Effect of Ni Addition on Corrosion Behavior of Weathering Steels Under Rust Layers in a Wet-Dry Cyclic Environment Containing Chloride Ions
Authors: Shimizu Motonari , Sugawara Yu , Omoda Masataka , Ootsuka Shinji 
Published: December 2020

Weathering steels are low alloy steels containing Cu, Ni, and Cr. Weathering steels form dense and adherent corrosion products during a long-term exposure to the outdoor environments. The protective rusts inhibit the diffusion of corrosive species such as chloride ions and oxygen to steel surfaces, resulting in reduction of the corrosion rate. However, it is well known that the protective rusts are not formed in the environments with a high chloride concentration, and weathering steels are not available in coastal areas. It is reported that Ni as an alloying element is effective to reduce the corrosion rate of weathering steels in chloride-containing environments.1 Although many researches showed that the protectiveness of rust layers was improved by the Ni addition to weathering steels,2,3 its detailed mechanism has not been clarified. In this study, we focused on the corrosion behavior of the weathering steels under rust layers. We investigated the corrosion resistance and the corrosion morphology of Ni-added weathering steel, conventional weathering steel, and carbon steel in a wet-dry cyclic environment containing chloride ions, and discussed the effect of Ni addition on the improvement of the corrosion resistance of weathering steels.
Carbon steel (CS: Fe-0.1 mass% C), weathering steel (WS: Fe-0.1 mass% C- 0.3 mass% Cu-0.2 mass% Ni-0.5 mass% Cr), and 2.5 % Ni-added weathering steel (NiWS: Fe-0.1 mass% C-0.3 mass% Cu-2.5 mass% Ni) were used as the specimens. The specimens were polished with a 1500 grit SiC paper, and cleaned ultrasonically with ethanol. The cyclic wet and dry corrosion test was conducted according to The International Organization for Standardization (ISO) 16539 Method A. The dew point of water vapor in the air was kept at 301 K. We used 0.017 mM NaCl as the electrolyte, and an electrolyte film was formed on the specimens every cycles. The thickness of the electrolyte film was 500 μm and the amount of chloride ion deposition was 0.3 g m-2. After 7, 14, and 28 cycles of the cyclic corrosion test, we removed the rust layers and measured mass loss of the specimens. In addition, we observed the cross section of the specimens with the rust layers using FE-SEM equipped with an EDS system.
WS and NiWS recorded lower mass loss rates in the cyclic corrosion test compared with CS. The especially lower mass loss of NiWS was observed. NiWS showed the highest corrosion resistance in the steels under the wet-dry cyclic environment containing chloride ions. After 28 cycles of the cyclic corrosion test, we observed the cross section of a rust steel interface of the specimens by FE-SEM. In the case of CS and WS, the corrosion test increased the roughness of steel surfaces under the rust layers. On the other hand, the surface roughness of NiWS under the rust layers was small compared with CS and WS. From EDS results of the steels, the migration of chloride ions to the rust/steel interface was observed. In the case of CS and WS, the signal of Cl was strongly detected at locally-corroded parts. The cross section of NiWS shows the signal of Cl was widely distributed near the rust/steel interface. It is thought that the Ni addition inhibited the enrichment of chloride ions in the rust/steel interface.

Reference:

1) Toshiyasu Nishimura, Hideki Katayama, Kazuhiko Noda, Toshiaki Kodama, Corros. Sci., 42, 1611(2000).
2) Wei Wu, Xuequn Cheng, Huaxing Hou, Bo Liu, Xiaogang Li, Appl. Surf. Sci., 436, 80(2018).
3) Wei Wu, Xuequn Cheng, Jinbin Zhao, Xiaogang Li., Corros. Sci., 165, 108416(2020).