Corrosion Prevention

The following is a brief summary of corrosion prevention tactics for metal boats. Primarily these strategies are aimed at the boat building process, at which time one has the very best opportunity to create a hassle free, low lifetime maintenance situation for any metal vessel. Vigilance and good workmanship are vital...!

Paint Systems

Small metal boats, unlike tankers and container ships, are not designed with an appreciable corrosion allowance. They must therefore be prepared and painted in the best way possible in order to assure a long life.

Current technology for protecting steel and aluminum boats is plain and simple: Epoxy Paint.

When painting metal, a thorough degreasing is always the first step, to clean off the oils from the milling process, as well as any other contaminants, like the smut from welding, which have been introduced while fabricating.

The next important step is a very thorough abrasive grit blasting on a steel boat, or a somewhat less aggressive "brush blast" on an aluminum boat. The process of sand blasting a metal boat is expensive and can in no way be looked at with pleasure, except in the sense of satisfaction and well being provided by a job well done.

While there is no substitute for grit blasting, there are ways to limit the cost of the operation. When ordering steel, it is very much to a builder's advantage to have it "wheel abraded" and primed. Wheel abrading is a process of throwing very small shot at the surface at high speed to remove the mill scale and clean the surface. Primer is then applied. Having been wheeled and primed, the surfaces will be much easier to blast when the time comes.

Ideally, a wheeled and primed surface will at least be "brush blasted" after fabrication is complete. Why? The shot used for wheel abrading creates a surface that is essentially a bunch of tiny smooth dents on the surface, whereas grit blasting creates a sharp contour that provides the paint system with a much better mechanical "bond" to the metal surface.

In terms of the paint system, aluminum boats are dealt with more easily than steel boats. Aluminum must be painted any place a crevice might be formed where things are mounted, and should also be painted below the waterline, if left in the water year-round. The marine aluminum alloys do not otherwise require painting at all.

On an aluminum boat, any areas which will be painted should receive the same aggressive preparation regimen used on steel: thorough cleaning, sand blasting, and epoxy paint. Aluminum is less hard than steel, so sand blasting aluminum is relatively fast compared to steel. The blast nozzle must be held at a greater distance and the blast covers the area more quickly.

Insulation

Many schemes are used to insulate metal boats. Insulation is mentioned here in the context of corrosion prevention primarily to point out that, even when using blown in polyurethane insulation, it is NOT to be considered an effective protection against corrosion. As with anywhere else on a metal boat, epoxy paint is the best barrier against corrosion.

Blown-in foam is an excellent insulator, and offers considerable sound deadening. Blown-in foam does offer some protection for the interior metal work in addition to the paint, but only if it is adhered well to the surface.

Sprayed in foam, while popular, does have drawbacks which are often overlooked. Urethane foam is not a completely closed cell type of foam. With time, urethane foam will absorb odors which become difficult or impossible to get rid of. This is especially a problem when there are smokers aboard.

Nearly all urethane foam will burn fiercely, and the fumes are quite toxic. Blown in foam should therefore be a fire retarding formulation, and should ideally be coated with a flame retarding paint.

An alternative to blown in foam is a good quality flexible closed cell cut-sheet foam to fit between the framing. Some sheet foams are fire retarding by composition, but if not, they should be painted just like the urethane foams.

The best choice among the foams for cut-sheet foam installation are Ensolite and Neoprene. There are several different varieties of each. The choice of insulation foam should be made on the basis of it being fireproof, mildew proof, easily glued, easy to work with, resilient, and if exposed, friendly to look at. Ensolite satisfies all these criteria. Ensolite is both better and more expensive than Neoprene.

Styrofoam or any other styrene type of foam should be strictly avoided. Purchase a piece at the lumber yard and throw it onto a camp fire.... You will be immediately convinced.

Sprayed in polyurethane foam is the best in terms of insulating value, since it nearly completely prevents condensation by sealing off the air from the metal hull surface. If the insulating value of the system is the paramount criteria, then sprayed in poly foam will be the preferred choice.

Zincs

Zincs are essential on any metal hull for galvanic protection of the underwater metals (protection against galvanic attack of a less noble metal by a more noble metal), as well as for protection against stray current corrosion.

In the best of all possible worlds, there would be no stray currents in our harbors, but that is not a reality. Regardless of the bottom paint used or the degree of protection conferred by high build epoxy paint, zincs must be used to control stray current corrosion, to which we can become victim with a metal boat, even without an electrical system, due to the possible presence of an electric field in the water having a sufficiently different potential at one end of your boat, vs the other end...!

The quantity of zinc and the surface area must be determined by trial and error by observing real-world conditions over time. However as a place to start, a few recommendations can be made. As an example, on a metal hull of around 35 feet the best scheme to start with would be to place two zincs forward, two aft, and one on each side of the rudder. With a larger metal boat of say 45' an additional pair of zincs amidships would be appropriate. As a vessel gets larger the zincs will become more numerous and / or larger in surface area.

Zincs will be effective for a distance of only around 12 to 15 feet, so it is not adequate to just use one single large zinc anode. Zincs will ideally be located near the rudder fittings, and near the propeller. The zincs forward are a requirement, even though there may be no nearby hull fitting, in order to prevent the possibility of stray current corrosion, should the paint system be breached.

Using the above scheme, after the first few months the zincs should be inspected. If the zincs appear to be active, but there is plenty left, they are doing their job correctly. If they are seriously wasted, the area of zinc should be increased (rather than the weight of zinc). During each season, and to adjust for different marinas, the sizes of the zincs should be adjusted as needed.

Aluminum Hulls

All manufacturers of out-drives and outboard motors provide for installation of zinc anodes of a specific type and size on their equipment, for protection of the aluminum and other metal components immersed in sea water.

I've posted additional information about online in a PDF entitled "Corrosion, Zincs & Bonding" based on the experience of the ABYC with regard to protection of underwater metals. Those recommendations are based on real-world experience in an electrolyte of salt water, with the relative sizes of aluminum (large) to zinc (small) being a relevant factor. Relative size of the surface areas is important. Temperature is important. Salinity is important. Specific alloys of zinc are important, as are the specific alloys of aluminum being protected.

During installation of any zincs, in particular on an aluminum hull, ABYC recommends using a silver / silver chloride reference electrode to assess the degree of protection being conferred, which is accomplished according to the procedure outlined within the ABYC guide book. ABYC additionally recommends that metal boats (in particular aluminum boats) have a permanently installed hull potential meter and a reference electrode so the hull / zinc potential can be continuously monitored.

The ABYC recommendations regarding the size of zincs and the measured degree of protection conferred primarily address the installation of zincs in order to prevent galvanic corrosion of the hull and its fittings. However a separate and often more important consideration is the possibility of there being stray currents present, whether their source may be onboard or onshore, or in the water due to faults on other boats, etc.

There are no specific hard and fast installation recommendations for zincs, only rules of thumb which then must be adjusted according to one's experience based on observed behavior in actual real-world usage.

Bonding

For maximum corrosion protection, metal boats will ideally NOT be bonded. This of course is contrary to the advice of the ABYC. Keep in mind that the ABYC rules are primarily aimed at satisfying the requirements of GRP vessels. Little by little, we are seeing the ABYC create special case recommendations for aluminum and steel boats. The situation is not as simple as it might at first seem. For an introduction to some of the issues with regard to bonding, please see our "Corrosion, Zincs & Bonding" booklet.

Electrical System Considerations

Aboard a metal vessel, the ideal would be to make use of a completely floating ground system. In other words, the negative side of the DC power will not permitted to be in contact with the hull nor any hull fittings, anywhere. With a floating ground system, a special type of alternator is used which does not make use of its case as the ground, but instead has a dedicated negative terminal.

This is contrary to the way nearly all engines are wired. Typically, engines make use of the engine block as a mutual ground for all engine wiring. Also, the starter will typically be grounded to the engine, as will the alternator. And typically the engine is in some way grounded to the hull, possibly via the coolant water, or possibly via a water lubed shaft tube, etc.

Needless to say, for the sake of preventing corrosion, there should not be a connection between the AC shore power and the hull. This includes that insidious little green grounding wire. Of course this is also contrary to the ABYC recommendations, which are primarily concerned with prevention of shock, rather than the protection of the hull itself.

At the very least, all AC power coming aboard a metal boat should be passed through a marine quality isolation transformer, and the wiring attached per manufacturer recommendations. Other "black box" devices should be avoided, including "zinc savers" or impressed current systems, etc. On a military vessel, commercial vessel, or large crewed yacht where these systems can be continuously monitored, those "active" protection schemes may have some merit. However on a small yacht, which may spend long periods with no-one aboard but which may still be plugged into shore power, an "active" system will not be attended to with any regularity, and could easily develop a fault that could cause rapid and considerable damage.

General Rules For Preventing Galvanic Corrosion:

Corrosion Prevention For Metal Boats