Marine projects present numerous obstacles for installing steel piles; but after completion, there are other factors that must be considered to ensure the longevity and safety of the structure. Steel piles are exposed to a host of natural conditions that can prematurely cause steel corrosion and lead to malfunction or breakdown.
The most common issues are caused by corrosion and can cause significant damage.
Damage that stems from corroded steel can lead to severe breakdowns, and implementing preventative measures can help deliver long-term protection. From corrosive elements, zones, and the different forms of corrosion, understanding corrosion is key to protecting sheet piles and ensuring longevity.
Corrosive Elements
The two main factors that will cause corrosion for steel pipe piles and other sheet piles are moisture and oxygen. The presence of moisture can also increase the steel’s electrical conductivity, which is associated with corrosion in carbon steel.
Some other sub-factors include:
- Chlorides
- pH Values
- Salinity
- Pollution
Chlorides are responsible for more aggressive corrosion found in saltwater, so they should be considered for any marine project where sheet piles are driven in seawater. The degree of acidity and alkalinity in seawater is measured in pH value; higher values can lead to more significant corrosion.
The salinity refers to the amount of salt in water and projects near freshwater runoff or in brackish waters can have less corrosive elements. Anaerobic conditions can be created in polluted waters where natural filters have been damaged.
High Corrosion Zones
Most marine projects have steel elements that are exposed to a variety of zones that can present corrosive conditions.
- Atmospheric Zone: This refers to the zone where bare steel is exposed to elements in the surrounding environment. This is also dependent on the duration of exposure to moisture and other elements.
- Splash Zone: This is often the area where the most corrosion occurs and refers to the zone above the tide line. An example would be a steel H-pile that is subject to frequent exposure to ocean waves splashing against it.
- Tidal Zone: These zones are associated with relatively lower corrosion as the metal in the tidal zone benefits from being cathodic to the metal that is a bit lower and submerged below the tidal zone.
- Continuously Submerged Zone: Both freshwater and saltwater steel piles that are fully submerged experience less corrosion because they experience less exposure to dissolved oxygen.
- Buried Zone: Structures composed of sheet piling, like interlocking sheet piles, gain their stability from being driven into the Earth and this is referred to as the buried zone. Piles driven into existing soil are exposed to less corrosive elements than piles that are installed in trucked-in fill or dredged sites. This is due to higher levels of oxygen found in disturbed soils.
Different Forms of Corrosion
- Pitting: This commonly occurs in salt water and can result in craters on the exterior of steel piles. This damage leads to reduced cross-sectional strength.
- Uniform: This refers to damage that occurs at the same rate on the steel and contributes to reduced overall strength. This can lead to catastrophic failure if a pile is overstressed. Typically uniform corrosion is found in atmospheric and freshwater immersion.
- Galvanic Action: This occurs when there is electrical contact of one metal with another metal that is more passive. It results in the anodic metal experiencing corrosion while the cathodic metal remains inactive.
- Stray Current: Improperly grounded electrical equipment can cause stray currents in marine projects that lead to damage in ungrounded structures. It’s estimated that stray currents can remove up to twenty pounds of steel a year.
- Fatigue: A common form of corrosion caused by repeated stress or repeated exposure to corrosive elements.
- Bacteria: Anaerobic bacteria can be released into marine environments by illegal dumping or a civic malfunction and cause corrosion at the midline.
- Fouling: Fouling is damage caused by biological organisms like barnacles attaching to steel structures.
How to Protect From Corrosion
The best way to prevent corrosion is to conduct a thorough site analysis and address how aggressive the conditions will be. The most common solution is to encase steel piles in concrete to protect them from corrosive elements.
Coatings
This is a routine method for protecting steel and includes metallic and non-metallic options. Non-metallic coatings include organic and synthetic materials. These offer a cost-effective solution that is more reliable than cathodic coating. Metallic coatings include galvanizing and flame spraying.
Metallic coatings include galvanizing and flame spraying:
- Galvanizing steel involves hot dipping steel into molten zinc. This coating is often cost-prohibitive for large mating projects.
- Flame spraying steel is an older process where metal wire is melted and sprayed directly onto the steel piles to provide a protective layer forming a tight bond.
No Permanent Protections
No matter what methods of protection are employed, the inevitable decay of steel is a fact. All steel piling will experience some level of corrosion in its lifetime; by accepting this and shifting the focus to identifying when the steel will be vulnerable, engineers and contractors can develop a lifespan and scope for repair and replacement.
The first step in preventing corrosion is understanding the causes along with their results and applying up-to-date methods to deliver a safe structure that will last for many years to come.
