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Steel Designers' Manual - 6th Edition (2003)
35.8 Weather-resistant steels
t.
Weather-resistant steels 1045
Fig. 35.2 Typical corrosion losses of structural (mild) steel and weather-resistant steels in
the UK
Weather-resistant steels are high strength, low alloy weldable structural steels which
possess good weathering resistance in many atmospheric conditions without the
need for protective coatings. They contain up to 2.5% of alloying elements, e.g.
chromium, copper, nickel and phosphorus. On exposure to air, under suitable conditions,
they form an adherent protective rust layer. This acts as a protective film,
which with time causes the corrosion rate to reduce until it reaches a low terminal
level, usually between 2 and 3 years.
Conventional structural steels form rust layers that eventually become nonadherent
and detach from the steel surface. The rate of corrosion progresses as a
series of incremental curves approximating to a straight line, the slope of which is
related to the aggressiveness of the environment. With weather-resistant steels, the
rusting process is initiated in the same way but the alloying elements react with the
environment to form an adherent, less porous rust layer. With time, this rust layer
becomes protective and reduces the corrosion rate (see Fig. 35.2).
Weather-resistant steels are specified in BS EN 10155: 1993, and within this
category Corten is one of the best known proprietary weather-resistant steels.These
steels have mechanical properties comparable to those of grade S355 steels to BS
EN 10025: 1993.
35.8.1 Formation of the protective oxide layer
The time required for a weather-resistant steel to form a stable protective rust layer
depends upon its orientation, the degree of atmospheric pollution and the frequency