A prototype vessel makes use of an internal ballast system whereby lead bars are placed in the hull, and a polyester resin is poured on top to prevent movement of the system (Figure 2). Various deposits were found on the top of the internal ballast system.
The corrosion deposits were analysed and the inside of the aluminium hull in the region of the fixed ballast system was inspected. The lead ballast and the resin were removed to conduct a more detailed inspection of the aluminium.
Compositional analysis found that the corrosion deposits were all rich in aluminium (Table 1), indicating that the aluminium hull of the vessel is corroding.
The polyester resin was found to be porous, allowing areas of local moisture concentration to cause corrosion of the hull with the corrosion products deposited in and on top of the porous resin.
The pitting corrosion observed on the hull was extensive in the region of the fixed ballast system. Most of the pits varied from 0-1mm deep, however a significant number were found to be 3mm deep and in some cases deeper. Pitting corrosion was observed on the port side of the vessel in the forward section of the exposed hull (Figure 3), on the cross members in contact with the fixed ballast system, and on the starboard side of the vessel (Figure 4).
Lloyd’s Register MSPM Part E Chapter 2 Section 4 outlines the allowable losses for various components on ocean going vessels. The tables for a category 3 vessel state that a maximum of 20% losses can be tolerated for hull and deck plating before the component must be replaced. This means that any part of the hull which shows pits of greater than 1.2mm would have to be filled by weld filling. It was recommended that all pits of greater than 1mm should be filled with the appropriate welding rod in the areas where pitting is excessive.
In the case where a number of pits exist in a cluster which have a depth of greater than 1.2mm, care must be taken that extensive weld filling does not cause excessive residual stresses in the hull material. In such cases the best solution would be to replace the whole plate so as to avoid the associated distortion as a result of successive welding.
The hull must be restored back to its original 6mm thickness. A reduction in section thickness of the hull will reduce the stress bearing capacity of the hull in that area. This reduction in section thickness therefore can only be repaired by weld filler or by the replacement of the plate. The aluminium hull must be coated so as to prevent contact with stagnant water if pitting corrosion is to be prevented. Further, the resin is required to insulate the lead bars from the hull, to prevent galvanic corrosion. Only epoxy resin should be used as it has good bonding properties to aluminium. It must be applied in layers to prevent porosity.
- The corrosion deposits on the ballast system were found to be rich in aluminium, indicating corrosion of the hull.
- Inspection of the inside of the hull in the region of the fixed ballast system shows that extensive pitting corrosion has occurred in this area. This is a consequence of moisture ingress between the porous resin and the hull. Thus the deposits found indicate that the hull was degrading.
- Pits greater than 1.2mm that developed as consequence of corrosion must be filled.
- If repairs cannot be effected by means of weld filling, those hull plates must be replaced.
- The corrosion of the aluminium hull can only be prevented if stagnant moisture at the hull surface is prevented. The best method to facilitate this is to coat the hull with epoxy resin which has good adhesion properties to aluminium. The hull must first be effectively cleaned before the resin is applied to facilitate optimal adhesion.