Corten steel in contact with other materials The design of exterior wall cladding in Corten steel requires special attention. Limeparts takes this into account in the design of its exterior walls: materials that are below the exterior wall such as plaster, sandstone, bricks, natural stone, … can be discoloured by water that runs off of the Corten steel cassettes. This water contains dirt and oxides that can cause a brown discolouration. We never use Corten steel in the vicinity of the sea. swimming pools and next to public roads because the presence of chlorine salts causes abnormal oxidation.
The Nyquist plots obtained in 3.5% NaCl solution after 2nd and 4th week of cyclic wet/dry salt spray test for all the steels are shown in Figure 3&4. The Nyquist plots for all the samples showing double time constants are represented by two semi circles. The corresponding equivalent circuit model used for the impedance data is RS (Qedl Rct). Here, RS is the solution resistance between reference electrode and metal surface, Qedl&Rct are the constant phase elements of the electric double layer (EDL) and the charge transfer resistance of the metal/electrolyte interface, respectively. As per equivalent circuit modeling as well as Figure 3 and 4, the value of Rct is highest for the high phosphorus steel (G12) followed by G11 and SCR. However, the trend of Rct value for G11 and SCR has been reversed although in a close range after 4th week of test. This may be due to the effect of higher Si content in SCR. Rct is the lowest for low carbon steel in both the cases as no alloying element is present in this steel. The results obtained from wet-dry cyclic corrosion test can be compared with the literature where similar types of studies are carried out. A. Nishikata et al.had reported that in a relatively mild environment, the corrosion rate of weathering steel is similar to that of C-Mn steel at the early stage of exposure. However, the corrosion rate was reduced after long term exposure due to the formation of dense and compact rust layer. This behaviour is also evident from the Nyquist plot in Figure 4 after 4 weeks of exposure. Rct has been increased for G11, SCR and G12 dramatically than that shown in Figure 3. The higher phosphorus containing steel G12 has shown more Rct value both after 2 and 4 weeks of exposure. This is attributed to the formation of some phosphatic intermediate layer as observed by Kihira et al.