FUNDAMENTALS OF CATHODIC PROTECTION

Transcription

1 FUNDAMENTALS OF CATHODIC PROTECTION Corrosion is the deterioration of a metal because of a reaction with its environment. For the purpose of this report, corrosion is the result of an electrochemical reaction that occurs between two different metal surfaces placed in contact with a common conductive environment. The reaction that occurs is commonly referred to as a reduction-oxidation reaction. Corrosion occurs because an electrical potential difference (voltage) exists between two sites on the metal surface in the environment. The difference may be the result of variations in the metal or in the environment. Variations in the metal may be the result of temperature, stress, metal composition or the presence of impurities. Corrosion may occur between two different sites on a single metal, or between two different types of metal placed in electrical contact with one another. Differences in the environment may be the result of variations in chemical composition, temperature, velocity and oxygen concentration. An electrolyte is a solution or substance that may conduct electrical current as an ionic charge. Water and soil are both electrolytes. The term "anode" refers to the metal surface at which corrosion occurs and from which current leaves the metal surface to enter the electrolyte. The reaction that occurs at the anode is called oxidation. The term "cathode" describes the metal surface from which current leaves the electrolyte to enter the metal. The reaction at this surface is called reduction. Typically oxygen reduction or hydrogen evolution occurs at the cathode. The term "electrolysis" refers to changes that occur in the electrolyte as a result of the corrosion process. During the corrosion process, a metal molecule leaves the metal surface and enters the electrolyte to combine with a free ion at a lower valence state. This is oxidation. Electron flow occurs in the metal between the anode and

2 cathode. Simultaneously, a reduction reaction occurs at the cathode. Electrical current flows onto the cathode surface and off of the anode surface by means of ion exchange. A corrosion cell is a circuit consisting of an anode, a cathode, an electrolyte, and an electrical contact between the anode and cathode. The diagram below represents a simple corrosion cell between a copper cathode and an iron anode placed in a beaker with electrolyte solution. CURRENT FLOW WIRE CATHODE (COPPER) ANODE (IRON) CURRENT FLOW IONIZED ELECTROLYTE CURRENT FLOW (- TO + IN ELECTROLYTE) In relation to the subject structure, the water is an electrolyte and the steel surface is the metal. Both anodic and cathodic sites exist on the steel surface due to variations in the alloy and in the water. Cathodic protection is an electrical means of mitigating corrosion on buried and submerged structures. CP involves the application of a DC (direct current) onto the surface of a metal structure. Since corrosion only occurs at locations where current discharges from a metal surface, corrosion control may be achieved by applying a net DC current flow onto the entire surface of a structure. In those areas where current collects, corrosion is controlled. Two types of cathodic protection systems exist. The first type uses galvanic anodes for protection. When metals such as magnesium or zinc are placed in the environment in contact with a more noble metal such as steel, a current flows from the more active anode to the nobler cathode. This is similar to the operation of a dry cell battery. Current flows because a potential difference

3 (voltage) exists between the two metals relative to their electrolyte. Galvanic systems are most suitable for use with low resistivity environments, well-coated structures and relatively small surface areas. No external power source is required. Galvanic systems tend to have a high initial cost with minimal maintenance costs. The diagram below depicts a galvanic anode installed on a pipeline with a test station. GALVANIC ANODE SYSTEM Test Terminal Board Test Lead Pipeline Anode The second type of system is an impressed current system. These utilize an external power source to develop a high potential difference between the surface to be protected and an anode. A series of anodes installed in the ground are referred to as a groundbed. Impressed current type systems are advantageous because high driving voltages can be developed with an external power supply. This makes it possible to achieve a much higher current output from an anode, than from an equivalent size anode on a galvanic system. Fewer anodes are required for impressed current systems than are required for a galvanic system of equal current capacity. Impressed current systems tend to have a low initial cost with higher operating and maintenance costs than a galvanic system. The following diagram depicts an impressed current system for a pipeline.

4 IMPRESSED CURRENT SYSTEM Anode Groundbed Rectifier AC Power Supply Positive Cable Negative Cable Pipeline Impressed current systems typically use a power source known as a rectifier. The rectifier converts AC power to DC power and provides adjustability to the system. The current output may be increased by increasing the voltage. A transformer is used to adjust the output voltage of the rectifier. This is accomplished by adjusting tap bars on the front panel of the unit. A rectifying element, such as a diode bridge circuit, is used to convert AC power to DC power (ripple). In place of standard diodes, silicon controlled rectifiers (SCRs) may be used in conjunction with a circuit card to maintain a specified level of protection. Automatic rectifiers with SCRs may be adjusted to maintain a set voltage, current output or structure potential. Rectifiers are commonly equipped with meters to read current and voltage outputs. Circuit breakers, lightning arrestors and fuses are used to protect the unit from power surges and faults. Shunts are calibrated resistors used to measure current flow. Filters, chokes and capacitors are sometimes used to increase efficiency and limit radio frequency (RF) interference. The anodes of both galvanic and impressed current type CP systems corrode and are eventually consumed. When conventional anode materials such as graphite and zinc corrode, the anode gets smaller and the resistance of the electrical circuit increases. By Ohm's Law (V=IR), an amount of current (I in

5 amps) will flow equal to the driving voltage (V in volts) of the circuit divided by the resistance (R in ohms) of the circuit. As the anodes corrode, the circuit resistance increases and the current output decreases. With impressed current systems, the voltage output may be adjusted in order to maintain protection. Adjustments are typically required on an annual basis. Dimensionally stable anodes, such as platinized niobium wire remain relatively consistent in size and may not require adjustment as frequently. The amount of current required to protect a structure is proportional to the surface area of bare metal being protected and environmental conditions. Coating quality influences the amount of bare metal exposed to the environment tremendously. A bare structure requires current flow onto the entire surface. A well-coated structure requires minimal current flow except at holidays (coating flaws). A well-coated tank may have a current requirement of less than 1% of an equivalent bare tank. As the coating deteriorates, the amount of current required to maintain protection increases. New systems frequently operate well below their maximum designed current output capacity, which is intended for later use in the structure's design life. If changes occur in the structural or environmental conditions, the current output and/or level of protection may be affected. Typical examples are changes in soil condition that vary the moisture content due to rain, and freezing of the ground, either around the anodes or structure. Factors which increase the soil resistivity (make it less conductive) reduce the current output; those which decrease the resistivity (make it more conductive) increase the current output. Examples of change in structural factors may be the amount of tank bottom in contact with the earth based on the fill level of a tank or changes in the electrical isolation of a pipeline system. The primary changes in a water tank system occur due to water level fluctuations. Reference cells are used to measure the level of cathodic protection being received on the structure surface. A half-cell consists of a metal rod immersed in a specific environment. The half-cell serves as a standard against which the

6 structure s potential is measured. The level of protection may be evaluated based on industry established criteria. Reference electrodes are metal rods placed in the environment for the same purpose; however, they have not been placed within a stable environment and respectively may not be as accurate. Reference cells may be permanently installed or portable. Reference cells may not function when frozen. The most popular type of reference half-cell utilized is the copper-copper sulfate (CSE) reference half-cell. It consists of a copper rod in a closed container filled with a saturated copper sulfate solution. The vessel has a porous plug that permits electrical contact when placed on the ground or in water, but does not permit the solution to be lost. A structure-to-electrolyte potential measurement is recorded by placing a reference cell in direct contact with the electrolyte (soil surface or water) and measuring the DC voltage between the reference cell and structure. If the negative meter lead is connected to the half-cell and the positive lead is connected to the structure, the proper polarity will be read. The voltmeter used should have high input impedance (2 mega-ohms minimum). The following diagram depicts the use of a reference cell. DC Volt Meter + - Half Cell Soil Pipe Potential measurements recorded with the cathodic protection current turned on include a component referred to as IR-drop. When CP current (I) flows through the environment (which has a resistance (R)) between the reference cell

7 and structure, a voltage drop occurs in the soil. Electrolyte IR-drop caused by CP always makes the structure-to-electrolyte reading appear more negative than the actual polarized potential of the structure. When the current is turned off, the current goes to zero (I=0 amps) and so does the IR-drop. IR-drop free potential measurements may be recorded immediately after turning off the CP current, but before the structure has time to depolarize. These are referred to as instant off potential measurements. They are considered more accurate than readings recorded with the current on, and are commonly used to evaluate the level of protection achieved. It is the job of the corrosion engineer to design a system which provides sufficient current, adequately distributed on the structure surface to control corrosion, while providing a reliable and safe system of suitable design life. It is the responsibility of the CP system operator to assure an adequate level of cathodic protection continues to be provided and the equipment is maintained in good working condition.