Pitting corrosion is one of the most destructive hidden failures of 316 stainless steel heating tubes. Unlike surface rust which can be observed directly, tiny corrosion pits often form inside the tube wall or beneath surface fouling, and they gradually expand inward under the continuous erosion of chloride ions and thermal stress. In the early stage of pitting corrosion, there is almost no obvious abnormal change on the outer surface of the heating tube, so routine visual inspection and thickness measurement are difficult to capture these potential defects. Once the corrosion pit penetrates the tube wall, sudden leakage will lead to system shutdown, environmental pollution and huge economic losses. Many heating tubes operate safely for several years and then fail abruptly, mainly because internal micro-pitting defects have not been discovered and repaired in time during regular maintenance. Eddy current testing, as a mature non-destructive testing technology, can efficiently scan the inner and near-surface areas of stainless steel tubes, accurately locate tiny pitting holes, micro-cracks and wall thinning defects. Establishing a regular eddy current inspection mechanism is an indispensable preventive maintenance measure to avoid sudden leakage accidents of anti-corrosion heating tubes.
The working principle of eddy current inspection is based on electromagnetic induction. When an alternating current passes through the probe coil, a changing magnetic field is generated, inducing closed eddy currents inside the conductive tube wall. If there are defects such as pitting, cracks or wall thinning, the flow path and intensity of eddy currents will change, which can be converted into visual signal waveforms through professional testing equipment. Inspectors can judge the position, depth and size of defects according to the amplitude, phase and waveform change of feedback signals. Compared with ultrasonic thickness measurement which only obtains local wall thickness data, eddy current scanning can realize full-length continuous detection of heating tubes, which greatly improves the coverage rate of defect screening. However, improper probe selection, unreasonable scanning speed and interference from surface attachments will cause missed detection or false alarm, so standardized operation procedures and defect grading criteria must be formulated.
Table 1 Defect Grading Standard and Disposal Scheme Based on Eddy Current Inspection Results
表格
| Defect Grade | Pitting Feature & Wall Thinning Degree | Corrosion Risk Level | Recommended Processing Measures | Maximum Allowable Service Cycle |
|---|---|---|---|---|
| Grade 1 (Intact) | No abnormal eddy current signal, uniform tube wall thickness | Negligible | Continue normal operation, next routine inspection | 12 months |
| Grade 2 (Minor Pitting) | Tiny shallow pits, wall thinning less than 10% | Low | Mark defect position, shorten inspection interval | 6 months |
| Grade 3 (Moderate Pitting) | Multiple scattered pits, wall thinning 10%–25% | Medium | Local polishing and passivation reinforcement, strict monitoring | 3 months |
| Grade 4 (Severe Pitting) | Deep continuous pits, wall thinning over 25% | High | Replace heating tube immediately to prevent penetrating leakage | Forbidden to continue service |
To guarantee high detection accuracy, enterprises must formulate unified eddy current testing specifications for stainless steel heating tubes. Before inspection, the inner and outer surfaces of the heating tube need to be thoroughly cleaned to remove scale, biological slime, rust and coating residues, preventing conductive attachments from interfering with electromagnetic signals. Appropriate high-frequency or low-frequency probes should be selected according to tube wall thickness; high-frequency probes are used for surface and near-surface pitting detection, while low-frequency probes are applied to identify deep wall thinning defects. The scanning speed must be kept uniform and slow to avoid missing small corrosion pits. All abnormal signal points should be marked with accurate position records, combined with ultrasonic thickness measurement to verify the actual residual wall thickness.
After grading the detected defects, maintenance personnel must implement corresponding disposal plans strictly. Grade 1 equipment can run normally according to the original maintenance cycle. Grade 2 and Grade 3 heating tubes need to strengthen water quality control and surface anti-corrosion treatment to slow down the expansion of existing pitting defects. Any heating tube reaching Grade 4 must be replaced immediately without delayed operation. Regular eddy current non-destructive testing realizes early warning of hidden pitting corrosion, complements raw material incoming inspection, factory anti-corrosion performance testing and daily water quality management, and builds a full-cycle closed-loop quality safety system for 316 stainless steel anti-corrosion heating tubes, effectively avoiding unexpected shutdown losses caused by hidden corrosion failures.
