Titanium is a highly reactive metal that is extremely susceptible to hydrogen absorption. When the hydrogen content within titanium reaches 0.008%–0.015%, needle-like hydride phases precipitate within the material-a phenomenon that can be directly observed via metallographic microscopy. As the hydrogen content continues to rise, the quantity and volume of these precipitates increase, compromising the stability of the titanium matrix and triggering various forms of corrosion and failure.
Corrosion and damage in titanium resulting from hydrogen absorption and subsequent hydride formation generally manifest in three ways:
1. Surface hydrogen-embrittlement spalling: If the rate of hydrogen diffusion is relatively slow, hydrides tend to concentrate at the surface. These surface hydrides become brittle and spall off, significantly accelerating the corrosion process on the material's surface.
2. Stress-induced hydrogen-induced cracking (HIC): Under the influence of external stress, hydrogen atoms migrate toward regions of stress concentration, where they precipitate as hydrides. Micro-cracks within the material expand and coalesce under sustained stress, ultimately leading to failure via hydrogen-induced cracking.
3. Bulk hydrogen embrittlement: Extensive, high-level hydrogen absorption throughout the titanium leads to widespread degradation of the matrix microstructure, resulting in catastrophic, bulk-scale hydrogen embrittlement and total mechanical failure of the material.
Key Characteristics and Progression Patterns of Hydrogen Embrittlement in Titanium
I. Response of Mechanical Properties to Hydrogen Embrittlement
Hydrogen-induced failure in titanium is a classic form of hydride-based hydrogen embrittlement, characterized by a pronounced sensitivity to deformation rate: brittle fracture occurs only under high-speed deformation, whereas the material exhibits virtually no susceptibility to hydrogen embrittlement under low-speed deformation conditions. In terms of compositional thresholds, a hydrogen content exceeding 0.03% in titanium significantly reduces the material's reduction of area. Conversely, when the hydrogen content is below 0.05%, conventional mechanical properties-such as tensile strength, yield strength, and elongation-remain largely unaffected. This indicates that standard mechanical testing is insufficient for effectively assessing the risk of hydrogen embrittlement in titanium materials.
II. Influence of Iron Impurities on Hydrogen Absorption and Corrosion in Titanium
During the processing of titanium, contact with iron tools-resulting in surface abrasions or the adhesion of iron particles-creates sites for the initiation of pitting corrosion. These exogenous iron impurities also serve as primary pathways for hydrogen atoms to penetrate the titanium matrix, thereby substantially increasing the likelihood of both hydrogen absorption and corrosion. In contrast, trace amounts of iron dissolved within the titanium matrix, as well as iron-rich secondary phases present in the microstructure, do not exacerbate pitting corrosion, nor do they exert any significant negative impact on hydrogen absorption or the occurrence of hydrogen embrittlement.
III. Influence of Surface Treatment on Resistance to Hydrogen Embrittlement
The susceptibility of titanium materials to hydrogen absorption and hydrogen embrittlement is closely linked to the surface pretreatment method employed. This primarily depends on the integrity and stability of the surface oxide film, with different processes offering varying degrees of protection:
1. Surfaces subjected to anodizing or thermal oxidation feature an intact and dense oxide film, providing the best resistance against hydrogen absorption and hydrogen embrittlement.
2. Surfaces treated via pickling (using a mixture of nitric acid and hydrofluoric acid) or annealing offer an intermediate level of protection.
3. Surfaces processed through mechanical polishing or sandblasting exhibit the highest level of chemical activity; the resulting oxide film provides poor protection, making these surfaces most susceptible to hydrogen absorption and subsequent hydrogen embrittlement. Thus, an intact and stable surface oxide film serves as the primary barrier against the ingress of hydrogen atoms and the prevention of hydrogen embrittlement in titanium, whereas an exposed, chemically active titanium substrate is highly prone to failure caused by hydrogen absorption.
IV. Primary Pathways for Hydrogen Absorption in Titanium
Hydrogen absorption in titanium typically occurs under the following four types of operating conditions:
1. Environments containing pure hydrogen or hydrogen-bearing gases at temperatures above 300°C.
2. Environments where nascent (atomic) hydrogen is generated during processes such as crevice corrosion or corrosion in reducing inorganic acids.
3. Environments where hydrogen atoms are produced as a result of galvanic corrosion or cathodic protection.
4. Seawater environments where titanium components act as a cathode and are maintained at a potential below -0.70 V. In industrial applications, to prevent failure due to hydrogen absorption, the operating potential of titanium equipment in seawater must be strictly controlled and maintained above -0.70 V.
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