Cathodic Protection Influencing Factors and Monitoring

Transcription

1 Lecture 17 Cathodic Protection Influencing Factors and Monitoring Keywords: Coated Surfaces, Protection Criterion, Anode Materials, Pipeline Protection. For large structures such as underground pipe lines, impressed current cathodic protection is used, while for smaller structures such as house-hold water tanks, ship s hull etc, sacrificial anodes can be effectively used. Painting of steel pipe lines and tubes can significantly reduce protection current requirements and thus save cost. Approximate current requirements for cathodic protection of steel pipes are given below: Uncoated in flowing sea water ma/ft 2 Well-coated in water ma/ft 2 Excellently coated and exposed to water or under soil or less ma/ft 2 As can be seen above, good surface coating significantly reduces protection current requirements. Electrochemical basis for protection criterion can be assessed: Protection of steel is taken as example: Fe = Fe e E 0 = V When polarized to half cell potential of above reaction, corrosion rate reduces to 0. Rate of forward and reverse reaction are same when net reaction rate is zero. Eh = log [Fe ++ ] 1

2 Fe OH - = Fe (OH) 2 Calculated potential (based on solubility product) is -0.59V (SHE) which corresponds to about -0.90V (vs Cu/CuSO 4 ). Accepted criterion for protection of steel in water is -0.85V (vs Cu/CuSO 4 ). Potential of structure to environment is generally measured using Cu/CuSO 4 reference electrode. Test coupons made of same metal and previously weighed can be electrically connected to protected structures. These coupons are also exposed to same cathodic current in the corrosive environment. Estimation of weight losses of such coupons is a better proof of cathodic protection. Table 17.1 Potentials for Cathodic protection (Cu/CuSO 4 electrode) Iron and Steel Lead Copper and alloys Aluminium to V -0.6 V -0.5 to V to -1.2 V 2

3 Anode materials that can be used as ground-beds in impressed current cathodic protection are given Table 17.2 Table 17.2 Anode materials for impressed current cathodic protection Material Average consumption rate kg/a-year Cast Iron 5 7 Steel scrap 5-8 Aluminium 4 5 Graphite Lead Platinum Magnesium, zinc and aluminium and their alloys can be used as sacrificial anodes. Design considerations for both impressed current and sacrificial anode systems have some common steps. a) Area to be protected Exposed areas of the structure in coated system, exposed area at breaks and deteriorated coatings. b) Polarised potential Current density based on area need be estimated. c) Current demand Current density demands depend on the environment and nature of surface coating. d) Anode consumption Required number and weights of anode materials determined from known consumption rates for the desired current demand. Anode number and distribution for the protected structure can be thus estimated. Anode resistance and design output current can then be estimated. 3

4 Monitoring of effectiveness of pipeline protection Most widespread method is based on potential measurements of a cathodically polarized structure with reference to a standard electrode. A potential of -0.85V (Cu / CuSO 4 ) is sufficient for protection of steel in soil and natural water environments. It may however be borne in mind that the above criterion is not optimum and situations may arise when more negative (upto 1.0V) may be required or even lower (-0.7V) potential may suffice for protection. Interference from IR components can introduce errors in pipeline potential measurements. Elimination of IR drop can be achieved using switch off method. Potential measurements in chosen control points in a pipeline are frequently insufficient to ensure effective protection. Close Interval Potential Survey (CIPS) is an intensive monitoring technique based on connecting a thin cable to a pipeline to monitor frequent potential readings all the way. Special computer software together with appropriate instrumentation can be used for gathering and processing the data. Another technique called Direct Current Voltage Gradient (DCVG) method enables protection evaluation and also detection of defects in insulation. Potential gradient is monitored in the soil with a sensitive potential measurement meter using two reference electrodes kept at both sides of the pipeline at shorter distances. Corrosion coupons (probes) are generally used for monitoring of cathodic protection. A schematic representation of a coupon probe connected to a cathodically protected pipeline is illustrated in Fig The arrangement allows measurement of switchoff potential without any interruption of pipeline protection. 4

5 Fig 17.1 Circuit for monitoring cathodic protection. Different types of simulation probes are available for determination of : a) Level of protection in sections in casing pipes. b) Polarization resistance and depolarization rate. c) Insulation coating resistance. d) Any interference on neighbouring underground installations. e) Corrosion rate of protected structures. Such probes need be located in various geological locations through a running pipeline. Recently kinetic cathodic protection criterion has been proposed to allow maintenance of metal corrosion rate at a desired level. There are several pipeline corrosion rate control methods including both physical and electrochemical techniques, which allow determination of effective protection in chosen regions of structures. 5

6 Table 17.3 Corrosion rate control in pipelines. Electrochemical Physical Impedance spectroscopy Electrochemical noise Harmonic synthesis Polarization curves Polarization resistance Electrical resistance Radiography Ultrasonic Weight loss determination There are several developments in cathodic protection instrumentation. Use of thyristor controlled rectifiers will enable automatic control of current output depending on corrosive environment requirements. There is also a possibility of controlled potential cathodic protection to suit specific structures. For example, in sea-going vessels, the hull is subjected to variations in flow velocities leading to alteration in limiting current density (with respect to oxygen reduction). Such limiting current fluctuations significantly influence cathodic protection current requirements from time to time. In such environments, controlling the potential (rather than current) would be more beneficial. Controlled potential protection is extensively used for ship hulls incorporating anode reference electrode attachment along with automatically controlled power supply unit. 6