Monday, June 1, 2020

Cathodic protection

Different chemical structure, paint thickness', aeration etc. can lead to one area of the hull becoming more cathodic than another.
Electrons will flow from the anodic area through the hull to the cathodic area. By hydrolysis negatively charged hydroxyl ions will form. At the anode electron depletion leads to positively charged Iron ions. Hydroxyl ions migrated through the water to the anode, here combining with the iron ions to form Fe(OH)2 which combines with dissolved oxygen to form Fe(OH)3 or rust. In this way the anodic area will corrode. To prevent this it would be necessary to make the entire hull cathodic

The anode is insulated from the hull, electrical connection is via cable and ships side gland box. It may be made of lead or Platinised Titanium. With the lead anodes, the hydroxyl ions turn the surface of the lead a rich brown colour (PbO2).
A D.C. voltage is applied to just overcome the natural galvanic voltage. If the current is allowed to become too great then the increased Hydroxyl release causes sponginess and flaking of the paint
The cathodic system should make the hull 200mV more cathodic i.e. 2oomV negatively charged. The system measures this by checking the hull voltage against an insulated reference anode which has a known value of galvanic voltage with the hull material. Typically this may be Zinc which is normally at a voltage 450mV more negative than the hull, or Silver which is 600 mV more positive than the hull. The Cathodic protection system will try to make the potential difference between the hull and the zinc reference anode 250 mV (Zinc anode 250mV more negative than the hull), and the silver anode 600mV(Silver anode 800mV more positive than the hull).

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