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In order to assess the protective properties of rust films formed on weathering steel, electrochemical AC impedance and harmonic current measurements were made in 0.1 M Na2SO4 solution, and an equivalent circuit for rusted weathering steel was obtained. Through analysis of the equivalent circuit, the protective properties of rust films were found to be dominated by ion migration resistance through the rust films.Based on these results, a portable instrument was developed for nondestructive and quantitative evaluation of the protective properties of rust films on weathering steel.Many existing structures and test pieces made of weathering steel exposed throughout Japan were inspected by this instrument, and a database of rust conditions were accumulated. Through the analysis of the database, an empirical criterion for the patina condition was clarified. The application of this instrument to a field structure is also reported.

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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 m3 and the rest of them (17 vol%) are smaller than 3.0×10-24 m3, the rust on weathering steel shows a continuous and wide volume distribution down to the ultrafine particle region (smaller than 3.0×10-24 m3) without a missing region, and 53 vol% of particles are smaller than 7.5×10-24 m3. 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.

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