The structure of corrosion products, also referred to as rust, formed on the
steel surface is of great interest, because the corrosion rate of steel is known to
depend strongly on the condition of such corrosion products covering the surface
as a protective film. For example, the corrosion rate of weathering steel in
the atmosphere is found to be considerably suppressed when an amorphouslike
rust layer is formed. This enables us to reduce maintenance costs and
also contributes to the beneficial utilization of iron natural resources.
On the other hand, corrosion products of iron and steel are formed by reaction
of iron and its alloying elements with reactive species, such as oxygen
and water, which originate from the atmosphere. Corrosion products are frequently
not describable by the typical structure of iron oxides. This indicates
that the structure of corrosion products is considered to be very complicated,
and some parts of such iron oxides are assigned to the amorphous state for
this reason. Thus, a method for characterizing the atomic-scale structure of
corrosion products is strongly required, because it is likely to be related to
a corrosion mechanism of metallic materials. Lifetime of steel is sometimes
dominated by environmental degradation, so that the structure of corrosion
products formed on the surface of steel during exposure to air for a prolonged
period is of great importance. For example, the Delhi iron pillar, which was
made in India about 1600 years ago, still stands without serious degradation,
whereas steel structures produced by modern manufacturing methods
are sometimes replaced after use for a few tens years.