The corrosion behavior of carbon steel coated with a zinc-rich paint containing two metallic compounds, Al 2 (SO 4 ) 3 and CaO, as anticorrosive additives was examined under wet and dry cyclic corrosion test conditions. The zinc-rich paint coating without the two metallic compounds formed a white corrosion product and red iron rust on the surface after the corrosion test, whereas the coating with the metallic compounds showed reduced surface corrosion products. The corrosion current density of the painted steel substrate decreased drastically due to the incorporation of metallic compounds in the paint. The zinc-rich paint coating modified with the metallic compounds contained dispersed simonkolleite (Zn 5 (OH) 8 Cl 2 •H 2 O) phase and possibly very fine CaSO 4 particles, which remarkably improved the protectiveness of the zinc-rich paint coating.

We have examined the corrosion behavior of a rusted carbon steel coated with a paint containing metallic salt under wet and dry cyclic condition. The rust layer of the carbon steel was already grown in a coastal region of Japan before the coating. It was found that the addition of metallic salt to the coating film decreased the corrosion rate of the coated steel. The X-ray diffraction analysis of the rust layer revealed that the addition of metallic salt resulted in drastic increase of α-FeOOH during the cyclic corrosion test. The rust layer under the coating film containing metallic salt suppressed penetration of Clto the steel surface. Electrochemical analyses indicated that the coating with metallic salt decreased cathodic reduction rate of the rust layer due to the formation of α-FeOOH in the rust layer.

In the present work, the modification of a rust layer on a carbon steel surface was examined during a cyclic corrosion test. The rust layer grown in a coastal region of Japan was used as a model rust layer. The x-ray diffraction analysis of the model rust layer revealed that the layer consisted of α-FeOOH, β-FeOOH, γ-FeOOH, and Fe3O4. During the cyclic corrosion test, an additional immersion of the surface in solutions containing metal cations such as Mg2+, Al3+, Cu2+, or Ni2+ was performed. The additional immersion in the cation-containing solutions modified the model rust layer, that is, the fraction of stable α-FeOOH increased in the rust layer. Furthermore, the modification of the model rust layer could suppress the corrosion of steel. The decreased corrosion rate was attributed to the suppressed cathodic reduction of the rust layers due to the presence of the stable α-FeOOH in the rust layers.

The present work proposed a simple approach to modify the morphology and composition of rust layers formed on carbon steel during wet/dry cyclic atmospheric corrosion, and thereby improve the protective properties of the rust layers. Rust layers were grown on carbon steel by a laboratory cyclic corrosion test. Additional immersion in sulfate solutions containing Mg 2+ , Al 3+ , Cu 2+ , or Ni 2+ was carried out during the cyclic corrosion test to modify the rust layers. The morphology of the rust layers on carbon steel in the reference specimen that was not subjected to additional immersion showed a plate-like structure. The rust layers on the Mg 2+ specimen consisted of plate-like and needle structures. On the other hand, the other specimens subjected to additional immersion exhibited particulate structures, although the rust layers on the Cu 2+ specimen consisted of finer secondary particles. Rust layers on the Al 3+ specimen and the Ni 2+ specimen were denser compared to the other specimens. XRD revealed that the composition of the rust layer was changed by additional immersion, that is, on all the specimens subjected to the additional immersion, the growth of ¡-FeOOH was enhanced whereas that of Fe 3 O 4 was hindered. Variations in the corrosion potential and corrosion current density obtained from potentiodynamic polarization measurements were strongly related to the morphology and composition of the rust layers.