into a long-lasting protective film, constant self-repairing of the
film by fusing particles together, by filling up defect spaces, or by
other means is necessary, like the passive oxide film that is selfmaintained
by repairing defects. The atmospheric corrosion of iron
and steel is composed of several reactions, e.g., formation of
hydroxide ions at the cathode, dissolution of iron(II) and alloying
metal(II) ions at the anode, air-oxidation of iron(II) to iron(III), iron(
III) hydroxo-complex formation, precipitation and dissolution of
iron(III) hydroxide, magnetite, and ferrites, transformation of these
compounds to rusts by aging, ion adsorption on rusts, and other
reactions. As a result, an atmospheric environment that brings
about the corrosion of iron and steel can be regarded to constitute
an aquatic system. It is important to assess these reactions proceeding
in aquatic systems quantitatively to elucidate the characteristics
and properties of rusts necessary to form a long-lasting
protective film. Kinetic and equilibrium calculations are made for
the reactions occurring during the rust formation, and the composition
of aquatic systems around the corroding region at different
stages of corrosion is obtained. A scenario of the corrosion process
is deduced based on the calculated results, and the observed results
for the atmospheric corrosion are explained from the scenario.
Further, the conditions where a self-maintaining protective
rust film develops are discussed. For the formation of an adherent
dense rust film, it is pointed out that the acceleration of air-oxidation
of iron(II) during corrosion is important. With rapid air-oxidation,
the deposition of iron(III) hydroxide completes within the
film before iron(II) ions can leak out from the film, which results
in the plugging of rust defects and hence the densification and
self-repair of the film. Possible mechanisms to enhance the oxidation
rate during corrosion are suggested. For rusts where defects
remain, it is noted that cation-selective permeability of the rusts
brings about protective properties because corrosive anions are
prevented from penetrating through the defects. The conditions
with which the cation-selective permeability develops in rusts
are discussed, and several compounds that are cation-selective
over a wide range of conditions are suggested as potential protective
components in rusts.
This paper studies the role of rusts in corrosion and corrosion
protection of iron and steel by analyzing the related chemical reactions
in aquatic systems based largely on the author’s previous
work. In the previous individual works, the chemical reactions
themselves were the interest of the studies, and the relation to
the corrosion of iron and steel was explored only superficially.
However, the investigation in this paper aims at the elucidation
of corrosion phenomena by integrating the author’s previous
works and offers insights into the mechanism of corrosion and
the role of rusts in corrosion and corrosion protection. The results
obtained advance the knowledge of corrosion and can be applied to
develop weathering steels.
2. Observed results for rust formation
‘‘The Research Group of Rust Chemistry” (Chairman: Prof. T.
Ohtsuka) of the Japan Society of Corrosion Engineering has carried
out the characterization of rusts formed on weathering steels and
inspected sites of corrosion in weathering-steel bridges. The results
of these activities as well as the findings for the corrosion of iron
and steel obtained so far [3–8] can be summarized as,
(a) In wet environments containing chlorides, iron and steel
corrode forming rusts with brownish colors.
(b) Occasionally rust surfaces are stained yellow and it appears
that ‘‘rust fluids” have leaked out, and the new yellow rust
formed is called ‘‘flowing rust”. The ‘‘flowing rust” indicates that
corrosion is not a static process, but that dynamic changes
occur during rusting.
(c) Rusts develop into a layer structure with repeating dense
and coarse layers.
(d) The chloride ions are concentrated at layer boundaries, the
most notably at the rust/substrate interface.
(e) The application of wet pH-test paper to rust surfaces
exposed by removing the surface layers clearly indicates the
presence of acidic (pH < 3) and alkaline (pH > 10) regions.
(f) Microscopically, many cracks form along the layer planes as
well as in the direction vertical to the layer planes, and voids are
also found in rusts.
(g) On steels constantly exposed to water, rusts keep growing
and finally detach as flakes or even as slabs at the substrate/rust
interface.
(h) Rusts formed on heavily corroded steels contain approximately
25% of Fe3O4, 10% of a-FeOOH, 5% of b- and c-FeOOH,
respectively, with the balance amorphous components.