Intergranular corrosion brittle fracture failure refers to titanium pipe welding and high-temperature heat treatment leading to segregation of alloy elements along grain boundaries and precipitation of brittle intermetallic phases. The grain boundary area becomes electrochemically active with much higher corrosion susceptibility than the grain interior. Under corrosive medium erosion, corrosion preferentially develops along grain boundaries to form continuous corrosion channels. The bonding force between metal crystal grains is completely lost, the material loses macroscopic ductility, and brittle splitting fracture occurs under slight internal pressure or external vibration without obvious plastic deformation. This failure mostly occurs in welded titanium heating pipelines operating in acidic and halogen-containing corrosive environments.
1. Grain Boundary Phase Precipitation and Intergranular Corrosion Expansion Mechanism
During welding thermal cycle, titanium material undergoes rapid heating and quenching. Impurity elements such as iron, carbon and nitrogen gather at grain boundaries to form brittle segregation layers. The electrode potential of grain boundaries is lower than that of the grain matrix, forming numerous tiny micro primary batteries spontaneously in electrolyte medium. Corrosion continuously dissolves the grain boundary phase, cutting off the intergranular connection. Different from uniform corrosion that consumes the whole pipe wall evenly, intergranular corrosion invades along the crystal boundary network inside the material. The outer surface of the pipe shows no obvious thinning, but the interior has formed a penetrating crack network, which is extremely concealed and prone to sudden brittle fracture.
2. Typical Positions Prone to Intergranular Corrosion Damage
Weld seams and welding heat-affected zones subjected to high-temperature welding thermal cycle;
Pipe sections with improper post-weld heat treatment leading to abnormal metallographic structure;
Bending forming areas with severe grain distortion and grain boundary element segregation;
Local overheating parts caused by uneven heating during equipment operation;
Long-term immersion areas where corrosive medium continuously accumulates and contacts the pipe wall.
3. Core Inducing Factors Accelerating Intergranular Corrosion Failure
Welding filler material is mismatched, introducing impurity elements that aggravate grain boundary precipitation;
Welding operation lacks inert gas shielding, resulting in oxygen and nitrogen contamination of the weld metal;
Post-weld annealing process parameters are unreasonable, failing to homogenize the metallographic structure;
Process medium contains fluoride, chloride and other corrosive ions that activate grain boundary corrosion;
Raw material titanium plate has excessive impurity content, with inherent poor intergranular corrosion resistance.
4. Full-Link Prevention and Control Technical Measures
① Select high-purity base material and matched dedicated titanium welding wire with low impurity content
Control raw material composition to reduce the source of grain boundary segregation elements.
② Implement full-range argon shielding for weld front and back sides during welding construction
Prevent high-temperature molten metal from being polluted by air oxygen and nitrogen.
③ Carry out standardized post-weld stress relief and homogenization annealing heat treatment
Eliminate uneven grain boundary element distribution and restore uniform metallographic structure.
④ Strictly control corrosive ion concentration in the conveying medium and add appropriate corrosion inhibitors
Suppress the electrochemical reaction driving force of intergranular corrosion.
⑤ Conduct metallographic inspection and ultrasonic flaw detection on key weld joints after welding and regular maintenance
Screen out early intergranular corrosion cracks before fracture failure.
5. Prevention Effect Comparison Table
表格
| Welding & Heat Treatment Mode | Intergranular Corrosion Risk | Application Suggestion |
|---|---|---|
| No backside argon protection + missing post-weld annealing + impure welding consumables | Continuous intergranular corrosion penetration and brittle fracture appear in medium-term operation | Re-cut defective welds and implement standardized shielding and heat treatment procedures |
| Double-sided inert gas shielding + normalized annealing treatment + qualified welding matching | Effectively inhibit grain boundary brittle phase precipitation and intergranular corrosion expansion | Standard welding construction specification for pressure-bearing titanium heating pipelines |
| Ultra-low impurity titanium base material + real-time medium ion monitoring + annual weld metallographic sampling inspection | Extremely low hidden danger of intergranular splitting rupture and sudden medium leakage | Preferred scheme for high-corrosion working condition titanium anti-corrosion heating pipe network engineering projects |
Conclusion
Titanium heating tube intergranular corrosion is triggered by grain boundary element segregation and impurity phase precipitation caused by welding thermal influence, leading to preferential corrosion along crystal boundaries and loss of intergranular binding force, eventually causing brittle fracture. Core prevention measures include optimizing raw material and welding material selection, improving welding gas protection quality, completing post-weld heat treatment to homogenize microstructure, regulating medium corrosive composition and strengthening weld nondestructive testing. Whole-process closed-loop management of raw material incoming inspection, field welding construction and post-process heat treatment acceptance can avoid pipeline brittle rupture and safety accidents induced by intergranular corrosion of titanium heating pressure pipelines.
