4.1.5 Student Book © 2004 Propane Education & Research Council Page 1

4.1.5

Determining the Proper Corrosion Protection Systems for

Underground ASME Tanks and Buried Distribution Lines

Corrosion of underground tanks and piping is a significant problem. However, the corrosion process is to a great extent prevented by using a corrosion protection system.

In this module you will learn to identify:(1) The cause of corrosion (2) Methods and procedures used to protect buried tanks and

metal distribution lines from corrosion

4.1.5 Student Book © 2004 Propane Education & Research Council Page 1

Identifying the Cause of Corrosion

Corrosion is an electrochemical reaction between a metal and its environment.

This damaging reaction occurs between the metal surface on the underground ASME tank or metallic piping and the soil that surrounds it.

4.1.5 Student Book © 2004 Propane Education & Research Council Page 1

Methods and Procedures Used to Protect Buried Tanks and Metal Distribution Lines

Methods commonly used to protect buried tanks and metal piping from the effects of deterioration by corrosion are:

1. Coatings 2. Insulated fittings 3. Impressed current (cathodic protection), typically,

sacrificial anode(s)

4.1.5 Student Book © 2004 Propane Education & Research Council Page 1

Methods and Procedures Used to Protect Buried Tanks and Metal Distribution Lines

Coatings— Specially formulated coatings protect buried tanks and metal pipe against corrosion.

Manufacturers typically coat underground tanks as part of the manufacturing process to ensure a long and safe life of the buried tanks.

Because tanks may be damaged during transport or installation, additional coating may need to be done at the plant or customer location to repair “holidays”. (A holiday is a void in the protective coating that exposes the metal surface of the tank.)

4.1.5 Student Book © 2004 Propane Education & Research Council Page 2

Requirements for Properly Applying & MaintainingTank Coatings

• Properly Cleaned Surface

• Proper Priming and Coating Materials

• Proper Application of Coatings

• Proper Handling and Storing of Materials

• Proper Handling of Coated Surfaces

• Thorough Inspection and Repair of Holidays

Pipe and tank coatings provide the principle and most effective deterrent to underground tank and steel pipe corrosion. A properly manufactured and applied coating will serve to provide approximately 99% of the protective needs of buried metal structures.

4.1.5 Student Book © 2004 Propane Education & Research Council Page 2

Methods Used to Protect Buried Tanks

Insulating Fittings – Insulating fittings are used when buried metallic distribution lines (especially copper tubing) are connected to underground tanks to isolate various parts of the pipeline system and for quality control in cathodic protection systems.

Figure 1. Insulating Dielectric Union

4.1.5 Student Book © 2004 Propane Education & Research Council Pages 2 & 3

Methods Used to Protect Buried Tanks

Impressed Currents

Sacrificial Anodes – Sacrificial anodes are used where

• Current requirements are low

• Metal structures to be protected are usually well-coated

• Localized protection is required, and

• Relatively low soils having low electrical resistance exist.

When a sacrificial anode is buried and connected to an underground tank (Figure 2), a galvanic cell is developed. The electrical current produced by the galvanic cell provides cathodic protection to the tank in areas where the coating is defective.

4.1.5 Student Book © 2004 Propane Education & Research Council Page 4

Methods Used to Protect Buried Tanks

Cad W eldLead W ire(to inside &top of dome)

SandTank SupportF ill Anode Bag

“S cotch-Lok” TwistW ire Connector(jo ins anode and cadweld leads; providesCuCuSO half cell testpoint)

4

Cad W eldon Tank ManufacturerSupplied Structure(Not on Tank Shell)

Anode LeadW ire (to inside& top of dom e)

“N ative” Soil Backfilled &Sloped Away from Topof Dome for D rainage

Figure 2. Galvanic Cell

4.1.5 Student Book © 2004 Propane Education & Research Council Page 4

Methods Used to Protect Buried Tanks

Corrosion engineers typically use two factors to determine sacrificial anode requirements for buried steel tanks:

• Mass of the tank, typically expressed in pounds

• Surrounding soil characteristics, primarily soil electrical resistance

When selecting sacrificial anodes for large steel underground tanks or steel piping systems, or when anode installations in a particular area are not producing satisfactory measured voltage outputs, it is recommended that a consulting corrosion engineer be contacted.

4.1.5 Student Book © 2004 Propane Education & Research Council Page 5

Methods Used to Protect Buried Tanks

Figure 3. Sacrificial Anode Requirements for Typical Underground Tanks

4.1.5 Student Book © 2004 Propane Education & Research Council Page 5

Methods Used to Protect Buried Tanks

Rectifiers — Rectifiers may be used for cathodic protection of large underground tanks and steel piping systems. The rectifier is an electrical device which changes alternating current (AC) into direct current (DC) to provide cathodic protection for large tanks and buried metallic piping systems.

Impressed Currents

4.1.5 Student Book © 2004 Propane Education & Research Council Pages 6 & 7

Time to See If You Got the Key Points of This Module…

• Complete the Review on page 6.

• See if you are ready for the Certification Exam by checking off the performance criteria on page 7.