Painting

The plate is suitably prepared, primed and finished off with a suitable top coat. This method of protection excludes water and oxygen from the plate surface and as long as the paint remains intact, this method of corrosion control is very effective and comparatively cheap. The problem however lies at the plate preparation stage. Invariably, traces of rust, mill scale, and dirt and water moisture get trapped beneath the paint coat and consequently corrosion commences below the film of paint.

Cathodic Protection

Cathodic Protection technique can only be used where the concerned metals are immersed in an electrolyte. The principle involved in this process is that the anodic corrosion reaction is either suppressed by the application of an opposing current or the anode is of a type which allows it to be consumed. Two means of cathodic protection are in general use on modern ships, mainly for the ship’s sides or in tanks.

Sacrificial Anode Type

Metals such as aluminum or Zinc are fitted to the ship side or in the tank at strategic positions which form the anode of the corrosion cell making the steel of the ship’s plating the cathode. The sacrificial anodes are gradually corroded away and have a life of about 2-4 years when they have to be replaced, usually at dry dock. This system is suitable for ship’s protection as well as in cargo and ballast tanks. Anodes can be typically seen in the vicinity of the propeller, in the vicinity of Engine Room inlets and discharges and in the forward portion near the bow thruster. In tanks, they are usually fitted in the bottom portion of the tank for two reasons.

1. The concentration of dissimilar metals i.e. valves, pipe lines etc is only in the bottom portion of the tanks.
2. When the anode has corroded away, often it drops down to the bottom of the tank. It is possible that when it falls, it would cause an intrinsic spark and possible explosion.

Impressed current type

These systems are fitted for protection of the immersed portions of the extent hull only. A voltage of difference is maintained between hull and fitted anodes, which will protect the hull against corrosion. The system consists of a source of direct current, anodes and apparatus foe measuring and controlling the current. The amount of the impressed current will vary with the area to be protected, ship speed. Salinity and condition of the paint work.

For normal operating conditions, the potential difference is maintained by means of an extremely mounted silver Chloride cell detecting the voltage difference between itself and the hull. The potential difference is fed back to a controller which impresses the same amount of current onto several anodes positioned on the hull. The anodes used are usually of permanent noble metals such as lead or silver. In this way any corrosion cells which might exist are exactly cancelled and corrosion is controlled. The propeller shaft and rudder stock have to be specially grounded in order to ensure continuity with the hull. Care is to be exercised that the controller only impresses the exact amount of current required. If the impressed current is too little, corrosion protection will be inadequate. If the current impressed is too high, the paint work on the hull will be damaged and blistering occurs.