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

Water Source	pH	Conductivity (μS/cm)	Recommended Anode	Service Life	Standard
Tap water (Bangkok tap)	7.0-7.6	200-400	Magnesium (Mg-Al-Zn alloy)	5-7 years	NACE SP0100, ASTM B843
Deep well water	6.5-7.2	300-600	Magnesium	4-6 years	NACE SP0100
Brackish well (high Cl)	7.2-8.0	800-2,500	Zinc (high-purity 99.995%)	6-8 years	ASTM B418 Type II, MIL-A-18001K
Sea water make-up	8.0-8.2	>30,000	Zinc or Aluminium (Al-Zn-In)	8-12 years	TIS 3359-2563, DNV-GL
Treated water + biocide	7.5-8.5	600-1,200	Zinc (Mg unstable at high pH)	5-7 years	NACE SP0100
RO permeate (chiller plant)	5.5-6.5	<50	Magnesium + ICCP backup	3-5 years	NACE SP0388
Soft water + caustic dose	9.0+	varies	ICCP only (galvanic does not work at high pH)	15+ years	NACE SP0388

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

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
• Standard sizes: 5 kg, 10 kg, 12 kg, 24 kg
• Example: Sump 100 m² → 5 kg Mg × 2 points (distributed) = 10 kg total, 5-6 year life

Zinc (salt/brackish water)

• Current density: 50-80 mA/m²
• Anode mass: 8-12 kg Zn / 100 m² — for 8 years
• Standard sizes: 5 kg, 10 kg, 14 kg, 20 kg, 24 kg, 40 kg
• Example: Brackish-water sump 100 m² → 10 kg Zn × 2 points = 20 kg, 8 year life

Aluminium (sea water)

• Current density: 80-120 mA/m²
• Anode mass: 4-6 kg Al-Zn-In / 100 m² — for 10 years
• Standard sizes: 5, 10, 14, 20, 24, 40 kg
• 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)

Anode	Material	Common sizes	Application	Standard
Magnesium Anode	Mg alloy AZ31B	5/10/12/24 kg	Cooling tower freshwater sump	NACE, AMPP
Zinc Anode	Zn 99.995%	5/10/12/24 kg	Brackish + treated cooling water	MIL-A-18001K, GL-Zn1
Aluminium Anode	Al-Zn-In	5/10/14/20/24/40 kg	Sea water make-up + offshore	TIS 3359-2563, DNV-GL
Concrete Anode (standard / high-output)	Zn in alkaline mortar pH≥14	60/100 cm strip	RC sump + concrete tower fill	TIS 3029-2563, ASTM B418 Type II, 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 B843
   - 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:

Item	No CP	With Mg Anode CP
Sump replacement cycle	7-10 years	15-20 years
Tower fill structure repair	every 2-3 years	every 5-8 years
Annual chemical treatment cost	+25% (offset corrosion)	baseline
Anode replacement cost	n/a	฿60,000-80,000 every 5-6 years
Net 15-year TCO	100% baseline	-28% to -35%

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.