Selecting Zinc Anodes for Marine Structures in Thailand — An Engineer's Guide

Steel Structures in Thai Waters Corrode Faster Than the Manual Assumes

The Gulf of Thailand maintains water temperatures of 28–32°C year-round — 10–15°C warmer than North Sea conditions. Steel corrosion rates in these tropical waters run 0.1–0.3 mm/year, roughly 2–3× the rate seen in cold-water environments.

Steel piles and jetty structures without cathodic protection typically begin showing serious deterioration within 5–8 years — well short of the 20–30 year design life.

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How Cathodic Protection Works in Seawater

In seawater, steel behaves as a natural anode (corrodes). Cathodic protection reverses this by connecting a more electrochemically active material that sacrifices itself preferentially, making the steel structure a cathode.

Protection criteria (ISO 12473 / DNV-RP-B401):

• Potential ≤ −850 mV vs Cu/CuSO₄ reference electrode
• Or ≤ −800 mV vs Ag/AgCl/seawater in marine environments

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Anode Material Selection: Zinc vs Aluminum vs Magnesium

Material	Driving Potential (vs Ag/AgCl)	Capacity (Ah/kg)	Relative Cost	Best For
Zinc	−1,050 mV	780	฿	General seawater, widest use
Aluminum	−1,050 mV	2,700	฿฿	Seawater and brackish water, highest capacity
Magnesium	−1,550 mV	1,230	฿฿	High-resistivity soil, freshwater — not for seawater

Material Selection for Thai Structures

Gulf of Thailand / Andaman Sea:

• Use Zinc or Aluminum — both provide sufficient driving potential in full-salinity seawater
• Aluminum offers 3.5× higher capacity per kg → fewer anodes or longer design life
• Zinc: specify ASTM B418 Type I (naval brass core) or Type II (mild steel core)

Brackish water at river mouths (e.g. Chao Phraya, Bangkok Port area):

• Zinc or Aluminum remain viable, but efficiency drops when salinity falls below 1%
• Test resistivity first — if above 200 Ω·m, consider Magnesium

Never use Magnesium in seawater — the high driving potential produces excessive current density and risks hydrogen embrittlement in high-strength steel components.

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Anode Sizing Calculation (DNV-RP-B401 Method)

Basic formula:

Number of anodes = (Surface area × Current density) / (Anode capacity × Utilization factor)

Reference current densities for Gulf of Thailand:

• Open seawater (submerged): 30–50 mA/m²
• Tidal / splash zone: 50–80 mA/m² (elevated oxygen)
• Buried in coastal mud: 15–25 mA/m²

Worked example: River jetty steel pile

• Round pile, 400 mm diameter, 8 m submerged length
• Surface area = π × 0.4 × 8 = 10.05 m²
• Current density = 40 mA/m² (Gulf of Thailand average)
• Required current = 10.05 × 0.040 = 0.40 A
• Using a 5 kg Zinc anode (780 Ah/kg, utilization 0.85): → Anode capacity = 5 × 780 × 0.85 = 3,315 Ah → Design life = 3,315 / (0.40 × 8,760 hr/yr) ≈ 0.94 years → insufficient; upsize or add anodes

For a 10-year design life, approximately 10 kg of Zinc anode per pile is required, or multiple smaller anodes distributed along the pile.

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Correct Anode Placement

Jetty / pier piles:

• Space anodes every 1–2 m along the submerged section
• Increase density at the splash zone (inter-tidal zone) — highest corrosion rates occur here
• Inspect potential every 6–12 months using a reference electrode

Subsea pipelines:

• Install bracelet anodes every 30–50 m
• Add extra anodes near field joints where coating may be defective
• For pipelines exceeding 1 km, consider impressed current cathodic protection (ICCP)

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Indicators That Anodes Need Replacement

• Anode has consumed more than 50% of its original mass → replace
• Measured potential more positive than −750 mV (underprotection)
• Rust spots appearing on the structure despite anode presence → check anode connections and placement

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Consult our cathodic protection engineering team: +66 2-096-2118 | +66 83-494-6958 Sahawatthanakit (1988) Co., Ltd. — Nonthaburi, Thailand Request a Corrosion Protection Quote →