Even if 316 stainless steel heating tubes are produced with qualified raw materials, standardized welding and perfect pickling-passivation treatment, long-term operation in poorly regulated circulating water will still trigger under-deposit corrosion, which has become one of the most common causes of medium-term equipment failure. Suspended solids, chloride ions, dissolved oxygen, pH value and microbial colonies in industrial circulating water jointly determine the fouling deposition rate on the inner and outer surfaces of heating tubes. When sediments adhere to the tube wall, a closed anaerobic microenvironment forms beneath the scale layer, breaking the self-repair ability of the chromium-rich passive film. Local enrichment of chloride ions under deposits accelerates anodic dissolution, gradually forming hidden pitting holes that expand until tube penetration occurs. Many enterprises only focus on the anti-corrosion performance of heating tubes themselves while ignoring regular water quality monitoring and chemical dosing maintenance, leading to frequent leakage accidents after one to three years of service. Therefore, precise control of key water quality indicators is an essential operational anti-corrosion measure to extend the service life of anti-corrosion heating equipment.
Different water quality indicators produce synergistic corrosion effects on stainless steel substrates. Excessively high chloride concentration is the core factor inducing passive film rupture and pitting propagation. Dissolved oxygen provides the cathode reaction condition for electrochemical corrosion and accelerates the metabolic reproduction of microorganisms. Too low pH value weakens the stability of the chromium oxide passive film, while overly high pH will promote the precipitation of calcium and magnesium scale, aggravating fouling accumulation. In addition, total suspended solids directly increase the attachment probability of scale on rough tube surfaces, and microorganisms secrete corrosive metabolites to destroy surface protective structures. Only by setting reasonable threshold ranges for each parameter can enterprises effectively slow down fouling accumulation and inhibit under-deposit corrosion development.
Table 1 Recommended Water Quality Control Standards for 316 Stainless Steel Heating Tube Circulating Systems
表格
| Water Quality Index | Safe Concentration Range | Corrosion Risk When Exceeding Standard | Main Corrosion Form | Target Control Purpose |
|---|---|---|---|---|
| Chloride Ion | ≤200 mg/L | High | Local pitting & crevice corrosion | Prevent passive film breakdown |
| Dissolved Oxygen | ≤6 mg/L | Medium | Accelerate electrochemical corrosion | Suppress cathode reaction rate |
| pH Value | 6.5–8.5 | Low & high both risky | Passive film dissolution or scale precipitation | Stabilize surface protective layer |
| Total Suspended Solids | ≤20 mg/L | Medium | Under-deposit corrosion | Reduce fouling attachment |
| Total Microbial Count | ＜10⁴ CFU/mL | High | Microbiologically influenced corrosion | Eliminate microbial metabolic erosion |
Enterprises must establish regular water sampling and detection mechanisms for heating circulation systems. Weekly sampling is required for chloride ion, pH value and suspended solid testing, while microbial and dissolved oxygen testing is conducted monthly. When chloride content exceeds the safe threshold, online water softening or partial water replacement should be adopted to reduce ion concentration. Regular dosing of scale inhibitors, oxygen scavengers and biocides can effectively restrain scale formation and microbial reproduction. Meanwhile, periodic online chemical cleaning or offline high-pressure water flushing should be arranged to remove deposited fouling before it forms a dense isolation layer. For heating tubes applied in high-chloride wastewater systems, water quality control standards should be tightened to further lower the limit of chloride ions and suspended substances.
Scientific water quality management constructs an operational anti-corrosion barrier after heating tube installation, cooperating with material optimization, process control and surface protection to form a full-life anti-corrosion guarantee system. Strictly implementing water quality threshold control can significantly slow down the occurrence of under-deposit corrosion, reduce the frequency of equipment shutdown maintenance and heating tube replacement, and realize long-term energy-saving and stable operation of industrial anti-corrosion heating systems.
