Environmental Engineering Reference
In-Depth Information
Mo alloy, and Duriron normally used for handling sulfuric acid. This particular
chart has been constructed by superimposing the individual isocorrosion charts
and selecting the 20 mpy curve for each material. For applications where a corro-
sion rate of up to 20 mpy can be tolerated, the suitability of the materials at
different concentrations and temperatures is indicated in the chart.
Yet another type of summary chart is available showing demarcated areas of
identical corrosion rate for a wide variety of materials as a function of concentra-
tion and temperature. The summary chart of corrosion rate of less than 20 mpy
for various materials is shown in Fig. 4.3 and the materials falling in each zone
are detailed in the table that follows.
It should be remembered that the corrosion rates indicated in these charts
correspond to the rate of general corrosion under the static condition of the me-
dium. The propensity for localized attack is not indicated. The flowing condi-
tions of the medium, particularly high velocities, tend to bring about a drastic
increase in corrosion rate. Also, the erosion corrosion behavior of steels (and
other materials as well) varies widely depending on their composition and heat
treatment. For example, a hot water sprinkler used in a rubber curing plant was
reported to have failed prematurely because of the widening of the orifices due
to erosion corrosion as the material used was a 0.1% carbon steel against the
prescribed material of 0.2% carbon steel, which proved too soft for the operation
concerned.
The corrosion behavior of some of the important metals and alloys of construc-
tion are discussed briefly in the following sections. It should be borne in mind
that nonmetallic materials like ceramics, polymers, and concrete perform better
than metals in some corrosion applications and are economical as well.
4.1.1 Cast Irons
Cast irons can be grouped into three categories on the basis of their alloy content:
unalloyed irons, low- and moderately alloyed irons, and the high-alloy irons.
The unalloyed irons of gray varieties inherently contain silicon from 1.5% to
3%. The corrosion behavior of cast irons differs from that of steel because of
their high silicon content and the presence of graphite flakes. Due to its cathodic
nature, graphite remains at the casting surface on selective dissolution of the iron
matrix (Section 3.7.3). The attack on the underlying metal retards subsequently
if the corrosion products are retained in the network of graphite flakes. The pres-
ence of silicon leads to the formation of a dense and adhering iron oxide-iron
silicate subscale that retards further attack. This behavior of gray iron contrasted
with mild steel is schematically represented in Fig. 4.4. Under conditions where
corrosion rate continues to be high, iron castings are still preferred to steel be-
cause these can be produced with thicker sections providing a satisfactory length
