Cathodic Protection for Cooling Towers in Factories + Data Centers — Choosing Among 4 Anode Types and a TOR Guide

Cooling tower systems are a weak point of corrosion control in industrial factories + data centers — water circulating continuously 24/7, temperatures of 28-42°C, contact with chemical treatment (biocide, dispersant, pH adjust), and constant exposure to atmospheric oxygen, give the sump (lower water basin), fill structure (water-distribution panels), and distribution piping a corrosion rate 3-5 times higher than a closed system.

Using cathodic protection (CP) is a way to extend service life by changing the operating metal from anode (corroding) → cathode (protected) through connecting a sacrificial anode (galvanic) or ICCP (impressed current).

This article summarizes:

1. The 3 types of cooling tower structure + the points that corrode most
2. Selecting an anode by water chemistry + structure
3. Anode sizing + service life + replacement schedule
4. An example TOR template for facility / data center procurement

The 3 Types of Cooling Tower Structure + Corrosion Points

Counterflow (Crossflow → Counterflow CT)

• Structure: water drips against the airflow direction — fill is vertical
• Corrosion hotspot: the bottom corner of the sump (water reservoir) + the dry-wet water level (waterline)
• CP approach: Magnesium anode suspended in the sump (galvanic) or ICCP for large towers (>1,000 TR)

Crossflow

• Structure: water drips against the air from the side — fill horizontal
• Corrosion hotspot: lateral water distribution channel + side wall
• CP approach: Multi-anode array — magnesium at 2-3 m intervals

Closed-circuit (Hybrid / Indirect)

• Structure: a heat exchanger separates the process water + tower water
• Corrosion hotspot: exchanger tube bundle (usually stainless but the shell is carbon steel)
• CP approach: Zinc anode at the exchanger inlet + Mg at the sump

Selecting an Anode by Water — Decision Table

pH rule: Magnesium works best in water at pH 6.5-9.0 — above pH 9 → passivates (MgO/Mg(OH)₂ surface film) → CP stops working; Zinc tolerates up to pH 7-12 but corrodes quickly at pH <6.

Anode Sizing — Calculated by Surface Area + Corrosion Rate

🧮 Run it automatically in the Anode Sizing Calculator — set the environment to fresh/circulating water, enter the steel surface area and design life, and you get the anode count plus the acceptance criterion.

Magnesium (freshwater)

• Current density required: 100-150 mA/m² (steel in aerated water)
• Anode mass: use the rule 5 kg Mg / 100 m² of metal-water surface — for 5 years
• Sizing: cast to drawing; the manufacturer publishes no standard size table
• Example: Sump 100 m² → 5 kg Mg × 2 points (distributed) = 10 kg total, 5-6 year life

Zinc (seawater)

• Current density: 50-80 mA/m²
• Anode mass: 8-12 kg Zn / 100 m² — for 8 years
• Sizing: cast to drawing; the manufacturer publishes no standard size table
• Example: seawater sump 100 m² → 10 kg Zn × 2 points = 20 kg, 8 year life (for a BRACKISH sump use aluminium instead)

Aluminium (sea water)

• Current density: 80-120 mA/m²
• Anode mass: 4-6 kg Al-Zn-In / 100 m² — for 10 years
• Sizing: cast to drawing; the manufacturer publishes no standard size table
• Replaces zinc for sea water make-up work — lighter weight + high EER

KPI for the Acceptance Test — Polarization Decay

Both NACE SP0100 and ISO 12696 use polarization decay (PD) as the principal measure of CP performance:

• PD = the potential difference between "with anode (instant off)" and "24 hr after removing the anode"
• Criterion: PD ≥ 100 mV = the CP system is working
• PD ≥ 150 mV = excellent system

Measured with a Cu/CuSO₄ reference electrode (CSE) for buried structures, or Ag/AgCl/seawater for salt-water work.

Service + SKUs Supplied by SAHA

Sacrificial Anodes (Thai-made anodes, ISO 9001:2015 · supplied + system-designed by SAHA per DNV-RP-B401 / ISO 12696)

Engineering Service (in-house team)

• Site survey + water chemistry analysis — pH, conductivity, alkalinity, chloride
• CP design report — current density calc, anode sizing, layout drawing, BOM
• Installation — anode mounting + reference electrode + monitoring junction box
• Annual inspection + replacement — PD measurement + remaining mass survey
• Annual report per NACE SP0100 — for audit / insurance

TOR Template — Example Spec for Cooling Tower CP

1. Scope of work: Galvanic Cathodic Protection for cooling tower sump + fill 
   structure of the chiller plant system, building [X], number of [N] cooling towers
   capacity [Y] TR/cell

2. Standards:
   - NACE SP0100-2008 — Cathodic Protection of Cooling Water Systems
   - ASTM B418 (zinc), B843 (magnesium), B418 Type II (aluminium)
   - ISO 12696:2022 (for concrete sump)

3. Material specs:
   - Zinc Anode: 99.995% purity, conforming MIL-A-18001K + GL-Zn1
   - Magnesium Anode: Mg-Al-Zn alloy AZ31B, ASTM G-97
   - Cable: PVC-jacketed copper, sun-resistant, IP68 splice
   - Reference electrode: Ag/AgCl Cl⁻ saturated, 5-year life

4. Performance acceptance:
   - Polarization Decay ≥ 100 mV per NACE SP0100 §6.2
   - Measure 30 days + 180 days post-installation
   - Spot check every 6 months, full survey every 12 months

5. Documentation:
   - As-built drawing + anode location + cable routing
   - Initial PD measurement report
   - Material COA (mill cert) for every anode
   - Warranty 5 years galvanic / 10 years ICCP

6. Service life expectation:
   - Sacrificial anode replacement ≤ design life ± 10%
   - 50% anode mass remaining at mid-life inspection

7. Optional add-on:
   - Remote monitoring (PD telemetry, weekly auto-report)
   - Annual PM contract + replacement scheduling

ROI — Comparing CP vs no CP

A 1,000 TR cooling tower factory installing CP:

Insurance benefit: some companies reduce the insurance premium by 5-8% if there is a certified CP system + annual NACE survey.

Recommendations for Data Centers

For water-cooled data centers using chiller plants of 2,000-10,000 TR, we recommend:

1. An ICCP system (not sacrificial) — current can be controlled precisely + no frequent anode replacement
2. PD telemetry + integration with the BMS (Building Management System) — alarm if PD < 100 mV
3. Annual NACE certified inspector survey — for uptime SLA documentation
4. Use a closed-circuit tower to separate the process loop + tower loop — reducing corrosion factors at the chiller heat exchanger

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This article is an engineering reference — for project-specific CP system design, contact the Sahawatthanakit (1988) Engineering team for a site survey + water chemistry analysis + CP design report with a ready-to-tender TOR.