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| Engineering > Power Magazine Back Issues > July/August 2001 | ||||||||||||||||||||
| POWER Magazine, July/August 2001 | ||||||||||||||||||||
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WATER TREATMENT Advanced amines cut condensate corrosion By Deborah M. Bloom, Ondeo Nalco Co. Virtually all steam generators use some type of neutralizing amine, or a blend of neutralizing and filming amines, to prevent corrosion in the condensate and feedwater systems. The selection of amines and dosage rates requires an understanding of three characteristics: molecular weight, basicity, and relative volatility | ||||||||||||||||||||
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Condensate-system corrosion is most commonly associated with CO2. The gas is not corrosive until it dissolves, when it forms carbonic acid in the condensate. Although CO2 is found in many waters in the free state, most of it is removed through pretreatment and deaeration prior to its use as boiler feedwater. The major source of CO2 in a steam cycle, therefore, is the thermal breakdown of bicarbonate and carbonate alkalinity present in the feedwater at boiler-water temperatures and pressures. In addition to this source, CO2 is ingested whenever condensate comes in contact with air--such as in the main condenser, receivers vented to atmosphere, condensate pumps, valves, and steam traps. Since condensate is extremely pure, even small quantities of carbonic acid or other acidic species can significantly lower condensate pH. As little as 1 ppm CO2 dissolved in condensate will result in a pH of 5.5 at typical condensate temperatures (Fig 1).
Keys: Temperature, pH Of the factors affecting condensate corrosion of mild steel and copper alloys--which include O2 content, the presence of ammonia, and water velocity--by far the most important factors are pH and temperature. A pH of 6.5 will be 100 times more aggressive to mild steel than a pH of 7.5, and 10,000 times more aggressive than a pH of 8.5. The ideal pH for minimizing corrosion of mild steel components is greater than 9.5. (Note: All pH values in this report are measured at a standard 25C, or 77F.) However, a lower pH often is selected as the control range because of the presence of copper alloys or to minimize chemical cost. Slightly lower pH values--down to approximately 8.5--may be acceptable, relative to equipment life expectations, but corrosion rates increase significantly at pH values below 7.0. The actual recommended control range for a specific system will depend on the amount of corrosion observed at the lower pH values (which is system specific), the percent of condensate reused, and the cost of the treatment chemicals. To minimize corrosion in systems where copper alloys are present, the pH typically is controlled between 8.8 and 9.2. Temperature. Corrosion of iron and copper surfaces also is highly dependent on temperature. Hot condensate is more aggressive than cool. Between the temperatures of 140 and 190 deg F, the corrosion rate increases approximately 2.5 times for mild steel. Corrosion product solubility also is dependent on temperature. Magnetite solubility, the concern for mild steel systems, peaks at approximately 300F (Fig 2). Release or solubility of copper oxides greatly depends on the specific alloy used, but generally plateaus over the range of 200 to 350F and increases again at 400F. Other factors The presence of O2, even in very low concentrations, is another factor in condensate corrosion. The rate of O2 attack on mild steel increases with temperature, approximately doubling as temperature increases from 140 to 190 deg F. Oxygen, if combined with CO2 in a mild steel condensate system, results in a corrosion rate that is 10-40% faster than the sum of either gas alone. Higher temperatures and the combination of CO2 and O2 are likewise more aggressive to copper, although the exact percent increase is not known. It is important to note that the corrosion of copper-based alloys will only take place in the presence of O2 or some other oxidizing agent. Ammonia is another factor in condensate corrosion, particularly for copper alloy components. In the presence of O2, ammonia is capable of forming highly soluble copper ammonium salts [Cu(NH3)2+], which are swept away with the condensate. The table shows a rough rule of thumb for minimizing copper corrosion by ammonia. Neutralizing amines--such as cyclohexylamine, methoxypropylamine, ethanolamine, morpholine, and diethylaminoethanol--also contribute to copper corrosion, particularly if fed in excessive amounts or if the amines thermally decompose into ammonia. Of the amines listed above, diethylaminoethanol is least thermally stable and suffers significant decomposition at temperatures above 850F. Sulfur, sulfides, and hydrogen sulfide (H2S, which may be formed from sulfite degradation, if sulfites are fed for O2 scavenging in systems operating at 900 psig or higher) are also corrosive to copper. Erosion. Copper materials are sensitive to erosion when they are exposed to water with high flow velocity, especially where the flow is disturbed so that turbulence occurs--at tube bends, collection headers, piping obstructions, etc. As a result of high water velocity, metal oxide corrosion products are removed from the surface as quickly as they form. There is no chance of forming a protective oxide layer and massive wastage can result. Conditions that otherwise are only mildly corrosive--such as slightly low pH or a few ppb dissolved O2--can aggravate the erosion mechanism. Mild steel also is susceptible to erosion/corrosion, though higher fluid velocities typically are necessary, compared to those required for copper erosion.
Neutralizing corrosion Neutralizing amines are the typical chemical treatments for
condensate systems. In the last 20 years, combinations of
neutralizing amines and O2 inhibitors also have gained
popularity for condensate treatment. Neutralizing amines function
simply by neutralizing the carbonic acid (or other acidic species),
thus raising Neutralizing amines should be fed to maintain a minimum pH of 8.5 in the condensate system. In systems containing all mild steel and no copper alloys, somewhat higher pH values will improve corrosion control. A pH range of 8.8 to 9.2 is recommended for copper alloy systems. Typical neutralizing amine programs are a blend of several amines to provide a combination of characteristics. Three critical characteristics must be considered in selecting the proper amine program for your specific powerplant: molecular weight, basicity, and relative volatility. Note: The basicity and volatility characteristics of amines are highly temperature-dependent.
Molecular weight. The molecular weight of an amine determines how many molecules of the amine will be present in 1 lb of chemical. On a pound-for-pound basis, lower molecular weight amines will neutralize more acid than a higher molecular weight amine of equivalent strength. The amines commonly used for boiler treatment range in molecular weight from 61 to 117 grams/mole. Basicity. The basicity of an amine describes its ability to neutralize acids and raise the condensate pH. This characteristic assures that the treatment provides effective corrosion protection. An amine with a larger basicity will produce more OH ions per mole than one with a smaller basicity and, as a result, will produce a higher system pH. In general, amines show a decrease in strength with increasing temperature; however, the magnitude of the decrease varies by amine. For instance, cyclohexylamine is the strongest amine at room temperature, but methoxypropylamine provides greater strength above 350F. This characteristic makes a comparison of amines based only on room temperature misleading (Fig 3). Volatility. Amines and CO2 will distribute throughout the steam, condensate, and feedwater systems according to their individual volatilities. Each volatile chemical or contaminant has a characteristic relative volatility or vapor-to-liquid distribution ratio (V/L ratio) that determines the amount that will be present in the vapor (steam) vs liquid (blowdown or condensate) at any point in the steam cycle. To neutralize carbonic acid or other volatile organic acids, the amine must be present in the condensate as the acid dissolves. Amines with V/L ratios less than one (log RV < 0) are desirable for protection of initial condensation points in turbines, heat exchangers, steam traps, and wet-steam piping. Amines with V/L ratios greater than one (log RV > 0) typically are used to treat secondary condensation points and flash steam (Fig 4). No 'gunk balls' A limitation of neutralizing amines is that they offer only indirect protection against O2 attack (by raising condensate pH to a range where corrosion byproducts are much less soluble). In contrast, filming amines form a non-wettable film on all metal surfaces in contact with the condensate. The film acts as a barrier between the metal surface and the corrosive condensate; thus filming programs protect against both CO2 and O2. As a result, many amine programs are comprised of a combination of neutralizing and filming amines. Recently, a new filming-amine technology has become available that can be used by itself, without the addition of neutralizing amines for condensate pH adjustment: the Nalco ACT (Advanced Condensate Treatment) program. Proper dosage for Nalco ACT and other filming amine programs depends on the size of the condensate system being treated, not on the amount of dissolved gases present in the condensate. System pH must remain below 8.0 for the Nalco ACT programs (or 9.0 for filming amines) to achieve desired results. At higher pH values, the film doesn't form or, in the case of filming amines, it is stripped off metal surfaces, causing sticky deposits sometimes referred to as "gunk balls." Filming amines are not able to effectively form a film below a pH of 6.5, while the Nalco ACT program performance remains effective at low pH. —Edited by David Daniels © Copyright Platts 2001 | ||||||||||||||||||||
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VP Business Development and Magazines, William C. Faust, bill_faust@mcgraw-hill.com Editor-in-Chief, Robert Swanekamp, swanekamp@aol.com Circulation Manager, Loretta Applegate, loretta_applegate@mcgraw-hill.com Reprint Manager, Doreen Bitowf, phone: 609-426-5129 | ||||||||||||||||||||
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