Question

(a) Surface preparation and painting of new ship plates

(b) Design of the ships structure and its maintenance

(c ) Cathodic protection by sacrificial anodes, of the internal and external areas of the ship.

Answer:

(a) If the plates are immersed in a weak solution of sulphuric acid or hyfrochloric acid for a few hours the majority of scale is removed. This is pickling. The pickled plate must be hosed down with fresh water on removal from the tank, allowed to dry before painting. During drying a light coat of rust is formed and must be removed before painting.

Flame clearing, oxy acetylene torch having several jets is used to brush the surface, burns the dirt and grease, loosens the surface rust, loosens the mill scale. The surface is immediately wire brushed and the priming coat applied while still warm.

Mill scale is removed by shot blasting. This removes dirt, rust and grease as well. This removes 95% to 100% mill scale and results in slightly rough surface which allows adequate adhesion of the paint. The plate is spray painted on emerging from the shot blasting machine.

(b) Direct connection between dissimilar metals should be avoided or effectively insulated from each other. Avoid trapping quantities of sea water or any electrolyte, crevices by selection of construction members of suitable radiused section or by sealing.

Larger enough drains to be provided to drain water and dirt. Tanks should be provided with drains to enable complete draining with out leaving any residue.

The metals should be protected prior to assembly and prevent water soaking into porous materials. Sharp edges where the paint draws away should be removed.

If two dissimilar metals are used paint all the surfaces or at least the more cathodic material. Weld metal of more noble quality than the parent metal -should be used. Traces of residual surface products (slag which is cathodic sometimes) of welding, should be cleaned before painting.

(c) Zinc or aluminium anodes are used as sacrificial anodes for the protection of external areas of the ship. Zinc is allowed corrode in preference to steel hull and propeller. And also protects hull due to differences in steel. The current is provided by sacrificial anodes attached to the ships hull which form a short circuited cell with the hull as cathode and sea water as electrolyte. This system does not require any maintenance but a large number of bulky anodes are required to give full protection until next drydock.

Deep water ballast tanks are also protected by sacrificial zinc anodes. Rust is to be removed and a film of the anode material is to be formed. This is achieved by using booster anodes having large surface area compared with their volume. To maintain the film main anodes having large volume compared to surface area are fitted. These are designed to last for three years. Protection is provide to the whole tank only if the electrolyte is in full contact, it is necessary to press up the tank when ballasted.