Titanium immersion heaters obtain broad application in acidic brine circulation units, metal‑pickling auxiliary heating loops and weak‑acid salt‑containing process tanks. The dense titanium dioxide film provides outstanding general corrosion resistance. However, local cathodic polarisation may form under specific electrochemical conditions. Nascent hydrogen generated by cathodic reaction permeates titanium matrix and forms brittle titanium hydride precipitates. Hydride‑enriched zones lose ductility, and micro‑cracks initiate under thermal‑cycling mechanical stress. Conventional general‑corrosion coupon evaluation ignores hydrogen‑embrittlement cracking risk under local cathodic polarisation.
In acidic brine environment, local cathodic sites can form through galvanic coupling, partial passive‑film damage or stray current influence. Water or hydrogen‑ion reduction generates large‑volume nascent atomic hydrogen. Part of hydrogen atoms diffuse into titanium metallic lattice, reacting with titanium to produce brittle titanium hydride phases. Hydride platelets distribute along grain boundaries. Under repeated thermal expansion‑contraction stress from heater operation, internal stress concentrates at hydride‑matrix interfaces. Micro‑cracks nucleate and expand along hydride aggregation zones. Outer sheath may still keep metallic gloss without obvious pitting holes.
In early service phase, surface observation shows almost no change. Subsurface hydride precipitation and micro‑crack propagation proceed continuously. After multiple thermal cycles, cracks penetrate the sheath wall, causing medium leakage and heater failure. Material selection documents frequently focus on uniform corrosion rate, while hydrogen‑assisted cracking risk under local cathodic condition is easily overlooked.
Targeted engineering measures suppress hydrogen‑assisted cracking risk. Eliminate stray‑current interference from surrounding equipment. Avoid galvanic coupling with more noble metallic components. Adjust solution pH value to reduce hydrogen‑evolution tendency. Perform metallographic sampling inspection focusing on hydride precipitation risk zones during overhaul cycles.
表格
| Heater Type | Local‑Cathodic‑Polarisation Induced Hydrogen‑Assisted Cracking Risk | Core Degradation Mechanism | Early‑stage Diagnostic Feature | Key Mitigation Engineering Measure |
|---|---|---|---|---|
| Titanium | Medium‑High | Local cathodic reaction generates nascent hydrogen; hydrogen permeates matrix to form brittle hydride; thermal‑cycling stress drives micro‑crack propagation | Metallic intact outer surface; subsurface hydride precipitation invisible visually | Eliminate stray current; avoid dissimilar‑metal galvanic coupling; optimise pH; periodic metallographic spot examination |
| 316L Stainless Steel | Medium | Hydrogen permeation exists, yet hydride‑brittleness sensitivity is far lower than titanium; main threat remains chloride pitting | Local pitting holes under aggressive brine condition | Control cathodic polarisation source inside system |
| Fused Quartz | Negligible | No hydrogen‑absorbing metallic matrix; only acidic salt fouling forms on tube surface | Acid‑salt crystalline residues on exterior wall | Regular chemical rinsing maintenance |
| PFA‑Jacketed | Low | Hydrogen gas may penetrate polymer layer; base‑metal hydrogen‑embrittlement risk decreases due to insulation barrier | No obvious early visual defect; slow insulation drifting may occur | Check system stray‑current level routinely |
Conclusion: Low uniform corrosion rate does not guarantee titanium heater safety in acidic brine circulation units. Local cathodic polarisation introduces hydrogen‑assisted cracking threat independent of surface pitting damage. Stray‑current control and galvanic‑coupling prevention represent core protective strategies.
