Effect of Cr on Corrosion Resistance of Corten Steel Pipe in Marine Atmospheric Environment

Under the action of alloying element Cr, the oxide film on the surface of corten steel pipe becomes more dense, which greatly improves its passivation ability. Yamashita et al. studied the corrosion resistance mechanism of Cr element. The results showed that with the increase of Cr content, alpha-FeOOH was refined obviously. When the content of Cr was higher than 5%, the invasion of chloride ions was effectively inhibited. In addition, the results of Kamimura et al. show that in the process of alternating wetting and drying, Cr can effectively prevent the reduction of trivalent iron in the drying stage, ensure the formation of stable rust layer and improve the corrosion resistance of corten steel pipe.

Through the analysis of the surface rust layer of corten steel pipe exposed to the sun for three years in Qingdao and Hainan, it is found that with the change of dry-wet alternating atmospheric environment, the different Fe, H, O products produced by electrochemical corrosion reaction on the surface of corten steel pipe will develop to thermodynamic stable state, showing different morphologies in turn, namely (FeXHYOZ) →γ-FeOOH→α-FeOOH→α-Fe2O3. In the analysis of the rust layer, it is found that the presence of Cr will obviously accelerate the transformation process and make α-FeOOH tend to distribute in the inner region of the rust layer. In addition, due to the formation of part of CrxFe1-xOOH, the rust layer has the ability of cation selection, which can effectively prevent the penetration of chloride ions and sulfate ions, and let the rust layer play a protective role.

Through the analysis of corten steel pipe rust layer exposed to the sea atmosphere, it is found that Cr can form infinite solid solution in Fe, so low solubility in oxide compound on the surface will inevitably lead to secondary distribution of Cr, some Cr forms ferrochromium alloy oxide in the rust layer, and some Cr is rich at grain boundary and microcracks in the rust layer. Collecting energy promotes the formation of denser rust layer, and some Crs are excluded from the matrix because they exceed the solubility and enriched in the matrix at the interface, thus improving the corrosion resistance of corten steel pipe in marine atmospheric environment.