A titanium tube, otherwise perfect, shows a deep, jagged groove right where it enters the tubesheet. This is crevice corrosion-a microscopically small gap at the joint has created an acidified, stagnant cell that devours the metal in secret. For heat exchangers in hot chloride service, this failure mode is predictable and preventable, but only when properly understood and addressed during fabrication and operation.
Understanding the Mechanism: Why Titanium Fails in a Tight Gap
Titanium's outstanding corrosion resistance in seawater and brines relies on a tenacious, self‑healing oxide film (TiO₂). This film requires access to oxygen or other oxidizers to maintain itself. In a wide, open surface, oxygen is plentiful, and the film remains intact.
However, inside a tight crevice-such as the annular gap between an expanded titanium tube and the tubesheet hole-the situation changes dramatically. The crevice is typically only a few micrometres wide. Fluid ingress occurs, but circulation is nearly impossible. Oxygen in the stagnant solution is rapidly consumed by corrosion processes and cannot be replenished by diffusion from the bulk fluid. The area inside the crevice becomes deaerated.
Once oxygen is depleted, the passive film breaks down. Chloride ions (if present) migrate into the crevice to maintain charge neutrality, forming metal chlorides that hydrolyze to produce hydrochloric acid. The pH inside the crevice can drop to 2 or lower, creating a highly aggressive, localized environment. Titanium then corrodes rapidly in a narrow, penetrating attack.
The crevice corrosion titanium tubesheet interface is therefore a classic example of a design‑induced failure: a necessary mechanical joint that inadvertently creates the worst‑case chemical micro‑environment.
Where and How to Detect Crevice Corrosion at the Tubesheet Interface
Detection during routine inspection is challenging because the attack is hidden inside the crevice. By the time visible signs appear, significant damage has already occurred.
Visual and Non‑Destructive Inspection Signs
External indicators – Discoloration, staining, or salt deposits around the tubesheet face near the tube ends. A narrow ring of corrosion product (often white or yellow) may weep from the joint.
Telescoping or bulging – In advanced cases, corrosion products may push the tube slightly outward at the tubesheet.
Eddy current testing – This is the most effective non‑destructive method. An eddy current probe run through the tube can detect wall thickness loss localized at the tubesheet plane. The signal shows a sharp, narrow dip at the joint.
Pull‑out testing – For failed or suspect bundles, a tube is extracted. The corroded area will appear as a deep, knife‑edge groove around the circumference, often with a pitted or scalloped surface.
When to Suspect Crevice Corrosion
Crevice corrosion is not random. It follows predictable boundaries:
Elevated temperature – For Grade 2 titanium in neutral chloride solutions (seawater, brackish water, many brines), the critical crevice temperature (CCT) is approximately 70–80 °C. Below this range, crevice attack is rare. Above 80 °C, the risk increases sharply.
High chloride concentration – Chloride levels above 1,000 ppm accelerate crevice corrosion once the CCT is exceeded.
Low pH – Acidic conditions (pH < 4) lower the CCT.
Stagnant or low‑flow conditions – Slow moving or stagnant fluid in the tubeside promotes the oxygen depletion that drives crevice attack.
If an exchanger operates with tube‑side brine at 85 °C, and Grade 2 titanium tubes are simply expanded (not seal‑welded), crevice corrosion is almost certain within months to a few years.
Prevention Strategies: Eliminate the Crevice, or Raise the Threshold
Because crevice corrosion requires a gap and a critical set of environmental conditions, prevention focuses on either removing the gap or modifying the environment to stay below the alloy's threshold.
1. Create a True Crevice‑Free Seal
The most reliable prevention is to eliminate the crevice entirely. Two proven methods exist:
Sufficiently tight tube expansion – Hydraulic or explosive expansion can produce a near‑perfect interference fit between tube and tubesheet. If the residual gap is less than the threshold for fluid ingress (effectively zero), crevice corrosion cannot initiate. However, achieving and proving such a tight seal across thousands of tubes is difficult.
Seal welding – A circumferential weld at the tubesheet face, sealing the tube‑to‑tubesheet joint, is the industry standard for critical chloride service. The weld fills the annular gap and prevents any fluid from entering the crevice. Seal welding is mandatory for Grade 2 titanium exchangers operating above 80 °C in seawater or high‑chloride brines.
Critical fabrication note – Seal welding of titanium must be performed with an inert argon purge on both the weld face and the tube interior. Without proper purge, atmospheric contamination leads to weld embrittlement (alpha case formation) and cracking. Welding procedures should follow ASME Section IX with qualification for titanium.
2. Apply Protective Coatings or Sealants at the Joint
For existing exchangers where re‑welding is impractical, or for lower‑risk applications, a flexible sealant or coating can be applied to the tubesheet face and the tube ends.
PTFE‑based coatings – A thin, sprayed or dipped layer of PTFE (or other fluoropolymer) at the joint prevents fluid from wicking into the crevice. The coating must remain flexible and adhere to both tube and tubesheet surfaces.
Anaerobic sealants – Thread‑locking type sealants (e.g., methacrylate‑based) can be wicked into the annular gap before expansion or as a post‑expansion treatment. They cure in the absence of air, filling the crevice. However, chemical compatibility with the process fluid must be verified.
These sealants are considered secondary measures; they are less reliable than seal welding, especially at high temperatures.
3. Upgrade to a Higher‑Alloy Titanium Grade
If seal welding is not possible and the operating temperature cannot be lowered, switching to a crevice‑resistant titanium grade is an effective solution.
Grade 7 titanium (Ti‑0.15Pd) – The addition of palladium dramatically enhances crevice corrosion resistance. Grade 7 has a critical crevice temperature in neutral chlorides above 95 °C, and often as high as 120 °C depending on test conditions. It is the preferred material for brine coolers operating above 80 °C when seal welding is impractical.
Grade 12 titanium (Ti‑0.3Mo‑0.8Ni) – Provides improved crevice resistance over Grade 2, though less than Grade 7. It is a cost‑effective intermediate option.
4. Operate Below the Critical Crevice Temperature
The simplest operational prevention is to keep the tube wall temperature below the CCT of the alloy used. For Grade 2, a conservative limit is 70 °C at the tubesheet interface (not the bulk fluid temperature). This may be achieved by:
Reducing the inlet brine temperature.
Increasing flow velocity to reduce boundary layer thickness and prevent heating of the tubesheet region.
Adding a cooling jacket or quench stream at the exchanger inlet.
Monitoring and Maintenance for Existing Exchangers
Where crevice corrosion has been detected or is considered a risk, a monitoring and maintenance program should be implemented.
Regular eddy current testing – Inspect a sample of tubes (e.g., 10% of the bundle) annually or semi‑annually, focusing on the tubesheet area. A baseline scan after commissioning is essential.
Temperature and chloride logging – Continuous recording of operating conditions can identify excursions above the CCT. Alarms should be set at 75 °C for Grade 2.
Proactive seal welding retrofits – If an exchanger has Grade 2 tubes with only mechanical expansion and the temperature routinely exceeds 70 °C, a retrofit seal weld program can be performed, though access and purge requirements make this expensive.
Bund replacement – When extensive crevice corrosion is found, the only reliable remedy is to replace the tube bundle with one that uses seal‑welded Grade 2 or upgraded Grade 7 titanium.
Summary of Prevention Measures
| Method | Effectiveness | Application |
|---|---|---|
| Seal welding (Grade 2) | Highest – eliminates crevice | New fabrication, all high‑temperature chloride service |
| Tight expansion only | Low – risk of residual gap | Low‑temperature (<70°C), non‑critical service |
| PTFE coating or sealant | Moderate – can degrade over time | Retrofit or low‑risk, low‑temperature |
| Upgrade to Grade 7 | High – raises CCT >95°C | When welding not possible, or extreme conditions |
| Operate below 70°C (Grade 2) | High – passive prevention | Existing plant with temperature control capability |
Conclusion: A Predictable Threat with Proven Solutions
Crevice corrosion at the tubesheet interface is a predictable and preventable threat to titanium heat exchangers in hot chloride service. It occurs when a microscopic gap, stagnant fluid, and temperature above the alloy's critical crevice temperature combine to destroy the passive film. Prevention is achieved through tight expansion combined with seal welding, the use of protective sealants, or upgrading to a higher‑grade titanium alloy. The finest materials still demand precise fabrication and conscientious operation. By understanding the mechanism and applying the appropriate prevention methods at design and fabrication stages, a titanium tube bundle can deliver decades of reliable service without a single crevice‑related failure.
