In an open cooling tower system, zinc (Zn²⁺) is used as a cathodic corrosion inhibitor to help protect mild steel and other ferrous metals from corrosion. It plays a key role in many low- or no-phosphate treatment programs, especially when paired with phosphonates like PBTC or molybdate.
How Zinc Reduces Corrosion
- Cathodic Site Blocking
- Corrosion in metals like mild steel involves:
- Anodic reaction (metal dissolves)
- Cathodic reaction (usually oxygen reduction)
- Zinc ions preferentially adsorb at the cathodic sites, blocking the oxygen reduction reaction, which:
- Slows down the overall corrosion rate
- Doesn’t stop metal dissolution, but greatly reduces the rate
- Formation of Protective Films
- Zn²⁺ ions react with hydroxide ions or orthophosphate/phosphonates to form a thin, adherent zinc-based film on metal surfaces.
- Typical protective compounds:
- Zinc hydroxide (Zn(OH)₂)
- Zinc phosphate or zinc-PBTC complexes
- This film acts as a barrier to oxygen and water, reducing further attack.
- Synergy With Other Inhibitors
Zinc works best when combined with:
Additive | Function |
A. PBTC | Stabilizes zinc, prevents precipitation, helps form smooth protective films |
B. Polyacrylate | Keeps zinc soluble and disperses precipitates |
C. Phosphate/phosphonates | Enhances film formation, though high phosphate may cause zinc phosphate scaling if not controlled |
Key Operating Notes
Parameter | Target Value / Comment |
Zinc level | 0.5–1.5 ppm (as Zn²⁺) in bulk water |
pH range | 7.0–8.5 ideal; >8.5 may reduce zinc solubility |
Chloride caution | High chlorides (>500–800 ppm) increase corrosion risks despite zinc |
Discharge limits | Zinc is regulated in many wastewater permits (e.g., <1.0 ppm in some cities) |
Things to Watch Out For:
- Precipitation at high pH or high phosphate → zinc may drop out of solution
- Overdosing → can lead to deposits on heat transfer surfaces
- Environmental discharge limits → often a limiting factor in high-cycle or zero blowdown systems