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Cathodic Protection
Cathodic protection uses direct electrical current to mitigate corrosion.
There are two types of systems for cathodic protection:
- Sacrificial anodes, which can be attached to a coated steel UST for corrosion protection. Sacrificial anodes are pieces of metal more electrically active than the steel UST. Because these anodes are more active, the corrosive current will exit from them rather than the UST. Therefore, the UST is protected while the attached anode is sacrificed. Depleted anodes must be replaced for continued corrosion protection of the tank.
- Impressed current systems use rectifiers to convert alternating current to direct current. This current is sent through an insulated wire to the anodes, which are special metal bars buried in the soil near the UST. The current then flows through the soil to the UST system and returns to the rectifier through an insulated wire attached to the UST. The UST system is protected because the current going to the UST system overcomes the corrosion-causing current normally flowing away from it.
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Maintenance and Testing Tips
General Tip 1: Cathodic protection systems require periodic maintenance and testing to ensure that they are functioning properly. Some checks require only visual observations. Others are more involved. For detailed information about cathodic protection operation and maintenance, see the NACE International SP0169 standard (Control of External Corrosion on Underground or Submerged Metallic Piping Systems). Another NACE standard for USTs is SP0285 (Corrosion Control of Underground Storage Tank Systems by Cathodic Protection). Both standards can be purchased on the NACE website.
Tip 2: Ensure that aboveground equipment such as rectifier cabinets, junction boxes, and test stations are screened and that there are no unused or unsealed openings that would allow insects such as wasps and spiders to access the equipment.
Tip 3: Allow only trained and qualified staff to perform the required maintenance and testing on cathodic protection systems.
Tip 4: Inspections for sacrificial anode systems:
- Monthly/bimonthly: Inspect exposed system components on a monthly basis to ensure that equipment is intact. Look for damage to test stations or junction boxes, and open them to verify that connections are intact. Clean out any debris found, and schedule repairs if needed. (Note: If monthly checks are repeatedly acceptable, you can go to a bimonthly schedule.)
- Detailed yearly inspections and testing must be performed by qualified personnel, and the records should be reviewed by a qualified corrosion professional. Make or schedule repairs as recommended by the corrosion specialist.
- Maintain yearly inspection records along with recommendations and actions taken.
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Tip 5: Procedures for impressed current systems:
- For safety of personnel, make sure the cabinet of the rectifier is grounded per the National Electric Code (NEC) and local standards. Train personnel to verify the case is grounded and not energized before taking hold of it.
- Padlock rectifiers and junction boxes prevent unauthorized tampering.
- Inspect the system on a monthly basis to:
- Verify that the rectifier is energized and no unauthorized adjustments have been made.
- Verify that the rectifier tap settings are in the positions recorded on the last inspection sheet.
- Verify that there is DC voltage and current output.
- Look for scorch marks and indications of excess heat.
- Look for damage to test stations and/or junction boxes. Open them to verify that connections are intact.
- Clean out any debris found in rectifier cabinets, test stations, junction boxes, and bond boxes.
- Make or schedule repairs.
- Note: If monthly checks are repeatedly acceptable, you can move to a bimonthly schedule. Be aware, however, of rectifiers losing power due to lightning or other power interruptions.
- Detailed yearly inspections and testing must be performed by qualified personnel, and the records should be reviewed by a qualified corrosion professional. Make or schedule repairs as recommended by the corrosion specialist.
- Maintain yearly inspection records along with recommendations and actions taken.
Tip 6: For USTs, the EPA requires that all cathodic protection systems be tested within 6 months of installation and every 3 years thereafter, and within 6 months of a repair according to a nationally recognized association code practice. UST systems with compressed current cathodic protection systems must also be inspected every 60 days to ensure equipment is running properly.
Tip 7: For USTs, the EPA requires that records related to corrosion be kept until the UST is closed and 3 years after.