API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks1.0 Scope:This covers the procedures. Practices and guidance for effective corrosion control of the above ground hydrocarbon storage tank bottoms using cathodic protection to new and existing storage tanks.

4.0Corrosion of Steel Storage TanksCorrosion is the deterioration of metal due to reaction with the environment. Corrosion occurs when;a. Areas with different electrical potentials exist on metal surfaceb. These areas are electrically connectedc. Areas are in contact with electrolyte like moist soil in contact with tank bottom. Water and sludge are the electrolytes for internal bottom surface.

Forms of Corrosion:General Corrosion leading to general metal loss and thinningPitting due to localized actions ( metal loss may be concentrated within relatively small area and other near area may be unaffected)

API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks4.1Condition which influence Corrosiona. Composition of Metalb. Differential electrochemical potentialc. Difference between the weld bead, heat affected zone and parent metald. Physical and chemical properties of the electrolytee. Difference oxygen concentrationf. Soil characteristics ( dissolved salts, moisture contents, pH)g. Clay and debris in bottom contact, suspended solids in product stored, pH level, dissolved gases like H2S, CO2 or O2.

API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks6.0 Cathodic Protection MethodsCathodic protection is the technique for preventing corrosion by making entire surface of the metal to act as cathode of an electrolyte cell. There are two mathods commonly used;a. Sacrificial Anodeb. Impressed current6.2Sacrificial Anode System (Galvanic System): This system involves an anode buried in soil, but electrically connected to the structure (cathode). The anode is thus corroded (sacrificed) and metal surface is protected. Metals commonly used as anodes are magnesium and zinc ( cast or ribbon type). They are either buried beneath the bottom are distributed around the perimeter of the tank.Advantages:a. No external power supply is requiredb. Easy to installc. Low capital investmentd. Minimum maintenance coste. Interference problems are lessf. Less monitoring requiredDisadvantages:a. Driving potential is limitedb. Current output is lowc. Suitable only for low Resistivity soild. Not very suitable for large bare structures

API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks6.3Impressed Current System This system uses Direct Current (DC) usually provided by a rectifier. DC flows from the rectifier to the buried impressed current anode.Power source include a step down transformer (reduces AC supply voltage) and rectifying element to provide DC output.Silicon rectifiers are more efficient. Selenium rectifiers are also used but have limited life span.Impressed current anodes are of graphite, steel, high silicon cast iron or mixed metal oxide on titanium. They are usually buried in conducting coke breeze backfill (to reduce circuit resistance) in remote ground beds distributed around and under the tank or installed in deep ground beds.Advantages:a. Large driving potential is availableb. High current output for protecting large structuresc. Capable for variable current outputd. Applicable to almost any soil resistivity.However they are costly, more frequent monitoring is required, and loss of AC power may cause loss to protection. Solar powered systems are also available.

API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks8.0Design Criteria for Cathodic Protection A negative (cathodic) potential of at least 850 mV is applied for the cathodic current.A minimum of 100 mV of cathodic polarization shall be measured between the tank bottom metallic surface and the stable reference electrode contacting the electrolyte.The standard method of determining the effectiveness of cathodic protection on a tank bottom is the tank to soil potential measurement. Measurement is performed using a high impedance (resistance) voltmeter and stable reference electrode contacting the electrolyte.

API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks11.Operation and Maintenance of CP Systems This system shall be subject to field inspection surveys to establish the effectiveness at intervals.Conditions that effect protection are subject to change with time. Hence changes will be required in parameters that give protection. If tank is empty, large area under bottom may not be in contact with underneath soil, hence potential survey in such conditions may be misleading. Such surveys have to be done when tank is under load and maximum bottom cushion contacts have been made.Initial surveys of new installation shall not be conducted until adequate polarization has occurred, which generally occurs after 6-18 months of system energization.Surveys shall include one or more of:Structure to soil potential, Anode current, Negative structure to soil potential, Piping to tank isolation ( if protected separately), effect of adjacent structure, rectifier DC volts, DC Amps, efficiency and tap settings.Proper records have to be maintained, reviewed and referred.