properties of resistance to sulfuric acid (H2SO4) and
graphitic corrosion, also called graphiti- ductile irons permit the manufacture of
zation. Graphitization of gray cast iron is atmospheric corrosion.
thinner-walled piping compared to gray
the selective dissolution of iron from the cast iron piping with similar strength.
iron/carbon matrix. The overlapping This article is adapted by MP
This offers a significant saving in material
Editorial Advisory Board Member
graphite flakes that remain provide little costs and in handling.
resistance to pressure surges or mechani- Norm Moriber from Corrosion Ba-
cal stresses; however, the graphitized pipe High-Silicon Cast Irons sics—An Introduction, Second Edi-
can often withstand normal operating When the silicon content of gray cast tion, Pierre R. Roberge, ed. (Hous-
pressures for many years. This character- iron is increased to more than 14%, it ton, TX: NACE International, 2006),
istic is partly responsible for the remark- becomes extremely corrosion resistant in pp. 288-290.
80 MATERIALS PERFORMANCE May 2009
