A chemical plant pulled a PTFE immersion heater from a 30% nitric acid bath at 85°C after five months of service. The heater surface showed a pattern of fine, intersecting cracks-a crazing pattern-across the entire heated section. The PTFE felt hard and brittle, not soft and flexible as it should. Insulation resistance had fallen from 500 MΩ to 30 MΩ. The nitric acid had oxidized the polymer, embrittling the surface and causing it to crack.
Hot nitric acid is one of the most aggressive oxidizing acids for PTFE. At moderate temperatures, the oxidation rate is slow and the PTFE remains flexible. At elevated temperatures-above 80°C-the oxidation rate accelerates, and the polymer surface begins to degrade. The polymer chains break, and the surface becomes hard and brittle.
How Nitric Acid Embrittles PTFE
Nitric acid oxidizes PTFE at the chain ends and at structural defects in the polymer. The oxidation creates carbonyl fluoride and other oxidized species that disrupt the polymer chain. The chain scission reduces the molecular weight of the polymer and its mechanical strength.
The oxidized surface layer has different properties than the bulk polymer. It is harder, more brittle, and less flexible. When the heater cycles thermally, the brittle surface cannot accommodate the strain. It cracks.
| Nitric Acid Concentration | Temperature | Oxidation Rate | Time to Surface Hardening | Time to Cracking |
|---|---|---|---|---|
| 10% | 60°C | Very low | >36 months | >48 months |
| 20% | 70°C | Low | 18–30 months | 24–36 months |
| 30% | 80°C | Moderate | 8–14 months | 12–20 months |
| 40% | 85°C | High | 4–8 months | 6–12 months |
| 50% | 90°C | Very high | 2–4 months | 3–8 months |
Nitric acid concentration and temperature effects on PTFE embrittlement
Why the Cracks Form a Crazing Pattern
The crazing pattern-fine, intersecting cracks-is characteristic of a brittle surface layer on a flexible substrate. When the heater heats, the metal sheath expands, and the brittle surface layer must stretch to accommodate the expansion. If the surface layer cannot stretch, it cracks. The cracks form in a random pattern because the surface layer is uniform and the stress is distributed.
The crazing pattern is different from the cracks caused by thermal fatigue or mechanical damage. Thermal fatigue cracks are usually concentrated at stress points. Mechanical damage cracks are localized. Crazing cracks are distributed across the entire affected surface.
Why Hot Nitric Acid Service Is Particularly Demanding
Nitric acid is an oxidizing acid. The oxidation potential increases with concentration and temperature. At 85°C and 30% concentration, the oxidation rate is high enough to embrittle the PTFE surface within months.
The nitric acid also permeates the PTFE. The permeation rate increases with temperature. The acid that reaches the internal insulation reduces the insulation resistance. The combination of surface embrittlement and internal permeation causes the heater to fail faster than either mechanism alone.
Preventing Surface Embrittlement in Hot Nitric Acid
The most effective prevention is to reduce the surface temperature. A lower watt density-0.8–1.0 W/cm²-keeps the surface temperature within a range where the oxidation rate is low. A heater at 0.9 W/cm² may run 10–15°C cooler than one at 1.4 W/cm², which can double the time to surface hardening.
A thicker PTFE coating provides more material to absorb the oxidation. The oxidized surface layer is sacrificial. A thicker coating has more polymer below the oxidized layer, so the heater retains its barrier properties longer.
A shorter service interval for inspection and replacement is a practical approach. In hot nitric acid service, the heater should be inspected every 2–3 months. If surface hardening is detected, the heater should be replaced before the cracks penetrate the coating.
| Prevention Measure | Effect on Embrittlement | Implementation |
|---|---|---|
| Lower watt density, 0.8–1.0 W/cm² | Slower oxidation | Heater design |
| Thicker coating, 2.5–3.0 mm | More sacrificial material | Coating process |
| Shorter inspection interval | Early detection | Maintenance schedule |
| Reduced temperature | Lower oxidation rate | Process control |
Prevention measures for surface embrittlement in hot nitric acid
When Surface Embrittlement Indicates a Design Problem
A heater that embrittles and cracks despite low watt density and a thick coating indicates that the heater material or the design is not suited to the hot nitric acid service. A custom PTFE immersion heater configuration with a very low watt density, a thick coating, and a temperature-resistant polymer grade addresses the root cause of embrittlement. The specification process requires the nitric acid concentration, operating temperature, and required wattage. For facilities with hot nitric acid baths, this application-specific approach reduces surface embrittlement and extends heater service life.
