TC4 titanium alloy holds an indispensable position in numerous fields such as aerospace, marine engineering, chemical processing, and medical applications due to its exceptional specific strength, outstanding corrosion resistance, and excellent biocompatibility. Existing researches indicate that the superior corrosion resistance of titanium alloys primarily stems from the stable, dense passivation film that spontaneously forms on its surface. This passivation film effectively isolates the corrosive medium from direct contact with the metal substrate. However, when titanium alloys serve as connecting components, such as bolts, pipe joints, and welds, crevice corrosion inevitably occurs at these interfaces. Researches indicate that in halide-containing solutions, particularly those with chloride ions (Cl-), crevice corrosion has become one of the most destructive forms of corrosion for titanium and its alloys. Fluoride ions (F-), also belonging to the halogen group, readily adsorb onto the passivation film of TC4 titanium alloy due to their small atomic radius. This poses a serious threat to the stability of the passivation film. Furthermore, F- sources are widespread, such as industrial wastewater discharges and the dissolution of seabed fluorite deposits, among others, generates F- in the environment. Consequently, the detrimental effects of F- on TC4 titanium alloy are difficult to avoid. Most existing studies have been conducted under oxygen-rich conditions. However, in actual service environments, particularly oxygen-depleted zones such as crevices, the corrosion process often becomes significantly more complex. When titanium alloy components are assembled or connected with other dissimilar metals, crevices form. These crevices provide channels for corrosive media in the marine environment, leading to crevice corrosion. This seriously threatens the safe operation of marine engineering. In this work, TC4 titanium alloy (Ti-6Al-4V) was employed to investigate the effect of crevice width and crevice depth on its corrosion behavior with a self-made crevice corrosion device. A combination of weight loss tests, local electrochemical measurements, and characterization techniques (including SEM, XRD and XPS) was used to investigate the corrosion rate, corrosion product composition and electrochemical characteristics. The results indicated that the corrosion products inside the crevice of TC4 titanium alloy were mainly TiF3, Na2TiF6 in the F--containing solution. With the increasing crevice depth, the content of TiO2 gradually decreased while the contents of TiF3 and Na2TiF6 increased. Within a crevice width range of 0.1 to 0.3 mm, the crevice corrosion was the most severe at 0.2 mm. At this specific width, the potential difference between the innermost region of the crevice and its opening reached a maximum and continued to increase with prolonged exposure time. The anodic behavior under oxygen-deficient conditions accelerated corrosion within the inner region, facilitating the formation of a semi-occluded cell. The accumulation and hydrolysis of Tin+ ions in the anodic region lowered the local pH, thereby promoting the inward migration of F- ions. A sustained corrosion current was established in this process, flowing from the inner region to the crevice opening, which resulted in the preferential initiate and growth of pits in the inner region of crevice. In the F--containing solution, the crevice corrosion of TC4 titanium alloy is most pronounced at the bottom of the crevice, with corrosion pits generated there.
Key words
TC4 titanium alloy /
F- ions /
crevice corrosion /
crevice width /
crevice depth /
localized electrochemical testing
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Funding
University and Research Institute Researchers' Service Projects for Enterprises (25GXKJRC00035)
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