Cathodic Protection for Steel Structures: A Practical Guide for Thai Industrial Facilities

The Hidden Cost of Skipping Corrosion Protection

A buried steel water main designed for 30 years of service began showing leaks at year 12. The repair bill — excavation, pipe replacement, surface reinstatement — came to over ฿8 million. A properly designed cathodic protection system installed at construction would have cost under ฿800,000.

This 10:1 ratio is not unusual. Corrosion is invisible until it isn't, and in Thailand's climate — high average temperatures, year-round humidity, and conductive coastal soils — steel infrastructure corrodes 2–3 times faster than in temperate climates. Cathodic protection is not an optional upgrade; for any buried or submerged steel structure, it is standard engineering practice.

---

How Cathodic Protection Works

Every metal surface in a moist or conductive environment forms a galvanic cell. Some areas act as anodes (oxidize, lose metal) and others as cathodes (are protected). Cathodic protection works by forcing the entire structure to behave as a cathode — either by connecting a more active metal (galvanic method) or by injecting external current (impressed current method).

Protection criterion (NACE SP0169): A structure is considered protected when its potential, measured against a Cu/CuSO₄ reference electrode, is −850 mV or more negative.

---

Two Systems: Galvanic vs ICCP

Galvanic Anode (Sacrificial Anode) System

A metal more electrochemically active than steel — zinc, magnesium, or aluminum — is bonded electrically to the structure. The anode corrodes preferentially, protecting the steel.

Pros: No external power required, self-regulating, simple installation, minimal maintenance, works in remote locations.

Cons: Limited current output, anodes must be replaced periodically, less effective in high-resistivity environments.

Best for: Buried pipelines (small to medium), jetty piles, ship hulls, underground storage tanks, heat exchangers.

Impressed Current Cathodic Protection (ICCP)

A DC rectifier drives current through the structure via inert anodes — typically Mixed Metal Oxide (MMO) coated titanium, platinized titanium, or high-silicon cast iron.

Pros: Precise current control, handles large structures, long-life anodes (20–30 years), adjustable as environment changes.

Cons: Requires continuous power, higher capital and maintenance cost, needs professional design and commissioning.

Best for: Long-distance pipelines, refineries, offshore platforms, large port structures.

---

Anode Material Selection

Material	Best Environment	Potential (vs CSE)	Capacity (Ah/kg)	Notes
Zinc	Seawater, low-resistivity soil	−1,050 mV	780	Most common for marine use
Magnesium	High-resistivity soil, fresh water	−1,550 mV	1,230	Highest driving voltage
Aluminum	Seawater, brackish water	−1,050 mV	2,700	Highest capacity, lightweight

Selection rules:

• Seawater / brackish water → Zinc or Aluminum
• Urban soil, low resistivity → Zinc
• Rural soil, high resistivity → Magnesium
• Large structures requiring low anode weight → Aluminum

All anodes must meet their applicable material standards: ASTM B418 (zinc), ASTM B843 (magnesium alloy).

---

Applications in Thailand

• Buried pipelines — water mains, fuel lines, gas distribution, sewer force mains
• Underground storage tanks (USTs) — fuel stations, chemical plants
• Submerged structures — jetty piles, submarine pipelines, offshore buoys
• Ship hulls — all steel vessels operating in Thai waters (ISO 12473)
• Cooling water systems — heat exchangers, condensers, power plant cooling towers
• Reinforced concrete structures — bridges, wharfs, coastal buildings (ICCP via embedded anodes)

---

Basic Current Requirement Calculation

NACE SP0169 formula:

Required current (A) = Surface area (m²) × Current density (mA/m²)

Current density reference values for Thailand:

Environment	Typical current density
Urban soil (resistivity < 50 Ω·m)	15–30 mA/m²
Rural soil (resistivity 50–200 Ω·m)	10–20 mA/m²
Gulf of Thailand seawater	30–50 mA/m²
Fresh water (rivers/canals)	20–40 mA/m²

Example: DN300 buried steel pipeline, 100 m length, surface area ≈ 94 m², urban soil at 25 mA/m² → Required current = 94 × 0.025 = 2.35 A → Specify 25 kg zinc anodes, quantity 4–5 units for 10-year design life

---

Common Design Mistakes

1. Ignoring coating breakdown factor — A new coating requires minimal current; as it degrades, current demand can increase 5–10x. Design must account for end-of-life conditions.
2. Anode spacing too wide — Creates unprotected gaps ("holidays") where corrosion continues.
3. No monitoring system — Reference electrodes should be installed every 300–500 m for potential surveys.
4. Wrong anode type for environment — Zinc in high-resistivity soil delivers insufficient driving voltage; protection fails within 2–3 years.

---

Relevant Standards

Standard	Scope
NACE SP0169	Underground/submerged piping (primary reference)
ISO 15589-1	Oil & gas pipeline cathodic protection
DNV-RP-B401	Offshore structure anode design
BS EN 13173	Floating offshore structures
ASTM B418	Zinc anode specification
ISO 12473	Marine cathodic protection principles

---

Get Technical Support

Cathodic protection design requires accurate soil resistivity data, structure geometry, and coating condition assessment. Our engineering team provides free technical consultation and sources internationally certified anode materials.

Call: 02-096-2118 | 061-541-6939 Sahawatthanakit (1988) Co., Ltd. — Nonthaburi, Thailand Request a Quote →