I. Corrosion Is the Number One Killer of Heat Exchangers
Two fluids are separated by a thin wall, and one or both sides are often corrosive (sulfides in flue gas, chloride ions in cooling water, acids and alkalis in process media). The thinner the wall and the more complex the medium, the more corrosion comes knocking. Common forms:
| Form | Feature | Cause |
|---|---|---|
| Uniform corrosion | Overall surface thinning | Full immersion, uniform medium |
| Pitting | Local deep pits | Chloride ions, surface defects |
| Crevice corrosion | Concentrated at seals/dead corners | Under gaskets, beside welds |
| Stress corrosion cracking | Cracking along stress direction | Tensile stress + specific medium |
| Erosion corrosion | Wear on impacted surfaces | High velocity, particulate |
II. How to Choose Material
- Stainless steel 304/316: General-purpose, resistant to atmosphere and water, but only 316 has better chloride pitting resistance;
- Titanium: Extremely chloride-resistant, first choice for seawater and high-chloride environments, but expensive;
- Nickel-based alloys (e.g., 825, C276): Strong corrosion, high-temperature conditions;
- Aluminum alloy: Lightweight first choice for air-air heat exchange, but not acid/alkali resistant;
- Coating/lining: Carbon steel + anti-corrosion coating, reduces cost while isolating the medium.
III. More Than Material Selection: Systematic Protection
- Cathodic protection: Sacrificial anode or impressed current to suppress electrochemical corrosion;
- Water quality management: Control pH, chloride ions, hardness; add corrosion and scale inhibitors;
- Structural pitfalls: Reduce crevices and dead zones; avoid direct contact between different metals (galvanic corrosion);
- Flow velocity control: Too high induces erosion, too low promotes deposition; take the economic range.
When selecting, give the material engineer the four elements of "medium composition + temperature + flow velocity + maintenance," far more reliable than guessing from experience.