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Underground Corrosion - Mitigating The Risks

08.23.18   Robert C. Rabeler, PE | More by this Author

Underground Corrosion - Mitigating The Risks

Installing metal structures below grade always comes with the risk of corrosion problems. As we have explored in this blog series, such problems can stem from aeration issues, galvanic corrosion, soil pH, as well as other causes.

So what can be done to mitigate corrosion problems when you install utilities, tanks or other metal structures below grade?

There are a number of approaches that can be used for different situations. Always get a professional opinion from an expert in underground corrosion before using these options.

The best way to prevent corrosion is simply to avoid installing metal structures below grade. Consider suspension or even using a material other than metal.

SUSPENSION
Pipelines can be installed overhead or in tunnels.

MATERIAL
Alternate materials such as concrete or fiberglass can be used in place of metal. Concrete should not be used if the soil has a pH less than 5.5 and if high concentrations of sulfate are present. Also, if the pipe fittings are metallic, it may be necessary to coat these fittings and use cathodic protection.

If installing metal structures below grade is unavoidable, consider these options:

THICKNESS
The thickness of the metal can be increased to extend the life of the structure.

MATERIAL
Alternate materials such as concrete or fiberglass can be used in place of metal. Concrete should not be used if the soil has a pH less than 5.5 and if high concentrations of sulfate are present. Also, if the pipe fittings are metallic, it might be necessary to coat these fittings and use cathodic protection.

BACKFILL
Special backfills can be used around buried metal to reduce the corrosion potential. Buried metal completely surrounded by a uniform well-drained sand will likely have a long life. Be careful that no foreign material is present in the backfill, such as organic soil or cinders/slag.

COATINGS
Protective coatings can be used to protect buried metal. Coatings such as cement, mortar, bituminous coatings, and plastic or polyethylene sleeves have proven effective. However, be aware that using a coating might result in more severe pitting-type corrosion at the location of breaks (holidays) in the coating.

GALVANIZED METAL
Galvanized metal (zinc coating on steel) combines the desirable effect of a corrosion-resistant coating with the protection of small areas of bare metals by rendering them cathodic to the larger areas of zinc coating. Initial corrosion produces an oxide coating which slows down the rate of corrosion. The extent of cathodic protection provided by the zinc depends on the conductivity of the environment. For zinc coatings on steel in low conductivity environments, small defects in the coating may result in corrosion. However, in environments with high conductivity, the zinc coating protects large areas of uncovered steel.

CATHODIC PROTECTION
Cathodic protection provides an external current so that there is no (or little) tendency for metal ions to leave the structure and move to the surrounding environment. Cathodic protection can be applied to steel, copper, lead, and brass. Cathodic protection can also be used to prevent pitting in stainless steel or aluminum, to eliminate stress corrosion cracking, to eliminate corrosion fatigue of most metals, and to prevent inter-granular corrosion.

There are two types of cathodic protection:galvanized cathodic protection using artificial anodes and impressed-current cathodic protection.

SACRIFICIAL ANODES
Galvanized cathodic protection using sacrificial anodes is one method of corrosion protection. Typically magnesium or zinc anodes are placed at frequent intervals alongside the buried pipe, and electrically connected by copper wire to the coated buried pile as shown below:

If properly designed, the coated metal becomes cathodic to the magnesium or zinc and thus is protected against corrosion. This type of system does not require any power source, but the anodes must be replaced when they wear out.

IMPRESSED CURRENT
Impressed-current cathodic protection systems are also common. Rectifiers (converting AC to DC current) are used to produce the right amount of DC current flowing from the anodes to the structure (such as a pipeline), as shown below:

Cathodic protecting systems, especially impressed current systems, can cause stray current-type corrosion on nearby unprotected metal structures. For this reason, impressed-current systems are not desirable in congested areas.

Cathodic protection is generally used in conjunction with a coated pipe. The coating improves the distribution of current to the pipeline, reduces the required current, and reduces the required number of anodes.

It is common practice to reduce the electrical resistance in the soil surrounding the anode. Special backfill materials are usually used. The backfill carries some of the current and reduces consumption of the anode. Sometimes the anode is immersed in a riverbed or lake, and then special backfill is not required.

The spacing of the anodes depends upon the design life of the structure, the size of structure, the expected current discharge per anode, the size of the anode, and the presence of adjacent utilities or buried metallic objects.

For more information or for assistance with underground corrosion issue, please contact Bob Rabeler.





TAGS: Geotechnical

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