The bottom plates of above-ground storage tanks are subject to corrosion. In some situations, the tank bottom may be protected from corrosion by oil sand, asphalt sand or impressed current cathodic protection with sand and/or electrochemical techniques.
Protection by oil or asphalt sand tales advantage of the dielectric, non-electrolytic properties of the oil or asphalt. Recently, however, the effectiveness of this approach has been questioned due to corrosive tank failures resulting from insufficient dielectric protection. In addition, water or rain intrusion from the edge of the tank bottom plate may accelerate corrosion in those areas.
Impressed current systems generally use an inert anode with a transformer rectifier (DC power supply) to generate cathodic protection current. The anode is typically embedded in sand. The cathodic protection current from the anode travels through the sand or soil electrolyte which contacts the steel plate and protects the tank plate from corrosion. So long as the tank plate contacts the sand electrolyte, the impressed current cathodic protection is effective.
However, when the product inside the tank becomes depleted or emptied, the tank plate may rise from the sand or soil, resulting in the development of air gaps in some areas. If this occurs, the cathodic protection current cannot reach the tank steel surfaces located over air Caps because the air cannot transfer the cathodic protection current. As a result, the effectiveness of the corrosion protection using an impressed current cathodic protection system is lost, and those areas are subject to corrosion.
In addition, since cathodic protection is continuously operating system, interruption or malfunction of the transformer rectifier or damage of any cathodic protection hardware stops the protection of the steel plate from corrosion. Therefore, the continuous maintenance of the transformer rectifier with all associated hardware is mandatory to protect the tank plate.
Published U.S. Patent Application Serial No. 2004/0238376 A1 discloses a method for cathodically protecting tank bottom steel plate using a sacrificial anode such as zinc or aluminum alloy sheets embedded in sand or soil. However, a disadvantage of this system is that the sacrificial anode passivates and becomes nonfunctional in a relatively short period of time.
To achieve sufficient cathodic protection current through sacrificial metals such as zinc, aluminum, or their alloys, the metals must corrode or oxidize to generate cathodic protection current. However, these metals only corrode in a very low or high pH electrolyte environment. If the electrolyte has a pH less than approximately 10 in the absence of high chloride concentrations, these metals do not corrode due to passivation.
Furthermore, when such metals corrode, oxide products build up at the interface between the sand and the anode. The pH of the aluminum or zinc oxide products is approximately 5 to 7. This means that the aluminum or zinc metal underneath the oxide products is exposed to a neutral pH. As a result, the aluminum or zinc metal strongly passivates and becomes stable and non-corroding metal. When this occurs, they cannot function as sacrificial anode to protect the tank steel plate.
To minimize the passivation of zinc or zinc alloys, calcium bentonite and gypsum based backfill materials may be used. The zinc anode is embedded in the backfill material in a cloth bag to minimize the zinc passivation. The bulk zinc anode in the cloth bag with the backfill material is not suitable for tank bottom plates, however, due to poor current distribution from the localized anode to the entire tank plate. By significantly increasing the number of the anodes to uniformly distribute current to the tank plate, cathodic protection using bulk anodes is feasible. However, the cost of such a system is significantly high. In addition, this type of the backfill material is corrosive to the steel plate, it make more difficult to protect the tank bottom.
Aluminum or aluminum alloys are generally used for seawater or brackish water electrolytes as a sacrificial anode because the high chloride concentration prevents the passivation of the aluminum. The aluminum anode has high electrical capacity (greater than 2900 amp-hours/kg) and high efficiency (greater than 90%). In sand or soil environments, however, the aluminum or aluminum alloy does not function as a sacrificial anode due to passivation. Another disadvantage is that because the steel plate is exposed to sand or soil, the passivated zinc or aluminum anode cannot produce a sufficient level of current to protect the steel plate.
Corrosion protection using magnesium sacrificial anodes is commonly used to protect the steel contacting to soil or sand environment because magnesium anode does not passivate in a neutral pH electrolyte. The electrical capacity of magnesium anode is approximately 1,230 amp-hours/kg with less than 50 percent efficiency. The magnesium anode is generally provided as a bulk anode in a cloth bag with a backfill material similar to that used with a zinc anode. As such, the cost of sacrificial magnesium anode cathodic protection is significantly high. Furthermore, as with impressed current cathodic protection current, sacrificial anode systems cannot protect the steel plate across air gaps. As a result, the effectiveness of the corrosion protection of steel tank plates is limited.
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