(Invited) Measurement of Salt Abundance on Weathering Steel Structures By Portable XRF
Authors: Takebe Masamichi , Ohya Makoto , Matsumoto Ayaka 
Published: December 2020

In suitable environments, weathering steels can be used for bridges without anticorrosion painting, because weathering steels have enough resistance to atmospheric corrosion by the protective rust generated on its surface. While, abundant supply of salt causes serious corrosion damage on weathering steels. Hence, deterioration due to corrosion is found in weathering steel bridges in conditions that salt abundantly supplied. When serious damages with corrosion are found in the components of weathering steel bridges, salt abundance on the components is measured to estimate reason of the corrosion and determine the repairing method. The salt abundance on the components is usually measured by surface salinity checker, which determines salt abundance by electrical conductivity of water that is in contact with an analyzed area. However, measurement of salt abundance by surface salinity checker is difficult for the component with serious corrosion damages, because porous rust or rough surface on the damaged component cause leakage of the contact water during the analysis. Therefore, measuring method for salt abundance without using water is needed to know salt abundance on corroded components for the maintenance of weathering steel bridges.

In order to discuss the possibility of salt abundance measurement by portable XRF, characteristic X-ray intensity of Cl-and abundance of Cl- on five types of test pieces of weathering steels were analyzed by portable x-ray fluorescence analyzer (XRF) and gauze wipe method with ion chromatography respectively, in this study. The two types of test pieces are new weathering steels with coating of A-5 or C-5. Three types of test pieces were artificially corroded previously. For the two types of the corroded test pieces, coating with Ra-III or Rc-I were applied after removing rust. The other type of corroded test pieces was used without rust removing and coatings. The rust thickness on the test pieces was about a few 100 µm. To make air-born salt accumulation on each test pieces, sheltered exposure test was performed in the test site on the roof of the three-story building of the National institute of Technology, Matsue College, which is about 10 km far from sea. After that, characteristic X-ray intensity of Cl- on the test pieces was measured by portable XRF. Cl- on the test pieces was sampled by gauze wiping method, and the Cl- abundance was analyzed by ion chromatography. The characteristic X-ray intensity and abundance of Cl- on test pieces with coatings show good correlation (Fig. 1), suggesting that portable XRF is possible tool for measurement of Cl- abundance on the steels. While, the intensity of characteristic X-ray of Cl- on the corroded test pieces without removing rust shows poor correlation with the Cl- abundance. The intensity of characteristic X-ray of Cl- on the corroded test pieces without removing rust is extremely higher than that estimated from its Cl- abundance by gauze wiping method, considering the correlation among the test pieces with coatings. Since Cl- on weathering steels are known to be also in rust [3], completely sampling of Cl- by gauze wipe method is probably difficult for the corroded test pieces. Considering to the correlation line among the test pieces with coatings, the X-ray intensity of Cl- on corroded test pieces without removing rust potentially suggests that Cl- abundance on the test pieces are a few 100 mg/m2. For more accurate measurement of Cl- abundance on weathering steels with rust, the clear relation between characteristic X-ray intensity and Cl- abundance on weathering steels covered with rust should be revealed. To found the relation, method for complete sampling of Cl- on weathering steels with rust is required. In addition, it is needed to investigate detection range of Cl- in rust of weathering steels by portable XRF in terms of depth direction from surface.

References

[1] T. Kamimura, S. Hara, H. Miyuki, M. Yamashita, H. Uchida, Composition and protective ability of rust layer formed on weathering steel exposed to various environments, Corros. Sci. 48 (2006) 2799–2812.

[2] S. Hara, T. Kamimura, H. Miyuki, M. Yamashita, Taxonomy for protective ability of rust layer using its composition formed on weathering steel bridge, Corros. Sci. 49 (2007) 1131–1142.

[3] A.Imai, M. Gagasawa, K. Ishida, M. Sano, H.Kihira, Study on surface state monitoring methods with supplemental rust controlling surface treatment, Journal of Japan Society of Civil Engineers, 70 (2014) 252-264.

Acknowledgement

We wish to express appreciation to Dr. Imai, Nippon Steel, Anti-Corrosion Co., Ltd. for supporting the analysis by portable XRF.

Figure 1