The Relationship Between Microstructure and Corrosion Behaviour of Titanium Under Simulated Crevice Corrosion Conditions

Morgan, Adam

The Relationship Between Microstructure and Corrosion Behaviour of Titanium Under Simulated Crevice Corrosion Conditions

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Morgan_Adam_PhD_2025_Thesis.pdf (11.1 MB)

Date

2025-10-21

Authors

Morgan, Adam

Abstract

Titanium is often selected over conventional materials for applications requiring superior corrosion resistance. The corrosion behaviour of a given Ti material is strongly linked to its microstructure and composition, where even small variations in the alloying element(s) and/or impurity content can result in major microstructural changes. Certain elements promote the precipitation of intermetallic particles (IMPs), producing locations that are heavily enriched in alloying elements and impurities relative to the nominal composition. The reactivity of IMPs has been shown to strongly influence the corrosion behaviour of Ti materials; however, further research is required to develop a thorough understanding of the influence of microstructure on corrosion behaviour. This thesis focuses on linking the microstructure and corrosion behaviour of ASTM (American Society for Testing and Materials) Grade-2 Ti (Ti-2), Grade-7 Ti (Ti-7), and Grade-12 Ti (Ti-12) under simulated crevice corrosion conditions, a common failure process.

Microstructural characterization determined that Ni, Mo, and Fe all exclusively localize to IMPs in Ti-2 and Ti-12. For Ti-7, Pd was present in the matrix and only slightly enriched at Fe-rich IMPs, provided the alloy contained sufficient Fe to form IMPs. Titanium hydride (TiHx) formation on Ti-2 was determined to initiate and propagate at TixFe IMPs before the hydride formed a uniform surface layer that impeded further hydrogen absorption. Mass spectroelectrochemistry (MSEC) measurements showed that Fe-rich IMPs are preferential sites of corrosion for all alloys, regardless of whether the specimen displayed a passive or active potential. Furthermore, Ti-7 displayed the lowest overall dissolution rates, attributed to the distribution of Pd within the matrix rather than its sole localization in IMPs. The MSEC results indicate that the role of Mo in Ti-12 is related to stabilizing the IMPs.

The findings in this thesis provide new insights into the roles of alloying and impurity elements, the compositional changes accompanying cathodic modification, and the role of microstructure on corrosion behaviour. A consistent finding was that IMPs are sites of enhanced reactivity that can have a significant effect on corrosion performance. These results can be applied toward improving alloy design and material selection criteria.

Summary for Lay Audience

Titanium is a metal that offers a unique combination of excellent corrosion resistance, a high strength-to-weight ratio, and biocompatibility, making it an attractive material for demanding applications. The properties of Ti can be altered by alloying with specific elements, leading to a wide range of available Ti alloys. Titanium and titanium alloys are often selected over more common structural metals like steel, aluminum, and copper for high-performance applications, including aerospace, medical, and chemical processing industries, where they are used in components such as engine parts, medical implants, and heat exchangers. Due to these extreme applications, failure of titanium components can have severe financial, environmental, and even human health consequences. Although titanium has outstanding corrosion resistance under a wide range of conditions due to the presence of a protective surface oxide, in certain instances, when the oxide is breached, it can experience significant corrosion. In industrial service, the two most common modes of failure are crevice corrosion and hydrogen-induced cracking. Crevice corrosion occurs in narrow gaps and occluded regions, such as under washers, bolts, clamps, and even under deposits/debris. Hydrogen induced cracking results from hydrogen absorption during crevice corrosion or when the titanium is in electrical contact with less noble metals, such as steel or copper. Despite significant progress in understanding these corrosion processes, the corrosion of titanium remains a complex process that is strongly influenced by many factors. In this thesis, electrochemical and surface analytical techniques were employed to investigate the corrosion behaviour of three titanium alloys to provide new insights into the relationships between alloy composition, impurity levels, service conditions, and corrosion behaviour.

Keywords

Titanium, Corrosion, NATURAL SCIENCES::Chemistry::Analytical chemistry::Electrochemistry, Titanium Hydride, Intermetallic Particles, Cathodic Modification, Passivity, Crevice Corrosion, Hydrogen Absorption, Mass Spectroelectrochemistry