I. DESCRIPTION OF THE SYSTEM:
The purpose of the cathodic protection system is to prevent corrosion on the submerged metallic surface. This is accomplished by a passage of D.C. current from sacrificial zinc and/or aluminum anodes, which are submerged, through the electrolyte to the structure. The system is energized by connecting the zinc and/or aluminum anodes to the submerged structure.
II. OPERATION OF THE SYSTEM:
The underlying purpose of cathodic protection is to stop the flow of direct electrical current from a metal structure (bare or coated) through the electrolyte in which it is submerged (corrosion cell). There are two well-known methods of cathodic protection. The type we will discuss is the galvanic anode system type.
The galvanic anode system, which requires no external power supply, incorporates the use of metallic anodes. The anode metal generates sufficient voltage in the water which acts as an electrolyte. The electrical current flows from one piece of the metal to the other anode structure because of a natural voltage difference between them in the "Electromotive Series". The current flows from the anode through the electrolyte to the structure thus applying protective current to the structure. Such metals as magnesium, zinc, or aluminum may be used as the anode.
The typical type of anodes used on small boats are zinc or aluminum in sweater. The zinc or aluminum anodes are attached to shafts, rudders, trim tabs, or mounted to the stern and connected to the structures by way of internal bond wires. A typical zinc or aluminum anode cathodic protection system is normally sufficient size to protect the submerged structures with the boat electrically isolated (A.C.) from any external power source. However, many boats utilize A.C. dock power continuously. The continual use of the dock power allows the boats protective current from the boats protective current from the zinc anode to be lost and/or shorted to the A.C. power electrical ground and any other boat also utilizing the A.C. dock power. This condition exists because the National Electric Code requires that the A.C. dock power has a continuous green ground wire running to every electrical receptacle box on the dock for protection from ground faults and electrical shocks. To back up the protection provided by the shore ground, the green grounding wire is also connected (per ABYC standards) to the boats underwater metal structure bonding system.
III. PROTECTION CRITERIA:
The protection criteria for dissimilar metals in seawater should be a voltage reading of a minimum of -.800 volts to a silver-silver chloride reference electrode placed near the structure being protected, NACE International Standard Criteria. A voltage potential of -.900 volts will assure that corrosion is completely mitigated and minimize zinc or aluminum anode consumption. The closer the structure potential is to (-1.1 volts) zinc or aluminum anode open circuit potential the lower the consumption rate of the zinc or aluminum anode.
Zinc anode consumption rate is 25#/amp-year, thus the lower the zinc anode out put the longer the anodes will last. Typical measurements on a typical 42' foot Cruiser can lose 0.40 amperes or more of D.C. current in the shore power grounding lead, representing the unnecessary loss of 0.77 pounds of zinc per month, in addition to a reduction in the boats cathodic protection potential level.
A Galvanic Isolator, installed on board in the green grounding lead from the shore-power receptacle blocks any significant DC current due to voltages less than about two volt. This covers the galvanic potential differences which may exist between a protected boat and the shore grounding connection. Yet in the event of a ground fault in the A.C. system on board, the voltage across the Isolator is enough for it to pass the full A.C. fault current with less than 2 volts drop, thus preserving full protection for those on board or alongside.
Any boat docked and utilizing the A.C. dock power should have an anode isolator installed in all A.C. ground circuits (single or multiple) to insure complete cathodic protection and extend zinc or aluminum anode life.
V. SUPPLEMENTAL ALUMINUM ANODES:
On Boats with potentials below -.900 volts to a silver-silver chloride reference electrode it is recommended that a 3"φ x 36" long 30# aluminum anode be installed, by hanging it from the dock cleat nearest the props, shafts and rudder, thus attaching it to the cathodic bonding system by way of a Mueller type clip.
This sacrificial aluminum anode will supplement the zinc anode system and will bring the protection level up within the range of -.900 volts. The open circuit voltage is -1.175 volts the consumption rate of 6.8#/amp-year. Supplemental aluminum anode protection will further extend the zinc or aluminum anode life.
VI. STRAY D.C. LEAKAGE CURRENTS:
If there are sources of stray D.C. power in the vicinity of your submerged structure, it is possible for leakage currents to flow onto your submerged structures. The submerged structure, therefore, can become a portion of the boats internal D.C. circuit. The structure-to-water potentials resulting from the stray currents must be interpreted differently from the potentials that generate galvanic corrosion currents on structures. In the interpretation of potentials where there are no effects due to outside stray currents, we have seen that where the structure-to-water potentials are more negative than those existing on the remainder of the structures are the areas where galvanic current is leaving the structure. This is true because the structure, together with the water, acts as a complete galvanic cell. However, when outside stray currents are present, areas where current is leaving the structure to enter the water have structure-to-water potentials that are less negative than the potential at those areas where current is flowing from the water onto the structure. This condition is true because in the latter case the structure behaves as one portion of an electrolytic cell.
Stray currents can be responsible for a great deal of corrosion in relatively short periods of time. If they are found to be present on a boat in any appreciable magnitude, their effect must be eliminated as quickly as possible. It is possible by proper use of cathodic protection to overcome or minimize the effects of these currents, or if the magnitude of the strays is very large, it may be necessary to trace the source of the strays and repair faulty equipment or wiring and/or to design measures which will enable the current to return to its source without doing any damage to submerged structures.