Mössbauer Studies on Particle Volume Distribution of α-FeOOH in Rust Formed on Weathering Steel

Rust formed on weathering steel and that on mild steel were studied by Mössbauer spectroscopy at various temperatures from room temperature to 12 K. The observed spectra were decomposed into the contribution of fine antiferromagnetic particles of α-FeOOH (85 at.%, T N =403 K) and that of γ-FeOOH (15 at.%, T N =73 K). Volume distributions of α-FeOOH particles obtained by analyzing the superparamagnetic relaxation effect on Mössbauer spectra show the following distinct characteristics: While 83 vol% of α-FeOOH particles in the rust on mild steel are larger than 7.5×10 -24 m 3 and the rest of them (17 vol%) are smaller than 3.0×10 -24 m 3 , the rust on weathering steel shows a continuous and wide volume distribution down to the ultrafine particle region (smaller than 3.0×10 -24 m 3 ) without a missing region, and 53 vol% of particles are smaller than 7.5×10 -24 m 3 . It is understood that this continuous volume distribution of α-FeOOH particles in the rust on weathering steel is favorable for accommodating a closely packed structure in the protective rust layer on weathering steel.