Avoiding corrosion in box cooler sea chest

Vessels with box coolers are particularily exposed to agressive galvanic corrosion on the cooler flanges and in the sea chest compartments.

There has been an increasing number of severe corrosion incidents related to box coolers. The problems typically appear from the stage following the 5 years classification, but is often first discovered 2-5 years later.

Corrosion arising from marine growth on cooler tubes

Marine growth on box cooler tubes creates several challenges. The most apparent is reduced cooling performance. The contamination of box coolers requires cleaning and maintenance, which ultimately will damage the phenol coating on the tubes.

This in turn will expose the cooler’s surface to seawater, creating a galvanic cell between the noble metal in the tube and the more reactive steel structure surrounding the box cooler.

When the phenol coating on the box cooler tubes is no longer capable of isolating the noble tube metal (usually CuZn20Al2) from the seawater, a galvanic reaction starts, and the ship’s steel will corrode. Read more about the process of corrosion.

When the noble box cooler tubes are coupled to the ship’s steel, the sea chest basically turns from being the cathode to a sacrificing anode.

Read more about the galvanic series that determines the nobility of metals.

How to avoid corrosion related to box coolers?

Among a number of measures, we recommend special attention to:

Proper setup of the MGPS/ICAF system. This system does not only protect against marine growth, but plays also a significant role in terms of preventing corrosion in the sea chests. Cathwell experiences that unfavorable settings and marginal design of MGPS/ICAF systems are the direct cause of a number of severe corrosion incidents.

Proper arrangement of sacrificial anodes. The need for sacrificial anodes in sea chests is often underestimated. Sacrificial anodes will consume significantly faster than expected if damaged cooler tube coating coincides with unproper setup of the MGPS/ICAF system.

Adequate ventilation. Special attention must be paid to the top plate and box cooler flange to prevent that air is being trapped inside the sea chest. Cathodic protection from sacrificial anodes and MGPS/ICAF systems will come short in air pockets, leaving the entire corrosion protection responsibility to the coating system.

Safety measures during operation

Box coolers are often assumed to be maintenance free. However, a number of incidents have shown that certain measures should be regularly followed up by the crew. Here are our recommendation for the most important ones, which are all related to corrosion:

• Look for signs of corrosion and water leakage in the mounting frame from inside.
• Ensure proper settings of the MGPS/ICAF system (be aware of many misunderstandings when it comes to operation of such systems).
• Keep long-term records of set points of the MGPS/ICAF system for eventual failure investigations.

You are welcome to contact our team of engineers for consultance regarding corrosion protection and marine growth prevention.