Surface-active amines and methods of using same to impede corrosion
Surface-active amines that impede corrosion on metal surfaces in industrial or other systems, corrosion resistant apparatuses comprising the amines, methods of identifying surface-active amines that impede corrosion by using electrochemical quartz microbalance (EQCM), and methods of using the surface-active amines in industrial or other systems are disclosed.
The present invention relates to surface-active amines that impede corrosion on metal surfaces, corrosion-resistant apparatuses comprising the surface-active amines, methods of identifying the surface-active amines using electrochemical quartz microbalance (EQCM) in combination with buffered solutions and methods of using the amines to impede corrosion of metals.
BACKGROUND OF THE INVENTIONContaminates such as dissolved oxygen and carbon dioxide promote corrosion of metal surfaces. For example, under laboratory conditions, iron has a tendency to corrode in water in the absence of oxygen because iron is less noble than hydrogen, however, the ferrous hydroxide formed by iron and water elevates the pH by providing hydroxide ions and ferrous ions. This reduces the amount of hydrogen ion which tends to retard corrosion. Therefore, under such laboratory conditions, the reactions that cause iron corrosion are self-limiting. In contrast, however, in industrial systems, where oxygen is present, ferrous hydroxide is unstable and ferric hydroxide is formed. Unlike ferrous hydroxide, ferric hydroxide is not a corrosion inhibitor.
Corrosion of metal surfaces can cause substantial damage to various components of a wide variety of products and systems. The damage is typically characterized by pitting and/or gouging of the metal surfaces and/or general wastage. Furthermore, in the case of steam generating systems, for example, oxides from corrosion of metals and thermally insoluble compounds from salt impurities introduced into the systems results in harmful and unwanted deposition of such products on performance critical components of the system, particularly within two phase regions of steam generating components. Such metals include steel (e.g. low carbon steel), iron, copper, nickel, chromium, aluminum and like alloying components, resulting in their respective oxides in various states and morphologies and further including binary and ternary mixed oxides, system design and thermodynamic conditions. Such impurities include, by way of example, calcium, magnesium, sodium, potassium, aluminum, lead, chloride, silica and sulfate and salts thereof.
The consequences of such corrosion may be quite dramatic. For example, in steam generating systems, failure of condensate return lines, feedwater piping, condensate receivers, pumps, heaters, and/or other equipment associated with steam generating and/or hot water heating systems may result from corrosion. Ultimately, the corrosion reduces the energy efficiency of the systems, as well as the water quality. Moreover, the damage caused by the corrosion increases the costs associated with operating and maintaining the systems while reducing the reliability and effectiveness of the systems. Additionally, various salts may be introduced as impurities into steam generating systems. For example, condenser cooling water inleakage may be the greatest or most common source. Impurities may be introduced in the form of chemical additives, ion exchange media, welding residuals, grinding operations, desiccants and related construction/maintenance functions. These introduced impurities include, for example, without intending to be limited thereto, calcium, magnesium, sodium, potassium, aluminum, chloride, silica, and sulfate, and insoluble salts, such as for example, without limitation, calcium sulfate and aluminum silicate. These impurities are commonly reported as solids in the surface analysis of steam cycle materials. Further, carbon dioxide that dissolves in water causes the pH to be depressed and results in the formation of carbonic acid. Carbonic acid promotes the iron corrosion reaction by supplying the reactant hydrogen ion.
Specific consequences of corrosion are particularly well illustrated in the case of pressurized water nuclear reactors. In a pressurized water nuclear reactor, fission of the nuclear fuel is used to heat water which is held in a pressurized loop so as to prevent the water from boiling. This pressurized loop is referred to as the primary coolant system. The heated pressurized water is passed through the inside of a number of, straight or U-shaped tubes in a steam generator, consistent with once-through or recirculating designs, respectively. Water in the secondary circulation system is passed over and around the outside diameter of the tubes containing the pressurized water. Heat is transferred to the water contained in the secondary circulation system. Because the water in the secondary circulation system is not held under similar high pressure, the transferred heat causes the formation of steam in the secondary circulation system. This steam is used to turn turbine-generator systems which produce electrical energy. Once heat has been transferred to the secondary circulation system, thereby cooling the pressurized water in the primary circulation system, the pressurized water is recirculated back to the reactor core where it is heated again. After turning the turbine, the steam created in the steam generator is cooled by a condensing system returning it to a single phase liquid state.
Corrosion products in the steam generator can lead to a variety of problems including reduction of steam production, loss of efficiency or increased heat-rate, and lower net electrical output. Furthermore, deposition and accumulation of corrosion products on steam generator components can promote locally high steam qualities or void fractions that permit extreme accumulation of other contaminants, such as sodium, chlorides, sulfates and other caustic or acid forming species, all of which can lead to tube cracking, pitting or degradation through intergranular and transgranular modes of attack. Tube degradation caused by corrosion, stress cracking and/or deposit formation occurs in a number of locations within the steam generator, including the junction of the tubes to the tubesheet, the junction of the tubes and support plates, at the top of tubesheets within sludge layers, and along the freespan tube surfaces within thick/and or dense tube scales.
Structural components of steam generators can be degraded by various modes of attack including abnormally high general corrosion and flow accelerated corrosion. Such components may include steam separator systems, structural plates, tube support plates and specifically the tube Ian areas limiting freedom of movement for the tubes. Degradation of the latter can give rise to enhanced degradation by flow induced fretting or wear of the thin-walled tube.
Steam carryover of steam formed oxides and compounds often result in deposit formation on high and/or low pressure steam turbines, giving rise to loss of efficiency and/or creating locally corrosive environments as steam expands across the turbine. Such carryover can be enhanced by high transport rates of metals and their oxides from corrosion in the feedwater to the steam generator, high mechanical carryover due to ineffective steam separators resulting from a fouled state, and from release of heavy deposits formed within the overall steam generator system.
In a nuclear reactor environment, the consequences of tube degradation range from loss of heating efficiency and increased operating costs to potentially untenable situations, such as leakage of radioactivity into the steam generated in the secondary circulation system. Indeed, maintaining the integrity of the pressure boundary between the primary and secondary circulation systems is one of the highest concerns in the nuclear power generation industry. Major tube degradations can result in substantial increases in maintenance and repair cost, extension of maintenance outages, unplanned outages, and overall reduction of capacity factors.
Steam formed deposits within the steam generator system have an adverse effect on the performance of periodic non destructive examination to verify tube integrity by methods such as eddy current, since the deposits can exhibit varying interferences due to magnetic susceptibility of the deposit compounds, deposit thickness, density, and deposit discontinuity on the outer tube surface. Therefore, such deposits can increase inspection requirements and give rise to ambiguity in the data interpretations. In contrast, inspections of clean tube surfaces improve inspection efficiency, enhance confidence in the data interpretation and minimize the probability of unidentified degradations.
Consequently, there is a significant need to minimize corrosion of metal surfaces, whether it be in an industrial setting or elsewhere. Specifically, there is a need for chemistries having benefits beyond conventional alkalizing treatments, such as chemistries exhibiting enhanced corrosion inhibiting properties, such as the surface active amines of the present invention.
In addition to the safety aspects of minimizing corrosion and deposit formation, there are significant cost benefits. For example, taking the case of steam generating systems as being merely illustrative, minimization or elimination of corrosion and deposit formation in the secondary circulation system can prevent unscheduled outages to clean, replace or repair steam generator components. Chemical cleaning operations of steam generators may cost up to ten million dollars and so, the ability to reduce the frequency of such cleanings can greatly decrease the maintenance cost of the steam generator. Flow accelerated corrosion within steam generators can also lead to large repair or replacement costs for components most susceptible to such corrosion, such as feedwater inlet baffles, steam separators, feedwater inlet headers and tube support plates. It is not uncommon for flow accelerated corrosion to result in expenditures of several million dollars in periodic replacement and repair of such components. Finally, accelerated degradation of steam generators and major components can require premature and untimely replacement decisions for the entire steam generator, which typically cost several hundred million dollars and require a lengthy plant outage.
Previously disclosed chemical treatments to reduce corrosion may generally be divided into two classes: (1) classic filming inhibitors, which form a protective barrier between the metal and the corrosive environment and (2) neutralizing amines, which increase the pH of the water and steam phases to reduce the corrosive (acidic) environment. Prior to the present invention, amine treatments fell into the second class of inhibitors, those used to raise the pH of the environment. For example, U.S. Pat. No. 4,192,844 discloses an amine formulation and methods for inhibiting condensate corrosion. The '844 disclosure describes the use of a combination of methoxypropylamine which functions as a neutralizing amine and hydrazine which reacts with and removes oxygen. G. Quandri, et al., “Use of Amines In Once Through Steam Generators”, EPRI, Workshop On Use Of Amines In Conditioning Steam/Water Circuits (September 1990), discloses the use of dimethylamine as a conventional alkalizing amine. Such previously disclosed techniques for limiting corrosion, however, have no known efficacy as classic corrosion inhibitors or in directly inhibiting deposition of corrosion products, from a mechanistic perspective. Indeed, conventional alkalizing amines often work in opposition to the minimization of deposit formation by enhancing consolidation of transported corrosion products.
General practice to minimize corrosion is to substantially increase the concentration of amine treatments to further increase the pH, however monovalent cations are removed from steam generating systems through the use of ion exchange systems. High concentrations of alkalizing amines tend to interfere with such removal by binding with the exchange media, in preference over the monovalent cations, causing the release of the cations, sodium, for example, back into the steam generating system. Therefore, optimization of corrosion control by alkalization has practical limits created by compromise of effective removal for corrosive species.
Another example of conventional amine limitations is found in the application of ethanolamine, ETA, due to degradation of the cation resin, particularly higher temperature condensate polisher, resulting in kinetic fouling of the anion resin within the mixed bed polisher system. The impact of the kinetic fouling includes high leakage of sulfates, another corrosive species, premature replacement of the resin system, and less than optimum control over corrosion and corrosion product transport as the amine concentration exacerbates the condensate polisher issues.
Other corrosion and deposit formation inhibitors, such as the amine formulations disclosed in U.S. Pat. Nos. 5,779,814 and 6,017,399 fall outside the classification system discussed above. That is, the methods of the '814 and '399 patents do not function by simple alkalization of the corrosive environment nor do they function by formation of a self assembled monolayer, as is common with classic filming inhibitors. The amines and methods of the '814 and '399 patents control and remove solid deposits from the surfaces of steam generating components by selective sorption of the amine by the solid deposits thereby displacing ions sorbed onto the deposits.
The functioning of the '814 and '399 methods rests upon the known fact that steam formed deposits typically exhibit acidic and basic centres which permit selective sorption of soluble ions, such as sodium and chloride ions. Whether the deposits exhibit acidic, basic or both types of centres depends upon the conditions and environment in which they are formed as well as the chemical composition of the deposit. Generally, the deposit reintrainment is enhanced to compete with deposition processes. (See public presentations listed in G. Quandri, et al. cited above)
The formulation and method of the '814 and '399 patents, however, are not known to exhibit classic corrosion inhibition properties within highly corrosive, strongly acidic, environments caused by concentration of corrosive anions such as sulfates or chlorides. The disclosure of the '814 and '339 patents for deposit control is not based on the intervention of the treatment at the site of corrosion release processes to preemptively preclude deposition, as disclosed in the present invention.
Several other approaches to controlling corrosion in system condensate systems have been previously attempted. For example, some methods have been devised to control acid induced corrosion in such systems. Under this approach, materials are added that are believed to adsorb to the metal surface to form a thin barrier between the metal and the acidic solution. Azoles, such as tolyltriazole, and long chain amines, such as octadecyl amine, have been used in this manner.
Further approaches involve the addition of amines to neutralize the carbonate and thereby increase the aqueous pH. Many different amines are utilized, but some commonly used materials include cyclohexylamine, morpholine, and methoxypropylamine. It has been believed that the high basicity of the amines allows attainment of a higher pH after acid neutralization, and low molecular weight allows greater molar concentration (and thus more neutralization).
Other methods for minimizing the effect of introduced impurities include exclusion and removal on ion exchange media. It is known by a person skilled in the art that steam cycles are treated with alkalizing amines directed at maintaining alkalinity of pH control and reducing agents to promote passivation and minimize corrosion from impurities inherent in the steam generating system such as, for example, iron. G. Quadri, et al., discloses the use of dimethylamine in utility fossil fueled power plant boilers in connection with the conventional manner of steam cycle alkalization, known by a person skilled in the art, to reduce corrosion of ferrous materials.
However, each of the previous approaches to impeding corrosion in steam generating systems is deficient. In particular, previous attempts directed to the use of amines have failed to identify compositions that are effective in industrial settings, and have failed to provide a means for identifying such compositions. Moreover, previous attempts have failed to identify ways to efficiently and cost-effectively impede corrosion in steam generating system.
For example, the use of alkalizing amines to minimize corrosion caused by impurities inherent to the steam generating system is not generally compatible with the objective to minimize the formation of solid deposits arising from impurities introduced into the steam generating system. Maintenance of high alkalinity with alkalizing amines such as ammonia, limits monovalent cation, such as sodium, removal on ion exchange systems such as condensate polishers. Salem, E., “Separation Technology Requirements for Operation in the Amine Cycle with Deep Bed Condensate Polishing”, EPRI, Workshop on the Use of Amines in Conditioning Steam/Water Circuits (September 1990). Thus, meeting the two objective requires a compromise that is economically undesirable because the ion exchange system service life is shortened.
Accordingly, there remains a significant unmet need for compositions and methods for effectively and efficiently impeding corrosion of metals, including metal surfaces and metal components (such as components comprising steel, e.g., low carbon steel), in a variety of settings including industrial settings, such as stream generating systems. For example, there is a need for compositions and methods for impeding corrosion of metal components during manufacturing, transport, storage and use. Further, there remains a significant unmet need for corrosion-resistant steam generating system apparatuses. Moreover, there also remains a significant unmet need for efficient and accurate methods of identifying compounds and compositions that are capable of impeding corrosion.
SUMMARY OF THE INVENTIONThe present invention provides surface-active amines that impede corrosion on metal surfaces, corrosion-resistant apparatuses comprising the surface-active amines, methods of identifying the surface-active amines using electrochemical quartz microbalance (EQCM) in combination with buffered solutions and methods of using the amines to impede corrosion in industrial and other settings.
An embodiment of the present invention provides an apparatus that resists corrosion comprising: a metal surface or component which is in the presence of a surface-active amine, said surface-active amine having a pKa of about 8 to about 14 at 25° C., and said surface-active amine having a molecular weight of less than about 1500.
A further embodiment of the present invention provides a composition comprising: at least one surface-active amine that impedes corrosion on the surface of a metal in a solution buffered at a predetermined pH having a pKa of about 8 to about 14 at 25° C. and a molecular weight of less than to about 1500; and at least one compound selected from the group consisting of dimethylamine (DMA), hydrozine, morpholine and combinations thereof.
An even further embodiment of the present invention provides a composition comprising: an effective amount of at least one surface-active amine having a pKa of about 8 to about 14 at 25° C. and a molecular weight of less than about 1500; a trace amount of oxygen in contact with said surface-active amine; and deionized water having an electrolyte content of less than about 500 ppm.
A still further embodiment of the present invention provides a method of identifying a composition effective at impeding corrosion of metal components comprising: measuring with an Electronic Quartz Crystal Microbalance (EQCM) the weightless of a metal deposit treated with a candidate amine in a solution buffered at a predetermined pH.
Another embodiment of the present invention provides a method of impeding corrosion comprising: adding at least one surface-active amine to at least one surface of a metal component or to a substance in contact with at least one surface of a metal component, said at least one surface-active amine having a pKa of about 8 to about 14 at 25° C. and a molecular weight of less than about 1500.
Yet another method of the present invention provides a method of impeding corrosion in a steam generating system comprising: adding at least one surface-active amine to feed water in the steam generating system, said at least one surface-active amine having a pKa of about 8 to about 14 at 25° C. and a molecular weight of less than about 1500.
Still another embodiment of the present invention provides a method of preparing a corrosion resistant apparatus for use in a system comprising: adding at least one surface-active amine to at least one surface of a metal component.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 28 to 30 are SEM micrographs showing steel samples exposed to plain steam.
FIGS. 31 to 33 are SEM micrographs of steel samples exposed to steam with DBU.
FIGS. 37 to 39 show the morphology for oxyhydroxides.
Contrary to previously disclosed methods, the formulation and methods of the present invention provide strong and direct, unexpected corrosion inhibition. For example, immediate inhibition of accelerated corrosion rates, in highly aggressive acid sulfate buffers at pH 2.6, has been demonstrated and is in clear contrast to the results for conventional amines in the same environmental conditions.
Direct evidence for inhibition of particle growth from the crystalloid or nanocrystalline state have been demonstrated, wherein normal coalescence and agglomeration steps to form larger deposit forming particles are retarded by the sorbed amine formulation. The sorbed amine fraction of the treatment formulation has been demonstrated, in contrast to conventional amines not demonstrating interaction with the metal oxides formed from solutions.
The efficacy of an advanced amine formulation was unexpectedly enhanced by the presence of dissolved oxygen, even in a most aggressive strong acid-oxygen saturated environment. This discovery is contrary to the teachings of the literature, wherein oxygen control has been strongly emphasized as essential to successful corrosion control practices. By contrast, little or no corrosion inhibition was shown, even for preferred amine formulations, in experiments where the dissolved oxygen was removed. Therefore, it will be understood by a person skilled in the art that the demonstrated corrosion inhibition of the present invention involves a strong interaction with the protective oxide, whose kinetics of formation and stability are dependent on the presence of dissolved oxygen. Accordingly, one embodiment of the present invention is to coordinate the corrosion inhibiting amine formulation with appropriate concentrations of dissolved oxygen to enhance efficacy but at less than the concentrations of oxygen sensitive materials.
For example, unlike smaller amine molecules, such as dimethylamine, it was not expected that DBU would adsorb into the micropores of deposits. Therefore, it was initially expected that DBU would exhibit less corrosion inhibition than smaller amines.
The present invention is directed to the unexpected discovery that surface-active amines, such as di- and tri-azabicyclo compounds, effectively impede corrosion of metal surfaces by adsorption strength rather than secondary pH effect. Specifically, the surface-active amine forms a chemical bond with the protective oxide on the surface of a metal, in contrast to the formation of an ionic bond between hydrophobic and hydrophilic moieties, for example. The invention is further directed to highly corrosion-resistant apparatuses that comprise the surface-active amines of the present invention. The surface-active amines are a class of amines having a low molecular weight, as identified according to methods of the present invention described below. It will be understood by a person skilled in the art that the methods of the present invention provide unexpected results based upon the understanding of sorptive effects not previously disclosed.
The term “sorption,” as used herein, is meant to include, without limitation, physical as well as chemical chemisorption effects and refers to the ability of a substance to hold or concentrate gases, ions, solids including particulates or dissolved substances upon its surface. In particular, as used herein, the term refers to the absorption of a surface-active amine to a protective oxide on the surface of a metal component. As used herein, the term “metal component” refers to any item or part of an item that is comprised partially or fully of one metal or any combination of metals.
The term “desorption,” as used herein, is meant to include the reverse of sorption and refers to the evolution or liberation of a material from a solution or a substance.
As used herein, the term “pKa” is the symbol for the logarithm of the reciprocal of the acid dissociation constant of an electrolyte. The reference temperature for pKa values is about 25° C. It will be understood by a person skilled in the art that pKa values decline with increasing temperature.
As used herein, the term, “passivation” refers to the formation of a film which lowers the corrosion rate of the metallic surface which is being treated.
“Passivation rate,” as used herein, refers to the time required to form a protective film on a metallic surface.
The term “surface-active”, as used herein, refers to the property of a molecule to form a chemical bond with a protective oxide on the surface of a metal component.
The phrase “effective amount of a surface-active amine”, as used herein, refers to an amount sufficient to cause the desired result as would be apparent to a person skilled in the art based upon the guidance provided herein.
The phrase, “in the presence of a surface-active amine” includes being in direct contact with the surface-active amine or being in contact with a substance (e.g., liquid, gas, mixture, etc.) that comprises or is in direct contact with a surface-active amine.
The surface-active amines of the present invention impede the corrosion of a metal component by chemisorption without pH effect. Unlike classic filming amines, the surface-active amines of the present invention form a chemical bond with the protective oxide on the metal surface. By way of theory, without intending to be limited thereto, corrosion is impeded directly as a result of this chemical bond between the protective oxide on the surface of the metal and the surface-active amines of the present invention, rather than through a pH effect as with the classic filming amines previously described. The examples which follow provide evidence that supports this mechanism of action. The following discussion describes in detail various embodiments of the apparatus, composition and method of the present invention.
The surface-active amines of the present invention include amines that form a chemical bond with the surface of a metal object. The means through which the bond between the amines of the present invention and the metal object is established is referred to herein as chemisorption. The surface-active amines of the present invention favor chemisorption over physisorption. This is in contrast to the classic filming amines that have been previously described by others.
By way of theory, without intending to be limited thereto, the surface-active amines interact almost exclusively with the oxide rather than the metal. When corrosion release occurs, a surface-active amine spontaneously interacts to (1) promote further oxidation and (2) establish corrosion protection through a sustained adsorbed layer.
The surface-active amines of the present invention include amines having a pKa of about 8 to about 14 at 25° C. Preferably, the amines of the present invention have a pKa of about 8 to about 13 at 25° C. More preferably, the amines of the present invention have a pKa of about 8 to about 12 at 25° C.
The surface-active amines of the present invention are characterized as small molecules (e.g., molecules having a low to intermediate molecular weight). The surface-active amines of the present invention may have a molecular weight no greater than about 1500. Preferably, the amines of the present invention have a molecular weight no greater than about 1000. More preferably, the amines of the present invention have a molecular weight no greater than about 500. Even more preferably, the amines of the present invention have a molecular weight no greater than about 250.
The surface-active amines of the present invention include amines characterized by the method of the invention for identifying amines that are effective at impeding corrosion. The amines of the present invention are amines that impede corrosion of a metal deposit in a buffered solution. The use of the buffered solution eliminates the impact of a pH change that would be caused by the highly basic amines. An Electronic Quartz Crystal Microbalance (EQCM) is utilized to measure weightloss of the metal deposit. In this manner, a class of amines is characterized by their ability to directly impede corrosion of a metal.
In accordance with an implementation of the present invention, the surface-active amine combines with a trace amount of oxygen to form an amine-oxygen complex. Preferably, the surface-active amines of the present invention are amines capable of promoting the oxidation of ferrous ion to ferric ion.
The surface-active amines of the present invention may be non-nucleophilic bases, non-nucleophilic organic bases, hindered and non-nucleophilic bases, non-nucleophilic amidines, amidine bases, amine based traditional bases, guanidines or sterically hindered amidine bases, in accordance with implementations of the present invention.
Non-limiting exemplary amines of the present invention include DBU (1,8-diazabicyclo[5.4.0]undec-7-ene); DBN (1,5-diazabicyclo[4.3.0]non-5-ene); DABCO, TED or DBO (1,4-diazabicyclo[2.2.2]octane); PMDBD (3,3,6,9,9-Pentamethyl-2,10-diazabicyclo[4.4.0]dec-1-ene), piperidine, N,N,N′,N′-Tetramethyl-1,8-napthalenediamine, TMG (1,1,3,3-tetramethyl-guanidine), 2-tert-butyl-1,1,3,3-tetramethyl-guanidine, N,N′,N″-tricyclohexylguanidine, TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene), MTBD (7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene), PBG (1,1,2,3,3-pentabutylguanidine), 1,3-diphenylguanidine, 1,2,3-triphenylguanidine and the like, including derivatives and/or substituted compounds thereof or any combination thereof. Any combination of any of these or other suitable surface-active amines is also contemplated by the present invention. Preferably, the surface-active amine is DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or a derivative or substituted compounds. Substituted compounds include compounds where one or more substitutent group of the specified molecule is substituted by another substitutent group while having the same reactivity as the specified molecule with regard to the present subject matter as would be known to persons of skill in the art without undue experimentation.
Specific amounts of a particular surface-active amine to use in the compositions and methods of the present invention may be routinely determined by a person skilled in the art based upon the guidance provided herein. For example, the concentration of DBU to use in the method of the present invention is preferably at least about 1 ppm, more preferably at least about 2 ppm, even more preferably at least about 10 ppm and still more preferably in the range of about 10 ppm to about 10,000 ppm. For instance, in a steam generating system, for example, without limitation, a selected concentration of DBU, e.g., 1 ppm, may be added directly to the feed water and that concentration in the feed water may be monitored and maintained as necessary. The present invention also contemplates distribution of the DBU or other surface-active amine to the steam phase as well as the liquid phase in a steam generating system, and particularly the secondary cycle of a steam generating system. The DBU or other surface-active amine may be added to the system in any suitable manner and at any suitable time and suitable place in the system as would readily be determined by a person skilled in the art without under experimentation, based upon the guidance provided herein.
The present invention also contemplates combining the surface-active amines of the present invention with other substances. For example, in addition to adding DBU or other surface-active amine to the feed water of a steam generating system, it is contemplated that other substances may be present in, added to and/or maintained in the feed water, such as substances that keep the level of oxygen in the system low, for example, without limitation. According to an embodiment of the present invention additional substances such as hydrozine, DMA (dimethylamine), mopholine or combinations thereof may also be added to the system in any suitable manner, such as by directly adding them to the feed water, for example without limitation.
The apparatus of the present invention demonstrates resistance to corrosion. According to an implementation, the apparatus of the present invention comprises a surface-active amine. In one embodiment, the present invention provides an apparatus that resists corrosion comprising: a metal component comprising at least one surface which is in the presence of an effective amount of at least one surface-active amine, said at least one surface-active amine having a pKa of about 8 to about 14 at 25° C. and a molecular weight of less than about 1500.
In an embodiment of the invention, at least one surface of the metal component is in contact with the at least one surface-active amine by a process comprising applying the at least one surface-active amine to the metal component prior to use or incorporation of said metal component into a system or apparatus for use. In this manner, the metal components of a system may be pre-treated with the surface-active amine to render the components corrosion resistant prior to use or even prior to installation or construction of the system, as well as application during use. Based upon the disclosure provided herein, a person skilled in the art would readily understand how to prepare metal components for systems, such as steam generating systems that comprise the surface-active amines of the present invention. According to an embodiment of the invention a metal component (which includes any component containing metal or having a metal surface, for example) may be treated for protection against, or resistance to, corrosion throughout the life of the component or at certain phases of its life. For example, a metal component (such as a low carbon steel component, for example) may be treated for protection during manufacture, storage, transport, use and the like or any combination thereof.
According to an embodiment of the present invention, the surface-active amines may be mixed, sprayed, injected or otherwise applied to the metal substance or composite or to a composition in contact with the metal substance or composite, e.g., feed water in a steam generating system, without limitation. The surface-active amine may also be coated onto the metal component at any time during or after the fabrication of the component and/or coated onto the mold for forming the component. Any techniques or combination of techniques that would be suitable for accomplishing the purpose of the invention are contemplated herein, as would be understood by a person skilled in the art.
Preferably, the metal surface or component of the present invention comprises steel, iron, nickel, copper, chromium, like alloying components or combinations thereof. More preferably, the metal surface or component is steel. Even more preferably, the metal surface or component is low carbon steel.
The present invention provides compositions for resisting and/or impeding corrosion of a metal component. According to an implementation, the present invention provides compositions comprising a surface-active amine-metal complex. In one embodiment, the composition comprises: at least one surface-active amine and at least one metal substance in contact with the at least one surface-active amine to form an amine-metal complex.
The metal substance of the compositions of the present invention may be any metal substance or metal-containing substance that is used in any system such as an industrial system, for example, a steam generating system, without limitation. According to an embodiment, the metal substance comprises steel, iron, nickel, copper, chromium, like alloying components or combinations thereof. Preferably, the metal substance comprises steel. More preferably, the metal substance comprises low carbon steel.
The present invention provides a method for identifying compositions effective at impeding corrosion of metal components. The method of the present invention employs an approach that excludes a pH effect of a candidate amine as an explanation for impeding corrosion so that the direct effect of the amine with regard to metal corrosion can be properly evaluated. In this manner, the method of the present invention identifies surface-active amines in accordance with the purposes of the invention.
According to an implementation of the present invention, a method of identifying a composition effective at impeding corrosion of metal components comprises measuring the weightloss of a metal deposit treated with a candidate amine in a buffered solution. In an embodiment of the invention, an Electronic Quartz Crystal Microbalance (EQCM) is used.
The quartz crystal microbalance (QCM), also referred to as the Electronic Quartz Crystal Microbalance (EQCM), is an ultra-sensitive weighing device. It consists of a piezoelectric quartz crystal, often in the form of a disk, which is sandwiched between a pair of evaporated electrodes. When these are connected to an electronic oscillator, the crystal can be made to oscillate in a very stable manner at its resonant frequency, f, due to the piezoelectric effect. If a thin, rigid film is deposited evenly over one or both of the electrode surfaces, the resonant frequency decreases proportionally to the mass of the film. By measuring the resonant frequency, masses well below 1 ng/cm2 can be gauged. Traditionally, the QCM has been used in gaseous or vacuum environments but has lately been employed as a mass sensor in liquids in electrochemical studies. In this application, mass gain or mass loss (corrosion) can be measured.
Through the Sauerbrey relationship, the estimated mass change is calculated via the corresponding frequency change:
Δf=−Δm×constant Sauerbrey Equation
Where: f=frequency and m=mass
At 6 MHz, the mass sensitivity reaches 2.5 ng per Hertz with a 0.2 cm2 active surface, which is equivalent to a fraction of absorbed oxygen onto the surface.
The properties of a quartz crystal microbalance (QCM) depend on the plane in which it is cut. The AT cut is normally employed which has an orientation of approximately 35° to the z-axis. It is known to have a particularly low temperature coefficient. In the manufacture of the sensor, a thin wafer about one tenth of a millimeter is cut from the quartz, polished and then electrodes deposited on the opposite faces of the quartz. The electrodes are deposited on the quartz wafer by evaporation or sputtering. In commercial devices aluminum is often used, however, for sensor application and higher tolerance devices gold and silver are preferred. The frequency of oscillation of the crystal is typically 10 to 30 MHz. For the following examples, 10 MHz crystals were employed which had typical dimensions of 10.2 mm by 10.2 mm and gold electrodes.
If a quartz crystal oscillator is coated with a material such as a gas chromatographic stationary phase the resonance frequency decreases at a rate quantified by the Sauerbrey equation provided the acoustic impedance of the coating material does not change and is similar to that of quartz:
DF=−2.3·106F02Dm/A
Where:
Dm is the mass of the crystal (g), A is the gas sensitive area (sm2), F is the related frequency change (Hz), and F0 is the initial frequency of the quartz crystal (MHz). The frequency decreases on application of a coating and subsequently decreases on exposure to a vapor. On complete desorption of the vapor the frequency returns to the coated frequency.
In one implementation, Quartz crystal microbalances (QCMs) are used as partition sensors that simply weigh the gas partitioning into the sensor coating. A wide range of coatings, often gas chromatographic stationary phases, are available whose sorbent properties are well characterized and stabilities known. Gases and vapors will partition into the sensing layer or coating in a reproducible fashion described by the partition coefficient K which is equal to the concentration of the vapor in the coating (CC) divided by the concentration of vapor in the gas phase (CG). K=CC/CG
Where:
K describes the liquid/liquid partitioning ratios.
A candidate amine is identified as a surface-active amine in accordance with an implementation of the present invention when the weightless measurement of the candidate amine-treated iron deposit was immersed in an Ar-purged pH 2.6 buffered solution is less than about 20 microns as measured by EQCM after exposure for about 6.5 hours. Preferably, the weightless measurement as determined by EQCM after about 6.5 hours is less than about 15 microns, more preferably, less than about 12 microns and even more preferably, less than about 8 microns, for a candidate amine-treated iron deposit immersed in an Ar-purged pH 2.6 buffered solution. The use of an Ar-purged pH 2.6 buffered solution is not intended to be limiting in terms of the methods for identifying suitable surface-active amines of the invention, but is intended merely to characterize suitable amines in accordance with an embodiment of the present invention.
The preparation of buffered solutions is well known to a person skilled in the art. As further explained in the examples below, measurement of weightless is preferably conducted in a pH buffered solution that is buffered at a pH appropriate to exclude pH effect as the explanation for corrosion inhibition or resistance. Preferably, a solution buffered at an acidic pH is used for these purposes. More preferably, the buffered solution for weightless measurements to identify suitable surface-active amines is buffered at a pH of 2.6.
A person skilled in the art could readily prepare a solution buffered at a predetermined pH in accordance with implementations of the present invention using conventional skills and techniques. For example, without intending to be limited thereto, a buffered solution of the present invention comprises borate, phosphate or combinations thereof.
A metal deposit suitable for the purposes of the present invention could be readily prepared or obtained by a person skilled in the art. The metal deposit may comprise any metal substance (e.g., any metal or material containing one or more metals), in accordance with implementations of the present invention, as would be understood by a person skilled in the art, without limitation. The metal substance is preferably one used in an industrial system, such as a steam generating system, without limitation. Preferably, the metal or metal substance comprises iron, nickel, copper, chromium, aluminum, metal alloy or the like or any combination thereof.
According to an implementation of the present invention, a method of impeding corrosion in an apparatus comprises: placing the at least one surface-active amine in a steam generating system. According to one embodiment, a surface-active amine, such as DBU, is added to the feed water in the secondary system of the steam generating system. According to an embodiment, the surface-active amine may be combined with other substances or added before, during or after adding other substances, such as substances to keep oxygen levels low, e.g., hydrozine. According to an embodiment, the surface-active amine may be added continuously, at predetermined times or intervals, or on an ad hoc basis. The surface-active amine may be added by an automated process or manually by a person. The surface-active amines of the present invention are described above.
The methods of the present invention contemplate the use of surface-active amine identified by a process comprising measuring with an Electronic Quartz Crystal Microbalance (EQCM) the ability of a candidate amine to impede corrosion on the surface of a metal in a buffered solution. According to an embodiment of the invention, the at least one surface of the metal component is in contact with the at least one surface-active amine by a process comprising adding the at least one surface-active amine to the metal component prior to placement (or replacement) of said metal component into a system or apparatus. The metal component may comprise any metal substance as described above. Persons of ordinary skill in the art are readily able to carry out the invention using conventional skills and techniques, based upon the disclosure provided herein.
In another implementation of the present invention, a method of preparing a corrosion resistant apparatus for use in steam generation comprises: admixing at least one surface-active amine with at least one metal compound. In this manner, a metal component with said at least one surface-active amine adhered thereto is formed. The metal component so formed provides corrosion resistance to an apparatus constructed therefrom. The metal component may comprise any metal substance as described above. A person skilled in the art would readily be able prepare a corrosion resistant apparatus, and the metal components thereof, in accordance with the present invention using conventional skills and techniques, based upon the disclosure provided herein.
The following examples are provided for purely illustrative purposes and the present invention is not intended to be limited thereto. All percentages are weight percent unless indicated otherwise. All units of measure are metric unless indicated otherwise.
EXAMPLES Example 1 Effects of Amines on the Corrosion of Iron in Sulfate Media: Weightloss MeasurementsCombined weightloss (using a quartz crystal microbalance) and pH measurements were performed to characterize the effectiveness of amines, such as diisopropylamine, DBU, MPA, piperidine, DMA, 5-amino-1-pentanol, morpholine, and N,N-dibutylamine, at impeding corrosion. Weightloss measurements were performed by immersing the amine-coated iron surface in 0.5 M sodium sulfate solution (pH=5.8), and comparing the results observed with those obtained in case of an unprotected iron surface. From the results achieved, the following four major conclusions follow:
1. The efficiency of an amine at impeding corrosion depends on its strength and also its molecular structure. Thus, the pKa values of different amines were correlated with their efficiency at impeding corrosion. A high pKa value (strong basicity) is a necessary but not alone sufficient condition for good anti-corrosion efficiency.
2. The pH measurements indicate that some amine desorbs initially, after the amine-coated surface is immersed in the sodium sulfate solution (pH increases from 5.8 to 11.0 in most cases upon immersion of the amino treated surface in Na2SO4). However, little or no change in pH is seen during the experiment when loss of weight is clearly detected. Thus, local variations in pH are not detected by the pH electrode.
3. Weightloss measurements are an accurate measure of the anti-corrosion efficiency of an amine, and the quartz crystal microbalance provides a viable approach for screening amines for effectiveness.
4. At least one amine (5-amino-1-pentanol) was observed to significantly reduce a thick Fe oxide scale.
pKa is a very useful measure of the acidity of a chemical species. The concept of pKa can briefly be summarized as follows:
The dissociation of an acid HA in a dilute aqueous solution can be written as:
Thus the dissociation constant Ka of an acid is its capacity to furnish protons in aqueous solution. The higher the Ka, or the lower the pKa, the more acidic the species is.
The pKa of an amine is the ability of the protonated form of the amine to dissociate in a dilute aqueous solution according to the following equation:
RNH3++H2O=RNH2+H3O+
where RNH3+ is the protonated amine, and RNH2 is the amine. Hence, the higher the pKa, the stronger the base.
Correlation of the pKa of an Amine and its Anti-Corrosion Effect:
Table I below correlates the pKa of an amine with the percentage weight loss observed (using the quartz crystal microbalance technique) when the amine-treated iron surface was immersed for 6.5 hours in 0.5 M Na2SO4 solution (pH=5.8). See pKa prediction of organic acids and bases, D. D. Perrin, B. Dempsey, E. P. Serjeant (1981), Chapman and Hall. Physical methods in Heterocyclic Chemistry Vol. 1, A. A. Katritzky (1963), Academic Press.
*blue flakes were observed when the iron deposit was immersed in the amine
†2% aqueous solution of the amine was prepared and used in the experiment
It is evident that effective corrosion inhibitors (percent weight loss reported over the duration of 6.5 hours in the corrosion inducing medium was <1%) like piperidine, diisopropylamine and dimethylamine have considerably high pKa values (>10.5) and are very strong bases.
Relatively weaker bases like morpholine (pKa=8.33, Group 2) are poor corrosion inhibitors compared to the amines listed in Group 1 in the table. 5-amino-l-pentanol (Group 2) was found to etch blue flakes when the Fe deposit was immersed in the amine solution.
However, N,N-dibutylamine (Group 3) is astonishingly an unfavorable choice for corrosion inhibition (percent weight loss detected was >9% and also the experiment failed after 5 hours), despite the fact that its pKa is very high (=11.25). This anomaly is probably due to the structure of N,N-dibutylamine.
The foregoing establishes that pKa is a determinant but not the only definitive determinant of the anti-corrosion capability of an amine. Amines with pKa values falling within a particular range may also effectively impede corrosion.
Weight Loss and Simultaneous pH Variation Measurements:
The fact that weightless is observed for some cases at high pH (
Referring to
The following conclusions are apparent based on the foregoing:
1. pKa values (and hence the basicity of the amine) can give us an idea of the efficiency of an amine as a corrosion inhibitor. These values are necessary but not conclusive.
2. Amine structure seems to play a vital role in its inhibition-efficiency.
3. diisopropylamine, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), MPA (3-methoxypropylamine), piperidine and DMA appear initially to be good corrosion inhibitors. (See
4. 5-amino-l-pentanol seems to etch Fe oxide. The blue flakes observed during immersion of the iron deposit in the amine solution will be analyzed to determine their composition.
5. pH variations in the case of an effective or ineffective corrosion inhibitor are similar, even though simultaneous weight loss measurements indicate that the two amines exhibit different protective effect, hence probes which can precisely sense local variations in pH are required to exactly know about the reactions occurring at the surface.
Example 2 Role of Dissolved Oxygen in the Inhibitor Protection of Iron Films in Sulfate_SolutionsThe following experimental results indicate that dissolved oxygen plays a cooperative role in the protective influence of corrosion inhibitors, by forming a passivating overlayer. The development of a hematite-inhibitor passivating overlayer hinders further corrosion in the medium to which the metal is exposed.
This conclusion was arrived at by examining the effect of DBU on the corrosion of iron in Ar-purged (no dissolved oxygen) pH 2.6 (acidic) sodium sulfate solution using the QCM technique, and comparing the results obtained with those recorded in “open” (oxygen containing) pH 2.6 and 5.9 solutions. The results in the former case displayed extensive weightless, and unlike the “open” solution, a dark blue colored oxide film characteristic of magnetite was detected. Thus the absence of dissolved oxygen (purged pH 2.6 solution) impedes the growth of a passivating overlayer and hence corrosion of iron is rapid, despite the use of an inhibitor.
Other amines—piperidine, diisopropylamine, dimethylamine and 3-methoxypropylamine (which were found to be excellent corrosion inhibitors in “open” pH 5.9 solution)—were also tested for effectiveness in the pH 2.6 (“open”) solution. The results demonstrated that piperidine is a good inhibitor even in the acidic solution, while the others are not. They also indicate that a pH of 2.6 is a more rigorous condition for testing the corrosion of iron in the presence of amine inhibitors. The reason for the difference in behavior among the amines is unclear. As all the amines selected for the screening examination at pH 2.6 are highly basic according to their reported pKa values, it is inferred that molecular structure may also govern the inhibition efficiency of an amine at a certain pH.
Experimental:
The apparatus for the experiments performed under purged conditions (as illustrated in
The following experiments were performed:
1. Immersion of DBU—coated deposit in the purged pH 2.6 solution.
2. Control experiment to test the corrosion of iron in the purged pH 2.6 solution.
3. Testing the inhibition efficiency of amines—piperidine, diisopropylamine, dimethylamine, 3-methoxypropylamine in the “open” pH 2.6 solution.
4. Blank to study the corrosion of Fe in the “open” pH 2.6 solution.
The results obtained are discussed below:
Results and Discussion:
The DBU-treated iron deposit was immersed in the Ar-purged pH 2.6 solution, and the weightloss and corresponding pH measurements recorded are plotted in
The results obtained in the above experiment, namely the rapid weightloss, is compared with those seen in previous experiments—DBU treated surface immersed in “open” pH 2.6 and 5.9 solutions in
Other amines—piperidine, diisopropylamine, dimethylamine and 3-methoxypropylamine were tested for inhibition efficiency in the “open” pH 2.6 solution, and the weightloss measurements recorded are shown in
The protective effect of piperidine in the pH 2.6 solution is analogous to that of DBU. A blue colored oxide (magnetite) which gradually changed to orange (hematite) was noted in the case of piperidine in the “open” pH 2.6 solution, as in the case of DBU.
Table II below shows a comparison of the effects of the various amines on the corrosion of iron in pH 2.6 (acidic) and 5.9 (mildly acidic) solutions. Diisopropylamine and dimethylamine which are effective inhibitors in the pH 5.9 solution exhibit significant weightloss in the acidic solution. In contrast, DBU and piperidine are good corrosion inhibitors in “open” 2.6 solution also. From the table, 3-methoxypropylamine does not seem to show considerable weightloss even in the pH 2.6 solution, and might be mistaken for a good inhibitor. However, it must be noted that the weightless of 11.19% was observed over a period of 60 minutes, after which the experiment was discontinued, due to the failure of the probe to respond to subsequent weight changes (
†2% aqueous solutions of the amines were prepared and used in the experiments.
Thus, diisopropylamine, dimethylamine and 3-methoxypropylamine are not effective inhibitors in acidic solution, compared to DBU or piperidine. The reason for the difference in behavior is not very clear, because all the amines tested in the acidic solution are highly basic (indicated by their high pKa values).
The molecular structures of DBU and piperidine might be responsible for their inhibitive effect even in the pH 2.6 solution.
Conclusions:
1. The results obtained in the experiments discussed above demonstrate the significance of dissolved oxygen in the preventive influence of amines like DBU. The absence of oxygen (purged conditions) obstructs the formation of an amine-oxide passivating overlayer, leading to enhanced corrosion. One of the observations made during the “open” experiments (with DBU) is the initial development of blue colored oxide (magnetite) at certain areas on the deposit, which is gradually transformed to an orange colored oxide (hematite). It must be noted that this is very different from observations made in the purged solution, where the blue colored magnetite was observed throughout the iron deposit, but hematite was not. This also proves that hematite-inhibitor interactions prevent corrosion.
2. The behavior of piperidine in the “open” pH 2.6 solution is similar to that of DBU. A dark blue colored oxide is initially observed, and the color gradually changes to orange (hematite). This demonstrates the feasibility of utilizing UV-visible absorption analyses to probe into the mechanistic details regarding oxide-amine/amine-surface interactions and the composition of the oxide formed.
3. Diisopropylamine, dimethylamine and 3-methoxypropylamine, which are excellent corrosion inhibitors in pH 5.9 solution show significant corrosion in the acidic solution. 3-methoxypropylamine causes the failure of the probe after 60 minutes of the experiment.
4. Apart from high basicity (pKa value), molecular structure of an amine may also be responsible in determining its efficiency as an inhibitor.
Example 3 EQCM Studies of Amine InhibitionThe following experiments were conducted to study the effect of amine/surface interactions under passivating and corrosive conditions, and to determine the relationship between the amine molecular structure and the effectiveness of its corrosion inhibition. Previous experiments using quartz crystal microbalance (QCM) have demonstrated that some amines such as DBU can stop iron corrosion in corrosive environments. However, one cannot differentiate the effect of increasing pH (upon addition of the amine) from that of the amine itself in such circumstances. Furthermore, we want to know the adsorption strength of various amines on the metal surface, relationship between adsorption and inhibition, and other factors that may affect inhibition of metal corrosion, such as presence of dissolved oxygen and chemical composition of metal surface.
In order to answer all these questions, suitable buffered test solutions were chosen: one in which the metal exhibits passivation (mildly alkaline pH), and another in which the metal corrodes (acidic pH). All the test solutions were buffered so that the effect of pH variation can be eliminated as much as possible. Thus, amine inhibition effectiveness is directly related to the strength of adsorption of the amine and its molecular structure, independent of pH effects. Here, borate buffer (pH 8.4) was used as passivating environment, whereas phosphate buffer (pH 5.7) was used as corrosive environment.
Various amines, including 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), dibutylamine (DBA), morpholine and aniline, were tested as potential inhibitors. Both adsorption and inhibition behaviors of these amines were investigated in aerated and aerated solutions in order to identify the effects of dissolved oxygen on inhibition/surface interaction. The results obtained show that DBU and DBA adsorb on iron surface in both aerated and deaerated solutions, but inhibition occurs only in the aerated solution. This demonstrates that dissolved oxygen plays a significant role in the inhibition of iron corrosion. Morpholine adsorbs a little on iron surface, and its inhibition effectiveness is not strong; aniline does not adsorb on iron surface, and has no inhibition effect. These results demonstrate that the strength of adsorption of an amine is directly related to its efficiency as an inhibitor, and the strength of adsorption varies with different amines.
Besides the study of iron corrosion inhibition, inhibition of nickel corrosion was also studied using aniline to observe amine adsorption and inhibition in relation to different metal substrates. The results showed that although aniline does not adsorb on iron surface and has no inhibitive effect, it does work on nickel surface. This can be explained by considering the ZPC (zero point charge) theory and pKa of aniline.
Experimental:
All chemicals (purchased from Aldrich Chemical Company) had a purity of 97% or higher, and were used without any further purification. The crystals (polished) and plating monitor of QCM were purchased from Maxtek, Inc.
Plating of Iron:
Plating of iron deposit (1 u thick) onto the polished gold crystal of the QCM electrode was accomplished from a 200 ml aqueous solution with the following composition:
Ammonium iron(II) sulfate hexahydrate: 16.0 g
10−5 M sulfuric acid: 2.0 ml
The conditions set for plating iron were: room temperature, current density=0.2 Amp dm−2.
Plating of Nickel:
Plating of black nickel deposit (1 u thick) onto the polished gold crystal of the QCM electrode was accomplished from a 200 ml aqueous solution with the following composition:
Nickel sulfate: 15.0 g
Nickel ammonium sulfate: 9.0 g
Zinc sulfate: 7.5 g
Sodium thiocyanate: 3.0 g
The conditions set for plating black nickel were: room temperature, current density=0.2 Amp dm−2.
The deposit was rinsed several times with deionized (Millipore, 18 mΩ cm) water to remove adsorbed ions, and then wiped with a clean towel before being subjected to corrosion and adsorption tests.
Adsorption/Corrosion/Inhibition Testing:
The buffer solutions used in these experiments were prepared according to the following procedures:
For adsorption studies, the deposit was immersed in a passivating buffered solution (borate, pH 8.4, prepared as cited above) for 40 rain. for equilibration, after which 15 mM amine was added to the solution. The variation in frequency upon addition of the amine is quantitatively proportional to the amine adsorbed on the surface of the metal (Sauerbrey equation).
For the corrosion inhibition studies, the deposit was immersed in a corrosive buffer solution (phosphate, pH 5.7) for 40 min. for equilibration, and then 15 mM amine was added to the solution.
Deaerated test solutions (no dissolved O2) are prepared by purging the test solutions with argon for 2 hours before the tests and also during the tests. The experiments performed are summarized in Table III.
Results & Discussion:
I. Adsorption & Inhibition on Iron Surface
1. In Aerated Test Solutions
The relationship of amine molecular structure with its interaction with a metal/metal-oxide surface can be understood by performing experiments in a corrosive buffer solution (pH 5.7). From
2. In Deaerated Solution
Adsorption and inhibition tests were also carried out in deaerated solution in order to find out the effect of dissolved oxygen.
II. Effect of Different Metal Surfaces
Aniline has no adsorption and no effect with respect to inhibition of iron corrosion, as discussed above. However, as shown in
These differences between the nickel surface and the iron surface may be qualitatively explained by considering the pKa of aniline and ZPC of the two metal surfaces. According to George A. Parks' ZPC theory, when the pH of a solution is greater than the ZPC of the metal, the metal surface is negatively charged. See George A. Parks, Chem. Rev., 65 (1965), 177. John Mcmurry, Organic Chemistry. (2nd Ed.), 937. G. A. DiBari, Metal Finishing—Guidebook and Directory, 1980, 48, 280. If the pH of the solution is smaller than the ZPC of the metal, the surface of the metal is positively charged. Assuming most oxide on the nickel surface is NiO, and that on the iron surface is Fe3O4, then ZPC of the nickel surface is 10.3, and that of the iron surface is 6.5. Because the pKa value of aniline is 6.15, we obtain the results shown in Table IV.
From Table IV, one can clearly note that in a pH 8.4 test solution, the iron surface is negatively charged, while the nickel surface is positively charged, and aniline exists mainly in its unprotonated form, i.e., as (1) Φ—NH2. Aniline cannot adsorb on the iron surface when pH is 8.4, because the iron surface is negatively charged and therefore repels the lone pair electrons on N in Φ—NH2. For a similar reason, aniline adsorbs on the nickel surface, because in pH 8.4, the nickel is positively charged, which attracts Φ—NH2.
In pH 5.7 test solution, the situation is a little complicated. Because the pKa of aniline is 6.15, which is close to 5.7 (considering the pH value will raise a little after addition of aniline, it is closer, in fact), aniline exists in both Φ—NH2 and Φ—NH3+ (anilinium ion) forms. And Φ—NH2 can be attracted by positive charged surface, whereas —NH3+ cannot. The reason why the —NH2 works on the nickel surface and not on the iron surface although both are positively charged in a pH 5.7 buffer test solution, is ZPC of nickel (10.3)>>ZPC of iron (6.5). This means, in this case, the nickel surface is much more positively charged than the iron surface. And considering the corrosion of nickel is negligible in pH 5.7 test solution (comparing with that of iron), it is reasonable aniline stops corrosion of nickel, but not that of iron, in this pH 5.7 corrosive test solution.
Summary & Conclusions:
1. Various amines show different effects on adsorption and inhibition of iron in buffered solutions. Increase of pH is not a factor in these effects, because solutions are buffered.
2. Effectiveness of amine inhibition is directly related to the adsorption strength of amine on the metal surface.
3. Dissolved Oxygen must be present to carry out the amine inhibition of iron corrosion.
4. Charges on metal surface (ZPC) affect amine adsorption and inhibition.
Example 4 Studies of Amine-Surface Interactions, Decomposition, and Corrosion Inhibition Anodic polarization scans were used to study the behavior of the amine/metal interface and to determine corrosion rate as determined by Tafel plot. A Tafel Plot is shown in
Referring to
Referring to
Referring to
The influence of DBU, morpholine, and DMA on oxidation kinetics, oxide formation/transformation in carbon steel coupons, when exposed to 250° F. steam was studied. This condition is relevant and of great interest to the power industry. Specifically in certain stage of the secondary cycle in steam powered plants when steam leaves the low pressure/temperature turbine and before entering the condenser where the working fluid has a temperature of about 250° F. and low quality. Hence, the results obtained in the following study would be relevant to that part of the secondary cycle. In this study, efforts were spent on kinetic studies, morphological analysis, and phase identification characterization using Fourier Transform Infrared Spectrophotometer (FTIR).
Kinetics Studies
Morphological Analysis Using Scanning Electron Microscopy:
In this research efforts were focused on surface characterization using SEM. The importance of surface topography originates from the fact that the dense and uniform oxide layers, that are chemically stable, provide an adequate protection of steel in a corrosive environment. A combined analysis of surface morphology and phase identification of the oxide formed on steel help corrosion engineers to assess the severity of corrosion and extent of resistance offered by the oxide layer. This research work utilized FTIR and SEM analysis to study interaction of mines with mild steel surfaces while forming an oxide layer during wet corrosion reactions taking place on mild steel surfaces inside an autoclave. SEM results obtained for AISI 1018 steel coupons exposed to plain steam followed by those for steam with DBU, morpholine and DMA are presented.
Morphological Analysis of Steel Samples Exposed to Plain Steam
Morphological Analysis of Steel Samples Exposed to Steam with DBU:
DBU presented more equiaxed and finer particles. It appears that these small particles slowed down metal dissolution when looking at weight loss results.
Similar observed morphologies, between
Morphological Analysis of Steel Samples Exposed to Steam with Morpholine:
An interesting observation is related to the presence of relatively dispersed grains on top of each flake. It appears that oxide formation takes place in sequential order. Once growth of a layer ends, formation of a new layer starts at the surface and by a process of nucleation and growth the oxidation proceeds to form multi layered structures seen in many cases in the literature (Nasrazadani S. et al. 1987, 1988). This case of thin platelets of hematite formed on layers of magnetite is clearly demonstrated in
Overall morpholine showed a tendency to form equiaxed grains initially and acicular grains at longer exposure times.
Morphological Analysis of Steel Samples Exposed to Plain Steam with DMA:
The morphology for oxyhydroxides is a characteristic long thin needle-like as is seen in most of
Small γ-FeOOH colonies are presented in
Thin sharp platelets are an extremely distinct character of goethite and it is clearly seen in almost all of the SEM micrographs related to steel samples exposed to steam with DMA. Based on morphological features of oxyhydroxides formed with this type of texture one can infer that presence of high porosity will facilitate transfer of harmful anions like chlorides to diffuse through the oxide reaching to the metal site, hence provoking higher oxidation rates. This fact does not eliminate DMA from consideration of being used in the power industry because all of the amines used in this research have been shown to have a pronounced effect in controlling pH of the circulating water in the secondary systems of power plants.
Summary of Morphological Features
The control set produced more crystalline grains especially at the longer exposure times (
Based on the observed images, it is noticed that control and DMA samples showed acicular structure, whereas DBU and morpholine showed more equiaxed grains. FTIR results indicated presence of magnetite in morpholine exposed samples (Nasrazadani S., and Raman A: 1991). However, those sets in some cases showed consolidation of acicular grains as well.
Higher oxidation rates were seen from consecutive exposure times. This can lead one to conclude that the inhibition effect of amines vanishes at longer exposure times. One should also note that these chemical are very volatile, and it is believed that after one loop circulation in the secondary cycle they are thermally broken down. This observation confirms what Feller B. et al (2001) stated about keeping a continuous feed along the line to maintain non-aggressive conditions.
Phase Formation and Transformation Analysis Using FTIR:
The main purpose of this analysis was to observe the relative delay in magnetite transformation to hematite. Delay in this phase transformation is favorable due to general belief that magnetite in crystalline form provides a passivating layer on steel surface that protects metal from accelerated corrosion. Extensive discussion is found on literature about magnetite's passivation characteristics. It forms a protective film that inhibits further metal dissolution (Nasrazadani S., 1997, Tsum T. et al, 1990, Novakovsky V., 1965). Another oxide that is highly close to magnetite is maghemite that in fact is a somewhat oxidized form of magnetite.
Differentiation of these two forms of iron oxide is not an easy task. These two phases usually form in a solid solution that is difficult to separate using x-ray diffraction technique that is commonly used by industry. Fortunately our research group has developed a method based on FTIR spectra of these two oxides so that one can examine the FTIR spectra and be able to get an idea of whether either one or a mixture of these two phases are present. Fingerprint FTIR spectra of different iron oxides and oxyhydroxides are unique, and this facilitates identification of these materials (Nasrazadani S., 1993).
Amorphous crystals will produce a variation in peak width, and a slight shift on IR values (Cornell R., and Schwertmann U., 1996). The main absorbance peak of magnetite is at 570 cm−1 whereas the characteristic peak of maghemite is 600 cm−1, and hematite shows a pair composed by a medium sharp peak at 470 cm−1 and a medium wide peak at 560 cm−1 (Nasrazadani S., and Raman A., 1993). Oxyhydroxides absorption hands are as follows: α-FeOOH is known to have a deep well defined pair of sharp peaks at 890 and 795 cm−1, γ-FeOOH presents an enunciated and deep peak at 1018 cm−1. β-FeOOH was not expected to be present in the experiment conditions.
FTIR Analysis of Steel Samples Exposed to Plain Steam:
FTIR Analysis of Steel Samples Exposed to Steam with DBU:
FTIR Analysis of Steel Samples Exposed to Steam with Morpholine:
FTIR Analysis of Steel Samples Exposed to Steam with DMA:
Although illustrated and described above with reference to specific embodiments, the invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. Thus, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims.
Example 5 Characteristics of DBU in Steam Generating SystemsThe influence of DBU, morpholine, and DMA on oxidation kinetics, oxide formation/transformation in carbon steel coupons, when exposed to 250° F. steam was studied. This condition is relevant and of great interest to the power industry. Specifically, in the stage of the secondary cycle in steam powered plants where steam leaves the low pressure/temperature turbine and before entering the condenser, the working fluid has a temperature of about 250° F. and low quality. Hence, the results obtained in the following study would be relevant to that part of the secondary cycle. In this study, efforts were spent on kinetic studies, morphological analysis, and phase identification characterization using Fourier Transform Infrared Spectrophotometer (FTIR).
Kinetics Studies
Morphological Analysis Using Scanning Electron Microscopy:
In this research efforts were focused on surface characterization using SEM. The importance of surface topography originates from the fact that the dense and uniform oxide layers, that are chemically stable, provide an adequate protection of steel in a corrosive environment. A combined analysis of surface morphology and phase identification of the oxide formed on steel help corrosion engineers to assess the severity of corrosion and extent of resistance offered by the oxide layer. This research work utilized FTIR and SEM analysis to study interaction of mines with mild steel surfaces while forming an oxide layer during wet corrosion reactions taking place on mild steel surfaces inside an autoclave. SEM results obtained for AISI 1018 steel coupons exposed to plain steam followed by those for steam with DBU, morpholine and DMA are presented.
Morphological Analysis of Steel Samples Exposed to Plain Steam
Morphological Analysis of Steel Samples Exposed to Steam with DBU:
DBU presented more equiaxed and finer particles. It appears that these small particles represented an oxide morphology that slowed down metal dissolution as indicated by the weight loss results.
Similar observed morphologies, between
Morphological Analysis of Steel Samples Exposed to Steam with Morpholine:
An interesting observation is related to the presence of relatively dispersed grains on top of each flake. It appears that oxide formation takes place in sequential order. Once growth of a layer ends, formation of a new layer starts at the surface and by a process of nucleation and growth the oxidation proceeds to form multi layered structures seen in many cases in the literature (Nasrazadani S. et al. 1987, 1988). This case of thin platelets of hematite formed on layers of magnetite is clearly demonstrated in
Overall morpholine showed a tendency to form equiaxed grains initially and acicular grains at longer exposure times.
Morphological Analysis of Steel Samples Exposed to Plain Steam with DMA:
The morphology for oxyhydroxides is a characteristic long thin needle-like as is seen in most of
Small γ-FeOOH colonies are presented in
Thin sharp platelets are an extremely distinct character of goethite and it is clearly seen in almost all of the SEM micrographs related to steel samples exposed to steam with DMA. Based on morphological features of oxyhydroxides formed with this type of texture one can infer that presence of high porosity will facilitate transfer of harmful anions like chlorides to diffuse through the oxide reaching to the metal site, hence provoking higher oxidation rates. This fact does not eliminate DMA from consideration of being used in the power industry because all of the amines used in this research have been shown to have a pronounced effect in controlling pH of the circulating water in the secondary systems of power plants.
Summary of Morphological Features
The control set produced more crystalline grains especially at the longer exposure times (
Based on the observed images, it is noticed that control and DMA samples showed acicular structure, whereas DBU and morpholine showed more equiaxed grains. FTIR results indicated presence of magnetite in morpholine exposed samples (Nasrazadani S., and Raman A: 1991). However, those sets in some cases showed consolidation of acicular grains as well.
Higher oxidation rates were seen from consecutive exposure times. This can lead one to conclude that the inhibition effect of amines diminishes with longer exposure times under the conditions of this test. A hypothesis for this observation could be due to the loss of the amine through volatility, thermal decomposition, and reaction with the metal oxide over time. This observation confirms what Feller B. et al (2001) stated about the need for a continuous feed of amine in the secondary system to maintain non-aggressive conditions due to loss of the amine.
Phase Formation and Transformation Analysis Using FTIR:
The main purpose of this analysis was to observe the relative delay in magnetite transformation to hematite. Delay in this phase transformation is favorable due to general belief that magnetite in crystalline form provides a passivating layer on steel surface that protects metal from accelerated corrosion. Extensive discussion is found on literature about magnetite's passivation characteristics. It forms a protective film that inhibits further metal dissolution (Nasrazadani S., 1997, Tsum T. et al, 1990, Novakovsky V., 1965). Another oxide with similar properties of magnetite is maghemite that in fact is a somewhat oxidized form of magnetite.
Differentiation of these two forms of iron oxide is not an easy task. These two phases usually form in a solid solution that is difficult to separate using x-ray diffraction technique, the most commonly used technique for phase identification. Fortunately, an alternative has been developed that is a method based on FTIR spectral analysis of the two oxides Such that these two oxides can be differentiated and identified. Fingerprint FTIR spectra of different iron oxides and oxyhydroxides are unique, and this facilitates identification of these materials (Nasrazadani S., 1993).
Amorphous crystals will produce a variation in peak width, and a slight shift on IR values (Cornell R., and Schwertmann U., 1996). The main absorbance peak of magnetite is at 570 cm−1 whereas the characteristic peak of maghemite is 600 cm−1, and hematite shows a pair composed by a medium sharp peak at 470 cm−1 and a medium wide peak at 560 cm−1 (Nasrazadani S., and Raman A., 1993). Oxyhydroxides absorption hands are as follows: α-FeOOH is known to have a deep well defined pair of sharp peaks at 890 and 795 cm−1, γ-FeOOH presents an enunciated and deep peak at 1018 cm−1. β-FeOOH was not expected to be present in the experiment conditions.
FTIR Analysis of Steel Samples Exposed to Plain Steam:
FTIR Analysis of Steel Samples Exposed to Steam with DBU:
FTIR Analysis of Steel Samples Exposed to Steam with Morpholine:
FTIR Analysis of Steel Samples Exposed to Steam with DMA:
Claims
1. An apparatus that resists corrosion comprising:
- a metal surface or component which is in the presence of a surface-active amine, said surface-active amine having a pKa of about 8 to about 14 at 25° C., and said surface-active amine having a molecular weight of less than about 1500.
2. The apparatus of claim 1, wherein the at least one surface-active amine is characterized by an ability to impede corrosion on the surface of a metal in a solution buffered at a predetermined pH.
3. The apparatus of claim 1, wherein the surface active amine is a di-azabicylo or tri-azabicyclo compound.
4. The apparatus of claim 1, wherein the at least one surface-active amine promotes conversion of ferrous ion to ferric ion.
5. The apparatus of claim 1, wherein the surface-active amine inhibits the corrosion of the metal component by chemisorption without pH effect.
6. The apparatus of claim 1, wherein the at least one surface-active amine is in the presence of a trace amount of oxygen.
7. The apparatus of claim 1, wherein the surface-active amine has a pKa of about 8 to about 14 at 25° C.
8. The apparatus of claim 1, wherein the surface-active amine has a molecular weight of less than about 500.
9. The apparatus of claim 1, wherein the surface-active amine has a molecular weight of less than about 250.
10. The apparatus of claim 1, wherein the at least one surface-active amine is DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).
11. The apparatus of claim 1, wherein the metal surface or component comprises steel, iron, nickel, copper, chromium, aluminum or combinations thereof.
12. The apparatus of claim 1, wherein the metal surface or component comprises steel.
13. The apparatus of claim 1, wherein the metal surface or component comprises low carbon steel.
14. A composition comprising:
- at least one surface-active amine that impedes corrosion on the surface of a metal in a solution buffered at a predetermined pH having a pKa of about 8 to about 14 at 25° C. and a molecular weight of less than to about 1500; and
- at least one compound selected from the group consisting of dimethylamine (DMA), hydrozine, morpholine and combinations thereof.
15. The composition of claim 14, wherein the surface active amine is a di-azabicylo or tri-azabicyclo compound.
16. The composition of claim 14, wherein the at least one surface-active amine promotes conversion of ferrous ion to ferric ion.
17. The composition of claim 14, wherein the surface-active amine impedes the corrosion of the metal component by chemisorption without pH effect.
18. The composition of claim 14, wherein the at least one surface-active amine combines with a trace amount of oxygen to form an amine-oxygen complex.
19. The composition of claim 14, wherein the surface-active amine has a pKa of about 8 to about 12 at 25° C.
20. The composition of claim 14, wherein the surface-active amine has a molecular weight of less than about 500.
21. The composition of claim 14, wherein the surface-active amine has a molecular weight of less than about 250.
22. The composition of claim 14, wherein the at least one surface-active amine is DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).
23. The composition of claim 14, wherein the composition additionally comprises a trace amount of oxygen.
24. The composition of claim 14, wherein the composition additionally comprises deionized water having an electrolyte content of less than about 500 ppm.
25. The composition of claim 14, wherein the metal surface comprises steel, iron, nickel, copper, chromium, aluminum or a combination thereof.
26. The composition of claim 14, wherein the metal surface comprises steel.
27. The composition of claim 14, wherein the metal surface comprises low carbon steel.
28. A composition comprising:
- an effective amount of at least one surface-active amine having a pKa of about 8 to about 14 at 25° C., and a molecular weight of less than about 1500;
- a trace amount of oxygen in contact with said surface-active amine; and
- deionized water having an electrolyte content of less than about 500 ppm.
29. The composition of claim 28, wherein the surface active amine is a di-azabicylo or tri-azabicyclo compound.
30. The composition of claim 28, wherein the at least one surface-active amine is DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).
31. A method of identifying a composition effective at impeding corrosion of metal components comprising:
- measuring with an Electronic Quartz Crystal Microbalance (EQCM) the weightloss of a metal deposit treated with a candidate amine in a solution buffered at a predetermined pH.
32. The method of claim 31, wherein the buffered solution is selected from the group consisting of borate, phosphate and combinations thereof.
33. The method of claim 31, wherein the metal deposit comprises steel, iron, nickel, copper, chromium, aluminum or a combination thereof.
34. A method of impeding corrosion comprising:
- adding at least one surface-active amine to at least one surface of a metal component or to a substance in contact with at least one surface of a metal component, said at least one surface-active amine having a pKa of about 8 to about 14 at 25° C., and a molecular weight of less than about 1500.
35. The method of claim 34, wherein the surface active amine is a di-azabicylo or tri-azabicyclo compound.
36. The method of claim 34, wherein the at least one surface-active amine promotes conversion of ferrous ion to ferric ion.
37. The method of claim 34, wherein the surface-active amine impedes the corrosion of the metal component by chemisorption without pH effect.
38. The method of claim 34, wherein the at least one surface-active amine combines with a trace amount of oxygen to form an amine-oxygen complex.
39. The method of claim 34, wherein the surface-active amine has a pKa of about 8 to about 12 at 25° C.
40. The method of claim 34, wherein the surface-active amine has a molecular weight of less than about 500.
41. The method of claim 34, wherein the surface-active amine has a molecular weight of less than about 250.
42. The method of claim 34, wherein the at least one surface-active amine is DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).
43. The method of claim 34, wherein at least one additional agent is added to the at least one surface of a metal component or the substance in contact with the at least one surface of a metal component.
44. The method of claim 43, wherein the at least one additional agent is dimethylamine (DMA), hydrozine, morpholine or a combination thereof.
45. The method of claim 43, wherein the surface-active amine is DBU (1,8-diazabicyclo[5.4.0]undec-7-ene); and wherein dimethylamine (DMA), hydrozine, morpholine or a combination thereof is added to the at least one surface of a metal component or the substance in contact with the at least one surface of a metal component.
46. The method of claim 34, wherein the metal component comprises steel, iron, nickel, copper, chromium, aluminum or a combination thereof.
47. The method of claim 34, wherein the metal component comprises steel.
48. The method of claim 34, wherein the metal component comprises low carbon steel.
49. A method of impeding corrosion in a steam generating system comprising:
- adding at least one surface-active amine to feed water in the steam generating system, said at least one surface-active amine having a pKa of about 8 to about 14 at 25° C., and a molecular weight of less than about 1500.
50. The method of claim 49, wherein the surface-active amine is DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).
51. The method of claim 50, additionally comprising adding dimethylamine (DMA), hydrozine, morpholine or a combination thereof to the feed water.
52. The method of claim 50, wherein the concentration of DBU in the feed water is at least 1 ppm.
53. The method of claim 50, wherein the concentration of DBU in the feed water is at least 2 ppm.
54. The method of claim 50, wherein the concentration of DBU in the feed water is about 10 ppm to 10,000 ppm.
55. The method of claim 50, wherein the DBU is distributed into the steam phase of the secondary cycle of the steam generating system.
56. A method of preparing a corrosion resistant apparatus for use in a system comprising:
- adding at least one surface-active amine to at least one surface of a metal component.
Type: Application
Filed: Feb 16, 2006
Publication Date: Aug 16, 2007
Inventor: Billy Fellers (Trent, TX)
Application Number: 11/355,199
International Classification: C23F 11/00 (20060101);
