A chemical plant pulled a PTFE immersion heater from a sulfuric acid pickling tank after fourteen months of service. The sheath surface facing the process was in good condition. The area behind the mounting brackets-the regions where the brackets contact the heater-showed dark staining and pitting. The metal beneath the PTFE had corroded, and in two locations the corrosion had penetrated deep enough to create a pinhole leak.
Crevice corrosion occurs in the narrow gaps between the mounting brackets and the heater surface. The gap is too small for fluid circulation but large enough for process fluid to enter. The fluid in the gap becomes stagnant, and the chemistry within the crevice changes in ways that accelerate corrosion.
Why Crevices Form Behind Mounting Brackets
Mounting brackets are attached to the heater by welding, bolting, or clamping. Each attachment method creates a crevice between the bracket and the heater surface. The crevice may be only a few thousandths of an inch wide, but it is large enough for liquid to penetrate by capillary action.
The crevice is shielded from the bulk flow. The fluid inside cannot circulate. Oxygen and other dissolved species are consumed by corrosion reactions but cannot be replenished. The chemistry of the crevice fluid changes.
| Attachment Method | Crevice Width | Fluid Exchange | Corrosion Risk | Best Practice |
|---|---|---|---|---|
| Welded bracket | 0–0.05 mm | None | High at weld defects | Full penetration weld |
| Bolted bracket | 0.05–0.2 mm | Very low | Very high | Seal with gasket |
| Clamped bracket | 0.1–0.5 mm | Low | High | Use solid PTFE pad |
| Slotted bracket | 0.5–1.0 mm | Moderate | Moderate | Flush with water |
Bracket attachment methods and their crevice corrosion risk
The Chemistry of Crevice Corrosion
In a crevice, the oxygen dissolved in the fluid is consumed by the initial corrosion reaction. The oxygen cannot be replenished because the crevice is shielded from the bulk flow. The oxygen concentration inside the crevice drops, while the concentration outside remains high. The difference creates an oxygen concentration cell.
The metal inside the crevice becomes anodic-it corrodes. The metal outside the crevice becomes cathodic-it is protected. The corrosion rate inside the crevice accelerates.
The metal ions produced by the corrosion reaction attract chloride ions from the bulk fluid. The chloride concentration inside the crevice increases, sometimes reaching several times the bulk concentration. The high chloride concentration accelerates the corrosion further.
The pH inside the crevice drops as the metal ions hydrolyze. The acidic environment accelerates the corrosion even more. The result is a rapidly deepening pit that can penetrate the heater sheath.
Why PTFE-Coated Heaters Are Still Vulnerable
The PTFE coating on a heater is intended to protect the underlying metal from the process fluid. But the coating does not extend into the crevice between the bracket and the heater. If the coating is damaged during bracket installation, or if the crevice is not fully sealed, the process fluid can contact the bare metal.
Even if the coating is intact, the crevice can form between the coating and the bracket. The coating may be slightly permeable, and the fluid that penetrates the coating creates a crevice between the coating and the metal.
Preventing Crevice Corrosion
The most effective prevention is to eliminate the crevice. A bracket that is welded to the heater with a full-penetration weld has no crevice. The weld seals the joint and prevents fluid from entering.
If welding is not practical, a gasket or sealant can fill the crevice. A PTFE gasket or a fluoropolymer sealant prevents fluid from entering the gap between the bracket and the heater.
A solid PTFE pad between the bracket and the heater eliminates the crevice entirely. The pad is compressible and fills the gap, and the PTFE is chemically inert.
| Prevention Measure | Effect on Crevice Corrosion | Implementation | Cost |
|---|---|---|---|
| Full-penetration weld | Eliminates crevice | Fabrication | Moderate |
| PTFE gasket | Seals crevice | Assembly | Low |
| Fluoropolymer sealant | Seals crevice | Assembly | Low |
| Solid PTFE pad | Eliminates crevice | Bracket design | Moderate |
Prevention measures for crevice corrosion behind mounting brackets
When Crevice Corrosion Indicates a Design Problem
A heater that develops crevice corrosion despite proper bracket design indicates that the coating or the bracket material is not suited to the application. A custom PTFE immersion heater configuration with welded brackets, PTFE gaskets, or solid PTFE pads addresses the root cause of crevice corrosion. The specification process requires the bath chemistry, operating temperature, bracket design, and required wattage. For facilities with aggressive chemistries, this application-specific approach reduces crevice corrosion and extends heater service life.
