How Should Materials Be Selected for Seawater Desalination Systems to Address Seawater Corrosion?| Insights by AQUALITEK

Introduction

Seawater desalination systems operate in one of the most aggressive corrosion environments encountered in industrial water treatment. High chloride concentration, dissolved oxygen, microorganisms, and continuous hydraulic stress make material selection a decisive factor in system reliability, safety, and lifecycle cost.

Inadequate material choices can lead to:

•Rapid pitting and crevice corrosion

•Stress corrosion cracking (SCC)

•Unexpected leaks or catastrophic failures

•Exponential maintenance and replacement costs

This article provides a Best-practice guide to selecting materials for pipelines, pumps, and pressure vessels in seawater desalination (SWRO) systems.

1. Why Seawater Is So Corrosive

Before selecting materials, it is essential to understand the corrosion mechanisms in seawater:

Key Corrosion Drivers

•High chloride concentration (~19,000 mg/L Cl⁻)

•Dissolved oxygen

•Microbiologically influenced corrosion (MIC)

•High operating pressures

•Temperature fluctuations

•Galvanic coupling between dissimilar metals

Conclusion: Ordinary carbon steel or standard stainless steel grades are usually unsuitable for long-term SWRO service.

2. General Principles for Material Selection in SWRO Systems

When selecting materials, the following principles should guide decision-making:

1.Chloride resistance first, strength second

2.Uniform corrosion resistance is not enough — pitting resistance matters

3.Avoid stress corrosion cracking under pressure

4.Minimize galvanic corrosion risks

5.Consider total lifecycle cost, not just initial CAPEX

3. Pipeline Material Selection for Seawater Service

Common Pipeline Materials and Suitability

1️⃣ High-Density Polyethylene (HDPE)

Best for:

•Seawater intake

•Low-pressure seawater transfer

•Chemical dosing lines

Advantages:

•Excellent corrosion resistance

•No galvanic corrosion

•Flexible and lightweight

•Long service life

Limitations:

•Pressure limitations

•Temperature sensitivity

2️⃣ Glass-Reinforced Plastic (GRP / FRP)

Best for:

•Seawater pipelines

•Large-diameter lines

•Intake and discharge systems

Advantages:

•Outstanding seawater corrosion resistance

•High strength-to-weight ratio

•Long service life

Key consideration:
Resin system must be seawater-grade (vinyl ester preferred).

3️⃣ Duplex / Super Duplex Stainless Steel

Typical grades:

•Duplex 2205

•Super Duplex 2507

Best for:

•High-pressure seawater pipelines

•Critical sections near pumps and membranes

Advantages:

•Excellent pitting and crevice corrosion resistance

•High mechanical strength

Caution:
Requires strict welding quality control to avoid corrosion initiation.

4. Pump Material Selection in SWRO Systems

Pumps are among the most corrosion-sensitive components due to turbulence, pressure, and cavitation risks.

Recommended Pump Materials

Wetted Parts

•Super Duplex Stainless Steel (2507)

•Titanium (Grade 2 or 5) for extreme conditions

•Nickel-based alloys (e.g., Hastelloy) for specialty applications

Shafts & Impellers

•Super duplex or titanium

•Ceramic or polymer coatings may be used for enhanced resistance

Key Design Considerations

•Avoid sharp edges to reduce cavitation

•Ensure sufficient NPSH margin

•Prevent stagnant zones where MIC may occur

5. Pressure Vessel Material Selection

Membrane Pressure Vessels (RO Housings)

1️⃣ FRP (Fiber-Reinforced Plastic) Pressure Vessels

Industry standard for SWRO

Advantages:

•Completely corrosion-resistant

•Lightweight

•Long lifespan

•Proven track record

Design note:
End closures and metallic inserts must be corrosion-protected.

2️⃣ Metallic Pressure Vessels (Less Common)

If used, materials must be:

•Super duplex stainless steel

•Internally coated or lined

Risk:
Higher cost and corrosion management complexity.

6. Fasteners, Valves, and Auxiliary Components

These “small parts” often cause big failures if ignored.

Best Practices

•Use super duplex or titanium fasteners

•Avoid mixing carbon steel bolts with stainless components

•Select seawater-rated valves with compatible trim materials

•Apply electrical isolation where dissimilar metals cannot be avoided

7. Galvanic Corrosion Control

When dissimilar metals are used:

•Install dielectric flanges or insulating gaskets

•Maintain proper grounding

•Use sacrificial anodes where appropriate

Ignoring galvanic corrosion can destroy even premium materials rapidly.

8. Lifecycle Cost Perspective

While corrosion-resistant materials cost more upfront, they deliver:

•Fewer shutdowns

•Longer service life

•Lower maintenance costs

•Reduced safety and environmental risks

Best practice: Always evaluate total cost of ownership (TCO), not just purchase price.

Conclusion

Material selection for seawater desalination systems is not optional engineering refinement — it is a core design decision. Proper choices for pipelines, pumps, and pressure vessels must address:

•Chloride-induced corrosion

•Mechanical stress

•Microbial activity

•Long-term operational stability

By prioritizing corrosion resistance, compatibility, and lifecycle economics, SWRO systems can achieve decades of safe, reliable operation.