It has well known that Ni improves the corrosion resistance of weathering steels in atmospheric corrosion environments with a relatively high chloride concentration, the improvement mechanism of the corrosion resistance of weathering steels by the Ni addition has not been fully understood. We focused on the corrosion behavior at the rust layer/steel matrix interface because it was expected that the solution chemistry of electrolyte permeating through the rust layer depended on the Ni content in the rust layer formed on weathering steels. In this study, a model interface between the rust layer and the steel matrix was fabricated and its corrosion behavior during the wet/dry corrosion test was observed. In addition, solution chemistry of the electrolyte permeating through the rust layer was evaluated to analyze the corrosion behavior of the model interface.

Commertial type weathering steels (WS: Fe-0.5Cr-0.3Cu-0.17Ni) and 2.5% Ni-containing weathering steels (2.5%Ni-WS: Fe-0.3Cu-2.5Ni) were used as the specimens. All the specimens were polished upto 100–150 µm in thickness. Figure 1a shows the schematic of the specimen to create the model interface between the rust layer and the steel matrix. After the polishing, the samples were masked by a masking tape, and fixed on a silica glass plate by an epoxy resin. The wet/dry corrosion test (ISO 16539 method A) of the specimen with the model interface was taken to form the rust layer on the specimen surface. The rust layer penetrated through the steel, the electrolyte also permeated through the rust layer (Fig. 2b). Then, the back side of the specimen was corroded by the electrolyte permeating through the rust layer. This corrosion behavior in the back side of the specimen seems to simulate that of the interface between the rust layer and the steel matrix.

Figure 1c shows the surface and back appearances of the WS specimen during the wet/dry corrosion test. The entire surface of the specimen was immediately covered by brown rusts. After 4 cycles of the wet/dry corrosion test, the rust layer penetrated the steel, and green and black rusts were formed in the back of the specimen. After that, the electrolyte permeated through the rust layer and spreaded over the back of the specimen. The steel matrix was corroded by the electrolyte permeating through the rust layer and brown rusts were formed after 6 cycles. In the case of 2.5%Ni-WS (Fig. 1d), the rust layer penetrated the steel after 6 cycles, and the electrolyte permeated through the rust layer. 2.5%Ni-WS was also corroded by the electrolyte permeating through the rust layer, black rusts were formed at the trace of the electrolyte. Raman spectroscopy measurements revealed that the brown and the black rusts indicated γ-FeOOH and Fe₃O₄, respectively. This suggested that Fe₃O₄ was formed in the inner side of the rust layer on the 2.5%Ni-WS. Solution chemistry of the electrolytes permeating through the rust layers was measured by ICP-OES and ion chromatography, and it was found that the pH of the electrolytes increased and the chloride ion concentration decreased with the Ni content in the weathering steels. The addition of Ni in the weathering steels promoted the formation of Fe₃O₄, this was thought to change solution chemistry of the electrolyte permeating through the rust layer.