Sea Chest Corrosion With Box Cooler Arrangement

The information contained within this post are referring a technical problem encountered in a number of fishing and offshore support vessels, which have reported sea water leakage at the top of sea chests, located in the engine room. The sea chest holds a box cooler and the leakage has been experienced at the bolt flange between the box cooler and the sea chest. As a result a number of vessels have needed to be taken into dry dock for unscheduled repairs.

2013.11.01 - Sea Chest Corrosion With Box Cooler Arrangement Figure 1

In a number of cases, severe corrosion damage has been observed in the carbon-steel mounting flange on top of the sea chest. In some cases the flange was found to be partially wasted and in a few cases the bolts connecting the box cooler to the bolt flange were also heavily corroded. Additionally, the aft shell plating of a sea chest itself was found to be nearly corroded through, in an area covering 200 mm by 200 mm. It is of utmost importance, when mounting a box cooler in a sea chest, that the watertight integrity of the vessel is maintained. A considerable sea water leakage in an undermined sea chest may cause large ingress of water in the engine room if not noticed in time.

A box cooler can be a preferred solution in many cases. The design eliminates the secondary cooling water circuit of sea water inside the engine room (pumps, filters, valves, pipes etc.). For the arrangement with box cooler, the sea chest is provided with grids and the cooling effect is achieved by natural circulation of the surrounding sea water.

The design of the box cooler arrangement has to address two main challenges, galvanic corrosion and marine growth. There are two main trends of modern box coolers, which give somehow
diverse concerns on the maintenance aspects of the box cooler. The main trends are:

  • U-tube bundle made of aluminium-brass (CuZn20Al2) and coated to prevent harmful galvanic effects on the carbon-steel sea chest.
  • U-tube bundle made of copper-nickel (CuNi10) and uncoated to prevent marine growth on the tubes.

A box cooler (see image below) made of coated aluminium-brass tubes is exposed to marine growth. The coating itself give some reduction of the thermal efficiency but this should have been compensated for in the design (size). However marine growth may reduce the thermal efficiency severely and an active anti-fouling system is often needed. One such common system utilises anodes of pure copper mounted under the box cooler, releasing Cu-ions when an impressed current is applied between the copper and the sea chest (actually between unpainted steel strips acting as cathodes and with electrical continuity to the sea chest, being installed next to the copper bars). A box cooler made of coppernickel tubes, which are directly exposed to the sea water in the sea chest, must be electrically isolated from the hull in order not to introduce galvanic corrosion of the sea chest (naked steel becomes the sacrificing material). Isolation must be made and maintained of all bolts and flanges connecting the box cooler to the sea chest and to systems in the engine room. A natural anti-fouling of the bundle is obtained when Cu-ions are released from the tubes due to a slow spontaneous corrosion.

2013.11.01 - Sea Chest Corrosion With Box Cooler Arrangement Figure 2

The probable causes to the corrosion of the sea chests reported are three-fold.

  1. The natural circulation around the cooling elements causes warm sea water to rise towards the top of the sea chest (top plating) arranging for a more corrosion friendly environment and marine growth (barnacles, shells, etc.).
  2. Exposed noble material (corrosion resistant) in the box cooler tubes may cause galvanic currents between the box cooler and the sea chest causing galvanic corrosion (of the sea chest if the steel is exposed) if proper insulation is not maintained.
  3. The corrosion protection systems applied (coating, sacrificial anodes and/or impressed current cathodic protection) are not able to suppress the corrosion ratio of the sea chest. This can be due to poor design and installation, or lack of maintenance of the system as intended.

2013.11.01 - Sea Chest Corrosion With Box Cooler Arrangement Figure 3

Lessons to be learned

Box coolers are sometimes assumed to be maintenance free. However, the ship’s crew should pay regular attention for signs of corrosion and leakage in the mounting frame and the corresponding sea chest.

The ship’s crew should be most aware of the corrosion protection arrangement of the specific box cooler and sea chest. Consult the relevant manufacturer’s instruction manual for guidance.

Rapid galvanic corrosion may appear if the components make a closed loop (tube-steel-saltwater) and appears strongest in the sea chest at the closest distance between the tubes and the sea chest (which normally is the top plate). A smaller area of damaged coating in the sea chest will concentrate galvanic corrosion to such area of exposed steel (paradoxically, the smaller
area, the more aggressive corrosion).

Introducing additional sacrificial anodes (most often zinc) inside the sea chest to protect the hull, may under some circumstances counteract the effect of the tube bundle anti-fouling and thermal behaviour (Zn deposits on the tubes). Consult the relevant box cooler manufacturer for guidance on the positioning of sacrificial anodes.

It is recommended that box coolers are taken out for inspection periodically (about every 5 years). Such inspection should in general focus on (consult the manufacturer for more specific
guidance):

  • Inspection of the mounting flange, bolts and gasket(s).
  • Inspection of the isolation parts (if applicable).
  • Marine growth (barnacles, shells etc) may cover local severe corrosion, especially in upper part of the sea chest.
  • Inspection of the sea chest inlet- and outlet grids (to ensure sufficient free flow area).
  • Cleaning of the tube bundle (do not harm the coating or take away the copper-oxide layer).
  • Inspection of anodes and cathodes as applicable.
  • Assessment of the sea chest coating condition (and if new coating is applied, do not coat tube bundle, anodes or cathodes, as applicable).

Source: DNV

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