Low-carbon structural steels 161atmospheres and weathering steels are not generally recommended for use in
such environments.
In spite of the large amount of corrosion data that has been gathered on
weathering steels, the mechanism of their superior performance is still relatively
obscure. Studies by the United States Steel Corporation t3 showed that Cor-Ten
steels rusted faster than carbon steel in the initial stages and it was only after a
period of eight days that carbon steel showed a greater gain in weight. After that
period, both materials had developed a continuous coveting of rust mounds but
those on the carbon steel grew to a larger size and eventually spalled from the
surface. With Cor-Ten A, splitting was less frequent and no spalling was observed.
X-ray diffraction work showed that the rust on both types of steel consisted
essentially of y- Fe203.H20 in the initial stages, but after about 30 days ~-
Fe203.H20 was detected. However, in addition to iron oxides, iron sulphates
(FeSO4.3H20, FeSO4.7H20 and Fe2(SO4)3) have been detected in the rust layers
formed on steel in polluted atmospheres. It is suggested tentatively that the bene-
ficial alloying elements render these sulphates less soluble and thereby retard the
penetration of air and moisture through the oxide layer to the steel interface.
Horton 14 has examined the effect of individual alloying elements on the
corrosion resistance of Mayari R steel (Bethlehem Steel Corporation). The base
composition used in this work was as shown in Table 2.5.
The results are summarized in Figure 2.14(a), which shows the effect of vari-
ations in a single alloying element in the above base. The corrosion penetration
of the base steel was 2.9 mm and is represented as a horizontal line. Copper
was not examined in this investigation but Horton lists the following order of
effectiveness for other elements:
Most beneficial P
Cr
Si
Ni
No effect Mn
Detrimental SHorton also analysed data on Cor-Ten A steel which involved the base composi-
tion shown in Table 2.6. These results are shown in Figure 2.14(b) and (c) which
deal with industrial and marine (Kure Beach, North Carolina) sites respectively.
Table 2.5
C% Si% Mn% P% S% Cr% Ni% Cu%
0.08 0.28 0.70 0.10 0.03 0.60 0.40 0.60
Table 2.6
C% Si% Mn% P% S% Cr% Ni% Cu%<0.10 0.22 0.25-0.40 O. 10 <0.02 0.63 '--0.5 0.42