By Anthony Chamy
It often takes many years before new technology is adopted within an industry or in a country. Such was the case with the GSM cellular phone networks technology, which was available in Europe years before it became commonplace in North America. Perhaps the same situation is occurring in the marina industry, which is currently in the initial phase of adopting Europe’s aluminum technology for its docks.
For many years, wood, steel, and concrete have dominated the North American commercial marina industry. What about aluminum? North Americans have traditionally questioned how aluminum reacts in a marine environment, especially in saltwater applications. Having developed advanced aluminum technologies decades ago, Europeans have understood that great advantages can be derived from using extruded aluminum in aluminum applications. Extruded aluminum offers a great many advantages that are unique to this material.
One major example of this can be found in NASA, which uses extruded aluminum for its satellites and space stations. The aerospace industry requires complex shaped modules that can withstand extreme temperatures and last many years. Similarly, the marina industry also requires such a material for its commercial docks, but it also needs a material that can withstand the corrosive effects of salt water, the heavy loads brought upon it by megayachts, the forces of large waves due to the ever decreasing amount of protected waterfront property available, and a host of other factors that make most commercial dock installations seem like real brain teasers.
So why have marinas in North America been reluctant to use aluminum for its commercial docks? Some of the reasons are: The prevailing wisdom within the marina industry was that aluminum was not a suitable material for use in the marine industry. There was a misperception that aluminum in marine environments would corrode. This stems from the fact that there is little known about aluminum alloy series. Although the industry has seen how lightweight aluminum products worked quite well in soda cans, golf clubs, and recent modern lightweight luxury cars, there has always been a hesitancy to use it in marina docks.
History shows that some people have tried their hands at extruding and welding aluminum docks in the past and have not been successful. Whether this was due to a lack of technical knowledge about aluminum welding or inexperience in this area, the fact of failure remains foremost in the minds of industry veterans.
The current story
Given this background, it might come as a surprise to many industry professionals that modern aluminum technology provides marinas with many advantages. These advantages are as follows: Marine-grade aluminum alloys are extremely versatile and have high strength-to-weight ratios, allowing for complex extrusions that place almost no limits on the manufacturing of products that are put under considerable stress. This means that instead of limiting docking systems to organic, porous, or corrosive materials, marinas can benefit from innovative structures that will last longer, withstand stronger wave action, and moor larger vessels.
These alloys also allow marinas to only use half the number of piles typically used for a traditional concrete floating dock system, a cost-savings that means a lot to the one paying the installation bill. It is important to note that all aluminum is not the same, and the industry needs to specify the 6000 series aluminum for marine use. To better understand why the 6000 series is commonly used as the series of choice for marine environments, a description of the different aluminum alloys follows: 3000 series: Coupled with manganese, this series has a good corrosion resistance and formability, but cannot be heat-treated. It is most used for nameplates, radiator, and HVAC components; interior panels and components; extruded condenser tubes. 5000 series: Coupled with magnesium, these non-heat treatable alloys provide moderate to high strength. It is extremely weldable and corrosion-resistant. It is used for the same functions as the 3000 series, as well as for truck bumpers, heat shields, structural and welded parts; wheels; engine brackets and mounts. 6000 series: This series uses both magnesium and silicon to make it heat treatable and extremely versatile. It demonstrates characteristics of formability, corrosion resistance, and high strength. It is used for body panels; bumper face bars; fasteners; body components (both extruded and sheet); driveshafts; bumper reinforcements; brake housings; fuel delivery systems; ship building and of course, commercial docks. 7000 series: Using zinc as its main ingredient, this series is heat treatable and has extremely high strength. It is used in seat tracks; bumper reinforcements and face bars; condenser and radiator fins; and headrest bars. It is also the alloy series of choice for the space station modules.
In reviewing these aluminum series, the most common semi-fabricated products (which happen to fall in the 5000 or 6000 series) have interesting mechanical characteristics, allowing them to be deformed at relatively low temperatures and resist corrosion. These properties are determined by the chemical composition, production process, and heat treatment they undergo during production.
As most people in the marina industry know, the salt water and sea air found in marine environments attack metals, wood, plastic, and masonry. “Marine grade” aluminum alloys, on the other hand, resist these factors and are ideal in marine settings, and here are a few reasons why:
• Aluminum’s excellent resistance to corrosion is due to the presence on the metal of a permanent film of naturally occurring oxide consisting of Al2O3. Although extremely thin—between 50 and 100 Angstroms (5 to 10 nanometers)— the oxide film forms a protective barrier between the metal and the surrounding medium as soon as the metal comes into contact with an oxidizing medium, such as atmospheric oxygen or water. For aluminum’s corrosion-resistance, the physical-chemical stability of the oxide film is of paramount importance. The characteristics of the medium, including its pH value, and the composition of the alloy itself determine its stability. The pH value actually determines the rate of dissolution of the oxide film. It dissolves very quickly in acid and alkaline media and very slowly in media that are close to pH-neutral (pH 5 to 9). Seawater has a pH of 8-8.2, so the oxide film is very stable in seawater and marine environments. However, the pH-value is not the only criterion that must be taken into account when predicting aluminum’s behavior in an aqueous medium. It is also important to note that the nature of the acid or base also plays an essential part.
• Galvanic corrosion, often misnamed “electrolysis,” is one of the most common forms of corrosion in marine environments. It occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. Aluminum docking systems are coupled with stainless steel nuts and bolts, because stainless steel is needed for some hardware pieces. However, in these instances, aluminum acts as the cathode, and stainless steel as the anode, and because the stainless steel nuts and bolts are small, the cathode-toanode (C/A) ratio is also small. If the situation were reversed, the potential difference would be the same, but the result would be very different. It could end up being a dangerously corrosive system not fit for marina applications. Therefore, the corrosion of the larger amount of aluminum will be negligible due to the small amount of stainless steel.
When it comes to aluminum applications in the marina industry, the future looks bright. This relatively light metal can be extruded into shapes with higher moments of inertia than its heavier counterparts. For example, there are tubular aluminum extrusions that can withstand large 150-ft. vessels’ wind loads, which many wooden, steel, and concrete docks can also withstand, but only if they are heavily reinforced. For all of its benefits, aluminum docks can actually bring substantial installation cost savings to marina owners. Apart from the fact that fewer and smaller cranes are needed to install an aluminum docking system, aluminum fixed piers can span longer distances between piles compared to wood, steel, and concrete piers. Compared to floating systems, a wellconceived aluminum dock might only need half the number of pilings of an equivalent concrete dock. As far as stability is concerned, the use of heavier aluminum is not necessarily better. The position and shape of the floats, the anchoring system, as well as the inter-section connections, all play large roles in the stabilization of an aluminum system, making it extremely stable in even the most precarious situations.
Many people do not realize that some high-quality aluminum marinas in Europe are now celebrating their 20th, 30th and 40th installation anniversaries. Perhaps now is the time for North American marinas to take a second look at aluminum and examine its usefulness and benefits. As the number of North American marinas installing aluminum docks increases, the industry will bring upon itself the task of better studying the advantages of this technology, so that it can make better, informed decisions in the future, and potentially reap the benefits.
MARINA DOCK AGE, MARCH 2006 43
Aluminum docks allow for greater
distances between pilings.
Heavy vessels such as tugboats can be
moored on aluminum docks.