1) Are all zinc anodes the same?
No, all zinc anodes are not the same. The chemical make-up of the alloy (or lack of alloy), has a significant effect on the anodes ability to protect a vessel.
2) Which zinc is best?
The best zinc to use is the “Mil-Spec” Alloy, which was developed by the US Navy back in the 1950’s and has continued to this day with the most recent specification A-18001-K.
3) Why should I use Mil-Spec zinc?
The reason you should use Mil-Spec zinc is that it will consistently keep working by sacrificing itself so your boat’s components will not corrode due to passivation.
4) What is passivation?
Passivation means the anode stopped working or has reduced effectiveness.
5) What causes passivation?
It is caused by a build-up of iron oxides that occur from the iron content in the anode.
6) How can the anodes avoid becoming passivated?
By insisting on Military Specification A-18001-K, you are insuring proper protection for your boat, as the alloy limits impurities and adds elements that allow the anode to properly sacrifice itself at optimum efficiency.
7) Certain Manufacturers claim the anodes they produce are 99.99% pure Zinc. Does that make sense?
No, it does not. The fact is Mil Spec Anodes are not 99.99% pure zinc. Mil Spec anodes are constructed with the following alloy specifications:
Cadmium .025% - .07%
Copper .005% Max
Iron .005% Max
Lead .006% Max
Aluminum .10% - .50%
Zinc Remainder
8) What would happen if I didn’t use Mil-Spec zinc?
If Mil-Spec zinc is not used on your vessel, you are risking components on your boat becoming “anodes”, hence sacrificing themselves because of contamination in the zinc that’s being used. The US Navy has too much of an investment, as you do, to take a chance with a product that may or may not get the job done. Their research has given them the proper alloy to consistently provide adequate protection and predictable results.
9) Some Anode manufacturers claim that their anodes last longer than all others. Does that make sense?
No, it does not. For Anodes to work properly they must be consumed. If Anodes do not sacrifice themselves they are not functioning correctly. If Anodes are passivated, serious corrosion damage could occur to the metal hull or other metal parts of the boat or ship causing several thousand dollars in damage. The last thing a boat owner should want is for their anodes to last a long time. The fact is, the anodes life span is governed by its weight. If the consumption of the anode is slower, there is a considerable risk that the minimum voltage relative to the most susceptible metal component on the hull, may not be met, resulting in corrosion.
10) Is it true some manufacturers use reclaimed Zinc in the production of their anodes.
Yes, apparently so. This is not a good idea as one has no idea where the zinc originates and the manufacturer is taking great risk of producing contaminated anodes that do not perform.
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 1950s, 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 aluminum 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 will readily slough off the anode's surface to expose fresh zinc alloy to the water, allowing the anode to be continuously electrochemically active. Thus, the zinc will not passivate. It is only through continuous electrochemical 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.
GALVANIC CORROSION
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 protection 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 anode..
As mentioned 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)
MILLIVOLTS METAL OR ALLOY
1100 Aluminum(with Indium)
750 Aluminum Stern Drive
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
possibility of overprotection as well as underprotection in certain situations.
Overprotection can cause damage in the form of alkali corrosion to aluminum,delignification to wooden hulls
(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.
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 -1050 mv
Aluminum Hull: -900 to -1100 mv
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 Outdrives.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.
Wood hull- with inboard,zinc is preferable
- with outdrive or outboard,zinc or aluminum anodes.(Aluminum stern drives prefer aluminum anodes.The type of aluminum for anodes is less noble than the drive.)
- In fresh water,aluminum anodes with aluminum prop : magnesium or aluminum with stainless prop.
Fiberglass,Aluminum,Steel Hull:-can use zinc or aluminum
- in fresh water,magnesium is preferred
DOCKED VESSELS
A potentially serious 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 alongside 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)
STRAY CURRENT
It is recommended to test your boat for DC voltage leaks.Stray currents can emanate from within a boat(faulty or exposed wiring),from shoreside fittings and /or cables,or from neighboring boats.A leak can have a devastating effect because of accelerated electron flow.Extreme cases 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.
MISC. CONSIDERATIONS
Do not mix anode types, as the lesser noble will protect the more noble metal.
Engine anodes are used with brass plugs, located in the water jackets.These also should be checked.
Anodes should make good electrical contact with boat surfaces.
A dielectric shield between the anode and protected metal will provide better current distribution
Never paint anodes
Remember to change anodes when half consumed so that the surface area remaining still gives adequate
current for protection.
