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I did the calculation for the surface area to anode ratio (zinc) for a project recently. It was my first time going through this exercise, and I ended up with a 7.2% number for surface area ratio (which was later confirmed externally by computer modelling through a different group). This brought me to think about aircraft carriers and large cruise ships, where this value is rather large.

If a ball-park number of 7% is correct for general applications, how are sacrificial anodes addressed on these large vessels?

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" Something different". Impressed current, very likely using titanium anodes , and the hulls have coatings/paint to reduce necessary current flow. Navy vessels can be relatively fast , so the turbulence of dozens of aluminium anodes on the hulls is unacceptable.On the other hand something like an oil platform where turbulence is not a factor hang many aluminum and aluminium/zinc anodes. Likewise slow moving oil tankers likely use aluminum anodes. For commercial vessels it is a cost decision; installation and operating costs ,including some factor for turbulence/drag of the sacrificial anodes. Protective coatings significantly reduce the current needed for cathodic protection below what you calculated. Also ,in service, stationary steel structures develop a "calcarous" coating due to cathodic protection, this reduces protective current requirements . In practice this means more anodes are initially installed than needed later in service.

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  • $\begingroup$ the anodes are on the inside of the hull. $\endgroup$
    – Tiger Guy
    Jul 7 '20 at 17:48
  • $\begingroup$ The corrosive sea water is,mostly , outside the hull, that is where the anodes are. Although there must be some in the bilge to protect from internal corrosion. $\endgroup$ Jul 8 '20 at 1:14
  • $\begingroup$ I see this now. My sub did not have anodes on the outside of the hull. $\endgroup$
    – Tiger Guy
    Jul 8 '20 at 20:54
  • $\begingroup$ I think a sub has such special coatings on the outside they may not need cathodic protection for corrosion protection. $\endgroup$ Jul 9 '20 at 1:04
  • $\begingroup$ we had them on the inside of the hull. Nothing that increases drag or noise is desirable on a sub. $\endgroup$
    – Tiger Guy
    Jul 9 '20 at 2:07
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Two forms of Cathodic Protection: Sacrificial anodes; and ICCP (Impressed Current Cathodic Protection).

Sacrificial anodes are passive and the anodes get eaten away as they protect the vessel. Aluminum anodes offer 3.4x the protection over zinc anodes, which means less are required but they cost more. Areas like rudder, propellors and thrusters, where water speed is higher, require a greater anode density. As anodes are consumed, protection disappears. All ballast/sludge tanks use sacrificial anodes + coatings for cathodic protection.

Impressed Current is active and protection can be adjusted to protect hull in all conditions, but is more expensive to install and run (fuel must be burned to generate power for system).

Some new vessels are using both, sacrificial anodes to offer base protection and impressed current to ensure hull is protected in all conditions. This is a good compromise between cost and protection.

Think offshore supply vessel running at low speed while towing, high speed during sailing and stationary while station-keeping. Cathodic Protection depends on a water temperature, salinity, etc, but mostly depends upon water speed, so an ICCP system can vary protection as required. Compare to a cargo vessel sailing at a fixed speed 100% of time.

The use of either method of cathodic protection ultimately comes down to cost and size of surface to be protected. US military use both, with larger vessels using ICCP (367 vessels) and smaller using sacrificial anodes (1800 vessels - feels like guesstimation).

Appendix A: Cathodic Protection NOD, Phase I Uniform National Discharge Standards for Vessels of the Armed Forces, Technical Development Document

ICCP systems are employed when the wetted surface of the hull and other underwater components requiring cathodic protection is large or a controllable system is required.

Current capacity, a sacrificial anode material property, is the total current available per unit mass over the life of the anode, commonly expressed as (amp-hr/kg) or (amp-yr/lb). The current capacity for zinc and aluminum anodes is 812 amp-hr/kg and 2759 amp-hr/kg, respectively. Current capacity should not be confused with the maximum output current of an anode, which is a function of the anode material, anode surface area, system resistance, and driving potential. For most common types of zinc anodes used on underwater hulls, the maximum output current is approximately 0.4 amps per anode.

This current capacity would apply to large commercial vessels also. 0.4A/anode for Zinc and 1.35A for Aluminum.

812 A-hr/kg means 0.0927A/kg for a year, which would give a Zinc anode weight of 4.3kg or 9.46lb (10lb anode) .

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When ships are in static storage for extended periods of time, impressed current is used to protect them from corrosion. The counter electrode is a carbon rod the size of a telephone pole and the power supply occupies a small building all by itself on land nearby. The rods are immersed in the spaces between the "mothballed" ships.

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