itanium heating devices often remain shut down for days or even months in low-temperature factory environments, with the whole heating assembly cooling down to ambient temperature before restarting. If operators directly activate full-rated power during cold startup, the titanium tube surface heats up sharply within a short time, while the surrounding process fluid is still kept at low temperature. Severe instantaneous temperature gradients generate tremendous thermal stress on the tube surface, leading to brittle cracking of the thin dense titanium dioxide passive film. Once the protective layer develops microcracks, corrosive ions such as chloride rapidly penetrate these defects, triggering pitting corrosion and crevice corrosion at crack sites. Implementing standardized low-temperature staged preheating specifications slows the temperature rise rate of titanium components, reduces transient thermal stress differences between the tube matrix and surrounding medium, safeguards the structural integrity of the surface passive film, and eliminates cold-start thermal shock as a common inducement for early-stage corrosion failure of industrial titanium heating equipment.

Gradual power step-up preheating serves as the core operating rule to mitigate cold-start thermal shock damage. Instead of applying 100% rated power at startup, the heating system is activated with low power load in stages. In the initial preheating phase, only 20% to 30% of the rated power is input to slowly raise the surface temperature of titanium heating tubes, allowing heat to conduct evenly from the heating wire to the titanium substrate and diffuse into the adjacent process fluid. After the medium temperature rises steadily and the temperature difference between the tube wall and bulk fluid falls below the safe threshold, the operating power can be increased step by step until reaching the designed working load. Sudden full-power startup creates localized overheating hotspots on titanium surfaces, where thermal expansion mismatch tears the continuous passive oxide film and forms countless tiny corrosion-sensitive microcracks that cannot self-repair in high-chloride industrial media.

Medium circulation activation prior to heating startup is an essential auxiliary measure to homogenize the temperature field around heating components. Static stationary fluid around idle titanium tubes easily forms localized low-temperature stagnant zones. Even with low-power preheating, heat cannot disperse quickly through static liquid, resulting in partial tube wall overheating and concentrated thermal stress. Starting the circulating pump or stirring system first drives the process medium to flow uniformly across all heating surfaces before switching on heating power. Continuous fluid convection eliminates local cold dead zones, balances the heat transfer rate on every section of the titanium heating assembly, and avoids uneven passive film stress caused by regional temperature deviation. This simple pre-operation step drastically reduces the probability of thermal shock cracking especially in high-viscosity fluids with poor thermal conductivity.

Low ambient temperature pre-insulation measures effectively narrow the initial temperature difference before cold startup. For titanium heating equipment installed outdoors or in unheated low-temperature workshops in winter, pre-wrapping temporary thermal insulation sleeves on exposed terminals and tank penetration sections prevents excessive temperature drop of titanium components during long standby periods. If the initial temperature of the heating tube is far lower than the operating medium temperature, the first preheating stage needs to extend the constant-temperature holding time to fully release thermal stress. After completing the whole staged power rising process and reaching stable operating conditions, the temporary insulation can be removed for routine temperature monitoring. Any passive film cracks generated by repeated thermal shock will gradually expand under cyclic startup-shutdown loads, eventually evolving into penetrating tube corrosion failure.

The following table displays targeted low-temperature preheating schemes for different cold startup service scenarios:

表格

Standardized low-temperature preheating specifications eliminate transient thermal stress shocks originating from improper cold startup operations. Even compact, well-formed titanium passive films cannot withstand instantaneous temperature gradient-induced brittle fracture. Staged power regulation, fluid pre-circulation and ambient temperature pre-insulation jointly stabilize the heating startup thermal environment, protect the continuous integrity of the titanium dioxide protective layer, reduce corrosion initiation risks at micro-defects, and extend the overall service cycle of anti-corrosion titanium heating systems under frequent startup-shutdown industrial operating modes.

(Word count: 684)

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42nd Full Original Article (No.042)

Title: What selective cathodic protection parameter calibration standards avoid over-protection hydrogen embrittlement of high-strength titanium heating pressure components

High-strength titanium heating pressure components widely used in high-pressure hydrogenation, pressurized fine chemical and sealed energy storage reaction devices often face combined risks of seawater salt fog corrosion, chloride medium pitting and crevice corrosion. Impressed current cathodic protection is frequently adopted as an auxiliary anti-corrosion measure to restrain local passive film breakdown of titanium structures. However, unreasonable protection potential setting, excessive current output and inaccurate reference electrode positioning easily lead to over-protection phenomena. Excessively negative cathode potential will drive massive electrolytic hydrogen evolution on titanium surfaces, allowing hydrogen atoms to permeate into the metal matrix, aggregate at grain boundaries and stress-concentrated welding and bending areas, forming brittle titanium hydride phases. Such metallurgical defects drastically reduce the tensile strength, fatigue resistance and stress corrosion resistance of titanium pressure heating parts, easily triggering brittle cracking and sudden pressure leakage accidents under cyclic thermal and pressure loads. Establishing standardized selective cathodic protection parameter calibration specifications can lock the potential within the safe protection interval, inhibit localized corrosion while avoiding hydrogen evolution over-protection, and realize safe and long-term anti-corrosion operation of high-strength titanium heating pressure assemblies in harsh pressurized corrosive service environments.

Determining the critical protection potential range through laboratory polarization curve testing is the core premise of parameter calibration. Before formally putting the cathodic protection system into operation, potentiodynamic polarization tests should be carried out on titanium base materials under simulated on-site temperature, pressure and medium salinity conditions to draw accurate polarization characteristic curves. The lower limit of the safe protection potential is set slightly negative to the pitting corrosion potential of titanium, which can passivate local active corrosion points and prevent passive film breakdown; the upper limit must be controlled above the hydrogen evolution potential of the medium to avoid electrolytic hydrogen precipitation on the titanium surface. For high-strength titanium heating components sensitive to hydrogen embrittlement, the potential safety window is relatively narrow, and a buffer interval of at least 50 mV should be reserved between the set protection potential and the hydrogen evolution threshold to resist transient potential fluctuation caused by medium flow velocity change, temperature drift and anode aging. Blindly referring to carbon steel protection parameters will inevitably cause over-protection and induce irreversible hydrogen embrittlement damage to titanium substrates.

Multi-point distributed reference electrode layout paired with real-time potential closed-loop adjustment avoids local over-protection dead zones. Single reference electrode measurement can only reflect the potential state of a small regional heating component, while pipeline bent sections, flange crevices and fluid stagnant zones often bear higher cathode current density, resulting in local potential exceeding the safe negative limit even if the main measuring point is within the standard range. Reference electrodes need to be arranged at high-risk positions such as liquid level fluctuation areas, heating coil welding seams and pipeline low-flow dead zones to collect multi-dimensional potential data. The control system adopts average potential feedback to dynamically adjust the output current of the impressed current anode, so that all monitoring points are stably maintained within the calibrated safe potential interval. Regular electrode potential calibration and anti-fouling cleaning of reference probes prevent measurement drift from leading to continuous over-protection operation, which is particularly critical for large-scale pressurized titanium heating network systems with complex fluid flow fields.

Regular hydrogen embrittlement performance inspection matched with seasonal parameter recalibration forms a closed-loop safeguard mechanism. Long-term service will cause anode consumption, medium water quality seasonal fluctuation and equipment operating parameter adjustment, which may gradually shift the actual potential of titanium heating components to the over-protection range. Quarterly potential spot verification and annual full-system parameter recalibration are required to recheck the polarization curve under the latest on-site working conditions and fine-tune the protection potential upper and lower limits. Meanwhile, hardness testing, ultrasonic metallographic inspection and slow strain rate tensile tests are implemented on representative high-stress titanium heating parts to detect hydride precipitation, grain boundary hydrogen enrichment and material ductility attenuation signals. Once hydrogen embrittlement signs are found, the protection potential must be positively shifted upward immediately, combined with vacuum dehydrogenation heat treatment for defective components to eliminate brittle hydride phases and restore the intrinsic mechanical and anti-corrosion properties of titanium substrates.

The following table presents classified cathodic protection calibration schemes for different high-strength titanium heating pressure service scenarios:

表格

Selective cathodic protection parameter calibration balances the dual demands of localized corrosion suppression and hydrogen embrittlement prevention for high-strength titanium heating pressure components. Titanium's excellent corrosion resistance cannot offset irreversible material brittle damage caused by electrolytic hydrogen permeation under over-protection conditions. Scientific polarization testing, distributed potential monitoring and periodic parameter recalibration lock the operating state within the safe protection interval, avoid human-induced hydrogen embrittlement failure, reduce pressurized equipment safety hazards, and realize full-lifecycle reliable anti-corrosion operation of high-strength titanium heating facilities in complex pressurized corrosive industrial environments.

(Word count: 686)

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43rd Full Original Article (No.043)

Title: How do regular stray current detection and drainage control specifications prevent subway and factory DC grid-induced stray current pitting corrosion of buried titanium heating pipelines

Buried titanium heating pipelines laid in industrial parks adjacent to subway lines, DC electrolysis workshops and large rectifier power stations are constantly exposed to DC stray current leakage. When direct current leaks from defective power cables, damaged rail insulation or ungrounded electrical equipment into soil, conductive saline soil acts as an electrolyte medium, causing current to flow into local sections of buried titanium pipelines. The positions where current leaves the titanium pipe to return to the earth serve as anodic zones; continuous anodic dissolution destroys the stable titanium dioxide passive film, triggering dense distributed pitting corrosion along pipeline outer walls. As corrosion pits gradually deepen under long-term stray current erosion, buried pipelines face perforation leakage and forced excavation maintenance risks. Implementing standardized periodic stray current detection, targeted potential monitoring and graded drainage control specifications can identify abnormal current leakage areas timely, guide the layout of drainage protection facilities, limit pipeline anodic polarization amplitude, and effectively block stray current-induced localized pitting failure of buried titanium heating pipeline networks in DC power-intensive industrial regions.

Grid-style stray current potential mapping is the core technical means to locate high-risk corrosion sections. Traditional single-point potential measurement can only reflect local electromagnetic interference intensity and easily omits scattered stray current abnormal zones formed by uneven soil conductivity and cable insulation aging. Inspectors adopt equidistant grid layout to carry out continuous on-site pipe-to-soil potential scanning along the entire buried titanium heating route, record transient potential fluctuation amplitude, potential reversal frequency and abnormal potential drift duration of each measuring point. Areas with frequent potential reversal and excessively negative or positive transient potential are marked as key stray current hazard segments. Combined with the distribution of nearby DC power facilities, insulation damage points of public pipelines and soil salinity distribution data, the root leakage source can be preliminarily traced, providing accurate positioning basis for subsequent drainage protection construction and insulation reinforcement transformation.

Classified stray current drainage design matched with pipeline insulating isolation restrains anodic dissolution of titanium passive films. For high-risk sections with severe transient anodic polarization, polarized drainage electrodes are buried near the titanium pipeline, connected via low-resistance copper drainage cables to guide stray current back to the original DC power negative pole or dedicated grounding grid, avoiding concentrated current discharging from the titanium pipe surface. Meanwhile, insulating flange joints are installed at the boundary between stray current interference zones and normal pipeline sections to block longitudinal stray current transmission along the titanium pipeline, preventing the expansion of anodic corrosion regions. It is necessary to strictly control the drainage current within the safe range, avoiding excessive drainage leading to cathodic overprotection and subsequent hydrogen embrittlement damage to buried titanium substrates. All drainage connection positions must adopt insulated transition terminals to prevent galvanic corrosion between copper cables and titanium pipeline fittings in humid underground electrolyte environments.

Periodic dynamic detection combined with seasonal interference correction optimizes long-term drainage operation parameters. Stray current intensity usually fluctuates regularly with factory production shifts, subway peak operating hours and seasonal soil moisture variation; rainy seasons increase soil conductivity and significantly aggravate DC leakage interference. Monthly fixed-period potential monitoring during typical peak and off-peak periods of surrounding DC equipment is required to update the stray current risk distribution map dynamically. When interference intensity rises sharply in specific seasons, the drainage system output parameters should be adjusted synchronously to limit pipe-to-soil potential within the safe range for titanium passive film stability. In addition, regular insulation resistance testing for pipeline anti-corrosion outer coatings and drainage cables is implemented to eliminate new leakage channels caused by coating aging, soil extrusion damage and construction excavation collision, ensuring the long-term effectiveness of stray current isolation and drainage protection systems.

The following table displays targeted stray current protection schemes for different buried titanium heating pipeline interference scenarios:

表格

Stray current drainage and regular potential detection fundamentally cut off the anodic polarization corrosion path induced by external DC electromagnetic interference. Titanium's inherent underground uniform corrosion resistance cannot resist rapid passive film breakdown caused by forced anodic current discharge. Scientific interference area positioning, graded drainage design and periodic parameter dynamic optimization stabilize the pipe-to-soil potential of buried titanium heating pipelines, prevent large-area scattered pitting accidents, reduce high-cost underground excavation maintenance losses, and guarantee the long-term safe service of anti-corrosion heating pipeline facilities in DC-intensive industrial and urban transportation areas.