Materials and Corrosion 2012. 63. No. 1 Electrochemical corrosion failure of leakage on titanium tubes Fe 2p3 oIs Ti 2p3 2p3 C T1 2p3 Ti 2p3 Binding Energy (ev) Figure 7. XPS results of the outside wall near the rupture on tube A:(a)all elements, (b)titaniu left less than 0.5 mm gaps in some places of the tubes due to the service temperature was only about 35-40C, much lower unsymmetrically hydraulically expanding, as seen in Fig. 2. As a than the critical temperature. So how could the crevice corrosion result, seawater was provided with access into the shell side from occur? This may be ascribed to the complicated environment in the inlet of the tubes, and simultaneously introducing chloride the gap of less than 0.5 mm. Let us review ions. Then, these chloride ions would particularly accumulate in mechanism of crevice corrosion at first. In this electrochemical the gaps between the 78 mm-thick tube sheet and the titanium corrosion, at the beginning, the metal exposed to the air acts as tubes, and gradually concentrate on the outside wall surfaces of the anode and will be oxidized to cations, while the oxygen within the titanium tubes under the gaps, eventually initiating crevice the gap serves as the cathode to be reduced. Once the oxygen is orrosion[23] as well as galvanic corrosion. exhausted, this dissolved oxygen reaction ceases. As a result, the tiani ctuaily, crevice corrosion is a common degradation on potential of the metal under the crevice decreases due to lack of ium, and has already attracted a wealth of researches to oxygen, and the potential of the metal exposed to the air still identify its influencing factors [24]. For example, He et al. [25] remains at a relatively high value. Then, an occluded corrosion observed that iron content between 0.042 and 0.078%(wt%) cell (OCC)is formed, and will keep attacking the metal under the would increase the possibility of crevice corrosion on titanium; crevice (i.e, the titanium tube in our event) in an autocatalytic while Schutz [26 designated the threshold temperature for mode with the assistance of the localizedly accumulating initiation of crevice corrosion on titanium as 70C. Come back to hydrochloric acid that is generated by both the hydrolysis of our event, in which the content of iron in the titanium tube was metal cations and the introduction of chloride ions from the about 0.07%, conforming to the opinion of He et al. [25: however, environment. All the relevant electrochemical reactions are www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheimleft less than 0.5 mm gaps in some places of the tubes due to unsymmetrically hydraulically expanding, as seen in Fig. 2. As a result, seawater was provided with access into the shell side from the inlet of the tubes, and simultaneously introducing chloride ions. Then, these chloride ions would particularly accumulate in the gaps between the 78 mm-thick tube sheet and the titanium tubes, and gradually concentrate on the outside wall surfaces of the titanium tubes under the gaps, eventually initiating crevice corrosion [23] as well as galvanic corrosion. Actually, crevice corrosion is a common degradation on titanium, and has already attracted a wealth of researches to identify its influencing factors [24]. For example, He et al. [25] observed that iron content between 0.042 and 0.078% (wt%) would increase the possibility of crevice corrosion on titanium; while Schutz [26] designated the threshold temperature for initiation of crevice corrosion on titanium as 70 8C. Come back to our event, in which the content of iron in the titanium tube was about 0.07%, conforming to the opinion of He et al. [25]; however, the service temperature was only about 35–40 8C, much lower than the critical temperature. So how could the crevice corrosion occur? This may be ascribed to the complicated environment in the gap of less than 0.5 mm. Let us review the familiar mechanism of crevice corrosion at first. In this electrochemical corrosion, at the beginning, the metal exposed to the air acts as the anode and will be oxidized to cations, while the oxygen within the gap serves as the cathode to be reduced. Once the oxygen is exhausted, this dissolved oxygen reaction ceases. As a result, the potential of the metal under the crevice decreases due to lack of oxygen, and the potential of the metal exposed to the air still remains at a relatively high value. Then, an occluded corrosion cell (OCC) is formed, and will keep attacking the metal under the crevice (i.e., the titanium tube in our event) in an autocatalytic mode with the assistance of the localizedly accumulating hydrochloric acid that is generated by both the hydrolysis of metal cations and the introduction of chloride ions from the environment. All the relevant electrochemical reactions are Materials and Corrosion 2012, 63, No. 1 Electrochemical corrosion failure of leakage on titanium tubes 13 Figure 7. XPS results of the outside wall near the rupture on tube A: (a) all elements, (b) titanium www.matcorr.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim