You can find a dock or seawall made of almost any construction material under the sun. However, certain materials have gained widespread acceptance while others have struggled to find their place.

Aluminum, while one of the most abundant metals on earth, has made slow inroads to the construction market due in large part to the lack of available technology to capitalize on its natural advantages. With modern manufacturing and engineering procedures for aluminum now firmly established, it is no longer an experimental metal. It has gained worldwide acceptance as a dominant marine construction material and for good reason. With aluminum's prevalence in such critical and demanding industries, why then has it seen such resistance in the U.S. for coastal applications? The answer lies in common misconceptions regarding the major causes of corrosion in the marine environment and how they effect aluminum specifically.

This paper will lay out the different electro-chemical processes that affect aluminum in the marine environment, the positive results, and the keys to overcoming negative outcomes.

You are probably familiar with aluminum's reputation as a highly corrosion-resistant material, but you may have also heard stories of, or actually experienced cases of extreme aluminum corrosion in your area. The first and single most important step towards capitalizing on aluminum's advantages is making certain that a marine grade alloy is used.

An inspection of the vast difference in corrosion-resisting abilities of stainless steel and plain carbon steel may give some insight. As its name suggests, stainless steel is very corrosion resistant, while plain carbon steel is attacked almost immediately when exposed to the atmosphere. Just as stainless steel alloys have specific additives and properties that provide an optimum combination of strength and corrosion resistance, certain aluminum alloys are formulated for similar results. Actually, the operation that allows stainless steel to perform as it does is nearly identical to that of aluminum. 6061 and 6063 are examples of "marine grade" alloys that can achieve high strength and corrosion resistance. Conversely, another common alloy, 7075, exhibits superior strengths, over 1.5 times that of the marine grade alloys, but is much more susceptible to corrosion. This alloy sees heavy use in the aircraft industry where the environment is typically mild and aluminum corrosion isn't likely to occur. While a high performance material in the aircraft industry, it would perform poorly in marine conditions.

There are hundreds of different aluminum variations. Like steel, each exhibits different qualities and is formulated for different and specific end uses. It is imperative that the proper alloy is selected to realize aluminum's reputation as a highly corrosion-resistant metal in marine applications.

Aluminum is actually a very active metal, meaning that its nature is to oxidize very quickly. While a weakness for most metals, this quality is actually the key to its ability to resist corrosion. When oxygen is present (in the air, soil, or water), aluminum instantly reacts to form aluminum oxide. This aluminum oxide layer is chemically bound to the surface, and it seals the core aluminum from any further reaction. This is quite different from oxidation (corrosion) in steel, where rust puffs up and flakes off, constantly exposing new metal to corrosion. Aluminum's oxide film is tenacious, hard, and instantly self-renewing.

According to the US Army Corps of Engineers, "Aluminum has excellent corrosion resistance in a wide range of water and soil conditions because of the tough oxide film that forms on its surface. Although aluminum is an active metal in the galvanic series, this film affords excellent protection except in several special cases."1

The Aluminum Association states, "Unless exposed to some substance or condition which destroys this protective oxide coating, the metal remains resistant to corrosion. Aluminum is highly resistant to weathering, even in many industrial atmospheres, which often corrode other metals. It is also resistant to many acids." 2

Although aluminum has a huge advantage when compared to other metals, it is not always completely impervious to corrosion. Its protective oxide layer can become unstable when exposed to extreme pH levels. When the environment is highly acidic or basic, breakdown of the protective layer can occur, and its automatic renewal may not be fast enough to prevent corrosion.

According to the US Army Corps of Engineers, aluminum's protective "oxide film is generally stable in the pH range of 4.5 to 8.5, but the nature of the compounds present is crucial.certain soils tend to be corrosive to aluminum, particularly non-draining clay-organic mucks. As a general rule, contact with clay soils should be minimized unless special corrosion treatment measures are instituted." 1

In the unlikely event that extreme pH levels or known corrosive chemicals are present and cannot be avoided, there are several simple solutions to avoid possible damage, such as annodization and cathodic protection.

Annodizing is a common process used to further increase aluminum's corrosion and abrasion resistance, as well as a method to chemically bond colorant to the surface. Anodization is achieved by artificially thickening the natural oxide layer. This film can be made many times thicker than what would otherwise be formed.

You may have noticed that you never see aluminum corrosion in lakes, pools, food packaging products, etc. Typically, if you have seen corroded aluminum, it was in or near the ocean. While it may seem logical to draw the conclusion that the salt water must be corrosive to the aluminum, it is not. Salt water does not corrode aluminum because of its neutral pH. A saltwater solution can, however, be a major facilitator for galvanic or dissimilar metal corrosion, a more complex corrosive process.

This is a basic version of a galvanic cell, much like the battery in your car. When two dissimilar metals are immersed in an electrolyte solution, a battery is created. The electrolyte solution serves as a bridge between the two metals and effectively closes half of an electrical loop.

When the two dissimilar metals come into contact, the electrical loop is closed, and the natural voltage differential between them causes electron flow. One metal will become the anode (negative) and one will become the cathode (positive).

In the simplest terms, this electrical circuit causes the anode to lose ions and the cathode to gain ions. This process slowly consumes the anode (galvanic or dissimilar metal corrosion) and effectively strengthens the cathode against corrosion.