The materials used in metal boat construction — primarily mild steel (as well as the venerable derivative Corten, which is what Coast Guard 44-foot lifeboats were made of) and aluminum alloys — have a long and well-established tradition in the boatbuilding business. Aluminum has gained considerable favor in recent years, used for everything from small utility craft to mega power and sailing yachts.
However, there are vast differences between these two distant metallic cousins. Mild steel, which is derived from iron by the addition of carbon and oxygen, is quite strong. The term “mild,” having been added to denote a lack of the brittleness that caused cracking in many early steel vessels, is used today to describe any low-carbon steel. The smelting process for steel was first perfected by Sir Henry Bessemer in the 1850s, and it is considered one of the most, if not the most, important contribution to the Industrial Revolution. Steel’s value for shipbuilding was quickly recognized and utilized. In addition to steel’s strength, its other attributes include its cost effectiveness: it is simply cheap to manufacture, trade tariffs not withstanding. Additionally, it is fairly easy to weld, its corrosion characteristics are very well understood and, if nothing else, it is predictable. Leave uncoated steel exposed to the environment and it will develop a layer of iron oxide, or rust. Keep paint on it and it will last almost indefinitely.
The actual rusting process is a result of electrochemical action. Mill scale — the slight blue hue that is visible on most “naked” steel — is actually iron oxide, the early vestiges of rust, which has a slightly different electrical potential than steel itself. This variation causes small galvanic cells — in essence small batteries — to be set up over the surface of the steel. Eventually the iron will form a solution, iron hydroxide, which is subsequently oxidized by oxygen in the air and water. The end product is the reddish-brown-colored substance hydrated ferric oxide, or simple rust. It is interesting to note that two ingredients are critical for this reaction to occur: an electrolyte (sea- or freshwater in this case) and oxygen. Remove either of these, and the rusting process slows considerably or stops entirely. Oxygen is consumed in the rusting process, so much so that salvage personnel have died upon entering heavily rusted, but hitherto air tight, lifting chambers.
As far as yachts are concerned, steel is practical for mid- to large-sized craft, and its strength is legendary. However, in order for it to remain long-lived, it must be properly prepared, primed and painted. This process invariably begins with thorough shot blasting (for health reasons, a non-silica-based blasting media is preferred over sand). If all surface impurities, mill scale, rust, oil, etc. are not removed, the coating will fail in short order.
The other metallic boat building material, aluminum, is a relative newcomer to the industry. Discovered by Sir Humphry Davy in 1808, a proper, albeit expensive, refining process was not perfected until the 1850s. Before 1888, aluminum was so difficult and expensive to manufacture that wealthy aristocracy of Europe preferred to impress their guests with cutlery and plates made of this “rare” metal rather than silver or gold.
Aluminum is an unusual metal in that it occupies a place in the lower regions of the galvanic scale. Only magnesium and zinc are less noble than aluminum alloy, and this term in itself deserves some explanation. Pure aluminum is too soft for any use other than ornamental. However, once alloyed with a small amount of magnesium, as are marine grade alloys, the tensile strength increases considerably to roughly 2/3 that of mild steel. In spite of its lowly position on the galvanic scale, aluminum possesses some rather unique properties that endear it to boat builders — primarily, its strength and low density (aluminum is only 1/3 as dense as mild steel, brass or copper). Strong and light, what more could one ask for in a construction material?
The low galvanic nobility is, to an extent, offset by one of aluminum’s additional attributes: corrosion resistance. This is accomplished by a “self-healing” skin of aluminum oxide, which forms as soon as aluminum is exposed to air or oxygen in water. Much like steel, aluminum needs to be exposed to air in order to corrode. However, unlike steel, the oxide formed on aluminum impedes rather than accelerates further corrosion. As long as the oxide film remains intact, the aluminum will remain corrosion resistant.
During a recent refit of a 16-year-old aluminum boat undertaken by the boatyard I manage, I gained a new appreciation for the strengths and weaknesses of aluminum as a boatbuilding material. Because the vessel was aluminum, tankage was integral, which saved considerable room while adding to the rigidity of the hull. However, as strong and durable as aluminum is, it is not the preferred material for use as a holding tank. The environment found in these tanks is usually oxygen poor, which inhibits aluminum’s ability to form its oxide skin. This ultimately led to severe pitting, corrosion and leakage.
Aluminum is susceptible to yet another woe known as poultice corrosion. This occurs when aluminum is exposed to a constantly wet and oxygen-poor atmosphere, which may be facilitated by anything from wet carpeting to wet wood. The aluminum brackets supporting the sole, which provide the interface between metal and wood, are a prime location for this scenario. In fact, this problem may appear any place wood encounters aluminum. The evidence of it is clear: In addition to the actual corrosion, a sticky, white aluminum hydroxide material (it looks a bit like plain yogurt) oozes forth from the interface zone. Wet bilge debris, such as sawdust and ordinary dirt, may lead to poultice corrosion as well.
The most insidious and potentially destructive attack aluminum may fall prey to is that of galvanic corrosion, caused by contact with incompatible metals. Because aluminum is so galvanically ignoble, it is anodic to nearly every other metal. However, the primary culprits in the boatbuilding environment are copper-based alloys such as bronze and brass. Remarkably, the aforementioned refit aluminum vessel originally had bilges that were laced with copper and bronze plumbing, and the results were nearly fatal. Galvanic corrosion had caused considerable damage to bilge plating and horizontal surfaces such as tank tops. In many cases, the corrosion did not occur because of direct contact between the aluminum and copper alloy. Condensation, as well as small fresh- and seawater leaks and other liquid run-off from these pipes carried with them salts of copper, which, like hot BBs melting through butter, wreaked havoc on any aluminum surface unfortunate enough to be stationed beneath them.
The refit involved the removal of copious amounts of copper and bronze plumbing, as well as scraps of copper, brass and bronze in the form of wire strands, screws and brass dust created by the repeated installation and removal of brass trimmed sole hatches, which had fallen into the bilges over the years. Additionally, the bronze quadrant and steering cable sheaves were swapped for aluminum and stainless units respectively. Any copper that was impractical to remove, such as some pump bodies, was coated with CRC Heavy Duty Corrosion Inhibitor, a waxy anticorrosion compound, to discourage copper salt run off.
What does this mean for those contemplating aluminum vessel construction or ownership, or for those who already own an aluminum vessel? Primarily, nearly every metal is potentially dangerous to your aluminum hull, but especially copper alloys. Something as seemingly benign as a penny could be as lethal to an aluminum vessel as Kryptonite is to Superman (technicians working on this refit were required to empty their pockets of all change before boarding). As previously mentioned, other metals may also be harmful to aluminum, but few would be as harmful as mercury. This liquid, albeit relatively rare, dissolves many other metals (it is used to remove lead fouling from the barrels of firearms) and it is, unbeknownst to many boat owners, found within some automatic float switches. These units shouldn’t be used on aluminum vessels. Obviously, mercury thermometers should also be left ashore.
Contributing editor Steve C. D’Antonio is a freelance writer and the boatyard manager of Zimmerman Marine in Mathews, Va.