Crevice Corrosion Of Titanium

Author : Xihua He
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (16 download)

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Download or read book Effects of Temperature, Impurities, and Alloying Elements on the Crevice Corrosion of Alpha Titanium Alloys written by Xihua He and published by . This book was released on 2003 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis is a study of the crevice corrosion behaviour of commercially pure titanium, Ti Grade-2 (Ti-2), and two Ti dilute alloys, Ti Grade-12 (Ti-12, Ti-0.8Ni-0.3Mo), and Ti Grade-7 (Ti-7, Ti-0.15Pd) in air saturated 0.27 mol/dm3 NaCI solution using a galvanic coupling technique. Various aspects of the crevice corrosion process were studied to understand the primary factors that influence titanium's susceptibility to this form of corrosion and to determine the conditions under which crevice attack is possible. First, the temperature dependence of crevice corrosion initiation and propagation on Ti Grade-2 has been studied in detail. Crevice corrosion experiments in which the temperature was increased and decreased in steps over the temperature range 55 degrees C to 95 degrees C have identified a temperature threshold of 65 degrees C for the initiation of crevice corrosion. This is based on the onset (increasing temperature) and disappearance (decreasing temperature) of film breakdown/repassivation microtransients generated inside the crevice. Consistent with the initiation of crevice corrosion, the passive film resistance dropped significantly at T>: 65 degrees C accompanied by water incorporation into the oxide film. Early initiation occurred at individual sites around the edge of the crevice, and the acidity that consequently developed spreads towards the central region of the crevice. Then, the microstructure and crevice corrosion behaviour of three Ti-2 materials containing different levels ofFe as impurity (0.04 - 0.12 wt%), were studied. The grain size decreased significantly with increasing iron content. Crevice corrosion was initiatedon all three materials at 70 ± 5 degrees C. Crevice corrosion of Ti-2 with an Fe content of 0.078 wt% (medium Fe content) was initiated at the lowest temperature and exhibited extensive intergranular attack due to the accumulation ofFe in the grain boundaries. Ti-2 with a low iron content (0.042 wt%) was free of intergranular attack. By contrast, corrosion damage was more localized along the periphery of the crevice for Ti-2 with an Fe content of 0.12 wt% (high Fe content), and the crevice corrosion resistance was greatly enhanced. By performing a series of crevice corrosion experiments combined with metallographic and image analysis techniques on Ti-2 with Fe 0.078 wt%, a damage function relating the maximum penetration depth to the duration of propagation was developed for crevice corrosion. The damage function showed two distinct stages. In the first stage, penetration was rapid due to the intergranular nature of the penetrating front on this material. On the second stage, penetration was greatly limited by IR (ohmic) effects at the corroding sites. Thirdly the crevice corrosion behaviour of Ti-12 was studied. Crevice corrosion on Ti-12 initiated a similar temperature to that observed for Ti-2, -70 degrees C, but showed a higher resistance to crevice propagation. The propagation process was independent of temperature. The crevice attack appeared in the form of rectangular-shaped pits with limited propagation. Secondary ion mass spectrometry (SIMS) mapping of elemental distributions on the surfaces of these pits suggested that the accumulation of Ni and Mo at the bottom of pits was the cause for the limited penetration depth. For this material, propagation driven by internal proton reduction was the predominant process. Lastly the high temperature corrosion of Ti-7 under both planar and creviced conditions was studied. Despite the presence of Pd, Ti-7 showed some general corrosion activity in the NaCI solution as indicated by oxide film breakdown/repassivation transients. The transient activity increased with temperature. Overall, however, these transients did not have a major degrading effect on the oxide film. The material remained passive under open circuit conditions. Ti-7 was also found not immune to crevice corrosion. For temperatures above 80 degrees C, it showed minor reactivity within the crevice. The crevice attack on Ti-7 appeared as very shallow pits with penetration depths