Steel Sheet Piling Corrosion and Protection

In marine construction environments, steel sheet pilings 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 steel 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 steel 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 steel.

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

Rain: When retained in crevices, rain can stimulate corrosion by maintaining damp conditions.

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 steel pilings 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 steel 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 steel 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.

Metallic Coatings

Galvanizing: Accomplished by hot-dipping the item in a tank of molten zinc, galvanizing has not factored into the protection of most sheet piling in the U.S. due to costs; however, galvanized steel has performed fine in corrosion tests.

Flame Spraying: This is an old procedure where metal wire is fed into a hand gun, melted, and then blown by compressed air onto the surface to be protected. The molten particles alloy to the surface layers of the steel to form a tight bond.

Aluminum flame-sprayed coatings with film thicknesses of 3 mils and 6 mils, unsealed, provide complete protection in seawater and severe marine environments, with thin coat films offering better guarding than thick films.

Zinc flame-sprayed coatings with film thicknesses of 12 mils and higher, both unsealed and sealed, allow for complete protection in seawater.

Cathodic Protection

This type of guard involves the use of the galvanic series, which determines the nobility of metals when submerged, to prevent corrosion of the sheet pile wall. Utilized only in the fully immersed zone, cathodic protection can be difficult to maintain over an extended time. When implementing this type of protection, all piles must be bonded together to insure electrical continuity.

Sacrificial Metal Protection Method

Considered uneconomical and less effective than coatings and cathodic methods, sacrificial metal protection allows for extra material to be designed into the sheeting.

Special Steels

Adding certain alloys to carbon steel can enhance its performance in some marine environments; however, the application of weathering steels to sheet piling projects has not been practical because of its limited benefits in relation to cost.

This article comes from pilebuck edit released