Corrosion is defined in different ways, but the usual interpretation of the term is an attack on a metallic material by reaction with its environment. Corrosion of metallic materials can be divided into three main groups.
1. Wet corrosion, where the corrosive environment is water with dissolved species. The liquid is an electrolyte and the process is typically electrochemical. 2. Corrosion in other fluids such as fused salts and molten metals. 3. Dry corrosion, where the corrosive environment is a dry gas. Dry corrosion is also frequently called chemical corrosion and the best-known example is hightemperature corrosion.
In general, the development of modern society and industry has led to a strongerdemand for engineers with specialized knowledge in corrosion. There are a number of reasons for this: a) The application of new materials requires new corrosion knowledge. b) Industrial production has led to pollution, acidification and increased corrosivity of water and the atmosphere. c) Stronger materials, thinner cross-sections and more accurate calculation of dimensions make it relatively more expensive to add a corrosion allowance to the thickness. d) The widespread use of welding has increased the number of corrosion problems. e) The development of industrial sectors like nuclear power production and offshore oil and gas extraction has required stricter rules and control. f) Considering the future, it should be noticed that most methods for alternative energy production will involve corrosion problems.
Cathodic Protection The basic principle of cathodic protection (CP) is a simple one. Through the application of a cathodic current onto a protected structure, anodic dissolution is minimized. Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection. The CP current requirements tend to be excessive for uncoated systems. The first application of CP dates back to 1824, long before its theoretical foundation was established. This chapter deals mainly with CP related to buried pipelines, an important application field. Other common CP installations include buried tanks, marine structures such as offshore platforms, and reinforcing steel in concrete.
Measurement of instant-off potentials, by interrupting the CP current supply(schematic).
Sacrificial Anode CP Systems Cathodic protection can be applied by connecting sacrificial anodes to a structure. Basically, the principle is to create a galvanic cell, with the anode representing the less noble material that is consumed in the galvanic interaction. Ideally, the structure will be protected as a result of the galvanic current flow. In practical applications a number of anodes usually have to be attached to a structure to ensure overall protection levels. The following advantages are associated with sacrificial anode CP systems: No external power sources required. Ease of installation (and relatively low installation costs).
Principle of cathodic protection with sacrificial anodes (schematic).
Anodes for the protection of underground structures are buried at intervals along the structure. They are installed in an upright position, if possible, deep enough to be in permanent moist soil. For pipelines, the top of the anodes will be usually be approximately level with the bottom of the pipeline. For underground protection, the anode may be packaged in a cotton bag with backfill around the anode. Alternatively, the backfill may be placed as a slurry around the anode during burial. The backfill ensures a uniform consumption of the anode and promotes a higher current supply. Anodes used to provide protection in water should be distributed as evenly as possible over the surface of the structure. They are mounted on brackets welded or bolted to the structure, suspended on galvanized cables, or placed on sea bottom alongside the structure.
The spacing between anodes used to provide pipeline protection may vary from one anode every 3m to one anode every few miles depending on current required by the pipeline. The normal distance from the structure at which the anodes are placed is approx. 1-1.5m. The design of a galvanic ground bed is similar to impressed ground bed Thedriving voltage available to force current from anode to electrolyte is the open circuit potential less the polarized pipeline potential.
In impressed current systems cathodic protection is applied by means of an external power current source (Fig. 11.7). In contrast to the sacrificial anode systems, the anode consumption rate is usually much lower. Unless a consumable scrap anode is used, a negligible anode consumption rate is actually a key requirement for long system life. Impressed current systems typically are favored under high-current requirements and/or high-resistance electrolytes. The following advantages can be cited for impressed current systems: High current and power output range Ability to adjust ( tune ) the protection levels Large areas of protection Low number of anodes, even in high-resistivity environments May even protect poorly coated structures The limitations that have been identified for impressed current CP systems are
Application: 1. Current from out side source is impressed on the pipeline by using of a groundbed and a power source. Advantage: 1. It can be used in high soil resistivity with large current demand.. Limitations: 1. Need power source and more maintenance work. 2. Corrosion interference can be a problem. Anode materials: 1. Silicon iron 2. Mixed metal oxide 3. Platinized titanium etc.
When impressed current is used for protection of the buried structure, the anodebed is buried at some distance from the structure. The positive terminal of the power source is connected to the anodebed and the negative terminal connected to the structure. The resulting current is from anode through the soil to the structure.transformer/rectifier is usually used to supply the direct current. T/R: convert AC to DC and supply power to the CP system. Anodebed: Transfer the current to the environment: Reference cells and cables. The apparatus is usually a silicon rectifier. Its output voltage depends on electrical resistance of the cathodic protection circuit. According to requirements, it can work under constant voltage; constant current, potential controlled output.
Polarization: The deviation from the corrosion potential of an electrode resulting from the flow of current between the electrode and the electrolyte Polarized Potential: The potential across the structure/electrolyte interface that is the sum of the corrosion potential and the cathodic Polarization Reference Electrode: A reversible electrode with potential that may be considered constant under similar conditions of measurement. (Examples: saturated copper/copper sulfate, saturated calomel, and silver/silver chloride.) Three primary criteria for CP of underground or submerged steel or cast iron piping are listed in Section 6 of NACE RP-169-96: -850mV CSE with CP applied without IR drop. A polarized potential of -850mV CSE 100mV of polarization
CATHODIC PROTECTION OF STORAGE TANKS
Prepared by: Fatih ÖKSÜZOĞLU Ergil Gruop Chemical Engineer (Cathodic Protection Engineer)