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Metal Parts for Boats

What is so important about the metal parts on our boats?

Metals are everywhere on our boats and are quite important, however metals when placed in salt water bring with them the dark side… Electrolysis & Corrosion - Oh my..!

With some fore-knowledge, we can avoid the often heard boat builder's mantra: "Don't use anything but bronze…" After all, plenty of good things are done with a variety of other metals, or even without metals…!

Consider the advice of the well known sailmaker, Carol Hasse, who says: "Wherever you can, use soft-ware rather than hardware." Carol's excellent advice regarding hardware is especially appropriate: There is no reason to use piece of 'hardware' unless there is no alternative. For example, often there will be a way to avoid using boom hardware, possibly by using a wire or rope strop. This 'soft-ware' approach will many times be cheaper to make, more fatigue resistant, easier to maintain, or to fix or replace than fabricated metal hardware will be.

Where metal fastenings or metal fabrications must be used, the following information should serve us well on the briny deep in order to keep us as intended: Afloat!
 

Galvanic Series - Kasten Marine Design, Inc.
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UNDERWATER METALS

When building a wooden boat, the choice of whether one should use all galvanized fastenings or use all bronze fastenings will have already been made for us once we've looked into our wallets and chosen our ballast material. Why is the ballast part of this question…?
 

BALLAST VS. FASTENERS (i.e. the biggest chunk of metal vs. the smallest)

First let's consider the basic choices. Whether on a wood or fiberglass boat, external ballast materials are usually one of three kinds:

1. All iron ballast, cast to shape.
2. All lead ballast, cast to shape.
3. Heavily built fabricated steel ballast box, welded so as to be water tight and filled with poured lead, or with lead pigs and cement, or with iron pigs and cement.

When making these choices, it is critical to understand the way dissimilar metals react to one another when placed near each other in seawater. Many tables are available which show the galvanic series for metals in sea water.  A few are illustrated on this page.

Note that there is a range of voltage associated with each metal. At one end of the table are the metals with the most negative voltage. These are the most anodic (least noble) metals. At the other end of the table are metals having the least negative, or possibly even positive voltages. These are the most cathodic (most noble) metals. We will notice zinc near the extreme of anodic metals and lead near the extreme of cathodic metals.

On a wood or 'glass boat, we must keep the two following concepts in mind when choosing underwater fasteners:

1) Anodic metals corrode in the presence of cathodic metals. The definition of anodic and cathodic is entirely relative. Any metal on the chart will behave as cathodic if it is in solution with and electrically connected to a less noble metal on the chart. It is the relative location in the series that matters. The recommended maximum voltage difference between such metals is 0.2 volts.

2) In an underwater collection of small parts and large parts (for example, fasteners and the external ballast) it is desirable that the fasteners be cathodic relative to the external ballast. If this relationship is reversed, and the large metal keel were to be cathodic, the much smaller individual fasteners will quickly corrode. Relative surface areas matter. Anodic metal areas should be large, individual cathodic metal areas should be small. In this example, fasteners with their small areas should be cathodic (more noble) than the massive area of the keel.
 

BALLAST ALTERNATIVES

Here in the Pacific Northwest, a welded steel ballast box seems to be a fairly common strategy when building a wooden boat. A steel ballast box provides an excellent way for a wooden boat to use external lead ballast to achieve a low center of gravity, yet to still allow the use of galvanized fastenings. The rationale here is that once the steel box is welded closed, the lead is no longer able to be in contact with sea water. As far as the other underwater metals are concerned, the ballast is "seen" as being steel.

Whether for a wood or a fiberglass boat, a fabricated ballast box would be built in essentially the same way. Heavy steel plate is used to create a box for the ballast - ordinarily starting with a half inch thickness on smaller craft, and slightly greater thickness on larger craft. Penetrations for keel bolts are made by welding in a section of heavy wall pipe that is large enough to allow the bolts to pass completely through and still keep the ballast box water tight.

Once the box has been ballasted, fully welded, and pressure tested, the outside of the box is then sandblasted and painted with epoxy barrier coats and made ready for the boat. Care must be taken not to ravage the paint job during installation, especially on the surfaces to be mated to the deadwood. Any nicks and gouges in the ballast box paint coatings must be repaired. The paint barrier must be as perfect as it is possible to make it.

The keel bolts for a steel ballast box should be galvanized steel or galvanized iron. The keel bolts and the pipes they pass through can be nicely preserved by injecting the pipes with heavy waterproof grease. In addition to the protection offered by the epoxy paint system, the ballast box itself will be best preserved in use by having one or more zinc anodes welded on, and then subsequently maintaining both the zinc anodes and the epoxy barrier coats. Copper bottom paint creates no undue problem with a steel ballast box.
 

GALVANIZED FASTENINGS

For a wooden boat, whether above or below the waterline, hot-dip galvanized fastenings will work very well. Once used however, one must "follow through" with that theme on the rest of the boat, especially below the waterline.

Having chosen to use Galvanized Fastenings means we should also use:

  • An iron keel or steel ballast box
  • Hot dip galvanized steel or iron keel bolts and drifts throughout the deadwood
  • Hot dip galvanized steel or iron fittings and fastenings below the waterline
  • Nails, if used, should be hot dip galvanized reinforced square-cut boat nails

'Drifts' are used where bolts cannot be. Drifts have a 'head' but are without threads, much like a huge nail, and similarly they are driven by an equally huge hammer…

Square cut boat nails have a 'fat spot' part way down the shaft as reinforcement. The position of the 'fat spot' is designed to coincide with the interface between the plank and the frame, in anticipation that this will be the location on the nail where corrosion will occur most rapidly.

On a wooden boat with galvanized fastenings the only place we can violate the above rules of thumb is with our choice of metals for sea cocks and propellers, where there is little choice other than to use bronze fittings.

We know that on wooden vessels 'bonding' should be strictly avoided. Bonding is the act of electrically tying the underwater metals together, ordinarily aiming the accumulated potential of those metals at one or more zincs. On a wooden vessel, bonding will result in extreme alkaline attack of the wooden members surrounding each of the 'noble' fittings.

In most cases, the wooden parts under attack via bonding are the vessel's primary structural members. These members are always the most difficult and costly pieces to replace, and they are nearly always the parts most critical to the safety of the vessel. Therefore, rather than use bonding anywhere on a wooden vessel, each underwater metal fitting should have its own separate zinc anode.

A stainless propeller shaft can be used successfully, will be of a special shafting alloy, and should also be fitted with a protective zinc anode (more about stainless below).

Note that even on fiberglass boats, the usual wooden backing blocks used with bronze thru-hulls on will experience this same type of alkaline attack when the thru-hulls and seacocks are connected to the bonding system and the amount of zinc protection in the water is excessive. Isolated bronze thru-hulls probably should not be protected at all, and an excellent case can be made for not having them connected to the bonding system.

In general, for the sake of maximum corrosion protection, all underwater metal fittings should have their own separate zinc anodes. On a wooden boat, bonding is an invitation to trouble.
 

BRONZE FASTENINGS

Whether on a wooden or fiberglass boat, the keel bolts must always be of a galvanically equal or more noble metal than the ballast they hold in place.

If an external cast lead ballast keel has been used, then all the fastenings, fittings, drifts, and the keel bolts themselves should be silicon bronze (Everdur), or monel. Phosphor bronze works well, but it is not ideal when being cold worked or welded (say, for large fastenings or drifts).

Galvanized steel or iron fastenings or fittings should be strictly avoided on a bronze fastened boat, regardless of the application.

No absolute strengths can be given for any one of the copper alloys, since within any given alloy group the strength of a piece depends on the temper it has been given. As an example for Alloy 655 (Everdur) the tensile strengths listed range from 56,000 to 145,000 psi. Yield strengths range from 21,000 to 70,000, and elongation can be anywhere from 3% for the hard tempers to 63% for the soft tempers! Therefore when buying any copper alloy, the temper should be known, and the size of the fastener gauged accordingly.

Aluminum bronze is often used for propellers since it resists cavitation well. It is possible though for the aluminum in the alloy to be leached out in sea water, so aluminum bronze can't be recommended as a fastening material. When used for a propeller, this de-zincification can be prevented by the use of a shaft or prop-nut zinc.

Manganese "bronze" is also often used for propellers. Even though it usually contains around 40% zinc, it can succeed using the same strategy: a shaft or prop-nut zinc. Actually, any alloy of copper containing zinc is not 'bronze' but is actually 'brass.' Although it can be very strong, 'brass' is not at all suitable for use as a fastening material due to the likelihood of extreme de-zincification.

An often overlooked copper alloy for underwater fabricated fittings is copper-nickel. The two common types, 70-30 and 90-10 refer to the amounts of copper and nickel respectively. 70-30 Cu-Ni is almost as strong as some alloys of silicon bronze, and is considerably more ductile. Copper nickel is readily cut and welded, is available in rod, sheet, and tube, and is not as expensive as monel.

As a fastening material, stainless is not even worth our consideration. The only suitable use for stainless below the waterline is as a propeller shaft, and it must be protected with its own zinc anode. We will consider stainless more thoroughly below…
 

ABOVE WATER METALS

Above the waterline, the metals that can be used successfully naturally include all the metals that can be used below the waterline. Above the water though, we can add both aluminum and stainless to our list, with caution.

Below the water, however, aluminum will behave much like a sacrificial zinc with regard to the other immersed metals. As noted below, stainless has a host of problems which keep it off the list of desirable metals for use below the water.
 

ALUMINUM

We may well find ourselves asking, "Why would anyone want to put aluminum fittings on a wooden boat? Or for that matter, on a fiberglass boat…?"

A quick look around any marina reveals that aluminum is indeed often used on wood and fiberglass boats. The most obvious case being masts and booms on fiberglass boats, and often for windlasses.

In the Pacific Northwest, the largely wooden Alaskan fishing fleet often uses aluminum for bulwarks, tanks, hatches, holds, spars, and even for cabins and pilot houses. The reason for this is that aluminum is light and strong. It is very durable if used wisely, and makes a highly practical contribution to the longevity of the fishing fleet. The use of aluminum can be a complement to the structure, to the aesthetics, and most importantly to the continued function of these fine wooden vessels.

With aluminum, it pays big dividends to know just what alloys and which methods will contribute to a long lasting marriage of aluminum with other materials.

When fastening anything to aluminum or when fastening aluminum to anything else, it must be kept in mind that all aluminum alloys are subject to "crevice corrosion". What this means is that while the marine aluminum alloys will do just fine in the corrosive marine environment, the metal surface must be exposed to oxygen in order to form a protective oxide layer. Where there is a crevice, say where an aluminum windlass is mounted onto the deck, the aluminum has lost its ready access to oxygen, so cannot repair itself, and begins to freely corrode. This applies equally to aluminum alloys in the 5000 series (plate) and the 6000 series (masts, pipes, and other extrusions).

In general, when fabricating aluminum parts, or when mounting various parts onto aluminum surfaces, whatever can be welded in place should be welded, rather than that part being attached with fastenings.

Why…?

When mating anything to an aluminum surface, the interface becomes a crevice. It makes no difference to the crevice area whether we've put the aluminum against a piece of wood, a piece of plastic, or even against another piece of aluminum. It is commonplace for example to observe serious corrosion on aluminum masts underneath plastic winch pads, originally placed there in hopes of protecting the mast from the winch materials. 

Equally bad corrosion will be found under cast zinc winch pads placed there for their supposed anodic protection. Regardless, the crevice is still there and it prevents the aluminum surface from having free access to oxygen. Salt water will be able to wick into the crevice and freely corrode the aluminum.

The left over by-product of the corrosion process is a white powdery fluff. In that spot, the aluminum mast will have become much like our modern society, "All fluff and no substance."

In the case of a bronze part directly mounted on aluminum we have the added insult of having created a battery in which the aluminum has become the sacrificial anode. Here, we have the double whammy of galvanic corrosion in addition to crevice corrosion.

Mounting a porous material such as wood onto an unprotected aluminum surface is asking for big trouble. Sea water will penetrate, oxygen will be quickly depleted, and corrosion of the aluminum will be rapid.

What to do…?

When we mount something onto an aluminum surface, or when we mount an aluminum part onto the boat (say on deck) the best and most permanent solution is as follows: First thoroughly roughen the aluminum surface by sand blasting or grinding, then thoroughly clean the aluminum of any traces of oil contamination (i.e. fingerprints), then paint the aluminum with a heavy barrier coat of epoxy paint, just as we would do with a piece of steel.

Cleaning and roughening the surface are the best preparation. Further preparation for paint might make use of the "Alodine" chemical anodizing system, but that treatment should not be regarded as being sufficient by itself, without both cleaning and roughening. After all, a poorly applied paint job will eventually lift and provide its own excellent crevice underneath the paint.... definitely not good.

Note that this treatment is only necessary in the area where the two surfaces mate and would thereby form a crevice.

In the interface between an aluminum part and another surface (aluminum or otherwise), the additional use of a plastic isolator at the mounting surface is very much to be recommended. This helps to protect the paint and to keep any dissimilar metals apart. Keep in mind that just the plastic alone is not sufficient as water can still wick into the crevice.

Among plastics, Micarta is an especially good choice as an isolator. Unlike most other plastics micarta is stable, and adhesives and paint will stick to it. PVC works well too, but it has a higher thermal expansion rate. Over a long length such as under a long hand-rail or under a sail track, the high thermal expansion rate of any plastic will tend to break the seal, and again we have a crevice. In those cases it will be better to use shorter lengths of plastic, well bedded, or possibly to rely solely on thorough barrier coatings of epoxy paint.

Along with painting and isolating aluminum fittings, aluminum parts should be thoroughly bedded in an adhesive caulking to prevent the intrusion of salt water. With small parts, for example a small stainless fairlead mounted onto an aluminum mast or boom, we will very likely succeed by thoroughly cleaning the spot then bedding the part in Sikaflex or 3M-5200. On a painted aluminum surface, especially for something small that we'll want to be able to remove easily, a less aggressive adhesive bedding like Thiokol (Boatlife) might be preferred. Most fittings on a large aluminum structure will be welded on anyway. This is easily accomplished, simple, strong, and completely avoids the crevice problem.

It should go without saying that vinyl covered, or plastic dipped aluminum fittings of any kind will eventually become nearly worthless. As soon as the plastic covering is nicked and penetrated, the part is immediately surrounded by a large crevice.

A heavy anodizing on aluminum parts will not prevent crevice corrosion. Anodizing the surface of aluminum parts does temporarily provide resistance to pitting in the open salt air, however anodized surfaces must be treated the same as plain aluminum surfaces when it comes to crevices formed where things are mounted.  Further, the aluminum oxide is cathodic (more noble) vs. the base metal.

The above-described protection regime applies to the interface of any aluminum parts mounted onto any surface where salt water can get into the crevice. The mounting surfaces of aluminum parts must be protected.

Large pieces fabricated out of aluminum, or the topsides, decks and house structures of an aluminum boat do not need paint above the water where they will have ready access to the air. In this case, the aluminum surfaces are best prepared simply by grinding, sanding, or sand blasting in order to remove the mill-scale (a heavy oxide coating). The surfaces then left to weather in order to allow an even oxide coating to be naturally formed. If it is sand blasted, the surface will weather to a uniform matte grey, which can be very handsome.

Oddly, many boat builders do not go to the trouble to do this, preferring instead to just allow the mill finish to weather. In the long run this approach produces an unsightly finish, due to local pitting and discoloration.

What fastenings can we use with aluminum…?

Galvanized fastenings are friendly companions to aluminum parts and pieces. Still, wherever they will exist crevices must be protected for the sake of protecting the aluminum. Any steel fastenings should definitely be hot dip galvanized as opposed to simply being electro-plated. The thickness of the galvanizing is the key to longevity.

Stainless usually works fine as a fastening material for aluminum structures. Stainless, though, is distant enough from aluminum on the galvanic scale to form a "battery" allowing galvanic corrosion to develop between them. As we will see below, stainless is also subject to attack by crevice corrosion, therefore any crevices must be strictly avoided, or must be treated as described above for protection of the interface.

Where the fastenings must be easily removed, a coating of 'never seize' works well. Why use it at all…? Without some means of preventing the intrusion of salt water, the fastenings will eventually seize due to crevice corrosion of the aluminum where the fastenings pass through the aluminum piece. The temptation here will be to just use something slippery, and we might end up with a material containing either copper or worse yet, graphite.

If you encounter any 'never seize' product that contains either copper or graphite, DON'T USE IT…!  Both of those metals are more noble than aluminum and will cause considerable trouble. If the 'never-seize' product contains zinc or Teflon, it should be fine.

For protection of bolt threads or other surfaces that need to move freely, Anhydrous Lanolin works well. The active ingredient here is sheep squeezings. (Actually, wool squeezings…) It is a greasy paste like Vaseline. A small tub of Anhydrous Lanolin can be bought from almost any pharmacy. It is cheap and very effective for preventing rust. Anhydrous Lanolin should only be used in those spots, such as bolt threads, where we don't want to use an adhesive. It pays to be cautious though since Lanolin will create an oily film to which nothing will stick (such as paint).

If the above tactics are not faithfully followed, any salt water finding its way into the crevice will do its dirty deeds where fasteners penetrate the aluminum: i.e. the paint will lift or the fastening will seize, and we'll be back to square one.

Is all of this too much trouble to go to?

When the eventual alternative could be having to replace an entire bulwark, mast, tank, or hull structure, thorough preparation, painting, and isolation are a small price to pay…! It is relatively easily accomplished and saves a terrific amount of work in the long run.

Just remember that the key to solving corrosion problems with aluminum is to prevent the crevice in the first place: "Say no to crack!"
 

STAINLESS

Stainless fittings and fasteners are used nearly everywhere. Stainless by its very name inspires confidence! Stainless however is also subject to crevice corrosion due to oxygen starvation. As a result, stainless parts should be treated in the same way as aluminum parts, as described above.

In general though we will be best advised to resist the temptation to use stainless, whether for fastenings or for other parts or fabrications.

Some alloys of stainless steel do work well when the surfaces are out in the open air and it can readily repair itself by allowing the chromium in the alloy to create a protective 'stainless' oxide coating.

Stainless shafts also work well primarily because shafts are ordinarily of special alloys, and because a shaft usually has reasonably good access to oxygen in the water which allows the protective oxide to be renewed. Having said that, most of us have observed longitudinal 'striations' in the surface of stainless shafts when they have been left to sit for long periods without moving. This is due to crevice corrosion where the surface of the shaft has been in contact with the rubber cutless bearing.

Shafts should always be protected with zincs, mounted in such a way as to be easily inspected and easily replaced.

Other than for shafting, stainless fastenings or other parts will ideally never be used below the water, regardless of whatever hype we might hear from whoever might be selling them…!

One very common, though extremely inadvisable use of stainless is as ballast-keel bolts on production fiberglass boats. In most cases, inspection of the keel bolts will reveal serious corrosion at the interface between the ballast and the boat, right at the place where there is the greatest stress.

This introduces yet another strike against stainless: it is subject to work-hardening and then to preferential corrosion in the stress-affected zone, referred to as 'stress-corrosion.'

Stainless sailboat rigging is extremely common. Most of us who have been around boats for any length of time have observed what appears to be 'rust' coloration around swaged fittings. If the rigging is new, this should not be too great a cause for alarm, however over time the combination of stress corrosion and crevice corrosion conspire to make stainless rigging inherently short-lived and prone to sudden failure. Here, one can do well to specify Type 316 stainless. In any case, rigging should be frequently inspected for signs of fatigue or cracking. Prudent sailors will replace all suspect rig components sooner rather than later.

In a few cases, there may not be a good alternative to stainless, such as with the usual preference for stainless through-hull valves aboard aluminum boats. In this case, one must use Type 316 stainless, and the part must be completely protected in the crevice area by sand blasting, epoxy paint, isolation, and proper bedding, then further protected with its own zinc.

Needless to say, such a regime would be an absurd endeavor with hundreds of stainless plank fastenings…! Except in the case of large bolts, for example to hold a windlass in place, stainless fastenings should ideally not be used anywhere on the exterior of a boat. Stainless fastenings should not be used for any structural uses on a wooden boat, nor should stainless fastenings be used in any part of the structure of any boat. Where it must be used though, stainless fastenings should always be Type 316.

Large stainless fastenings like bolts that might be used to attach bigger aluminum parts to a wood boat should be thoroughly protected with paint, and further isolated using sealant. On a wooden vessel, the surrounding wood makes a perfect environment for crevice corrosion of the stainless, even in the deck structure.

Any fabricated assemblies aboard a boat that are built out of stainless must necessarily make use of a specialized alloy. When ordinary stainless is welded, a heat-affected zone is created next to the weld where temperatures allow carbides to precipitate out of the alloy and become concentrated at the grain boundaries of the metal as the metal cools. Carbide precipitation can be controlled to a large extent (but not entirely prevented) by the use of low carbon stainless, most commonly Type 316-L.

Type 321 and Type 347 are much less common (and much more costly) stainless alloys that prevent carbide precipitation via the addition of other alloying elements.

Type 2205 is a new type of stainless that is gaining acceptance for fabricated assemblies and also for rigging (though it is still very hard to find).

Type 2205 is a duplex (ferritic-austenitic) stainless having combination of high strength, very good corrosion resistance, stress-corrosion cracking resistance, and good pitting resistance. In addition, it is highly resistant to stress and provides a low level of thermal expansion (a welcome attribute when fabricating).

Overall, stainless has become popular because it is relatively inexpensive (as compared to bronze), relatively easily fabricated, and usually stays bright and shiny looking. If the specific alloy of a stainless part is not known or cannot be discovered, the best advice is to stay away from it..!

In general, with stainless fastenings or stainless fabricated assemblies, if we use any less than the precautions outlined above, we'll be rewarded with choosing between an expensive repair, a cheap cover up, or a lame excuse for the bad corrosion offered by a tradesman who should have known better...

Preventing crevice corrosion with either stainless or aluminum is like most other things with boats: The goal is to keep the water out..!
 

OTHER METALS, OTHER PLACES

A few general rules can be sifted out of the above comments:

Below the water on a wooden boat or a fiberglass boat, we can use copper nickel, monel, or silicon bronze; or alternately we can use a regime of hot dip galvanized steel or iron. Stainless shafting alloys can be used for propeller shafts.

On deck we can use the above metals, plus Type 316 stainless, the 5000 series or 6000 series aluminums or manganese "bronze."

Inside the boat we can use the above metals, plus brasses and other types of stainless except as noted above for critical structural parts. The obvious exception however is any place where interior metals may come in contact with sea water, such as the salt water plumbing system for engine cooling. Parts such as these must be viewed as though they are in fact exterior underwater metal parts, and the general rules for under water metals applied.
 

SUMMARY

Most of the above may well be plain common sense to the majority of boat builders and to many boat owners, however even a casual stroll through just about any boat yard or marina will quickly reveal the need for considerably more awareness of metals and how they should be used aboard.

The following list of what metals are suited to specific uses should be copied or cut out and placed in a prominent location…
 

APPROPRIATE METALS AND THEIR USES ON-BOARD

Galvanic Series Chart - Kasten Marine Design, Inc.
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