Metal Sheet Pile Corrosion and Protection

In marine construction environments, metal sheet pile are exposed to a host of natural influences that can impact their useful service lives.

Corrosion represents one inevitable factor which engineers and marine contractors must consider when designing, installing, or working around these structural sections.

Because damage stemming from corroded metal sheet pile can be significant, an understanding of corrosion factors, zones, and preventive measures is paramount to sustaining a piling's long-term life.

Corrosion Factors

Moisture and oxygen represent the two main factors that cause corrosion, with the presence of moisture increasing the electrical conductivity of the environment in contact with the metal surface.

A consortium of other sub-factors influence the type and rate of corrosion, including:

Chlorides: This is the principal reason for more aggressive attacks on metal sheet pile and other metals in seawater, as chlorides escalate electrical conductivity and corrosion currents.

pH Value: This is the degree of the acidity of alkalinity in seawater, which typically ranges from 7.2 to 8.2.

Salinity: In the open sea, water has a salt content of about 3.5 percent. In unpolluted harbors and other seacoast areas, dilution with freshwater run-off occurs, yet the proportion of salts relative to each other remains the same.

Pollution: In harbors, pollution can include domestic sewage, complex industrial water, oil well brines, and spilled oil. When pollution destroys oxygen-dependent fouling organisms, corrosion rates can be unfavorably altered through permitting the survival of anaerobic bacteria. Contaminants that introduce sulphides or reduce pH at the sites of piles can elevate the corrosion rate of metal sheet pile.

Wind: Gusts can alter wave action and carry salt-laden mist, with the residue of dried salt attracting moisture and continuing corrosion.

Protection

How aggressive a marine environment is will determine the extent of protections required, with all portions of a structure shielded in areas where the rate of corrosion is high.

Encasement of metal sheet pile in concrete is a common way to provide protection for both faces of sheet piling in the area extending from grade down to the average low water level, providing defense for the atmospheric, splash, and tidal zones. Encasement also provides a smooth face for the working zone of the bulkhead.

Protective coatings are routinely utilized and applied by brush, roller, or spray, with brushings of prime coats onto the metal sheet pile achieving surface penetration. Protective coatings can be specified for one side, both sides, or selected parts of the pile, with the interior of the interlocks remaining uncoated.

When attempting to reduce the amount of current required for protection or to shield portions of the pile not covered in the splash and atmospheric areas, coatings can be used in conjunction with cathodic protection.

Non-Metallic Coatings

Most metal sheet pile located in hostile environments is protected with non-metallic coating, with many organic and synthetic coatings and systems available. Non-metallic coatings are known for being cheaper on a first cost basis than metallic coatings, and more reliable than cathodic protection.

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