Titanium rods are widely used in numerous industrial fields due to their excellent properties, such as high strength, low density, and excellent corrosion resistance. However, in certain electrolyte molten liquid environments, titanium rods can experience localized corrosion, which in turn can lead to hydrogen embrittlement, severely impacting their service life and performance stability. According to Titanium Home, in-depth research into localized corrosion and hydrogen embrittlement in titanium rods in electrolyte molten liquids is crucial for optimizing titanium rod processing and improving product quality.
Titanium Rod Characteristics and the Localization of Hydrogen Embrittlement
Analysis of hydrogen embrittlement cases in titanium rods demonstrates that hydrogen embrittlement is clearly localized. During the use of titanium rods, various corrosion forms, including crevice corrosion, pitting corrosion, galvanic corrosion, and stress corrosion, can all contribute to hydrogen absorption and localized hydrogenation. These corrosion forms, in different environments, damage the titanium rod's surface structure in unique ways, creating conditions for hydrogen intrusion.
Mechanisms of Localized Corrosion-Induced Hydrogen Absorption and Hydrogenation
(I) Crevice Corrosion and Pitting Corrosion
Titanium rod manufacturers point out that although the causes of crevice corrosion and pitting corrosion differ, their development process follows the principle of a closed cell. In the initial stages of corrosion, tiny crevices or pitting pits form locally on the titanium rod surface. As corrosion progresses, hydrolysis reactions occur in these areas, producing large amounts of hydrogen ions (H⁺). Due to the large amount of H⁺ formed, the pH of the solution within the crevices or pitting pits drops significantly, reaching values between 1 and 1.6, or even as low as 0.7. This low pH further promotes the dissolution of titanium, creating an autocatalytic effect and causing localized corrosion to progress at an extremely high rate. Simultaneously, hydrogen ions (H⁺) precipitate in the cathode region, some of which are absorbed by titanium, causing localized hydrogenation.
(II) Stress Corrosion
Stress corrosion is similar to the above. Under stress, defects on the titanium rod surface are more susceptible to corrosion reactions, generating H⁺ and lowering the local pH, promoting titanium dissolution and H⁺ precipitation, and absorption. Some scholars believe that stress corrosion is essentially a form of hydrogen embrittlement, which fully demonstrates the close connection between stress corrosion and hydrogen embrittlement. During titanium rod processing, residual stress or external stress can easily trigger stress corrosion, leading to hydrogen embrittlement.
(III) Galvanic Corrosion
Galvanic corrosion-induced hydrogen embrittlement is common in industry. For example, in high-temperature seawater desalination equipment, the contact between titanium and steel forms a galvanic couple; in power plant condensers, the connection between titanium tubes and copper alloy tube sheets also forms a galvanic couple. In these galvanic systems, the potential difference between titanium and the dissimilar metals causes current flow, leading to galvanic corrosion. The outcome of galvanic corrosion is closely related to the stable potential, contact area, and polarization characteristics of the contacting metals. When titanium comes into contact with certain metals, its potential decreases. For example, the potential of titanium in seawater is 0.14 V/SCE, but it drops to -0.79 V/SCE when in contact with zinc and -0.8 V/SCE when in contact with steel in a 6% sodium solution. When titanium's potential drops to -0.65--0.7 V/SCE in seawater and in contact with these metals, it absorbs hydrogen, leading to localized hydrogenation.
The Influence of Various Factors on Hydrogen Absorption in Titanium Rods
(I) Influence of Tubesheet Material
A titanium alloy rod manufacturer studied the hydrogen absorption of expanded specimens made of different tubesheet materials in a 1M Fe(OH)₂ solution. The results showed that steel tubesheets increased titanium hydrogen the most, regardless of whether or not they were pickled after expansion. Admiralty brass tubesheets also exhibited significant hydrogen absorption even when not pickled after expansion. This suggests that contact between tubesheets of different materials and titanium rods can affect hydrogen absorption through galvanic corrosion and other mechanisms. When selecting tubesheet materials, it is important to fully consider their electrochemical compatibility with the titanium rod to minimize the adverse effects of hydrogen absorption on the rod's performance.
(II) Influence of Surface Condition
The results of cathodic treatment of samples with different surface conditions in 0.1M H₂SO₄ at 30°C showed that titanium rods with scratched surfaces exhibited significant hydrogen absorption, while those in as-sold condition were less affected, indicating that their surfaces were also contaminated to some extent. The surface condition of titanium rods significantly influences their corrosion resistance and hydrogen absorption behavior. During processing, damage to the titanium rod surface, such as scratches, should be minimized. Appropriate surface treatment processes, such as cleaning and passivation, should also be implemented to improve the surface condition, enhance corrosion resistance, and reduce the potential for hydrogen absorption.
Implications for Titanium Rod Processing
(I) Processing Optimization
During titanium rod processing, residual stress should be minimized. For example, appropriate heat treatment processes, such as annealing, can eliminate residual stress generated during processing and reduce the risk of stress corrosion. Furthermore, careful control of processing parameters is crucial to avoid surface damage caused by excessive processing and to reduce the incidence of pitting and crevice corrosion.
(II) Selecting Appropriate Complementary Materials
When selecting materials for use with titanium rods, full consideration should be given to electrochemical compatibility. Avoid direct contact between titanium rods and metal materials with large potential differences that are prone to galvanic corrosion. If dissimilar metals must be used, insulation measures such as insulating gaskets and coatings can be implemented to block the flow of current and reduce the impact of galvanic corrosion.
(III) Strengthening Surface Treatment
Appropriate surface treatment of titanium rods is an effective method for improving corrosion resistance and reducing hydrogen absorption. Chemical conversion coatings, such as chromate and phosphate treatments, can be applied to form a protective film on the titanium rod surface, preventing the intrusion of corrosive media. Alternatively, electroplating or chemical plating can be used to coat the titanium rod surface with a corrosion-resistant metal or alloy to improve its surface properties.
Localized corrosion of titanium rods in electrolyte melts can lead to hydrogen embrittlement, seriously affecting their performance and service life. Various forms of corrosion, including crevice corrosion, pitting corrosion, galvanic corrosion, and stress corrosion, are the primary causes of hydrogen absorption and localized hydrogenation. The surface condition of the tubesheet and titanium rod materials can also significantly affect the amount of hydrogen absorbed. During the processing of titanium rods, measures such as optimizing processing technology, selecting appropriate supporting materials, and strengthening surface treatment should be taken to reduce the occurrence of local corrosion and hydrogen embrittlement, improve the quality and reliability of titanium rods, and meet the needs of different industrial fields.
