By Adolfo Rosendo, Technical Service Engineer, Belzona Inc.

Corrosion can have financial consequences. In the United States corrosion causes yearly losses of $47.9 billion in the utility industry, $29.7 billion in the transportation industry, $22.6 billion in the infrastructure sector, $20.1 billion on government structures and $17.6 billion in the production and manufacturing industry. This totals $275.7 billion yearly because of corrosion.

Research on corrosion has shown two main mechanisms of corrosion: electrochemical and chemical oxidation. Both of these types have many forms, but the mechanism with which they degrade a substance can always be traced back to one of these two types.

Electrochemical Corrosion

Electrochemical corrosion is a process through which a metal returns to its lowest energy oxidation state. To properly understand electrochemical corrosion one has to understand how ores are found and turned into metal.

Raw ores are picked up from nature where some of them exist in a chemical compound with other elements in a thermodynamically stable state. Iron, for example, is found along with oxides, hydrates and sulfides amongst others. In order to extract or purify the iron, energy (heat) is forced into the ore to turn it into a metal. This causes the iron ore to reduce and become thermodynamically unstable. The fundamental laws of thermodynamics dictate that an unstable system eventually returns to its stable form, equilibrium.

Some metals, however, are found in nature in a metallic form already, such as gold, and therefore are already stable in metallic form (which is why gold doesn’t corrode). This is not to say that only those metals that are found in pure metallic form in nature are the only ones that won’t corrode; there are ores that when oxidized produce a metal that won’t corrode. The difference is that the amount of energy (heat) needed to oxidize the ore in this case is minimal.

Unstable metals become stable throughout the years by electrochemical corrosion if left unprotected. In order for electrochemical corrosion to take place a corrosion cell must be present. A corrosion cell is a combination of four elements, anode, cathode, electrolyte and a metallic pathway connecting the anode and cathode. Without any of these four elements corrosion will not take place, all elements must be present. Modern studies have shown that oxygen can greatly affect corrosion and so it’s considered to be an important factor.

The anode is where the actual metal loss takes place. By ionizing into the electrolyte the ions release electrons through an elaborate series of chemical reactions producing ferric oxide (Fe2O3) and oxygen. Ferric oxide is what we commonly refer to as rust.

• The cathode is an extremely important element of the corrosion cell as it determines the rate of corrosion. Electrons from the anode travel to the cathode and are accommodated there. The more electrons a cathode can accommodate the faster the anode will corrode. This is the primary relationship between the anode and the cathode in a corrosion cell.
• The electrolyte is also an important element of the corrosion cell as it is the solution that surrounds the anode and the cathode. The electrolyte also has an effect over corrosion rate since the conductivity of the electrolyte will either allow electrons to freely move from the anode to the cathode or restrict their flow, thus reducing corrosion rate.
• Metallic pathway is what some might refer to as the internal circuit as it is provided by the metal where the anode and cathode reside. The metallic pathway closes the corrosion circuit.

Oxygen is considered an important factor to corrosion since, without oxygen, corrosion slows down and eventually stops. Oxygen is responsible for reacting with the hydrogen ions that are released at the cathode due to the electron accommodation. In the lack of oxygen, hydrogen ions accumulate on the cathode and prevent electron accommodation, thus stopping the corrosion process. This phenomenon is called polarization.

Although electrochemical corrosion refers to a corrosion mechanism it can be manifested in many forms. Galvanic, pitting and crevice corrosion are the most common ones.

Galvanic Corrosion

Galvanic corrosion occurs when two different metals are submerged in a solution and joined together by a metallic pathway. An example is an iron ship with aluminum propellers. The electrolyte is the sea and the metallic pathway is the ship’s structure. The anode and the cathode are determined by looking into the galvanic series. Table 1 shows a simplified galvanic series.From Table 1 it can be seen that on an iron ship the aluminum propellers would be the most active metal (or the anode) and therefore corrode. Note that the farther apart the two metals are on the galvanic series the more accelerated the rate of corrosion will be.

On this specific case it’s also worth noting the size of the anode vs. the size of the cathode. The bigger the cathode is, the more electrons it can accommodate from the anode, causing it to ionize at a faster rate, thus corroding much faster. This relationship between the two is linear, the bigger the size difference between the cathode and the anode, the faster the anode will corrode. This only applies if the cathode is bigger than the anode. Should the anode be bigger than the cathode, the corrosion rate slows down. Therefore, the relationship is now inverse.

Pitting Corrosion

Pitting is a form of corrosion that often relates to the size relationship between anode and cathode explained above. The most common cause of pitting is inhomogeneity in metals. Sometimes metals are not consistent in their content; pot metals or other grains of easily corroded metals can be included in them. If there are inclusion of this nature in the metal, that are more anodic than the rest of the metal, these impurities corrode at a faster rate and cause pits. The inverse is also possible, the impurities can be cathodic to the rest of the metal, this will induce pits in the surrounding area of the impurity.

Inhomogeneity is not the only cause of pitting. Protective coatings can break and expose a portion of the substrate that it was meant to protect. This causes the rest of the coated metal to act as a cathode while the small discontinuity becomes the anode. As per the size relationship between anode and cathode, the rate of corrosion on this small anode will be high, thus causing pit.

Crevice Corrosion

This is a particular form of corrosion that happens in very small crevices on metals. Crevice corrosion is a very interesting form because at some point it involves one of the two main mechanisms, electrochemical and chemical. Crevice corrosion is often located in places where the electrolyte becomes stagnant. These places often include lap joints, under gaskets and under insulation. In these places crevice corrosion usually starts out as electrochemical corrosion with the ingression of an electrolyte into a crevice. Due to the small size of the crevice, oxygen that is needed to maintain electrochemical corrosion is soon depleted and the electrolyte becomes acidic. This happens by the hydrolyzation of the metal ions produced by the electrochemical corrosion. At this point the type or mechanism of corrosion is through chemical attack.

Chemical Corrosion

Chemical corrosion, unlike electrochemical, can occur in the lack of oxygen and does not need a complex cell to be in place. In chemical corrosion, the corrosion is caused by a substance, either an acid or an alkali. A general rule is that the more acidic a substance is, the more corrosive it will be and the more alkaline the less corrosive.

The level of acidity or alkanity of a substance is usually measured by it’s pH. pH is defined as “the negative decimal logarithm of the hydrogen ion activity in a solution.” According to this definition the higher the pH of a substance, the more alkaline, the lower the pH the more acidic it is. Table 2 displays the pH of common substances.

The difficulty of protecting against chemical attack is that acids can become vapor, travel to metals, react quickly with moisture and corrode the metal. Acids such as Nitric can not only evaporate and move through the air but it can also penetrate organic materials such as coatings when under immersion and attack the metal behind it. Alkalies can also become corrosive though the right environment. Sodium hypochlorite is a manufactured salt that is highly alkaline and extremely corrosive. Note that although an oxidizing salt might be alkaline, it can still be highly corrosive under the right circumstances. Some examples of chemical corrosion are concrete corrosion, heat corrosion and microbial corrosion.

Concrete Corrosion

Corrosion in concrete can happen two ways, through the reinforcement bars and through chemical attack. Corrosion of the concrete though the reinforcement bars is a form of electrochemical corrosion because of the moisture that makes its way into the bars through the concrete. Chemical, however, is much more complex form, it attacks by either leaching the calcium hydroxide or by penetrating it. Concrete is a highly alkaline building material that is essentially composed of a hydrated calcium silicate, a cement paste matrix. The calcium is the element that is often “leached” out of the concrete, weakening it.

When an acid such as sulfuric acid attacks a concrete the ensuing effect is the deterioration or complete dissolution of the cement paste matrix. This deterioration weakens the concrete and produces that “edged” or “exposed concrete” look that is often reported. Concrete can also be affected by carbon dioxide in the air through a process called carbonation. In this process the carbon dioxide reacts with the calcium hydroxide to form calcium carbonate. This process reduces the thickness of the concrete and also lowers the pH, allowing for electrochemical corrosion to be easily initiated should any reinforcement bars be present.

Microbial Corrosion

Microbial corrosion is caused by microorganism, bacteria, on not just metals but concrete, plastics and other materials. This form of chemical corrosion can happen when oxygen is present, through aerobic bacteria and in the lack of oxygen through anaerobic bacteria. The most common type of microbial corrosion is caused by Acidithiobacillus, where the bacteria acts as a sulfide-reducing agent that produces sulfuric acid, deteriorating the surface. Other bacteria, while in the presence of oxygen, can actually oxidize iron into iron oxide. Bacterial corrosion can also promote electrochemical corrosion by producing oxygen concentrations and causing pitting. Microbial corrosion is a subject that is currently undergoing a large amount of research. Bacteria that can grow and corrode on both salt and fresh water as well as bacteria that can utilize the hydrogen formed during electrochemical corrosion processes thus eliminating the need for oxygen

Heat Corrosion

Heat corrosion, also known as high temperature corrosion, is the degradation of a metal through the scaling of salts and other compounds from hot gases. As the title suggests, this form of chemical corrosion occurs on environments where temperatures are high, such as the hot gas path in a gas turbine. A common sequence of heat corrosion is the carburization — dusting — green rot sequence. This sequence starts when the high temperatures are in the presence of carbon compounds that cause the carbon content of the metal (usually a Chromium-Nickel alloy) to increase on the surface. The resulting effect is the hardening of the surface of the metal which leads to embrittlement, cracking and eventual failure. Further exposure to this environment with high carbon content leads to carbides forming in the metal structure and decomposing into graphite. This graphite acts as a catalyst for the decomposition of local carbon monoxide in carbon and oxygen. This second stage is known as metal dusting. The third stage happens only if the metal is, as listed above, a chromium-nickel alloy. This rather catastrophic form of corrosion is when a rapid cyclic between carburization and the discharge of oxides on the metal surface takes place. This usually results in a greenish residue of chromium oxide and a significant loss of metal. This last stage is known as green rot. All these stages can occur in a temperature range between 572 F and 1,922 F.

All these different forms of corrosion can always be traced back to one of the two main types of mechanisms, electrochemical and chemical. The forms discussed here are the ones most commonly seen in the industry. Research on corrosion is still ongoing and new things are discovered everyday. Corrosion is a wide and serious problem in today’s industry, causing loss of productivity on plants and sometimes tragedies. Not just metals corrode, but plastics, ceramics, glass, concrete and wide range of materials degrade because reactions with their environments. Corrosion is the ongoing life cycle of our equipment and materials, the ongoing life cycle of our industrial world.

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Hydroelectric Plant Benefits from Ultrasonic Clamp-on Measurement

By Jonas Norinder, Business Development Manager, Siemens AG Industry Sector

Pumped-storage hydroelectricity is a method of large-scale power generation that continues to gain in popularity due to its highly cost-effective nature, ability to lessen variation in the power grid and rapid response time to sudden load changes. However, as one pumped-storage plant discovered, the large amount of water required to create electricity in this manner means that major flooding can occur in the event of an accident such as a pipe breakage. After thorough consideration of all options, the plant concluded that clamp-on ultrasonic flow measurement was the most viable way to minimize the impact and cost of any future accidents.

The plant experienced the problem when a pipe broke after it was sand-dusted during maintenance. The pump location, including the turbines and control room, was completely flooded. As a consequence, the plant started to look for a cost-effective flow measurement solution to drive an emergency shut-down valve in case the pipe should break again, or in case the flow rate should exceed a certain value.

Plant Overview

The plant, which serves over 125,000 households and companies, provides electricity according to demand by cycling water between two reservoirs. The upper reservoir is located at an elevation of 1,289 feet (392.9 meters) and holds almost 2 million cubic yards (1.5 million cubic meters) of water. The lower reservoir and turbines are located at an elevation of about 305 feet (93 meters), resulting in a drop of 984 feet (299.9 meters). There are two pipes that are ¾ mile (1.2 kilometer) long, 11.3 feet (3.4 meters) in diameter and with a wall thickness of ½ inch (1.3 centimeters). These pipes are used to pump the water from the upper reservoir to the 4 turbines near the lower reservoir, and then back to the upper reservoir. Each pipe serves two turbines with a maximum flow rate of approximately 65 cubic yards (49.7 cubic meters) per second.

The turbines produce a total of 220 MW, and the total amount of energy that can be stored is 940 MWh. At night, when demand for electricity decreases, low-cost off-peak electric power is used to pump water uphill from the lower reservoir into the upper reservoir via the turbines. During the day, when there is higher demand, the stored water is released back into the lower reservoir to generate electricity.

Although the off-peak pumping process results in net electricity consumption, pumped-storage hydroelectricity nevertheless generates revenue by selling excess electricity when demand and prices are highest. Another benefit is that pumped-storage hydroelectricity allows baseload power plants, which provide electricity at a continuous and constant rate, to operate at maximum efficiency while reducing the need for more expensive peak power plants, which only run during periods of highest demand. Pumped-storage plants are also better able than thermal plants to respond to sudden shifts in demand, such as when the outside temperature changes suddenly or during major television broadcasts. This is because pumped-storage plants can be brought online much more quickly, often within 15 seconds, which reduces the potential for frequency and voltage instability within the power grid. All of these factors make pumped-storage hydroelectricity the most economical method of storing large quantities of electrical energy available today.

The requirements of the plant presented a significant challenge. The plant needed a system that would be able to measure high-velocity bidirectional flow while accounting for deposits on the inside of the pipe. The flow system would also need to provide an alarm in case of a broken pipe and trigger an emergency shutdown mechanism at the top of the pipe. In addition, the system would need to monitor and compare pump performance.

Weighing the Options

When considering the various metering technologies, the plant discovered that ultrasonic clamp-on flowmeters offer several advantages. These meters have sensors mounted externally to the existing pipe wall, eliminating the need to interrupt the flow and cut the pipe to install very large and expensive intrusive type meters. In addition, they are versatile enough to work in most configurations, and little or no maintenance is required due to the fact that there are no moving parts to wear out or break.

Clamp-on meters also offer a high level of accuracy through the use of bidirectional wide beam technology, in which the resonant frequency of the pipe wall is utilized to achieve a strong ultrasonic signal. Once installed, the sensors broadcast signals with varying frequencies to find the frequency that best matches the pipe wall. When found, the signal is transmitted into the flowing medium with the wall of the pipe acting as a waveguide. The result is a focused, coherent signal that increases precision by reducing sensitivity to any changes or anomalies within the medium.

A Non-intrusive Solution

Ultimately, the plant chose a non-intrusive solution based on the Sitrans FUS1010 ultrasonic clamp-on flowmeter from Siemens. During the trial and simulation phase, a modem was used for diagnostics. This modem allowed the user to record data or make adjustments via a computer from each of the company’s telephone connections. This feature allowed the power generating plant to analyze data and respond accordingly, especially during commissioning.

Two flowmeters with dual-path measuring and high-precision sensors were ultimately installed, one on each pipe. Because of the safety benefits provided by the new system, the customer is confident that, in the event of another accident, the cost savings would be in the hundreds of thousands.

While advantageous in many ways, pumped-storage hydroelectricity can become extremely costly in the event of an accident. Following a pipe breakage that led to serious flooding, a pumped-storage plant researched all options for safeguarding the facility and made the decision that a Sitrans FUS1010 ultrasonic clamp-on flowmeter from Siemens would best suit this important need. This solution provided the plant with more accurate monitoring of their pipes and a way to respond much more quickly in the event of another emergency.

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Enhanced In-Line Mill Technology Saves Utilities Millions

By Jon Norman, Sales and Technology Manager, Dry Sorbent Injection Division, United Conveyor Corp.

Coal-fired power plants are being driven by the EPA to assess various technologies and capabilities for enhanced SO₃ removal. Since 2005, United Conveyor Corporation-Dry Sorbent Injection has conducted numerous Dry Sorbent Injection (DSI) tests to demonstrate SO₃ removal performance versus trona particle size. The test described here confirms that low SO₃ stack emissions are achievable with dry sorbent injection technology.

A key discovery is that milling technology, specifically in-line pneumatic milling systems, provides the only way to achieve very low SO₃ stack emissions when injecting trona. Additionally, results show that a fine milled particle size can reduce sorbent usage by half.

Dry Sorbent Injection Test Overview

The DSI test compared SO₃ removal performance of as-delivered unmilled trona, in-line coarse milled trona and in-line fine milled trona. DSI is an economical and effective technology that injects selected sorbents into the flue gas to control SO₂, SO₃, mercury and other acid gases. When trona is the appropriate sorbent choice, in-line milling decreases trona usage by increasing surface area per ton injected to enhance reactivity and improve dispersion throughout the flue gas. The test had three primary objectives:

• Verify known removal performance of as-delivered unmilled trona and UCC fine milled trona.
• Determine the SO₃ removal performance of in-line coarse-milled trona.
• Use the SO₃ removal curves to compare the life cycle sorbent costs of each approach.

Implementation and System Configuration

Trona injection was performed with one positive pressure pneumatic conveying system splitting into eight injection lances located downstream of the air preheaters and upstream of the electrostatic precipitator (ESP), a common configuration for injection systems, as shown in Figure 1.

The United Conveyor Corp. (UCC) splitter system was designed to ensure ±10 percent sorbent distribution across each injection lance and was verified to achieve this performance during laboratory testing. The lance placement was determined using computational fluid dynamics (CFD) modeling.

The patent-pending UCC in-line mill system was used to mill the trona. The UCC in-line mill system operates with the positive pressure pneumatic conveying system and utilizes only the conveying air stream in the milling process. The mill size reduction is adjustable, and was reduced by design during some test runs, intentionally increasing the sorbent particle size to closely match competing mill performance and corresponding SO₃ removal rates.

The median trona particle sizes evaluated:

• As-delivered unmilled trona: 30-40 microns
• Coarse milled trona: 16-18 microns
• UCC fine milled trona: 12-14 microns 

Plant Operation

The UCC test was performed at a plant with a 700 MW unit with the following environmental controls:

• Selective catalytic reduction (SCR) system for nitrogen oxide (NOX) removal
• Cold side ESP for particulate removal
• Wet flue gas desulfurization (FGD) for SO₂ control

The baseline for the plant SO₃ concentration levels were:

• Boiler and SCR converting approximately 2 to 2.5 percent total SO₂ to SO₃
• ESP outlet SO₃ concentration in the range of 28 to 33ppm without DSI Injection 

Methodology and Testing Tools

SO₃ concentration was measured by both a dew point monitor and controlled condensate testing (EPA Method 8A). All testing samples were taken between the ESP and FGD. The dew point monitor provided continuous monitoring of the flue gas dew point which was converted to an approximate SO₃ concentration. SO₃ concentrations calculated indirectly from dew point compared conservatively to SO₃ concentrations determined by EPA Method 8A. SO₃ concentrations well below 5ppm were measured using interval and event sampling techniques.

At concentrations well below 5ppm, the measurement tolerance of current technology and test methods may not provide the accuracy necessary to consistently validate emission limits due to testing tolerance bands exceeding absolute measurements.

As-delivered unmilled, coarse milled and fine milled trona samples were taken passively by a pitot tube placed in the center of the conveying stream. This sampling method was validated by taking 100 percent material samples at multiple injection lance sites concurrent with the pitot tube samples. All samples were analyzed using a Beckman Coulter laser analyzer.

The test results, as seen in Figure 2, showed:

• Predictable SO₃ removal performance curves for each particle size measured/tested.
• Smaller particle size allows lower SO₃ removal while significantly lowering trona usage.
• Fine milled trona is projected to save $4.5 million and $2 million over five years compared with as-deliver unmilled trona and coarse milled trona respectively.

Table 1 shows a comparison of the annual trona costs and five year aggregated savings associated with achieving 5ppm SO₃ concentrations at the ESP outlet using as-delivered unmilled, coarse milled (competitive mills) and UCC fine milled trona. The annual trona cost comparison assumes a capacity factor of 90 percent for a mid- to large-sized power plant and a price of $180/ton for as-delivered unmilled trona.

UCC DSI technology delivers effective SO₃ control and is capable of achieving SO₃ emission limits well below 5ppm as a result of in-line milling technology producing sorbent particle size of 12 to 14 micron. This small particle size will reduce sorbent usage up to 50 percent.

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