MXPA97006812A - Inhibition of metros corrosion by carb dioxide - Google Patents

Abstract

The present invention relates to a method for the inhibition of carbon dioxide corrosion of ferrous metals in the presence of an aqueous salt environment containing dissolved carbon dioxide and having substantially pH, which comprises in addition to the saline environment an amount of polyaspartic acid anticorrosive having an average molecular weight in the range of about 1,000 to about 10.0

Description

INHIBITION OF THE CORROSION OF METALS BY CARBON DIOXIDE Field of the Invention This invention relates to the use of polyaspartic acid and salts thereof to inhibit the corrosion of ferrous metals by carbon dioxide in the presence of a different corrosive aqueous saline environment. BACKGROUND OF THE INVENTION Corrosion of metals and formation of mineral scale are common problems in a variety of industrial installations, especially in water treatment systems and oil fields. In corrosion, an electrochemical or chemical reaction between a material, usually a metal, and its environment produces a deterioration of the material and its properties. This corrosive attack can be uniform or located above the surface of the metal but generally results in the undesirable decrease in the useful life or usefulness of the metal surface. An example of chemical attack is the oxidation by air of hot steel, which forms a coating of iron oxide. In order to have electrochemical corrosion, it is necessary to have one (1) anode; (2) cathode; (3) electrolyte and (4) external connection. The presence of water is essential to reduce the processes of corrosion due to temperature. However, pure water that does not contain dissolved substances is only very slightly corrosive to iron. Water containing impurities or dissolved substances can be corrosive or non-corrosive, depending on the nature of the dissolved substances. Chromates and phosphates are dissolved in the water to inhibit or reduce corrosion. Other substances such as salts, acids, hydrogen sulfide, carbon dioxide, and oxygen can increase the corrosivity of water. Generally, water found in oilfield operations, in particular, contains one or more of these substances, which increase their corrosivity. The dissolved carbon dioxide also influences the solubility of magnesium and calcium carbonates. These salts sometimes precipitate on the surface of a metal tube and form a protective coating. However, water containing "aggressive" carbon dioxide (ie, excess carbon dioxide dissolved in water) will not deposit this protective coating. The salts dissolved in the water can act as regulators, thus preventing the pH from reaching a low enough value to produce a serious corrosion. In addition to impurities, which are commonly found in water, temperature and velocity also influence the corrosivity of water. Rarely is a corrosion problem where only one of these contributory factors is present. Consequently the problem is complex due to these various influences and the way in which they can interact with each other. In this way, the material continues to need new and improved methods of inhibiting the corrosion of metals in various aqueous environments using environmentally acceptable chemicals. In certain industries, the economy often determines which metal construction materials are selected for the equipment associated with the industry. The oil and gas production fields of Alasa and North Sea are typical commercial examples. For example, mild steel is generally the metal of choice for equipment and long pipes. The waters of oil fields, such as saline waters and reservoir waters, present in mild steel tubes, provide a corrosive environment, which can cause electrochemical corrosion that occurs in the solid-liquid interface. In this corrosive environment, carbon dioxide is dissolved in an aqueous solution of brackish to saline with associated hydrocarbons from oil or gas production but this will generally not contain dissolved oxygen. As a result, chemical corrosion seldom occurs but electrochemical corrosion occurs in solid-liquid interfaces on almost every occasion where oil from oil fields makes contact with steel equipment. The need for specialized corrosion inhibition, is known by people in the field of internal corrosion control of mild steel surfaces associated with the production of oil and gas and their transportation. Currently, the protection of metal surfaces against corrosive deterioration is achieved through the use of multi-component corrosion inhibiting systems, which are nitrogen and aromatic compounds, such as amine and compositions containing organic sulfur. Accordingly this combination of corrosion inhibitors, raise environmental concerns due to their persistence or to the impact of biota and their hazardous nature on the surrounding environment and public health. Heavy metals, chromates, phosphates, silicates and materials that form a persistent layer are typical inhibitors to reduce the corrosion of iron and steel in aqueous solutions. All these inhibitors have a negative environmental impact, such as toxicity, eutrophication, environmental persistence. In addition, the removal of these materials from the environment requires expensive and complicated processes. Accordingly, there is a need and desire for environmentally friendly (biodegradable) chemicals, which provide equal or better inhibition of carbon dioxide corrosion in different corrosive aqueous saline environments than currently available inhibitors. The search for environmentally acceptable carbon dioxide corrosion inhibitors for metal surfaces in contact with aqueous saline environments is well known to those skilled in the art of aqueous corrosion inhibition. Polyaspartic acid and its salts have previously been shown to inhibit scale formation and possess dispersancy properties for calcium carbonate and phosphate in U.S. Patent No. 5,152,902 to Koskan and Cois, and of calcium sulfate and barium sulfate in U.S. Patent No. 5,116,513 to Koskan and Cois. These characteristics make polyaspartic acid and its salts desirably compatible with the deposition control chemicals used in the oil and gas production industries. It has been found generally amino acids and notably aspartic acid to have a small tendency to inhibit corrosion effective for commercial use. In addition, aspartic acid is known to be inherently corrosive under slightly alkaline pH conditions, as effectively reported, accelerating corrosion to a pH of about 8. Thus, amino acids, such as aspartic acid, despite having properties desirable non-toxic biodegradable, have generally been avoided as corrosion inhibitors. Research has reported that the thermally produced polyaspartate, a synthetic polypeptide consisting of about 20 aspartic acid residues (of apparent molecular weight from about 2000 to about 5000) was a mild inhibitor of corrosion of mild steel samples exposed to water of synthetic seawater at a pH of 8 under static conditions of use. However, the maximum inhibition reportedly achieved was less than 30%. See, Little and Cois; "Inhibition of Poliaspartate Corrosion", Peptides and Surface Reagent Polymers: Discovery and Marketing. Sikes and heeler (Eds), Symposium ACS series No. 444 (1990); and Mueller and Cois; "Polypeptide Inhibitors of Steel Corrosion in Seawater", Document 274 presented at the Annual Conference of NACE and Corrosion Exhibition (1991). U.S. Patent No. 4,971,724 to Kalota and Cois; teaches that aspartic acid and polyaspartic acid demonstrate corrosion inhibiting properties on mild steel samples in deionized, carbon dioxide-free, aerated water, under conditions of static use that provide total ionization above a pH of 8.9. However, crater-like corrosion remained a concern up to a pH of 10. Surprisingly, polyaspartic acid has not been found useful as an inhibitor of carbon dioxide corrosion of ferrous metals in an aqueous saline environment it is substantially free of dissolved oxygen. Summary of the Invention Polyaspartic acid has been found effective in inhibiting the corrosion by carbon dioxide of ferrous metals in an aqueous saline environment having a substantially acidic pH. The term "polyaspartic acid" as used herein includes the salts of polyaspartic acid. In particular, polyaspartic acid was found to effectively inhibit the corrosion of mild steel by carbon dioxide in contact with saline waters, which are substantially free of dissolved oxygen and have a pH in a range of about 4 to about pH below 7. Surprisingly, this inhibition of carbon dioxide corrosion can be practiced with a relatively low amount of polyaspartic acid of about 10 parts per million (ppm) based on the volume of the aqueous saline environment contacting the surface of the ferrous metal, under conditions of dynamic flow use. from mild to moderate. Polyaspartic acid prepared by any method can be used. A preferred polyaspartic acid has an average molecular weight (Mw) in the range of about 1,000 and about 10,000. Particularly preferred is β-polyaspartic acid (ie, one having β-form greater than 50% and less than 50% α-form) prepared as described in U.S. Patent No. 5,284,512 to Koskan and Cois. The beneficial benefits of using polyaspartic acid as a corrosion inhibitor on commercially available current inhibitors are their environmentally friendly and biodegradable nature. In the oil and gas production industries, in particular, the beneficial compatibility of polyaspartic acid with its chemical deposit control requirements, further increases its commercial importance and value in these applications. Other advantages and additional features, and the like will be apparent to those skilled in the art from the present specification and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 graphically compares the effectiveness of the inhibition of carbon dioxide corrosion by polyaspartic acid which varies in average molecular weight from about 1000 to about 5000 relative to that of the commercial reference inhibitor plotted as a function. of time under simulated conditions of use of soft dynamic flow; Figure 2 graphically compares the effectiveness of the inhibition of carbon dioxide corrosion by polyaspartic acid varying in average molecular weight from about 1000 to about 5000 plotted as a function of pH and time under simulated conditions of soft dynamic flow use; Figure 3 graphically compares the effectiveness of carbon dioxide corrosion inhibition by polyaspartic acid varying in average molecular weight from about 1000 to about 5000 relative to that of the commercial reference inhibitor plotted as a function of time under simulated conditions of use of moderate shear dynamic flow; Figures 4 and 5 graphically compare the effectiveness of the inhibition of carbon dioxide corrosion by polyaspartic acid ranging in average molecular weight from about 1000 to about 5000 in relation to the combination of type inhibitors and commercial inhibitors plotted as a function of time under simulated conditions of use of soft dynamic flow; Figure 6 graphically compares the effectiveness of carbon dioxide corrosion inhibition by polyaspartic acid varying in average molecular weight from about 1000 to about 2000 relative to that of (polyHIS) plotted as a function of pH and low time simulated conditions of use of soft dynamic flow; Figure 7 graphically depicts the effectiveness of the inhibition of carbon dioxide corrosion by polyaspartic acid having an average molecular weight of about 1000 in saline waters varying in calcium ion content to a pH of about 5.6 under simulated conditions of use of soft dynamic flow; Figure 8 graphically depicts the effectiveness of the inhibition of carbon dioxide corrosion by polyaspartic acid having an average molecular weight of about 1000 in saline waters varying in calcium ion content to a pH of about 4.5 under simulated conditions of use of soft dynamic flow, - Figure 9 graphically represents the effectiveness of the inhibition of carbon dioxide corrosion by calcium aspartate in calcium-free saline waters and saline waters containing 2800 ppm calcium ion traced as a function of time under simulated conditions of use of soft dynamic flow; Figure 10 graphically depicts the effectiveness of the inhibition of carbon dioxide corrosion by polyaspartic acid having an average molecular weight of about 1000 in saline water containing from about 1 ppm to about 1000 ppm of ferrous ion traced as a function of time under simulated conditions of use of soft dynamic flow; and Figure 11 graphically represents the effectiveness of the inhibition of carbon dioxide corrosion by polyaspartic acid having an average molecular weight of about 4400 relative to that of the commercial inhibitor plotted as a function of time under simulated conditions of moderate dynamic flow. . Description of the Invention The term "aqueous saline environment" is used herein for convenience to include saline to saline water, sea water and aqueous solutions, which contain sufficient dissolved carbon dioxide salts and electrolytes to corrode metal surfaces in contact with these, and in particular ferrous metal. The term "ferrous metals" is used for convenience to include iron, steel and similar iron metals, which are susceptible to corrosion by the oxidation of iron to ferrous ion. Briefly described, in the practice of this invention, polyaspartic acid is added to an aqueous salt environment, which is normally corrosive to mild steel, which is susceptible to dioxide corrosion. of carbon. Polyaspartic acid is a copolymer that contains two forms of aspartic acid L. The alpha form (a-) is acetoacetamide. The (ß-) form is 3-carboxypropionamide. Analysis of polyaspartic acids by Nuclear Magnetic Medium (NMR) indicated that the polyaspartic acids used to illustrate this invention have greater than about 50% of β form and less than 50% of form a. Preferred polyaspartic acids are from about 65% to about 80% of β form and about 35% to about 20% of form a. More preferably, the polyaspartic acid is from about 70% to about 80% of β form and more preferably about 70% to about 75% of β form. Preferably, the polyaspartic acid has an average molecular weight (Mw) in the range of about 1000 to about 10,000. The polyaspartic acid prepared from any known process can be used to practice the corrosion inhibition processes described. A preferred polyaspartic acid used in this invention was prepared from hydrolyzed polysuccinimide. The polyaspartic acid may also be in a water-soluble salt form having a counterion selected from the group consisting of alkali metals, alkaline earth metals, ammonium and quartanary amino amino groups having from 1 to about 4 carbon atoms. carbon in each alkyl radical thereof. Gel Permeation Chromatography (GPC) was used to determine the molecular weight of polyaspartic acid. This technique is known in the art and a description of the process can be found in U.S. Patent No. 5,152,902, the disclosure of which is incorporated herein by reference. Briefly described, the molecular weights were determined using polyacrylic acid reference standards (Rohm & amp; amp;; Haas) that has molecular weights of 2000 and 4500. Since molecular weights based on GCP can vary with the patterns used, they are reported as average molecular weight (Mw). Thus, the polyaspartic acids used in the illustration of this invention were within the range of about 1000 Mw to 10,000 Mw. For purposes of illustrating the beneficial effectiveness of polyaspartic acid as a carbon dioxide inhibitor, the corrosive aqueous saline environment free of dissolved oxygen typically present in water from oil fields was simulated and utilized. In the field, the corrosivity of ferrous metals can be impacted by the composition of saline water, crude oil / gas type, the ratio of water phase to hydrocarbon phase and shear by fluid flow in the pipeline. Carbon dioxide and hydrogen sulfide gases also impact the type and amount of corrosion possible in this environment. In this way, the internal corrosion of the gas and oil pipes by the transported fluid is complex and difficult to simulate in the laboratory. The absolute simulation of field conditions in a single laboratory test technique is not possible in a real way. Laboratory tests have not been able to duplicate exactly every aspect of field operation conditions. Typically, laboratory corrosion tests attempt to simulate the most important conditions, such as chemistry and temperature, by using either a "bubble test" or a "recirculating flow cycle" test, both of which are known by those Expert people in the field. A description of the methods and apparatus used for this bubble test can be found in the literature. See, for example, Webster and Cois; "Selection of Corrosion Inhibition for Oilfield Piping", Corrosion / 93, Document No. 109, NACE International Conference, Houston, TX (1993), relevant disclosures of which are incorporated herein by reference. A description of the methods and apparatus used for a small recirculating flow cycle of approximately 3 liters (L) capacity can also be found in the literature. See, for example, White and Cois; "A Historical Case for the Selection of the Corrosion Inhibitor for the Export Pipeline of the Atlantic Storm Zone between 39 and 50 degrees of latitude" Science of Corrosion. 35, Nos. 5-8, 1515-1525 (1993). The "bubble test" is a relatively simple test cell, of a substantially low shear spray analysis vessel, which can be established reasonably quickly. This is ideal for quickly transporting a large number of tests such as, for example, in the first step of selecting the corrosion inhibitor, or for selecting a wide range of field conditions. A bridge of several test cells connected to an automated corrosion rate measuring system can also be used for convenience. A useful bubble test cell is briefly described, which is a vessel for obturable analysis adapted (1) to introduce the control liquid, (2) to introduce a corrosion probe into the liquid, (3) to spray the dioxide carbon to maintain a carbon dioxide countercurrent to prevent the ingress of air during insertion of the feeler and to substantially strip dissolved oxygen, preferably less than about 10 parts per billion (ppb) and (4) with an agitator to produce a relatively low wall of shear stress to substantially simulate the conditions of use of smooth dynamic flow. For convenience, the results obtained from these bubble tests will be referred to herein as "soft dynamic flow conditions of use". A useful corrosion probe is a linear polarized element resistance 2 or 3 of the corrosion sensor. The elements are preferably made of mild steel. The polarized resistance electrochemical technique is used to measure the absolute corrosion rates, and is also usually expressed in mils per year (mpy). Polarized resistance measurements can be done very quickly, usually in less than about 10 minutes. The excellent correlation can be made often between the corrosion rates obtained by means of the polarized resistance and the conventional weight waste determinations. Polarized resistance is also referred to as "linear polarization". A detailed discussion and description of the theory of electrochemical corrosion and polarized resistance can be found in the literature. See, for example, Notable Applications Corr 1: Basic Corrosion Measurements published by the Applied Search Analytical Instrument Division of Princeton EG &G (1980). A useful stirrer for a test cell analysis vessel having a volume of at least about 140 cubic centimeters (cm 3) may be a magnetic stirrer bar of approximately 3.5 centimeters in length (cm), which, when rotated in approximately 300 revolutions per minute (rpm), produces a shear rate at an outer edge of approximately 1.2 Pascals (Pa), and similarly lower than that at the electrode. In a typical export pipeline, such as in commercial oil fields, the average wall shear stress is about 3 Pa to about 8 Pa. However, the main limitation of the bubble test is that the shear stresses in the Agitated control liquid are significantly lower than those experienced in a pipeline. Test laboratory corrosion using a recirculating flow wave simulates the turbulent flow rates of high shear to substantially medium present in the equipment and pipes. Increasing the shear stress can have a significant adverse effect on the function of certain corrosion inhibitors. For example, at approximately 7 Pa of shear stress, the absorption of an inhibitor may become negligible. Additionally, the shear stress may adversely affect the persistence of an inhibitor film on a steel surface. A recirculating flow wave laboratory apparatus, however, can simulate turbulent flow rates similar to those in the field from a pressure of 4 bars up to approximately 100 bars by respectively changing the glass construction material to metal. The feasible shear stress depends on the geometry and flow rate but is typically similar to that experienced in the pipes above about 20 Pa. It is briefly described, the principal units of a useful recirculating flow wave which may consist of (1) ) one or more receivers where the control liquid can be conditioned before starting the test, (2) a centrifugal pump with a flow rate control valve, (3) a means for heating or cooling the control liquid and (4) a cell for preserving the test electrodes. The control liquid can be pumped either around a passage to aid in deaeration and conditioning or be diverted through the test cell for corrosion measurement. For convenience, the results obtained from the recirculating flow wave tests which produce average wall shear stress substantially of what is referred to herein as "conditions of moderate dynamic flow use". A useful test cell can be constructed from nylon (for a low pressure wave) or metal (for a high pressure wave). The test electrodes preferably comprise three identical test samples worked from grade steel or mild steel to simulate tube wall conditions. This allows corrosion rate behaviors to be determined by means of conventional electrochemical measurements, such as the linear polar resistance test tube (LPR), the AC impedance and the complete polarization. The effectiveness of the polyaspartic acid as an inhibitor of carbon dioxide corrosion according to the present invention was determined under simulated conditions of use of soft dynamic flow using the bubble test and under conditions of moderate dynamic flow using a beaker. capacity of 3 liters of recirculating flow wave and a flow velocity of approximately 1.6 meters / second (m / s) to simulate a wall of shear stress of approximately 7 Pa. For convenience, the control liquid used to illustrate the effectiveness of the polyapartic acid as an inhibitor of carbon dioxide corrosion was artificial salt solution of high ionic strength. The artificial salt solution had the salt content in grams per liter set forth in Table I, below.

TABLE 1 ARTIFICIAL AQUEOUS SALTS SOLUTION Conc salt (g / 1) Na2S04 0.016 NaCl 74.14 NaHC03 0.68 MgCl26H20 4.21 CaCl26H2o 17.19 KCL 0.71 Deionized water in 1 liter The artificial saline solution is first prepared preferably by dissolving all the chloride salts. The solution is then preferably saturated with carbon dioxide followed by the addition of the bicarbonate and the sulfate salts previously dissolved in small amounts of water. This method of preparation reduces the amount of precipitation to scale. The artificial aqueous saline solution prepared as described herein mimics the composition of saline solution of the North Sea, as it is presented in the export pipe system of the Atlantic stormy zone between 39 and 50 degrees of latitude. The saline solution of the Atlantic stormy zone between 39 and 50 degrees of latitude is known to know about 2800 ppm of calcium ion (Ca **), of about 496 ppm of NaHCO3 and has a natural pH of about 5.6. All tests for corrosion inhibition were carried out on ferrous metals, preferably mild steel (C1008) under simulated field and conditions of use of functional temperature from about room temperature to about 150 ° C, preferably at about 25 ° C to greater than about 80 ° C and more preferably about 50 ° C. The test solutions were completely stripped with nitrogen then saturated with carbon dioxide, at a pressure of about one bar (absolute) to give a pH of about 5.6. An effective amount of polyaspartic acid anticorrosive can be from at least about 10 ppm to about 5000 ppm as polyaspartic acid, based on the volume of the liquid aqueous saline environment. The polyaspartic acid effectively inhibited corrosion by corrosion of mild steel carbon dioxide in saline solution, substantially free of dissolved oxygen, as acid substantially pH range from about 4 pH to less than about 6.6 pH, more preferably in the range from about 5 pH to about 6 pH and more preferably at about 5.4 pH at about 5.9 pH. The polyaspartic acid was found in a relatively low concentration of about 25 ppm to inhibit speed corrosion by mild steel carbon dioxide under substantially use conditions of smooth dynamic flow over a temperature range from about room temperature to over approximately 80 ° C. For example, at about 50 ° C, the polyaspartic acid having an average molecular weight of about 5,000 (PA-5) reduced the corrosion rate from about 104 mpy to about 27 mpy in about 1 hour, which represents a greater reduction to approximately 70%. At room temperature, the PA-5 reduced the corrosion rate from about 27 mpy to about 15 mpy in about 1 hour, which represents a greater reduction to about 40%. The corrosion rate is generally defined as the corrosion effect on a metal per unit of time. The type of unit used to express the corrosion rate depends on the technical system and type of corrosion effect. However, the corrosion rate can be expressed in variable units. A description of the relationship between the units commonly used for corrosion rates can be found in literature. See, for example, David, (De), ASM Materials Engineering Dictionary, published by the Materials Information Society. Here, the corrosion rate is expressed as an increase in depth per unit of time as thousandths per year (mpy) of penetration speed. Figures 1 to 11 in the following examples illustrate the effectiveness of the polyaspartic acid having an average molecular weight of about 1., 000, of approximately 2,000 and approximately 4,400, which, respectively, are referred to as PA-1, PA-2, and PA-5 as an inhibitor of mild steel carbon dioxide corrosion. For convenience and not by limitation, 25 ppm of polyaspartic acid was used, without considering the Mw, and it was added in all tests to the test solution in artificial saline solution at a use temperature of approximately 50 ° C with pressure in approximately 1 bar of carbon dioxide, except as indicated otherwise. For convenience, the inhibition of corrosion rate by carbon dioxide will be referred to simply as corrosion rate inhibition and refers to the artificial saline solution or saline solution should be understood to be dissolved oxygen-free artificial salt solution. EXAMPLE 1 This example illustrates the properties of inhibition of carbon dioxide corrosion of polyaspartic acid in three different average molecular weights of about 1000 (PA-1), about 2000 (PA-2) and about 4,400 (PA-3) about mild steel (1018) in artificial saline at a temperature of approximately 50 ° C. The effectiveness of corrosion inhibition was compared to that of a proprietary commercial corrosion inhibitor. Reserve solutions were used for each inhibitor. The corrosion inhibitor was introduced into the solution in saline at a concentration of about 25 ppm in one volume to a volume basis (V / V). The appropriate amount of polyaspartic acid buffer solution (as received) was introduced to the test kit using a microliter syringe. The corrosion of the inhibition was measured using a bubble test and a mild steel electrode (element 2) (corrator test tube LPR). The results of the test simulate the inhibition of carbon dioxide corrosion of mild steel under dynamic flow conditions. The corrosion rate expressed in thousandths per year (mpy) is shown in the following Table II, and graphically compared in figure 1 for untreated mild steel and for mild steel in the presence of the inhibitors over a period of about 12 hours .

TABLE II CORROSION SPEED FOR CARBON DIOXIDE (MPY) AT APPROXIMATELY 50 ° C INHIBITOR Time in hours None PA-5 PA-2 PA-1 Commercial 0 105 105 105 105 110 1 110 22 22 22 2 6 110 18 16 15 2 12 110 16 4 4 2 As shown by the results, the corrosion rate for mild steel in a saline solution of ionic strength at approximately 50 ° C was typically greater than 100 mpy. The commercial corrosion inhibitor reduced the corrosion rate by approximately 98% in about 1 hour, and this reduction remained constant above about 12 hours. The polyaspartic acids, are to consider the Mw, reduced the corrosion rate by approximately 78% in about 1 hour and gradually continue the further reduction of the corrosion rate with time. Specifically, the PA-E reduced the corrosion rate by approximately 83% in approximately 6 hours, by approximately 85% in 12 hours and gradually continued the effects of the additional corrosion rate reduction over 24 hours. Likewise, polyaspartic acids PA-2 and PA-1, each reduced the corrosion rate by approximately 85% in approximately 6 hours and by approximately 96% in approximately 12 hours, also gradually continued with speed reduction of additional corrosion in a period of approximately 24 hours in which the test was completed on time. EXAMPLE 2 The inhibition of corrosion rate by carbon dioxide of mild steel by PA-5 of 25 ppm in artificial solution under use conditions of about 50 ° C over a range of about 4 pH to one less than about 7 pH. A series of artificial saline solutions were prepared for this test having the pH adjusted to about 4 pH, about 5.1 pH about 5.4 pH about 5.9 pH about 6.3 pH and about 6.6 pH through the addition of NaHCO3. The inhibition of the corrosion rate of PA-5 in each solution was then determined using the bubble test described in example 1. The results of corrosion rate mpy of the bubble test are shown in the following Table III; and they are compared graphically in Figure 2 over a period of about 6 hours.

TABLE III CORROSION SPEED (MPY) INHIBITION IN Time in hours pH 4 pH 5.1 pH 5, .4 pH 5. .9 PH 6.3 pH 6.6 0 118 107 96 82 88 55 1 38 34 29 20 20 17 6 8 31 24 10 As shown by the results, at about 4 pH the initial corrosion rate of about 118 mpy was reduced by about 68% in about 1 hour and by about 93% in about 6 hours. At about 6.6 pH, the initial corrosion rate of about 55 mpy was reduced by about 69% in one hour and by about 71% in about 6 hours. EXAMPLE 3 The effectiveness of the inhibition of carbon dioxide corrosion in artificial saline of approximately 50 ° C by a PA-5 was determined and compared with that of a patented inhibitor each in concentrations of 25 ppm using the apparatus of 3 liter recirculating flow wave to substantially simulate the conditions of moderate dynamic flow use. The corrosion rate measurements were made on mild steel with a standard element 2 linear polymerization (LPR) resistance of corrosion specimen. A flow velocity of approximately 1.6 meters per second (m / s) was best simulated by a desired liquid shear stress of approximately 7 Pa in the pipe wall, which is typically found in commercial pipes. The results of the corrosion rate mpy of this test over a period of time of about 24 hours are shown in the following Table IV, and are compared graphically in Figure 4. TABLE 4 CORROSION SPEED (MPY) INHIBITOR Time in hours Nincruna PA-5 Commercial 0 120 120 120 1 120 22 5 6 120 3 2 24 120 3 2 As shown by the results, the corrosion rates of petrolatum in the solution in artificial saline solution over a period of time of about 24 hours were in the range of about 120 mpy to about 150 mpy for untreated steel. The reference commercial corrosion inhibitor reduced the corrosion rate by approximately 96% in approximately one hour and by approximately 98% in approximately 6 hours, remaining at that level above approximately 24 hours. The polyaspartic acid, PA-5 reduced the corrosion rate by approximately 82% in approximately 1 hour and by approximately 98% in approximately 6 hours maintaining that corrosion range above approximately 24 hours. However, substantially the same level of inhibition of the corrosion rate as that carried out with the commercial inhibitor was carried out by the PA-5 over a period of about 6 hours. These results show that polyaspartic acid approximates the effectiveness of the commercial inhibitor in reducing the corrosion rate from mild steel carbon dioxide in saline under conditions of substantially moderate dynamic flow use. EXAMPLE 4 The inhibition of the effective carbon dioxide corrosion of each polyaspartic acid, PA-1, PA-2 and PA-5 was compared against (1) commercial polyaspartic acid having an Mw of about 5000 (Chemical Sigma); (2) a patented commercial inhibitor; (3) a polypeptide composition of about 40% aspartic acid and about 60% asparagine; (4) a polypeptide composition of about 60% aspartic acid and about 40% asparagine; (5) a polypeptide composition of about 80% aspartic about 20% valine and (6) a polytyrosine. Each of the inhibitors was examined in a bubble test present at approximately 25 ppm in artificial saline. A bubble test cell having a volume capacity of approximately 140 cubic centimeters was used to determine the corrosion rate inhibition at a temperature of about 50 ° C over a period of about 14 hours. The artificial saline solution had a pH of approximately 5.6 and each of the inhibitors was introduced separately to the test cell by transferring the appropriate amount of the buffer solution (as received) on a volume basis (v / v). ) to the equipment using a microliter syringe. The test was handled under a pressure bar by carbon dioxide. The data of the inhibition of the corrosion rate mpy are compared graphically and shown in figures 4 and 5. The data show that the polyaspartic acids without considering Mw had a development of efficiency in the reduction of corrosion rate of approximately 70% at approximately 85% after approximately 6 hours compared to the reduction of approximately 98% for the patented commercial inhibitor. In addition, polyaspartic acids, are considered Mw were more effective during this period of 6 hours than the four inhibitors of polypeptide composition (Nos. 3-6). The polypeptide composition inhibitors (nos. 3-6) appeared to improve after about 10 to about 12 hours, but this improvement was not found to be reproduced. However, there is some suggestion that surface films may have some form over a period of time, which may have value for batch inhibition treatments. EXAMPLE 5 The general procedure of the bubble test described in Example 4 was followed except that the inhibitors were PA-1, PA-2 and polyhistidine L (polyHIS) and in each case the inhibitors were examined at a pH of about 5.1 and at approximately 5.6. The natural pH of the saline solution was approximately 5.6. For the test at about pH 5.1, the artificial salt water pH was adjusted with NaHC03 The results of the corrosion rate of the bubble test are compared graphically in Figure 6 marked as a function of time over a period of time of approximately 18 hours. The results of the data indicate that reducing the pH of the saline solution below about 5.6 pH tends to reduce the effectiveness of inhibiting the corrosion rate of PA-1 and PA-2. However, both polyaspartic acids, each was substantially more effective in reducing the corrosion rate than polyhistidine. EXAMPLE 6 This example illustrates the effect of the calcium ion present in the saline in the inhibition of the corrosion rate of polyaspartic acid, PA-1. The bubble test procedure described in Example 4 was followed, except that the artificial salt solution was prepared containing zero ppm, approximately 1000 ppm and approximately 10,000 ppm calcium ion (Ca **). The inhibition of the corrosion rate by carbon dioxide per 25 ppm of PA-1 was determined at a pH of about 5.6 over a period of about 8 hours. Duplicate determinations were made. The result of the corrosion rate mpy is shown in Figure 7 marked as a function of time. The data show the effectiveness of the inhibition of the corrosion rate by a PA-l beneficially increased as the ion of calcium content in bulk The bubble test was then repeatedexcept that the pH of the artificial solution was decreased to about 4.5 pH upon addition of NaHCO 3, and the rate of corrosion inhibition was determined over a period of about 14 hours. The results of the corrosion velocity mpy of the duplicate tests are shown graphically in Figure 8 marked as a function of time. These data indicate that the beneficial inhibition of PA-1 in the presence of the calcium ion becomes less apparent at the lower pH than if it were at the higher pH shown in Figure 7. It is believed that this decrease is because few aspartate groups ionize below the known pKa value of approximately 4.7 for polyaspartic acid. A third test was carried out in a similar manner, except that the calcium free saline solution (0 ppm Ca **) and normal saline water (containing polyaspartic salt 2800 ppm Ca **) were used. A calcium polyaspartate salt (calculated as a 25 ppm polyaspartic acid) was used as the inhibitor. The results of the corrosion rate data mpy are compared graphically in the figure or marked as a function of time over a period of about 6 hours. These data illustrate that calcium polyaspartum was less efficient as an inhibitor of carbon dioxide corrosion in calcium-free saline water than in calcium-containing saline water.

EXAMPLE 7 This example illustrates the effect of ferrous ion (Fe **) on the inhibition of carbon dioxide corrosion of polyaspartic acid, PA-1, in artificial saline solution. The bubble test described in Example 4 was followed, except that saline solutions were prepared containing ferrous ion of about 1 ppm, about 10 ppm, about 100 ppm and about 1000 introduced as FeS04. The corrosion rate (mpy) was determined using 25 ppm PA-l. The results of the corrosion velocity mpy are shown graphically over a period of about 14 hours in FIG. 10. The data show that the effectiveness of the polyaspartic acid, PA-1, as an inhibitor of carbon dioxide corrosion was substantially unaltered by the amount of ferrous ion present in the saline solution. EXAMPLE 8 The inhibition of the corrosion rate by means of polyaspartic acid, PA-5, on mild steel under conditions of moderate dynamic flow use was determined in three series using the three liters of recirculating flow wave described in example 3 with a flow rate of approximately 1.6 meters per second (m / s) at a temperature of about 50 ° C over a period of time of about ten hours.

The data obtained by the corrosion rate (mpy) are graphically represented in figure 11 compared to that made with a proprietary commercial inhibitor, present at a concentration of about 35 ppm, the amount commonly used in the field. The data show that the effective corrosion and development of the PA-5 inhibition was substantially equivalent to that of the patented commercial inhibitor after a period of time of about 3 hours. EXAMPLE 9 In this example, the effective inhibition of carbon dioxide corrosion of polyaspartic acid, PA-1 was determined in saline of about 50 ° C with a pressure in a carbon dioxide bar. The bubble test described in Example 1 was followed except that saline was prepared with amounts of sodium bicarbonate ranging from none to about 10,000 ppm to provide saline solution having a pH range of about 4 pH to about 6.6 pH. The test was developed over a period of about 11 hours as follows. Six test cells were prepared. Each test cell contained saline having a sodium bicarbonate (NaHCO3) content in ppm of either (a) zero (b) 125; (c) 375; (d) 1250; (e) 3750; or (f) 10,000 to provide artificial saline solution having a pH, respectively, of about 4.0 pH; of about 5.1 pH; of about 5.4 pH; of about 5.9 pH; of about 6.3 pH and about 6.6 pH. A petrolatum was first determined before the addition of the corrosion rate (mpy) of NaHCO 3 (Sequence A) after a total lapse of approximately 5 hours, and approximately 1.5 hours. After a total time lapse of approximately 2 hours (Sequence B), the sodium bicarbonate was then added in the amounts listed in Table V below. The corrosion rate was determined again after a total lapse of approximately 2.5 hours and about 3.5 hours. After a total lapse of approximately 4 hours, 25 ppm of PA-1 was added to each of the test cells (Sequence C). The corrosion rate was determined again after a total lapse of about 5 hours, about 7 hours, about 9 hours and about 11 hours. The determined corrosion rate (mpy) is shown in the following Table V.

TABLE V CORROSION SPEED (MPY) Lapse NaHC03 in ppm in saline (Hrs.) 0 125 375 1250 3750 10000 A. Base Line of the Corrosion Speed 0.5 117 117 122 118 124 103 1.5 110 111 111 107 110 103 2.0 B. NaHC03 added to each cell 2.5 112 100 93 85 95 97 3.5 118 107 96 82 88 55 4.0 C. 22 ppm of PA-1 added to each cell 5.0 38 34 27 20 20 17 7.0 18 32 27 17 18 17 9.0 10 33 25 13 16 16 11.0 8 31 24 10 15 16 The data indicates that as the pH of the solution increases, this tends to reduce the initial corrosion rate of Vaseline. The corrosion inhibition efficiency of polyaspartic acid, PA-1, appears to reduce the corrosion rate and appears to be adjusted by approximately 80% corrosion rate reduction at a pH of about pH 5.4 to about 5.9 pH. This Ph value is substantially in the range of natural saline environment found in the oil field of the Atlantic area between 39 and 50 degrees of latitude. The present invention has been described with respect to the preferred embodiments but is not limited thereto. It would be apparent to a person skilled in the art that the illustrations of the above method are subject to numerous modifications, which do not depart from the principle and purpose of this invention.

Claims (8)

1. NOVELTY OF THE INVENTION Having described the present invention it is considered as a novelty and therefore the property described in the following claims is claimed as property. 1. A method for the inhibition of the corrosion by carbon dioxide of ferrous metals in the presence of an aqueous saline environment containing dissolved carbon dioxide and having a substantial pH, which in addition to the salt environment comprises a anticorrosive amount of polyaspartic acid having an average molecular weight in the range of about 1,000 to about 10,000.
2. 2. The method according to claim 1, characterized in that the saline environment is saline solution substantially free of dissolved oxygen.
3. 3. The method according to claim 1, characterized in that the pH is in the range of about 4 to about 6.6.
4. The method according to claim 1, characterized in that the amount of polyaspartic acid is at least about 10 parts per million based on the volume of aqueous saline solution in contact with the ferrous metals.
5. The method according to claim 4, characterized in that the polyaspartic acid is present in a concentration of about 25 parts per million.
6. 6. The method according to claim 1, characterized in that more than 50% of the polyaspartic acid is in the ß form. The method according to claim 1, characterized in that the polyaspartic acid is in salt form and that it has a counter ion selected from the group consisting of an alkali metal, an alkaline earth metal, ammonium and an alkaline quaternary ammonium group. having from 1 to about 4 carbon atoms in each alkaline half thereof. The method according to claim 1, characterized in that the polyaspartic acid has an average molecular weight of from about 1,000 to about 5,000.