Effect of Cations on the Growth of Atmospheric Rusts on Carbon Steel in Wet/Dry Environments

Weathering steels, manufactured by the addition of small amount of alloying elements such as Ni, Cr, Cu and Al to carbon steel, exhibit high resistance to atmospheric corrosion due to a protective rust layer formed in atmospheric environments1-3. Weathering steel exhibits the similar corrosion rate to carbon steel in the early stage of the exposure to atmospheric environments, but after several years of the exposure, weathering steel can form the protective rust layer of mainly α-FeOOH and thus corrosion rates of weathering steel becomes significantly lower compared with the carbon steel. Due to this characteristic, weathering steel has been used in large-scale structures such as bridge. Recently we proposed a novel surface treatment to facilitate the formation of stable α-FeOOH without the addition of alloying elements to carbon steel. The present work reports the effects of the surface treatment on the growth of atmospheric rust layers on carbon steel. Materials were carbon steel sheets (ss400) with the size of 20x27x3 mm3. The steel surfaces were polished with SiC abrasive papers up to #600, then rinsed in methanol and dried in air. The laboratory cyclic corrosion test of SAE J23344 was applied to the samples. The SAE J2334 simulates a severe atmospheric corrosion environment and is extensively used to evaluate atmospheric corrosion resistance of steels. In the SAE J2334 test, polished samples were first kept for 6 h in the box where temperature and relative humidity(R.H.) were controlled at 50℃ and 100%, respectively. Then the samples were immersed in a mixture of 0.5 mass% NaCl, 0.1 mass% CaCl2 and 0.075 mass% NaHCO3 for 15 min, finally followed by holding the samples for 17 h 45 min under the dry condition at 60℃ and R.H. of 50%. These three steps were repeated for 50 cycles. From the 2nd cycle the immersion in a solution of 1M NiSO4, CuSO4, Al2(SO4)3, or MgSO4 for 5 s was carried out at the beginning of each cycle. The morphology and structure of atmospheric rusts formed during the cyclic corrosion test were characterized with SEM, Raman and XRD, respectively. X-ray diffraction (XRD) and Raman spectroscopy indicated that the rust layers after 3, 6, 9, 18, 30 and 50 cycles in the cyclic corrosion test consist of α-FeOOH, γ-FeOOH, β-FeOOH and Fe3O4. Based on peak intensity in XRD pattern and the Raman spectra of the rust layers, it was found that the immersion of a carbon steel in Ni2+, Al3+, Cu2+ containing sulfate solutions facilitated the formation of stable α-FeOOH. Stable α-FeOOH were found as major constituents of corrosion products on treated samples in Ni2+, Al3+ or Cu2+ containing sulfate solutions. The surfaces of the samples were observed by scanning electron microscopy (SEM). The rust layers formed without the immersion in the sulfate solutions and MgSO4 solution consist mainly of γ-FeOOH whereas the samples immersed in Al2(SO4)3, NiSO4 and CuSO4 were composed mostly of α-FeOOH. That is, the findings by SEM observations are in well accordance with the XRD and Raman results. Reference [1] I. Suzuki, Y. Hisamatsu, N. Masuko, J. Electrochem. Soc. 127 (1980) 2210-2215. [2] H. Schwitter, H. Böhni, J. Electrochem. Soc. 127 (1980) 15-20. [3] Y. Zhou, J. Chen, Y. Xu, Z. Liu, J. Mater. Sci. Technol. 29 (2013) 168-174 [4] SAE J2334, Cosmetic Corrosion Lab Test, 1998. SAE International, Warrendale, PA.