ABSTRACT
Traditional cathodic protection (CP) systems inside water storage tanks often sustain ice damage during cold weather. The CP system described in this paper uses ceramic-coated wire anodes and a floating support system to keep the anodes submerged underneath the surface ice. Its design, implementation, and performance will be discussed.
INTRODUCTION
Elevated steel potable water storage tanks and their associated piping have been identified as critical infrastructure at military installations. Soldiers, their families, and civilian employees depend on them to provide a safe and reliable source of water for drinking, bathing, washing, and cooking. Water storage tanks and piping also provide water storage capacity and pressure to the distribution system for firefighting including aircraft deluge systems, building fire suppression systems, and fire hydrants.
Water storage tanks and piping should have a service life of 50 to 75 years. However, severe corrosion inside the bowl of the tank and in the associated piping shortens the service life to as little as 20 years due to leaks and/or structural problems. Throughout the tank's life, corrosion products (i.e. "rust") will enter the potable water and will cause water quality problems including discoloration ("red water"), taste, and odor. In some cases the water may exceed the maximum contaminant level for iron of 0.3 mg/l as specified in the Environmental Protection Agency's National Secondary Drinking Water Regulations. Ladders that are used by maintenance personnel to access the inside of the water tank will corrode and become a safety hazard. The unplanned loss or failure of a storage tank and/or piping usually results in the catastrophic loss of adequate fire suppression capability and can endanger lives and property.
It is standard practice to install cathodic protection (CP) systems inside water storage tanks to prevent corrosion from occurring. However, conventional CP systems are often prematurely damaged or destroyed when surface ice forms in the tank in very cold environments. This is a persistent problem at DoD installations with water storage tanks located in areas with cold winters. The CP system must be repaired or replaced immediately to prevent corrosion damage inside the tank.
Protective coatings alone are insufficient to prevent corrosion; in fact, accelerated corrosion can take place at defects in the coating if there is not a properly functioning CP system to prevent it. Virtually all coatings have defects, so if there is no CP, accelerated corrosion is likely to occur.
ICE FREE CP SYSTEM TECHNOLOGY
Impressed current cathodic protection (CP) systems represent a mature technology to prevent water-side corrosion of potable water storage tanks. CP is an electrical method for preventing corrosion in which direct current (DC) is continuously supplied to a submerged or buried metallic structure to stop the natural corrosion processes from occurring. An impressed current CP system generates direct current by using a rectifier. The rectifier is connected to anodes that discharge the current through the soil or water and onto the protected structure.
In the past, large and heavy (approximately 40 pounds each) iron-silicon or graphite anodes were used for CP systems in water storage tanks.
