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Corrosion Prevention For Metal Boats

Copyright 1990 - 2015 Michael Kasten
 

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 mainly to point out that regardless of the type used, insulation 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.

Sprayed-on foam is not to be recommended. While popular, sprayed-on foam has many drawbacks that are often overlooked:

A much better insulation system is to use a Mastic type of condensation / vapor barrier such as MASCOAT, which adheres well to painted steel surfaces, as well as unpainted aluminum surfaces. It creates a barrier to water penetration, and an effective condensation prevention system. Applied to recommended thicknesses of around 60 mils, it is effective as insulation. Further, it is quite good at sound deadening, is fire proof, and will not absorb odors. Mascoat DTM is used for insulation, and Mascoat MSC for sound attenuation, very effective on engine room surfaces and above the propeller. Both are effective whether on a steel or an aluminum boat.

These mastic coatings can be painted if desired. In more severe climates the mastic coatings can be augmented by using a good quality flexible closed cell cut-sheet foam to fit between the framing. The best choices among these flexible cut-sheet foams 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 better than Neoprene in most respects, but is slightly more expensive. One brand offering good quality flexible foam solutions for boats is ARMAFLEX.

Styrofoam or any other styrene type of foam should be strictly avoided. Go get a piece at your local lumber yard and throw it onto a camp fire.... You will be immediately convinced. The same applies to any of the typical rigid or sprayed-on urethane foams. They are an extreme fire hazard and cannot be recommended.
 

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.

Good electrical connection between the zinc and the hull must be assured.
 

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

Bonding is the practice of tying all of the underwater metals together with wires or bonding strips. It is done in order to 'theoretically' bring all of the underwater metals to the same potential, and aim that collective potential at a single large zinc. It is also done in order that no single metal object will have a different potential than surrounding metal objects for the sake of shock prevention.

However 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 though that the ABYC rules represent the consensus of the US Marine Manufacturers Association, and are therefore primarily aimed at satisfying the requirements aboard GRP vessels, about which the MMA is most familiar. Naturally, aboard a GRP boat the boat's structure is electrically inert and not subject to degradation by corrosion, therefore aboard a GRP boat there is no reason to recommend against bonding - except perhaps the fact that bonding all underwater metals using a copper conductor invites the possibility of stray current corrosion of those underwater metals due to the possible potential differential in the water from one end of the boat to the other.

Little by little though, the ABYC is learning more about the requirements aboard metal and wooden vessels, and recommendations for aluminum and steel boats have begun to appear in the ABYC guidelines. Even so, the corrosion vs shock hazard conundrum aboard metal boats is not 'solved' since the solutions are not as simple as they might at first seem. For an introduction to some of the issues with regard to bonding, please see our "Corrosion, Zincs & Bonding" booklet. 

However, before you read that booklet and possibly take on the design of an onboard electrical system, please thoroughly study the materials at the Electroshock Drowning web site..!!  The following is a brief summary of the issues involved.
 

Electrical System Considerations

Aboard a metal vessel, purely for the sake of preventing corrosion the ideal will 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, or the engine mounts, or even a direct bonding wire, etc.

Needless to say, for the sake of preventing corrosion, there should not be a direct connection between the AC shore power and the hull. This includes that insidious little green grounding wire. This whole issue is avoided if a proper marine grade Isolation Transformer is installed, which has as its duty to totally isolate all direct connections between shore power and the onboard wiring. This is done by 'inducing' a current in the onboard circuits, thus the electrical energy generated has been created entirely within the secondary coils, and is therefore entirely separate from the shore side power.

The purpose of the green grounding wire is to return any leakage current back to ground onshore, rather than to leak away through the hull and its underwater metals into the water, seeking an alternate path to ground. If a leakage current of greater than 10 milliamps exists onboard (not at all uncommon), it presents an EXTREME hazard to swimmers nearby. This is especially dangerous in fresh water where a swimmer's body provides much less electrical resistance than the surrounding water, and the swimmer thereby becomes the preferred path for any stray currents in the water. With a leakage current above 20 milliamps, death can (and has) become the result. Above 100 milliamps, and the heart stops. Serious business.

The shore side green grounding wire must be brought aboard and connected to the primary side of the Isolation Transformer. It creates a 'fail safe' return path for the AC current seeking ground. But on the secondary side of the Isolation Transformer it serves no purpose onboard because the secondary side will have created an entirely independent electrical system, generated onboard, and not tied to shore power.

Separately, there should ideally be a green grounding wire in the onboard electrical system, however it should not be tied to the shore side green grounding wire. Recommendations differ here, and the Isolation Transformer should be chosen on the basis of providing COMPLETE isolation of the onboard electrical system from the shore power system... What this means is that if any Isolation Transformer's wiring diagram recommends connecting the shore side green grounding wire to the onboard green grounding wire (effectively defeating its very purpose) that particular Isolation Transformer should be rejected as a candidate for placement onboard.

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, such "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 fail and develop a fault that could potentially cause rapid corrosion, resulting in considerable damage.
 

Is There An Ideal Electrical System..?

Carrying the above notes to their logical conclusion, we find that the ideal electrical system onboard will be entirely 12v or 24v DC, energized via a large battery bank.  If shore power comes aboard, it must be via an Isolation Transformer.  Once onboard, the secondary side of the transformer can then be connected to marine quality battery chargers. Some battery chargers are available that have a built-in isolation transformer, however they should be screened on the basis described above. If the only thing the Isolation Transformer connects to onboard is a large battery charger, then there is no real connection between the onboard DC system and the shore side AC system.

Using such a system, it is possible to have onboard AC power provided by inverters, directly energized by the large battery bank. This provides yet another barrier between the onboard AC electrical system and the shore power system. It also provides other considerable advantages....

For one, some types of isolation transformer can be switched in order to accept either 110v AC or 220v AC, and to output either voltage, depending on what the onboard equipment requires (essentially just the battery charger in this case). Since the isolation transformer and the battery chargers are also frequency agnostic, if all onboard AC is generated by inverters, you then have a truly shore power agnostic system where all onboard equipment will be either DC, or will be AC generated onboard by the inverters at the requisite frequency and voltage required by the onboard equipment.

Where this scheme gets defeated rather quickly is where there must be an air conditioning system, and / or a washer / dryer, both of which are very power hungry. But we can still avoid bringing shore power onboard to directly serve those items by using the above described system (i.e. shore power -> isolation transformer -> battery charger -> battery bank -> inverter -> onboard AC system) in combination with an onboard AC generator. In this way, all AC current onboard will be generated onboard, either via the inverters for low current draw items, or by the AC generator when high current draw items are used.  With this kind of AC electrical system, the frequency and voltage of shore power suddenly become a non-issue...

The whole point is to keep shore power OFF the boat by limiting its excursion only to the Isolation Transformer, where it stops completely. With all onboard power being generated entirely onboard, there is no hazard to swimmers posed by stray currents attempting to seek ground onshore, because the onboard "ground" is in fact self-contained onboard...!

I know there are those who will disagree with the above statements about electrical systems. Whether you agree or disagree, please don't come all unglued over these matters and instead, for much more complete information on these topics, please see the resources mentioned below...

General Rules For Preventing Galvanic Corrosion:

Further Reading

Please see the following resources that I originally prepared for the Metal Boat Society during my editorship of the Metal Boat Quarterly:

Our "Marine Metals Reference" originally published as a supplement to the Metal Boat Quarterly is a brief guide to the appropriate metals for marine use, where they will be most appropriately used. It also contains welding information and a complete list of the physical properties of marine metals.

The second booklet, "Corrosion, Zincs & Bonding" also first published as a supplement to the Metal Boat Quarterly, offers a complete discussion of electrical systems, corrosion, zincs, and bonding. On a relatively complex vessel, having its own onboard generator and a variety of electrical appliances, this guide should be required reading - but it is not the whole story.

For the most up to date information and recommendations, it would be prudent to study the most recent ABYC guidelines regarding zincs, bonding, and the installation of electrical systems onboard.  And by all means before you take on the design of any onboard electrical system, please thoroughly study the materials at the Electroshock Drowning web site..!! 
 

Summary

In general, our efforts should be aimed at safe and hassle free boating. Toward that end, it always seems that simplicity is its own reward…!