Steels_ Metallurgy and Applications, Third Edition

158 Steels: Metallurgy and Applications

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Figure 2.11 Basis prices of steel plates as of March 1997
lower rate than plain carbon steels and, under favourable climatic conditions, they
can develop a relatively stable layer of hydrated iron oxide which retards further
attack. Very often, reference is made to the brown 'patina' that develops on the
surface of these steels and the attractive appearance that it presents in buildings
and bridges. However, regardless of their aesthetic qualities, weathering steels can
provide cost savings by eliminating the initial painting operation and subsequent
maintenance work.
These steels were first introduced in 1933 by the United States Steel Corpo-
ration under the brand name Cot-Ten and have been licensed for manufacture
throughout the world. However, they have not been used extensively in the UK
because frequent rain inhibits the formation of the stable oxide layer and rusting
continues, albeit at a relatively slow rate. Ideally, long dry summer periods are
required to develop the adherent oxide layer.
Corrosion resistance
The mid-specifications of the chemical composition in the Cor-Ten series are
given in Table 2.4. Cot-Ten A is the original high-phosphorus grade whereas
Cor-Ten B and Cor-Ten C were introduced in the 1960s. These later steels have
normal levels of phosphorus and are micro-alloyed with vanadium to provide
high strength.