Characteristics and Patterns of Crevice Corrosion in Titanium
Crevice corrosion is a localized corrosion phenomenon that typically occurs in tight-fitting gaps. These gaps may arise from structural design (such as flange connections, gasket surfaces, tube-to-tube sheet expansions, and bolted or riveted joints) or due to scale formation and deposits covering surfaces. Early studies suggested that titanium does not undergo crevice corrosion in seawater and salt spray environments. However, later research revealed that titanium equipment could suffer from crevice corrosion in high-temperature chloride media (e.g., seawater heat exchangers), wet chlorine gas (e.g., wet chlorine gas shell-and-tube condensers), oxidizing inhibitor-containing hydrochloric acid solutions, formic acid, and oxalic acid solutions.
Titanium's crevice corrosion is influenced by several factors, including environmental temperature, chloride type and concentration, pH value, crevice size, and geometric shape. Moreover, crevices formed between titanium and non-metallic materials (such as PTFE or asbestos) are more susceptible to crevice corrosion than those formed between titanium surfaces.
Characteristics and Patterns of Titanium Crevice Corrosion
1. Presence of an Incubation Period
Crevice corrosion typically undergoes an incubation period, the duration of which depends on various factors such as environmental temperature, chloride type and concentration, oxidizer concentration, contact materials, solution pH, and crevice dimensions. In sodium chloride solutions, higher chloride ion concentration, increased temperature, and lower pH shorten the incubation period, making corrosion more sensitive.
2. Changes in Crevice Solution Composition
The composition of the solution inside the crevice differs from that of the bulk solution. Generally, oxygen concentration is lower inside the crevice, while chloride and hydrogen ion concentrations are higher, leading to a significant decrease in pH (which can drop below 1). Additionally, the electrode potential inside the crevice becomes more negative, making titanium more active. Electrochemical studies indicate that the crevice corrosion susceptibility of titanium follows the order: Cl⁻ > Br⁻ > I⁻, meaning that chloride environments pose the highest risk, contrary to titanium's pitting corrosion behavior.
3. Localized Nature of Corrosion
Crevice corrosion usually occurs in specific areas within the crevice rather than across the entire surface. Once the incubation period ends, corrosion rapidly progresses due to an autocatalytic mechanism, eventually leading to localized perforation and failure.
4. Hydrogen Absorption Phenomenon
During crevice corrosion, hydrogen absorption is often observed, and microscopic examination may reveal needle-like hydrides in titanium. As hydrogen content increases, surface hydrides accumulate, accelerating corrosion. Meanwhile, hydrogen diffuses into the metal, and internal hydride precipitation may serve as a crack initiation site for stress corrosion cracking, increasing the risk of material embrittlement and fracture.
5. Stages of the Corrosion Process
Titanium crevice corrosion occurs in two stages:
Incubation Period: Initially, oxygen is consumed equally inside and outside the crevice through cathodic reactions. As oxygen is depleted inside the crevice, cathodic reactions only proceed externally, while anodic dissolution of titanium dominates inside the crevice.
Active Dissolution Period: With the continuous accumulation of titanium ions in the crevice, chloride ions migrate inward to maintain charge balance. Titanium ions hydrolyze, forming titanium hydroxide (Ti(OH)₄), which dehydrates to TiO₂. The hydrolysis reaction lowers the pH, further disrupting the passive film and accelerating corrosion.
6. Influence of Crevice Geometry
Crevice corrosion is affected by geometric factors such as crevice length, width, and the ratio of internal to external surface area. Experimental results show that narrow crevices (widths below 0.5mm) are significantly more prone to corrosion than wider ones. These effects must be determined through specific experimental studies rather than theoretical predictions.
7. Prevention Measures
To improve titanium's corrosion resistance in reducing inorganic acids and reduce crevice corrosion susceptibility, titanium alloys such as Ti-Pd and Ti-Ni-Mo are commonly used, as they offer superior performance compared to commercially pure titanium, especially Ti-Pd alloys. Additionally, the following surface treatments can enhance titanium's resistance to crevice corrosion:
Palladium Coating: Applying a palladium coating on crevice areas enhances corrosion resistance.
Thermal Oxidation Treatment: Forms a stable oxide layer, improving corrosion resistance.
Anodic Oxidation: Enhances the passivation film, increasing corrosion resistance.
Conclusion
Titanium crevice corrosion is influenced by environmental factors, solution composition, and crevice geometry, progressing through an incubation and active dissolution phase. The autocatalytic nature of crevice corrosion allows it to rapidly develop once initiated, leading to equipment failure. For high-risk environments, selecting appropriate alloy materials, optimizing structural design, and employing suitable surface treatments can effectively mitigate the risk of titanium crevice corrosion.
