SUBJECT : METB113 ENGINEERING MATERIALS

PROJECT : TYPE OF CORROSION AND

CORROSION PREVENTION

TECHNIQUES USED IN POWER

GENERATING INDUSTRY

TITLE : POWER PLANT

NAME : PRAVIIN JAYAKUMAR

STUDENT ID : ME092616

Lecturer : MADAM SITI ZUBAIDAH BINTI OTHMAN

DEFINITION OF CORROSION

Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly what gases that are in contact with the metal, determine the form and rate of deterioration. Corrosion of metals in power plants is a commonly occurring phenomenon due to the continuous contact of the metal with a corroding environment. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

IMPORTANCE OF CORROSIONThe three main reasons for the importance of corrosion are: economics, safety,and conservation. To reduce the economic impact of corrosion, corrosion engineers,with the support of corrosion scientists, aim to reduce material losses, aswell as the accompanying economic losses, that result from the corrosion ofpiping, tanks, metal components of machines, ships, bridges, marine structures,and so on. Corrosion can compromise the safety of operating equipment bycausing failure (with catastrophic consequences) of, for example, pressure vessels,boilers, metallic containers for toxic chemicals, turbine blades and rotors, bridges,airplane components, and automotive steering mechanisms. Safety is a criticalconsideration in the design of equipment for nuclear power plants and for disposalof nuclear wastes. Loss of metal by corrosion is a waste not only of themetal, but also of the energy, the water, and the human effort that was used toproduce and fabricate the metal structures in the fi rst place. In addition, rebuildingcorroded equipment requires further investment of all these resources metal, energy, water, and human.

CORROSION IN POWER GENERATING INDUSTRY : POWER PLANTS.

Corrosion in Nuclear Power Plants

Nuclear plants are designed for decades of operation. One of the challenges in their maintenance is how to predict three types of phenomena related to corrosion:

i) Stress Corrosion Cracking Stress corrosion cracking is the growth of cracks in metallic materials, enhanced by both stress and corrosion. In this phenomenon, too, it is the properties of the oxidised layer that affect the pace at which the degradation proceeds. As a result of international collaboration and research carried out in Finland it has been possible

to develop increasingly accurate methods for predicting the progress of phenomena related both to corrosion and stress corrosion cracking.

ii) Activity Build-upDeposition of activated corrosion products onto the surfaces of the reactor cooling system. Metal pipes, tubes, valves and other components housed in a nuclear power plant can be dramatically affected by corrosion. Typically, the metal develops a protective coating that is under constant attack by chemical fluctuations in the nuclear power process. Over time, the protective coating is eroded. Localized pitting and structural weaknesses occur. If these are not detected, catastrophic failure may result. CorrTran MV corrosion transmitters monitor online and real time corrosion rates, and effectively measure localized pitting and corrosion, thus protecting the nuclear power plant.

iii) Transpassive CorrosionAnother corrosion-related problem is a mechanism known as transpassive corrosion, which affects in particular boiling-water reactors. In these reactors the coolant water at the reactor core is strongly oxidising. Transpassive corrosion involves the dissolution of chromium from the surface of stainless steel and nickel-base alloys. It is suspected that transpassive corrosion affects the initiation of irradiation assisted stress corrosion cracking.

Prevention Techniques of Corrosion in Nuclear Power Plants

Keeping Cracking in Check

A few years ago stress corrosion cracking of welded pipes made of stainless steel was a worldwide problem in boiling-water reactors. As a result of international collaboration this problem was detected at an early stage in Finland, and the cost of the required renovation work was relatively low. The greatest stress corrosion cracking challenge of today concerns parts that are affected by radiation in the reactor. Irradiation assisted stress corrosion cracking is a process that advances very slowly, taking years or even decades.

If indications of cracking are found during the annual refuelling outage, a decision has to be made whether maintenance work will be carried out right away or whether it can be postponed so that it can be included in the planned repair work

for the following year. The decision has to be based on sufficient knowledge of the rate at which the crack grows.

Factors determining the speed of cracking are one of the main subjects of stress corrosion research. This research is carried out in simulated power plant conditions and facilitates the making of decisions about the urgency of maintenance tasks at nuclear power plants.

Reducing Activity Build-up By Optimised Water Chemistry

Possible accumulation of activated corrosion products into the oxide films is a slow process that may take years or even decades. It is usually monitored by measurements carried out during the annual refuelling outage.

Monitoring techniques have been developed recently that can be used also while the plant is producing power. Some of these methods have been commercialised and delivered to power plants and process industries in several countries. At Finnish nuclear power plants these systems are being used in monitoring tasks that will last for several years. In laboratory studies the monitoring system is used in designing optimal water chemistry conditions for a plant. Similarly, the system is used to determine how changing water chemistry conditions affect a plant’s materials during the shutdown and startup processes.

Vital Protective LayerIn nuclear power plants the temperature of the cooling water reaches about 300oC and the pressure up to 120 bar. The pressure bearing components in contact with the cooling water are made of stainless steel or nickel base alloys. Oxygen in water reacts with the outermost layers of these metals, forming a thin oxide layer that slows down further corrosion. Minor alloying elements added to the steel may enhance the protectiveness of this layer. Corrosion products are released from the thin metal oxide layer by the flow of the cooling water. These particles become activated as they pass through the core of the reactor, and are deposited on the inner surfaces of the pipes. The resulting activity build-up tends to make the maintenance operations of nuclear plants more costly in the long run.

Corrosion in Hydropower Plants

A hydropower plant station uses the kinetic energy of falling water to produce electrical energy. Water is stored behind a massive wall of concrete which retains water ina a valley to a high level-called a dam. The water then released and flows downhill through turbines which drive

electrical generators. Gravitational potential energy is changed to kinetic energy, and then to electrical energy

Hydropower converts kinetic energy from falling water into elec-tricity. As an environmental benefit, the generation of hydropow-er produces no greenhouse gases or other waste pollutants, nor does it generate any waste product which requires special han-dling or disposal. However, the “falling of water” itself results inthe release of hydrogen sulfide, a highly odorous gas which can be corrosive to metals in high concentrations. Concentrations of hydrogen sulfide are so high at Paraiso, that employees working near the settling ponds are required to wear gas masks to prevent toxic gas poisoning. Also affected are the concrete walk-ways surrounding the settling ponds, which are deteriorating as a direct result of the corrosive gas. When it was decided that the plant would undergo a modernization, the potential threat hydrogen sulfide posed to new distributed control systems was not taken into consideration. Within three days of installation, all corrosion failed due to corrosion.

Prevention Techniques of Corrosion in Hydropower Plants

The demolition of the old deteriorated steel cooling water pipeline, fabrication of the new HDPE spool units and their installation had to take place in a way that was both timely and organized – another reason prefabrication of the spools was an essential part of the project. Also, the demolition and installation could not disrupt the power plant from its main function, therefore only one of the plant’s generating units could be down at one time.ISCO Industries served as the lead on the job, working in conjunction with a mechanical contractor and another contractor to remove the old steel cooling pipe and then to replace it with the new prefabricated HDPE pipe spool system.Demolition began on the existing pipe as soon as one of the power generating units was shut down, secured and locked/tagged out. The fabrication of the new replacement HDPE pipe took place at ISCO’s facility during the time the existing steel pipeline was demol-ished. The fabrication was timed in such a way that the spools arrived on site for installation as the old pipe was removed. The HDPE pipe was installed above ground so it was necessary to fully use the pipe’s bending radius to handle the unique situation. Unlike steel pipe, HDPE pipe is very flexible.Once a new HDPE pipeline was installed, the unit was watered down and then brought

back online. This happened for each of the three spool units, one at a time, to avoid shut-ting down more than one power generating unit during installation.All three spool units came online without any issues or leaks. The project was completed safely and ahead of schedule. Now, the power plant has a new, fully functioning HDPE pipe cooling system that will not rust or corrode.

Corrosion in Thermal Power Plants

Cold End CorrosionUsing fuels with sulfur in steam generating units yields a potential hazard of sulfur corrosion at the cold end of the boiler. The severity depends on many factors like percentage of sulfur in the fuel, excess air, moisture in flue gas, etc. Many options are available to contain cold end corrosion

Boilers generating steam for use in power generation and process power plants use different type of fuels. These fuels contain sulphur to differing percentages. The higher the percentage of sulphur, the higher will be the risk of cold end corrosion in the boiler. The sulphur in the fuel during combustion gets converted to sulphur dioxide. Depending upon the other impurities present in the fuel and excess air levels, some portion of the sulphur dioxide gets converted to sulphur trioxide. The presence of moisture in the flue gas due to moisture in fuel and air, sulphur dioxide, and trioxide, combines with moisture and forms sulphuric acid and

sulphuric acid. These acids condense from around 115 degree centigrade to slightly higher than 160 degrees, depending upon the concentration of SO3 and water-vapour.

Some of the other types of corrosion generally encountered in themal power plants are:

i) Water/gas/acid corrosionii)High temperature corrosioniii)Low temperature corrosioniv)Oil-ish corrosionv)Stress corrosionvi)Fatigue corrosion

Prevention technique of cold end corrosionThere are many methods used world over to contain cold end corrosion. These methods fall in the category of in-combustion reduction and post-combustion reduction.

The in-combustion reduction methods include:

Burning low sulphur fuel Low excess air burners Fuel additives Fluidized bed combustorsGoing in for low sulphur fuel sometimes become economically unviable for the process for which the steam generators are used. Today many low excess air designs are available in the market. These burners adopt many ways to reduced excess air requirement without affecting the unburnts in the flue gas after combustion. Fuel oil additives like simple magnesium oxides are used to contain cold end corrosion due to sulphur. The magnesium oxide is injected in to the furnace or mixed with fuel which combines with sulphur oxides to form magnesium sulphate. In fluidized bed combustors, lime addition is a simple method used to reduce sulphur corrosion.

MOST COMMON TYPES OF CORROSION OCCUR IN POWER PLANTS

Uniform Corrosion Uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surfacearea, or a large fraction of the area of the metal under attack. Uniform corrosion results in loss of material until failure. This is the most widespread form of corrosion that is observed.

Pitting Corrosion Pitting corrosion is a localized form of corrosion by which pits or "pin holes" are produced in considered, to be more dangerous than uniform corrosion damage because it is more difficult to predict and design against. Corrosion products often cover the pits making the detection often very difficult. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system.

Crevice Corrosion Crevice corrosion is a localized form of corrosion that occurs in the presence of stagnant solution in a small (micro) crevice. Local chemistry change areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps, can result in crevice corrosion.

Galvanic Corrosion Galvanic corrosion refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. Either (or both) metal in the couple may or may not corrode by itself (themselves) in seawater.

Microbiologically Induced Corrosion (MIC) Microbiologically Induced Corrosion or MIC refers to corrosion caused by biological organisms or microbes. These microbes are categorized by common characteristics such as their by-products (i.e., sludge producing) or compounds they effect (i.e. sulfur oxidizing). Theyall fall into one of two groups based upon their oxygen requirements; one being aerobic (requires oxygen) such as sulfur oxidizing bacteria, and the other being anaerobic, (requires little or no oxygen), such as sulfate reducing bacteria.

MOST COMMON CORROSION PREVENTION TECHNIQUES USED IN POWER GENERATING INDUSTRY

Electrochemical Characterisation methods Linear sweep voltammetry (LSV) Linear sweep voltammetry or LSV is one of the most commonly used methods for characterising corrosion phenomenon. It involves sweeping the potential of the working electrode and measuring the current response. With LSV one can obtain valuable information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in various environments.

Electrochemical impedance spectroscopy (EIS) In recent years Electrochemical Impedance Spectroscopy or EIS has been successfully applied to the study of corrosion systems. One of the advantages of EIS over DC techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured.

Electrochemical Noise (ECN) During localized corrosion electrochemical noise isgenerated by a combination of stochastic (random) processes, such as breakdown of passive films and repassivation. ECN involves the measurement of the current and/or potential noise and analysis of the data using Fast Fourier Transform (FFT).

CONCLUSION

Hot corrosion & erosion are serious problems in power generation equipment, in gas turbines for ships and aircrafts and in other energy conversion and chemical process systems and should be either totally prevented or detected at an early stage to avoid catastrophic failure. Application of a proper combination of preventive approaches should lead, in practice, to a significant decrease in the number of failures due to hot corrosion. Hot corrosion and Erosion preventive methods to the existing environment are (a) change of metal i.e. use of super alloy (b) use of inhibitors and (c) use of coatings.

The development of modern coal fired power generation systems with higher thermal efficiency requires the use of construction materials of higher strength and with improved resistance to the aggressive service atmospheres. These requirements can be fulfilled by protective coatings. At present, methods to minimize the extent of hot corrosion and erosion have been identified; however considerable research effort is needed for application and quantitative evaluation of these methods under consideration of interest in the coal-gasification processes.

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