SEA WATER COOLED MULTI-TUBULAR HEAT EXCHANGERS - MATERIALS, CORROSION, PREVENTION

• Tubes: Copper-nickel alloys (e.g. 90/10 Cu-Ni, 70/30 Cu-Ni) are most common for sea water service; aluminium brass (Al-brass) is also used; sometimes titanium or stainless steel for aggressive service. Cu-Ni is preferred for its corrosion resistance and biofouling resistance.
• Tube plate (tube sheet): Naval brass, aluminium bronze, or Cu-Ni; sometimes carbon steel with a corrosion-resistant cladding/ overlay. The tube plate must be compatible with the tube material to avoid galvanic corrosion.
• Water boxes (headers): Cast iron, cast steel, or fabricated steel, often with a corrosion-resistant lining/ coating (e.g. rubber, epoxy, or a sacrificial anode system); sometimes bronze or Cu-Ni for small units. The water box is the sea water inlet/outlet header.

• Galvanic (bimetallic) corrosion: Where dissimilar metals are in contact in sea water (e.g. steel water box with Cu-Ni tubes, or brass tube plate with steel), the more anodic metal corrodes preferentially.
• Pitting corrosion: Localised attack on the tube surface, often initiated by chloride ions, deposits, or biofouling, leading to perforation.
• Erosion-corrosion (impingement): Caused by high-velocity sea water and turbulence, especially at the tube inlets, bends and behind the water box, eroding the protective film and accelerating corrosion.
• Crevice corrosion: Under deposits, at tube-to-tube-plate joints, and under gaskets, where stagnant sea water creates a differential aeration cell.
• Dezincification (of brass): Selective removal of zinc from brass, leaving a porous copper-rich layer, weakening the tube.
• Stress corrosion cracking: Under tensile stress in a chloride environment (e.g. in brass/ stainless).
• Biofouling/ microbiologically influenced corrosion (MIC): Marine organisms and bacteria attach to surfaces, creating localised corrosion cells.
• General/ uniform corrosion: On the steel water box and tube plate where the protective coating is damaged.

• Material selection: Use compatible materials (Cu-Ni tubes with Cu-Ni/ bronze tube plates and lined water boxes) to minimise galvanic couples.
• Cathodic protection: Fit sacrificial anodes (zinc, aluminium or magnesium) in the water boxes to protect the steel/ tube plate; or use impressed-current cathodic protection.
• Protective coatings: Apply epoxy/ rubber/ paint linings to the water boxes and tube plates to isolate them from sea water.
• Velocity control: Design and operate within the recommended sea water velocity (e.g. 1.5-3 m/s for Cu-Ni) to avoid erosion-corrosion; avoid excessive flow and turbulence.
• Filtration/ strainers: Fit sea water strainers/ filters to remove debris and reduce impingement and deposit formation.
• Biofouling control: Use anti-fouling coatings, chlorination/ electrolytic anti-fouling systems, or periodic cleaning to prevent marine growth.
• Corrosion inhibitors: Add corrosion inhibitors to the sea water/ cooling water where appropriate.
• Regular cleaning and inspection: Clean the tubes and water boxes, remove deposits, and inspect for pitting/ erosion; replace damaged tubes.
• Proper drainage and venting: Ensure the water box is fully drained when idle to avoid stagnant sea water corrosion.
• Sacrificial/ replaceable tube ends: Use ferrules/ inserts at the tube inlets to protect against impingement erosion.