Question

Sketch a cross section of a bulk carrier with either deep or shallow D.B. showing the types of framing used

Describe the corrosion problem experienced with ballast tanks

State how such tanks are protected against corrosion.

With the reduction in the number of new ships built, the older vessels in service have extended lives and these have shown some severe corrosion problems in ballast tanks.   The problems are made worse by the neglect of maintenance and the use of thinner^ scantling when high tensile steel is used.   The most serious problems ha^.e been found in permanent ballast tanks.   Galvanic corrosion can rapidly eat into the structure and rates increase with temperature and differential aeration (bottom of the tank is lower in oxygen concentration and hence anodic to die top).   

The presence of sulphate reducing bacteria, cause further   microbiological degradation.

The muds of some river beds are particularly rich in SRB. The bacteria can cause local pitting which has been known to penetrate the bottom shell.

Protection of ballast tanks is by complete paint coatings supplemented by sacrificial anodes. Some older ships have main anodes and booster anodes. The booster anode have large surface area to volume ratio (flat discs) causing rapid cation formation at cathode (steelwork) which cleans the surface of rust and scale.  A protective film is 'formed but this can quickly be removed in surface therefore a main anode which has a large volume to surface area ratio (hemispherical) are fitted and should last about 3 years.                

Hydrogen gas is formed on the cathode (steelwork) of the tanks, hence the tanks should be pressed up to prevent the formation of hydrogen gas pockets in the vapour space of slack tanks.   The tank venting system should be operational when emptying the ballast tank with precautions taken as for a low flash point cargo. The  tank should be considered non gas free until tested.

Ballast tanks that have been empty for some time may have atmospheres deficient in oxygen which is consumed in the corrosion process.   Deaths of crew entering such spaces have occurred (see M.910) therefore the tank must be fully vented before entering and 'Entry to enclosed spaces' procedures observed.                                                            

Maintenance includes cleaning and re-coating with pits filled or welded:   Serious corrosion over an extensive area would need plate renewal.