Corrosion and Protection of High-Pressure Piping Systems in Reverse Osmosis Seawater Desalination

Against the backdrop of global water scarcity, seawater desalination has gradually become an essential solution to alleviate freshwater shortages. Particularly in regions such as the Middle East, North Africa, and South Asia, where water scarcity is severe, reverse osmosis (RO) desalination has become the mainstream technology due to its low energy consumption, operational flexibility, and relatively lower investment costs. However, RO systems must treat highly saline and strongly corrosive seawater, which poses significant challenges to equipment materials. Among these, the high-pressure piping system is both the core component ensuring stable membrane performance and the most vulnerable to corrosion. Material selection and protection strategies directly affect the safety and economic viability of desalination projects.

The operating environment of RO seawater desalination is complex:

• High salinity: Chloride ions in seawater can easily break down passive films on metals, triggering pitting corrosion.
• Temperature fluctuations: In regions such as the Persian Gulf and the Red Sea, seawater temperatures in summer can exceed 35 °C-above the critical pitting temperature of many stainless steels.
• Residual chlorine: Even trace amounts of residual chlorine from biocides (e.g., hypochlorite) used in pretreatment can accelerate the corrosion of austenitic stainless steels.

As a result, early systems that adopted 316L, 317L, and 904L austenitic stainless steels often suffered from pitting and crevice corrosion. Even duplex stainless steels such as 2205 and 2507 have experienced localized corrosion and failures under extreme high-temperature and high-salinity conditions.

Today, seawater desalination plants generally adopt a segmented material strategy:

• Low-pressure sections: Non-metallic materials such as UPVC, FRP, and PTFE are widely used to minimize chlorine-related corrosion.
• High-pressure piping: Main headers are typically made of 2205 duplex stainless steel, while branch pipes may use 2507 duplex steel or high-alloy austenitic stainless steels.
• Critical components: Parts such as RO pressure vessel end plates and quick couplings require higher-grade alloys such as 2507 or 6Mo stainless steel.

In northern China, where seawater temperatures are relatively low, duplex stainless steels perform reasonably well. However, in southern waters and high-temperature, high-salinity regions such as the Middle East and North Africa, duplex steels continue to experience failures-including brine leakage incidents that compromise system safety.

Extensive field experience has confirmed that titanium (e.g., TA2 commercially pure titanium) is the ideal solution for extreme corrosion environments. Titanium offers:

• Outstanding corrosion resistance: Titanium naturally forms a stable oxide film, making it highly resistant to pitting and crevice corrosion in chloride-rich environments.
• Long-term reliability: In a desalination plant on a South China Sea island, titanium piping and valves remained intact and corrosion-free after eight years of continuous operation.
• Suitability for high-temperature waters: Even in regions such as the Persian Gulf and Red Sea, titanium maintains excellent stability.

Although titanium has a higher upfront cost, its low maintenance requirements and extended service life compensate for initial investment, making titanium piping systems increasingly adopted in high-pressure desalination applications.

Beyond material selection, multi-layered protection is essential:

• Operational practices: During shutdowns, flushing with fresh water prevents stagnant seawater from accelerating corrosion.
• Chlorine removal: Adding reducing agents before seawater enters the high-pressure section protects both piping and RO membranes.
• Cathodic protection: Sacrificial anodes (e.g., zinc) can reduce corrosion risks in duplex steel piping, though large-scale use remains limited.
• Coatings and surface modification: Well established in low-pressure piping, their long-term stability and cost-effectiveness in high-pressure systems require further validation.
• Green additives: Future developments may include eco-friendly chemicals with both corrosion inhibition and scale control functions, balancing equipment safety with environmental sustainability.

As global demand for freshwater continues to rise, RO seawater desalination will be deployed more widely across coastal cities, islands, and industrial bases. Industry trends will likely include:

Adoption of advanced materials: Titanium alloys and composites will see broader use, particularly in extreme marine environments.

Lifecycle cost assessments: Project planning will shift focus from initial investment to balancing long-term operational and maintenance costs.

Smart monitoring and protection: Sensors and big data will enable real-time corrosion monitoring and predictive maintenance.

Green protection technologies: Development of eco-friendly scale and corrosion inhibitors will support sustainable growth in seawater desalination.