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GALVANIC CORROSION

What is it and why does it happen?

• The word Galvanic means “ ...of, or producing an electric current by chemical action”.
• When any metal is placed in an electrolyte – (an electrically conductive liquid) it develops an electrical potential or voltage.
• No two metals have the same potential, so that when two different metals are placed in the same electrolyte, a measurable voltage difference is produced.
• If the 2 metals are then connected together by an external cable, a current will flow from the higher voltage metal via the bonding cable to the lower voltage metal, and then via the electrolyte to complete the path.
• Analogies with Batteries, electro-plating.
• This is a natural fact of nature, and the slow erosion of valuable underwater boat metals WILL take place, unless we take steps to control it.

• Galvanic action cannot be prevented, but it can be controlled by connecting a cheaper “sacrificial” metal to the boat metals we wish to “protect”. This is the principal called “CATHODIC PROTECTION”, where the “sacrificial” metal is the “Anode”, and the “protected” metal(s) is the “Cathode”.
• By attaching the correct size of Sacrificial anode, normally in the case of salt water, ZINC, or Magnesium in fresh water, to the more valuable underwater metals of our boat (prop, shaft, rudder stock, P-brackets etc., the RATE of “erosion” of these metals can be reduced to a negligible level.
• All that is required, is that the rate of erosion of the Zinc Anode(s) is monitored, to ensure that there is always sufficient metal content left to provide continued protection.
• If the “Zincs” are allowed to erode away completely, then there is no further protection for the boats more valuable
• (3)
• metals, and the “Galvanic Action” which is always present, will then start to destroy firstly the more “active” metals in the prop, shaft, P-brackets etc, and finally the more “Noble” metals until they become brittle or corrode away altogether.
• Sacrificial Anodes
• There is a formula for calculating the amount of weight required to provide adequate protection, and it should also be remembered that Zincs come in different qualities and the better qualities have other trace metals in them in order to give better protection and to make them more durable.
• MONITORING
• There are several products available which allow you to monitor the condition of the “Zincs” and also the effectiveness of the cathodic protection provided – from within the boat.
• (4)
• CONTROL OF THE RATE OF EROSION
• This can be achieved by installation of an “Impressed Current” system, and when fitted together with a monitor, can give very accurate control over the rate of erosion of the sacrificial Zincs, so that they are neither over protecting or under protecting. The disadvantages of these systems is that they require very frequent monitoring, and they are expensive, and for these reasons, they are normally found on larger vessels where they can be checked daily by permanent crew.
• Aluminium and steel hulls will also be better protected by being painted with a top quality epoxy paint.

• Bonding         (5)
• All metal underwater fittings (with the exception of thru-hulls) should be connected (BONDED) together inside the boat with a cable of not less than 4mm࿹, with correct terminals, either crimped or soldered to ensure a sound and low resistance connection, and all connections should then be further protected with a compound such as silicone grease to prevent any corrosion which may damage the integrity of the connection.
• Through hull fittings should be left unbonded, as they are generally small enough and provided they only have plastic or rubber hoses connected internally, they will have no path for galvanic current.
• The AC ground wire together with all DC negatives, should be connected to the boats bonding system at a common grounding point. (ABYC & BMEA directives)

(6)

STRAY CURRENT CORROSION

What is it

• SCC is a much more damaging situation than ordinary galvanic corrosion. Whilst galvanic currents are normally measured in milliamps, and driving voltages in millivolts, faulty electrical circuits can produce damaging currents hundreds of times greater, and are capable of destroying underwater boat metals in a matter of days or even hours!
• The conditions required for SCC are:-
• Boat connected to 220v AC shore power
• AC earth wire connected to DC bonding providing path for the destructive stray current from the faulty equipment on your boat or another vessel.
• Lack of protection in the form of an Isolation Transformer.
• Poor or faulty ground protection to 220v AC shore power sockets.
• Checks and Prevention     (7)
• Checks can be made to identify the following:
• With boat in water
• Integrity of shore power ground wire
• Level of galvanic current flowing through shore power ground wire on individual boats
• Integrity of boats grounding system
• With boat of water
• Check between zinc anode(s) and all other underwater metals to check integrity of bonding
• Inspection of anodes
• Inspection of condition underwater metals

Galvanic isolators are devices consisting of a number of diodes and are used to prevent small amounts of current flowing through the shore power ground wire, and causing accelerated erosion of the sacrificial zincs. However, these devices must be correctly rated to the shore power supply and they are only effective for preventing the passage DC currents up to a voltage difference of about 1.3v. Beyond this figure, the diodes will conduct and DC current will then bypass the galvanic isolator. Galvanic isolators will not generally prevent damage caused by stray current from faulty on board appliances, as the voltages involved are usually much greater than 1.3 volts.

Isolation Transformers       (9)

By far the best method of isolating shore power supplies (including the ground wire) from any of the boats DC grounding is by the use of an isolation transformer correctly rated for the total AC load of the vessel with some to spare.

This transformer will also protect against stray current corrosion, but should not be considered a cure for stray current problems. Any time stray current problems are identified the source should be located and the problem fixed.

Steel and aluminium boats should always be fitted with an isolation transformer.

Residual Current Device (R.C.D.)

ALL boats which have 220v AC power on board should be fitted with an RCD correctly rated for the boats on-board AC load, and with a trip current of 30m.a. and should also be of the trip free type.

There are no exceptions to this rule and it may one day save your life.