SHIPS (INTERNAL PROTECTION) Cathodic protection of tanks
ZINETI Zinc or Aluminum anodes for the protection of ballast tanks and ballast and cargo tanks.Since the Second World War, the classic anode used for the protection of ballast tanks was Magnesium. This metal was extraordinarily well suited, having an electronegative force of 700mV over that of polarized steel.
The greatest advantage was its rapid power of polarization. Its problem was an overproduction caused by the emission of hydrogen and its electrochemical power, approximately 55%.
Today this metal is rarely used due to the restrictions imposed on it by the classification associations. However it is still used in tanks with platforms.
Zinc and Aluminum show a relatively low potential, the condition voltage being in the order of 230 to 300 mW over Polarized Steel; however they have an efficiency of 80%. Experience shows us with regards to Zinc Anodes that the importante of contamination of the steel is very small.
The result of the Aluminum Anode depends largely on the additives (Indium and Zinc), which immunize the tendency of the steel to form a rust film with passive effect.
Aluminum, as well as Zinc, has a large and satisfactory history as a cathodic protection materials. One of the advantages of Aluminum is that, in its installation, only a third of the weight is used compared with Zinc, which is important when considering the dead weight of the ship, and also taking into consideration that the cost of the installation is calculated by the shipyard according to the installed weight. One disadvantage, in agreement with the classification associations, is the possibility of sparks, which gives rise to the situation whereby the aforementioned associations impose certain restrictions on the use of Aluminum Anodes, but not those made of Zinc. The distribution of these Aluminum Anodes, due to the possibility of sparks, must be studied so that they are placed in low areas of the tanks.
Precise data needed to make a calculation for the protection of tanks:
1. General layout, the master frame, the longitudinal profile, the principal screens, and the plan of capacity.
2. Type of tanks, if only ballast, cargo and ballast, or segregated ballast in petrol tankers.
3. The density of the current required.
4. The life expectancy of the anodes.
5. Specify if the tanks are painted and if so, what type of paint.
6. The type of expected ballast.