ZINCS 101

What are zincs?
Most boat owners and divers use the word “Zincs” to refer to a sacrificial anode. Various metals, such as aluminum and Magnesium are used as sacrificial anodes besides zinc. Zinc or sacrificial anodes are used to protect a boats’ underwater metal fittings, such as propellers, shafts, thru-hull fittings, rudders, trim tabs etc., from corrosion. The zinc must be attached or wired (bonded) to the metal fitting it is to protect from corrosion. Corrosion, like rust, is where the metal that is being corroded, gives up it’s electrons and is basically falling apart or disintegrating. It is electricity in the water which causes corrosion and is more prevalent in saltwater than fresh water, as saltwater is also a better conductor of electricity. The electricity can come from your boat, your neighbors boat or from shore power. Zinc is able to give up it’s electrons more readily than bronze or stainless and thus is used as a sacrificial anode. Keep in mind that for this electrical/Chemical reaction to take place, the zinc anode and the metal fitting it’s connected to must stay intact and must both be submersed in the same body of water for the circuit to be completed. Aluminum anodes are also used in some cases, mainly on aluminum sterndrives when in saltwater. Magnesium anodes are used on boats kept in fresh water and should never be used in saltwater because magnesium is extremely active.

Zincs 102

How much zinc to use?

The amount of zinc needed is proportional to the amount of BARE underwater metal fittings being protected. Both zincs and metal fittings are measured in surface area. If the zinc does not have enough surface area it cannot give up it’s electrons fast enough and the metal that the zinc is intended to protect will give up it’s electrons and corrosion will result. The amount of zinc needed can be determined by an experienced Marine Electrician performing a Corrosion Control Survey, but more often by the diver replacing the zinc anodes that originally came on the boat from the factory. A diver can determine how much zinc to use and where it should be placed based from experience. If the zinc anode disintegrates too fast, the diver uses a larger zinc. If the zinc anode disintegrates too slow, less zinc or surface area of zinc is called for. Zincs which disintegrate too slow may grow marine growth over their surface, which will disable the zinc. Too much zinc can also cause a problem by increasing the amount of electricity flowing through the underwater metal fittings, causing something similar to Metal Fatigue.

What quality of zincs to use?

The quality of zinc used is important because of its effectiveness. Mil. Spec. (Military Specification ) is always recommended, being 99% pure zinc it is very active and because of this, the zinc anodes surface is easily cleaned of ionized zinc. With Mil. Spec. Zinc, in most cases, ionized residue will slough off by itself, maintaining corrosion protection at all times. A non Mil. Spec. Zinc or anode will build up a hard crusty ionized material which has no electrons and does not conduct electricity; The hard crusty ionized material needs to be removed for the zinc or anode to offer corrosion protection. the non Mil. Spec. zinc will only provide adequate corrosion protection when cleaned by a diver. if the build up disables the zinc in 2 weeks and the boat is serviced or cleaned every 4 weeks, then the non Mil. Spec zinc is only offering protection for first 2 weeks. Thus, always use Mil. Spec. Zinc, Aluminum or Magnesium anodes.

Zinc attachment

generally there are two ways to protect a boat’s metal fittings from corrosion; where a zinc is attached to each metal fitting underwater and where all metal fittings are attached to (usually) a transom mounted zinc plate, (attached on the outside of the boat by bronze or stainless steel studs). These studs are bolted through the hull and into the bilge. The metal fittings are also attached to the hull by thru-hull bolts. To achieve protection from the zinc plate, the studs from the zinc plate are connected (bonded) by wires to the thru-bolts of the metal fittings within the bilge. Most boats use a combination of the 2 previously mentioned zinc attachment systems.

Good Contact

It is also important that there be good contact between the zinc and the metal fittings in order to protect them from corrosion. A zinc must be tightly mounted on a clean and paint free surface. If the zinc is connected to a metal fitting by a bonding wire, all connections must be clean and tight. A zinc that does not have a good connection is Inactive and is exemplified with marine growth over the zinc. Marine growth will not grow over an active Mil. Spec. Zinc because The constant sloughing of ionized zinc will give it nothing to adhere to. If a Mil. Spec. zinc is accumulating marine growth, it is not making a connection. This same scenario cannot be said for a non Mil. Spec. zinc. if marine growth is attaching to the zinc anode, The diver cannot determine at first inspection whether the problem is the quality of zinc or a bad connection.

When to replace Zincs?

A zinc should be replaced a month before the zinc eats away from around the fasteners or before the zinc anode is too small to provide adequate corrosion protection with regard to Surface Area of the remaining zinc.

Correcting the Corrosion Control System

Sometimes there is corrosion on a metal fitting and the boat’s existing zincs are working and all bonding wires are clean, tight and intact. If this is the case a zinc needs to be attached to the corroding metal fitting or the metal fitting needs to be connected by bonding wires from the metal fittings thru-hull bolt to a zinc plate or to the nearest bonded metal fitting.
Sometimes there is corrosion on a metal fitting below the waterline that is difficult to connect to a zinc, or is difficult to reach with a bonding wire inside the bilge. In such cases an external bonding wire may be considered. External meaning on the waterside of the hull by which A wire is attached to the corroding unprotected metal fitting to one that is protected by a zinc.
Any underwater metal fitting which marine growth fails to grow upon is probably not protected by a zinc anode, whether corrosion is visible or not. The corrosion may be unseen by the human eyes presently, but the lack of marine growth never fails to indicate a corrosion control problem.

Hire A Diver

Always hire a competent diver to inspect the bottom of your boat once a month; Every time your boat goes out of her slip a zinc anode can come loose. Every time something is stowed in the bilge a bonding wire can come loose or break. Water in the bilge can also damage bonding wire connections. A competent Diver under your boat can spot the subsequent corrosion due to loose zincs, broken or poor bonding wire connections before your underwater metal fittings and running gear become to damaged to save.

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