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It is recommended to test your boat for DC voltage leaks stray currents can emanate from within a boat (faulty or exposed wiring), from shore side fittings and / or cables, or from neighboring boats. A leak can have a devastating effect because of accelerated electron flow. Extreme causes can destroy hardware in a matter of hours. It doesn't take much current to overwhelm the low level protection of the cathodic system.
Basically, any metal that is feeding a current into the water will be ruined. Wiring systems cannot have a path back to the source of power.
Keep DC wires above water level in the bilge to avoid stray current and AC wires to avoid a shock hazard.
Bonding system-to help prevent stray currents, all electrical and underwater metal components should be connected to the battery's negative terminal or it's bus thus equalizing the voltage between them.
A potentially servious problem can arise with a boat at dockside using shore power to the vessel. The ground wire will connect all boats galvanically. This means, if a boat along side yours is also connected to shore power and he doesn't have anodes on his boat, then your boat will protect his as well. After the anodes are consumed, the metal components which are higher on the Galvanic Scale will now start being consumed. A galvanic isolator will remedy this by blocking the low voltage DC current flow that leads to this condition. (Attaches to the ground wire).
The ability to attain the -200 mv negative shift is dependent on the amount of current (amperage) the anode generates in relationship to the area of coverage. This is determined by the anodes surface area and proximity to the metal being protected.
Factors that cause current requirements to be higher are exposed areas (uncoated), water speed, and water temperature. Pure zinc has a theoretical capacity of 372 Ampere Hours per pound. That means at 1 amp, it would take 372 hours to consume a pound of zinc. Now, zinc operates at about 95% efficiency so it would actually be approx. 353 AmpHrs.
On a steel hull, the current requirements for protection could be between .3 ma (milliamp) and 6 ma. per sq. ft. for submerged protection depending on coating quality or lack thereof and very little water flow.
For aluminum hulls, this range could be between .5 ma. and 8 ma.
So an example would be a Z26 (25lbs) putting out 2 amps would last approx. (25lbs x 353 amphrs/ 2 amps/24hrs) or 8825/2=4412.5/24 = 183.8 days. Keep in mind that this is all theoretical and has many variables involved. A good rule of thumb is to be ready to change the anode when it reaches 50% consumption.
Types of Anodes and Applications
The best anodes on the market are ones that conform to Mil-Spec (Military Specification) Standards. They have been designed to provide maximum performance and reliability.
Magnesium - these are the most active anodes on the Galvanic Scale ( least Noble) and are recommended only for pure fresh water. Can be used with Fiberglass or Steel Hulls with Inboard Drives or Wood, Fiberglass, Aluminum and Steel Hulls with Out drives. These anodes can easily overprotect in other kinds of water with the resulting damage as explained above.
Zinc and Aluminum - these are generally suitable for all water conditions but there are preferences depending on hull type/drive type.
In fresh water, aluminum anodes with aluminum prop: magnesium or aluminum with stainless prop.
Fibreglass, Aluminum, Steel Hull
Approximate Recommended Range of Cathodic Protection:
Wood Hull: -550 to -600 millivolts
Fiberglass Hull: -550 to -900 mv
Steel Hull: -800 to -1050 mv
Non-metallic w/Aluminum drive: -900 to -1050mv
Aluminum Hull: -900 to -1100 mv
Example: If a zinc was protecting a brass fitting, the "driving"(or protecting) voltage would be
-.6v(-1.050vZn minus - .450vNaval Brass) or 600 mv.
A cathodic protection general rule of thumb is to provide a negative voltage that is at a minimum of -.2v(200mv) relative to the least noble metal being protected. There is, however, the posiblity of over protection as well as under protection in certain situations.
Over protection can cause damage in the form of alkali corrosion to aluminum, delignification to wooden hull (the breakdown of the fibers), and hull coatings (blistering). This can occur, for example, if a magnesium anode is used on aluminum, in water that is polluted, brackish (in between fresh and seawater) or seawater resulting in too much voltage. Another cause is stray DC current from defective wiring or equipment.
MILLIVOLTS METAL OR ALLOY
1100 Aluminum (with Indium)
750 Aluminum Stern Drive
CONSIDERATIONS FOR BOAT PROTECTION
When two or more dissimiliar metals are in contact with each other and immersed in an electroyte (a liquid that can conduct electricity), the more active metal electrically (less noble) will sacrifice itself by electron flow from the more negatively charged metal (the anode) to the more positively charged metal (the cathode). Hence, for protection of metal fittings on boats as well as hulls and drive units, a comprehensive plan of protection needs to be employed.
By utilizing sacrificial anodes, these components will be protected because they are not corroding themselves with the loss of material as is the anode. For example, on a steel hulled boat with brass fittings submerged underwater, the two metals provide the anode (steel) and cathode (brass). The water completes the circuit just like a battery. In this arrangement, the steel will deteriorate as it is less noble than the brass. To protect both metals, a third metal is introduced that is less noble than the other two. The metals widely used for this cathodic production are zinc, aluminum and magnesium. Each element has characteristics that make them suitable for certain applications.
Factors that affect galvanic corrosion are the salinity of water, the pollutants present, the water flow rate, cavitation, oxygen content, temperature, etc.
Anodes are supplied in varying weights and sizes. The surface area determines the amperage (current), which governs the amount of protection, and the weight determines the service life of the anodes.
As mentions before a metal that is more active electrically is less noble. On the Noble Scale these are the approximate negative voltages from Least to Most Noble:(referenced with a silver/silver chloride half cell).
The current U.S. Military Specification, A18001K is the result of extensive studies and experimentation carried out by corrosion scientists for more than forty years. Prior to the mid 1950's, corrosion prevention for underwater hulls and fittings, based on zinc sacrificial anodes. was not particularly reliable. No one could explain why one vessel using these anodes would suffer little to no corrosion of its underwater metals while another similar vessel using zinc anodes that appeared to be the same would receive extensive corrosion damage. Subsequent studies indicated that some zinc anodes did not remain electrochemically active, becoming relatively inert (passivated) over time due to a buildup of a dense, tightly adherent film on the zinc's surface. The passivating film's principal constituents were oxides of iron and the source of the iron was the elemental iron present in the zinc when the anodes were manufactured.
These studies determined that the maximum allowable iron content for reliable sacrificial zinc anode performance was 0.0014 percent. Hence, the first U.S. Military zinc anode specification, A18001A, was born. This specification also limited several other contaminating elements such as copper and silicon, but excessive iron was the main cause of anode passivation. To produce anodes that met the specification, manufactures had to start with the purest grade of zinc available from the smelters (Special High Grade) and not contaminate it while producing the anodes. This was, and is, very difficult to do.
Work continued on the problem over the years with metallurgists seeking an alloy that would perform as or more reliably than Super High Grade zinc and be easier to produce. They discovered that by alloying the zinc with small amounts of the elements and cadmium, an increased amount of iron could be tolerated. So the sacrificial anode specification was changed to allow an increase in the iron content to 0.005 percent. Zinc anodes meeting this new specification and all of the subsequent specifications form a loosely adherent film that is principally zinc oxide. This film readily slough off the anode's surface to expose fresh zinc alloy to the water, allowing the anode to be continuously electro chemically active. Thus, the zinc will not passivate. It is only through continuously electro chemical action at its surface that a zinc anode can provide corrosion protection for the metal structure to which it is attached. Installing zinc anodes that are not certified to meet the current U.S. Military Specification runs the risk that the anodes will be contaminated and will fail to protect the metals to which they are attached.