Characterization of rust on Fe-Cr, Fe-Ni, and Fe-Cu binary alloys by Fourier transform infrared and N2 adsorption

The structure and composition of the rust formed on the binary Fe-Ni Fe-Cr. and Fe-Cu alloys exposed at different situations for 2 years and 3 years were investigated by N-2 adsorption, Fourier transform infrared (FTIR), transmission electron microscopy (TEM), and inductively coupled plasma-atomic emission spectrometry (ICP-AES) to explore the role of alloying metals in the corrosion resistance of weathering steels. The rust particles formed in all exposure sites were strongly aggregated and microporous. The specific surface area (SA) obtained by N-2 adsorption decreased and then increased by increasing the contents of Ni and Cr. The increase of SA, that is, the reduction of particle size of rust, was remarkable in the exposure site with a high airborne salinity. This result verifies that alloying metals are effective for the corrosion restraint of steels in Cl- environments such as coastal and marine districts. The content ratio of alpha-FeOOH to gamma-FeOOH in rust (alpha/gamma) showed a maximum at alpha/gamma = ca. 0.5, and the corroded amount was very small at alpha/gamma > ca. 1.5, indicating that the alpha/gamma ratio reflects the stability of rust at alpha/gamma > ca. 0.5.