Metals are ordered in what is known as the galvanic series. The galvanic series is a list or table that describes how reactive or noble (less reactive) different metals are likely to be. The metal nearer the noble end acts as the cathode and the metal nearer the active end acts as the anode. The farther away the two metals are from each other in the series, the larger the voltage potential, and the more intense the reaction. The intensity of the reaction is dependant on many other things as well (for example, the particular electrolyte involved and the anode to cathode ratio).

Many versions of the galvanic series exist, and although it is a good indicator of the roles that each metal will play, and the relative intensity of the reaction, it cannot be used to predict actual reaction rates due to the many variables at play. Furthermore, there are many exceptions to the predictions made, due to real world conditions.

To complete a galvanic cell we need metal-to-metal contact and an electrolyte solution. Unfortunately, salt is a very effective and abundant electrolyte. Mix it with water (the ocean) and you basically have a permanent, electrolyte solution presence in coastal areas.

Galvanic corrosion is common and a very real concern for most metals in coastal applications. In this environment, all it takes is contact between two dissimilar metals to initiate a galvanic reaction. This reaction results in galvanic corrosion for one and effective corrosion protection for the other. Severity of corrosion depends primarily on where the two metals in contact fall in the galvanic series, contact area, and the anode to cathode ratio.

Galvanic corrosion is the most common cause of aluminum corrosion due to its inherent resistance to more natural forms of corrosion and a lack of preventative measures taken.

Aluminum is typically a reactive metal in the galvanic series. This means that in many cases, when it is in contact with other metals, aluminum will act as the anode and may begin to corrode. Although this process may take many years to yield signs of degradation, it is very easy to preclude. As a preventative measure, whenever possible, aluminum should be isolated from other metals with a non-absorbent, non-conductive, insulator like bitumastic paints or polymer sleeves and washers. Some common interactions between aluminum and other dissimilar metals include pressure treated wood (which contains copper), steel fasteners, and some aggregates.

This is not to say that contact with these or any other metals will always cause a noticeable reaction over the life of a structure. Isolating dissimilar metals is simply a very easy and very inexpensive way of minimizing the concern of a galvanic reaction altogether. In fact, The Aluminum Association approves stainless steel as well as HDG fasteners for use with aluminum structures. Stainless steel is, however, highly preferred in coastal environments due to its superior corrosion resistance and durability over HDG.

Additionally, aluminum structures typically only see peripheral contact with other metals, at worst. Contact is usually limited to fasteners and supporting members. When this is the case, the anode to cathode ratio (aluminum to dissimilar ratio) is typically very large which minimizes the negative effects of the galvanic reaction and greatly increases the life of the aluminum.

So far we have addressed the negative effects of being the anode in a galvanic reaction. From before, we discovered that the material acting as the anode is subject to induced corrosion, while the cathode is additionally protected from corrosive effects. If we can force the aluminum to be the cathode, not only will it minimize the corrosive effects of the galvanic reaction, it will show even greater corrosion resisting characteristics than when left to its own devices. This principle is executed in what is known as cathodic protection or a sacrificial anode system.

In the case of an aluminum seawall, rods or plates made of magnesium are electrically connected and buried, forcing a galvanic reaction, but this time the aluminum is the cathode and thus the beneficiary while the magnesium is sacrificed, hence the term sacrificial anode.

In situations where soils are a concern, or major dissimilar metal contact cannot be avoided, this is a relatively inexpensive and very effective method of aluminum corrosion protection.

When the term electrolysis is used, typically one of two phenomena is intended: a galvanic reaction, or stray current action. Although the method of corrosion between these two is similar, stray current action is induced by an external electrical current produced by improper grounding or insufficient isolation of large electric systems. Stray current corrosion is rare in aluminum waterfront structures, and in those cases, results only through improper design or installation of electrical systems in close proximity to a dock or seawall.

When aluminum's corrosion characteristics are properly understood, it becomes easy to predict how it will behave in the field. When negative effects are anticipated, solutions are inexpensive, effective, and easy to include in the design phase. Here are several tips to ensure a long lasting aluminum structure.

· Always use a marine grade alloy.

· Test to ensure backfills and/or native soils have a pH of 4.5-8.5, and, when possible, avoid clays or highly organic soils.

· If poorly compatible soils are anticipated or dissimilar metal contact cannot be avoided, use cathodic protection. The sacrificial anode should be checked at regular intervals, and may need to be replaced every 10 to 20 years.

· Insulate contact with other materials that may have significant metal content (steel fasteners, concrete, pressure treated wood, etc.).

· Use stainless steel fasteners. If stainless steel is not an option, use HDG fasteners. It is always a good idea to separate steel fasteners from the aluminum structure with polymer washers.

· When economically feasible, use anodized aluminum components because of their increased corrosion protection.

· Avoid grounding electrical circuits to aluminum structures when possible.

· Avoid unusually high stresses or surface damage. Residual stresses may amplify corrosion.

Over the last several decades the aluminum industry has developed, tested, and proven in field applications, hundreds of aluminum alloys with a variety of characteristics engineered and analyzed specifically for a given application. From aircraft to electronics to nuclear boilers to food packaging, aluminum has proven its versatility and longevity.

1. United States Army Corps of Engineers. EM 1110-2- 1614 Design of Seawalls and Bulkheads. Washington DC: USACE, 1995

2. The Aluminum Association. Specifications & Guidelines for Aluminum Structures. 8th ed. Arlington, VA: The Aluminum Association, 2005