DE69033634T2 - COMPOSITIONS AND METHOD FOR INHIBITING CORROSION OF IRON METALS - Google Patents

Abstract

Compositions comprising certain amino acids such as aspartic acid, when fully ionized at alkaline pH, function effectively as corrosion inhibitors for ferrous metals in the presence of an aqueous medium. This effect is enhanced with increased fluid velocity.

Description

Field of the invention

The present invention relates to new and improved corrosion inhibition compositions, an unexpected and new use of biodegradable corrosion inhibitors, and improved methods for corrosion inhibition of ferrous or iron-containing metal surfaces (which are susceptible to corrosion) in the presence of an aqueous medium. More particularly, the invention relates to corrosion-inhibiting amino acids and methods for using such corrosion-inhibiting amino acids that inhibit corrosion of ferrous metals under conditions of use in the presence of an otherwise corrosive aqueous medium.

Description of the state of the art

An important mechanism for protecting the metal against corrosive destruction is achieved through the use of inhibitors. Unfortunately, certain common corrosion inhibitors, such as nitrogen-containing and aromatic compound-containing formulations, which are widely used as corrosion inhibition additives in aqueous heating and cooling systems, have been found to be hazardous to public health and the environment. The removal of such hazardous compounds by precipitation or other treatments is complicated and expensive. The use of other corrosion inhibitors, such as chromium salts, has been banned because they are suspected of being carcinogenic. Consequently, it is desirable to investigate the inhibiting properties of biocompatible and/or biodegradable compounds. Such compounds, if nontoxic, easily prepared in high purity, and biodegradable, can greatly reduce the burden of disposal or recycling. Measures facilitate. Amino acids have been suggested for limited use.

For example, Nippon Kokoh in Japanese patent J50091546-A, dated July 22, 1975, described that mixtures requiring both amines and amino acids or their salts inhibit atmospheric corrosion of various ferrous and non-ferrous metal foils when dissolved in water to form 20% aqueous solutions. It is believed that the pH of the moisture absorbed on the foils is about 5.5 or less.

JP-A-56-98482 describes an anti-corrosion agent which is particularly suitable for food production, namely for use with calcium chloride salt solution which consists of at least one polyhydric alcohol, an amino acid or a saccharide and an alkali hydroxide.

However, extensive studies on common amino acids alone have not shown promise. For example, in V. Hluchan et al. "Amino Acids As Corrosion Inhibitors in Hydrochloric Acid Solutions", Materials and Corrosion, ~, 512-517 (1988) 22 of the best known amino acids were tested as corrosion inhibitors for iron in 1.0 M hydrochloric acid at a pH of about 0. In general, those that showed inhibiting properties at an acidic pH did not show corrosion inhibiting activity that would be useful for immediate industrial application. The amino acids with longer hydrocarbon chains and those with additional amino groups or with groups that could increase the electron density of the amino groups were the only ones that tended to show effective corrosion inhibition.

Namely, aspartic acid, the amino acid preferably used in the present invention, and glutamic acid do not show this "tendency". The conclusion was that such amino acids are poor inhibitors, particularly due to the single amino group, the short carbon chain and the additional carboxyl group.

Furthermore, it is considered disadvantageous by the art to use aspartic acid as an inhibitor under the aforementioned acidic pH conditions, since aspartic acid is known to be inherently corrosive under slightly alkaline pH conditions. See K. Ramakrishnaiah, "Role of Some Biologically Important Compounds on the Corrosion of Mild Steel and Copper in Sodium Chloride Solutions", Bulletin of Electrochemistry, 2(1), 7-10 (1986). It was reported that aspartic acid at pH 8 actually accelerates corrosion (inhibition effect -25.4%). Even in combination with an excellent mild steel corrosion inhibitor such as papaverine, the presence of aspartic acid maintained the corrosiveness of the solution.

A related problem in industry is that it is known that the movement of fluids increases the rate of corrosion of ferrous or ferrous metals when exposed to an aqueous environment. Whatever corrosive effect amino acids such as aspartic acids may be expected to have in aqueous media, one would therefore expect a deterioration in practice when such amino acids are present in automotive, cooling or heating devices where such media are set in motion.

Therefore, amino acids such as aspartic acids, although non-toxic and biodegradable, have been avoided as corrosion inhibitors.

A method for inhibiting corrosion of ferrous or ferrous metals using amino acids having only a single amino group and having an additional carboxyl group (such as aspartic acid) under conditions where such amino acids are fully ionized would represent a surprisingly unexpected discovery satisfying a long-standing need in industry. Likewise, a corrosion inhibitor for ferrous metals that reduces the rate of corrosion even under conditions of increased agitation of aqueous fluids would represent a significant improvement in the art.

Summary of the invention

It is a primary object of the present invention to provide a new and improved composition for corrosion inhibition of ferrous or ferrous-containing metals in the presence of an aqueous medium.

It is the primary objective of the present invention to provide new and improved methods for corrosion inhibition of iron or iron-containing metals in the presence of an aqueous medium.

Another primary object of the present invention is to provide new and improved methods for corrosion inhibition of iron or iron-containing metals in the presence of an aqueous medium under static conditions.

Yet another primary object of the present invention is to provide new and improved methods for corrosion inhibition of ferrous or ferrous-containing metals in the presence of an aqueous medium under dynamic fluid motion conditions.

It is a further object of the present invention to provide new and improved methods for using amino acids having a single amino group as corrosion inhibitors for iron or iron-containing metals in the presence of an aqueous medium.

Other and further objects of the present invention will become apparent from the appended description and claims.

It has been found that aspartic acid, polyaspartic acid and salts thereof, which were previously known to accelerate the corrosion of metal in slightly alkaline, aqueous media, unexpectedly act effectively as corrosion inhibitors when fully ionized under the conditions of use. Such amino acids cause a 100-1000-fold decrease in the corrosion rate of iron or iron-containing metals. Surprisingly, this corrosion-inhibiting effect is improved with increasing fluid velocity.

Short description of the drawings

Fig. 1 shows a plot of the impedance spectrum for a mild steel electrode, plotted in real versus imaginary coordinates, rotating at 200 rpm in an aqueous solution containing 1000 ppm aspartic acid at 90ºC and pH 10.

Fig. 2 shows a plot of the impedance spectrum for a mild steel electrode, plotted real versus imaginary coordinates, rotating at 200 rpm in an aqueous solution at 90ºC and pH 10 without aspartic acid, but whose conductivity is adjusted with sodium sulfate.

Fig. 3 shows a plot of the impedance magnitude versus the logarithm of the frequency for the mild steel electrode in Fig. 1 and 2.

Fig. 4 shows a plot of the phase angle versus the logarithm of the frequency for the mild steel electrode in Fig. 1 and 2.

Description of the preferred embodiments

The claimed technology is represented by a composition for corrosion inhibition of iron or iron-containing metals in the presence of an aqueous medium, the composition comprising:

(a) an amino acid selected from the group consisting of aspartic acid, polyaspartic acid and salts thereof in an amount sufficient to provide a concentration of from 100 ppm to 5.0% by weight in the aqueous medium under conditions of use, and

(b) a base in an amount sufficient to provide a pH of at least 8.9 in the aqueous medium under conditions of use.

The technology according to the invention also includes a method for corrosion inhibition of iron or iron-containing metals in the presence of an aqueous medium, wherein the method comprises adding to the aqueous medium

(a) adding an amino acid selected from the group consisting of aspartic acid, polyaspartic acid and salts thereof in an amount sufficient to provide an amino acid concentration of 100 ppm to 5.0 wt.% in the aqueous medium under conditions of use, and

(b) adding a base in an amount sufficient to provide a pH of at least 8.9 in the aqueous medium under conditions of use.

Preferably, the amino acid compounds have an excess of carboxyl groups over "free" amino groups, for example 2 carboxyl groups and one amino group, although a carboxyl group/amino group ratio of 1 is suitable. Suitable amino acids are represented by the following formula:

n is an integer indicating the number of repeating aspartic acid units.

Non-limiting suitable salts include, for example, alkali metal, soluble alkaline earth metal and C₁-C₄ alkylamine salts.

These compounds are readily available from a number of sources and can be produced by either chemical synthesis or microbial fermentation.

These compounds are ineffective as corrosion inhibitors when in the fully protonated cationic form and even contribute to the deterioration by actually accelerating corrosion as the pH increases from acidic to alkaline. However, once fully ionized using sufficiently alkaline conditions, they have been found to greatly reverse the corrosion rate of ferrous or ferrous metals. In general, alkaline pH values of at least about 8.9 are suitable, depending on the temperature and the particular compound used. Under such application conditions, the corrosion rate is reduced by 100 to 1000 times compared to the corrosion rate of ferrous or ferrous metals under comparable pH conditions in the absence of these compounds.

The corrosion inhibitors of the present invention can be used at low concentrations such as 100 parts per million and also at high concentrations such as 5.0% by weight and above (in the aqueous medium). It is particularly preferred to use the corrosion inhibitors of the present invention at a concentration of from about 1000 ppm to about 3.3% by weight. However, it is apparent that concentrations greater than 5.0% by weight of the corrosion inhibitors can be used if desired, so long as the higher amounts are not detrimental to the system in which the corrosion inhibitors are used.

The corrosion inhibiting effect of the compositions of the invention can be found at low temperatures such as room temperature or about 25°C or below and at high temperatures such as about 90°C and above.

Although it is known that temperature or heat accelerates the corrosion of metals, it is important to note that increasing the temperature does not affect the corrosion inhibiting properties of the present invention beyond the effect that temperature has on pH. For example, the pH of the system may decrease by 1 unit from the value measured at 25°C compared to the value measured at 90°C. The pK of the fully ionized form of the amino acid also decreases with increasing temperature. However, as long as the temperature does not cause the pH to decrease below the point at which the inhibitors are fully ionized, the compositions of the invention remain effective.

In a particularly preferred embodiment, the compositions of the invention are used in dynamic, flowing systems. Surprisingly, the corrosion rate of ferrous metals in such systems does not increase with increasing fluid velocity. In fact, there is a tendency for the corrosion rate to decrease significantly with increasing fluid velocity. In the presence of the compositions of the invention, an increase in fluid velocity from, for example, 200 revolutions per minute (rpm) to about 1000 rpm in a rotating cylinder electrode will typically result in an increase in the corrosion rate of ferrous metals in the presence of such an aqueous medium for a period of at least 24 hours. This increase in corrosion rate is common for steels in water and other aqueous systems because the reduction of oxygen is often the rate-limiting factor. That is, the rate of mass transfer of oxygen to the corroding surface increases with increasing fluid velocity.

Under the conditions of use for the corrosion inhibiting compositions of the present invention, the pH of the aqueous medium may vary from about 8.9 to about 14, preferably from about 9.5 to about 12, measured at ambient or room temperature (about 25°C). It is particularly preferred to use compositions of the present invention at a pH of about 10 or greater, measured at ambient or room temperature. However, it will be appreciated, as previously noted, that the pH will vary depending on the temperature at which it is measured.

Therefore, when an aqueous medium is inherently acidic, it is a preferred embodiment of the invention to use a suitable amino acid, preferably aspartic acid, in compositions comprising an effective amount of base to raise the overall pH of the aqueous medium to above about 9.5, most preferably above about 9.9-10, at which pH the amino acid is in the fully ionized form (corresponding base).

The pH of the aqueous medium can be adjusted by adding any suitable base, such as an alkali metal hydroxide, e.g., sodium hydroxide and potassium hydroxide. Other bases that can be used in the present invention include alkali metal carbonates, hydrocarbyl amines, alkaline earth metal hydroxides, and ammonium hydroxides.

The pH of a corrosive environment may be inherently alkaline, such as aqueous solutions in contact with lime deposits, concrete and fertilizers, and automotive antifreeze solutions. In such systems, corrosion inhibition can be achieved by merely adding a suitable amino acid or salt thereof in an amount sufficient to provide the previously described concentrations in the aqueous medium, without the need to add extraneous bases.

It is within the scope of the present invention that the corrosion inhibitors can also be used in aqueous media containing various inorganic and/or organic materials, in particular all components or substances used in the water treatment industry, the automotive industry and others such as antifreeze compositions, metal cleaning compositions and radiator flushing compositions.

The effectiveness of corrosion inhibition for metal surfaces is usually determined by measuring the corrosion rate of the metal in question under specific conditions. Two types of corrosion rate measurement are used here. For simplicity, these are referred to as (1) standard metal coupon mass loss test, also called static immersion test and (2) electrochemical impedance technique.

In the standard metal coupon mass loss test mode, metal coupons of known mass are immersed in an aqueous solution whose corrosion inhibiting properties are to be determined. The aqueous medium is maintained at specified conditions for a specified period of time. After the exposure time is complete, the coupons are removed from the aqueous solution, cleaned in an ultrasonic bath with soap solution, rinsed with deionized water, rinsed with acetone, blotted dry with a lint-free paper towel, purged with a stream of nitrogen blown and weighed to determine the mass loss and examined under a stereoscope of appropriate magnification to determine the penetration of the metal surface due to corrosion.

However, corrosion is an electrochemical process rather than a strictly chemical reaction. Electrochemical techniques, such as electrochemical impedance, therefore provide a useful and convenient indication of the corrosion rate. In electrochemical impedance engineering, it is helpful to remember that a corroding metal surface consists of a large number of local anodes and a large number of local cathodes, the locations of which may actually be shifted or may be at the same location where the corrosion reaction occurs. At the anode location, the metal is oxidized, while at the cathode location, reduction occurs, namely the reduction of hydrogen ions in acidic solutions. The magnitude of the current, in amperes per square centimeter (A/cm2), at rest voltage, measured with respect to a reference electrode, is a measure of the tendency for the reaction in question to occur. This corrosion current density is referred to as the "corrosion rate." In many cases, the corrosion rate is converted to a "penetration rate" of corrosion in milliinches per year (mils per year, mpy) or in mass loss, for example by assuming two electrons per ionized iron atom.

The "electrochemical impedance technique" is applied, varying the frequency at an electrode interface using an amplitude wave of low voltage, say 5-10 millivolts (mv). The response is used to determine the corrosion rate and to draw conclusions about the corrosion mechanism. By analyzing the impedance spectra, one obtains a term called the "polarization resistance," measured in ohm-centimeters squared (ohm-cm2), which is inversely proportional to the corrosion current density (corrosion rate). Accordingly, according to Ohm's law (i = V/Rp), the corrosion rate is equal to the proportionality factor (for the metal in question), measured in volts, divided by the polarization resistance. For example, a common proportionality factor for carbon steels is 0.025 volts. And since polarization resistance is inversely proportional to corrosion rate, the relative degree of polarization resistance is used to determine the degree to which different compositions exhibit either lower or higher corrosion rates. A polarization resistance of 100 ohm-cm2 is therefore by a corrosion rate that is about 100 times faster than a corrosion rate that has a polarization resistance of 10,000 ohm-cm². A polarization resistance of 100 ohms/cm² represents a corrosion rate on the order of about 100 mpy, while one of 1000 ohm-cm² represents a corrosion rate on the order of about 10 mpy. Conversion of polarization resistance to corrosion rate (as mpy) can be done by assuming a proportionality constant of 25 mV and Faraday's law.

For a guide to electrochemical impedance engineering, see D. C. Silverman, "Primer on the AC Impedance Teclnnique," in Electrochemical Technigues for Corrosion Engineering (R. Baboian, ed.), National Association of Corrosion Engineers, Houston, 1986, pp. 73-79.

The following specific examples illustrate the best mode presently known for carrying out this invention and are described in detail in order to facilitate a clear understanding of the invention. It is to be understood, however, that the detailed explanations of the application of the invention, while referring to preferred embodiments, are given for the purpose of illustration only and are not intended to be limiting of the invention, since various changes and modifications pertaining to the invention will become apparent to those skilled in the art from this detailed description.

In the following examples, unless otherwise indicated, all parts and percentages are by weight, all temperatures are in degrees Celsius (ºC), pH was measured at 25ºC, and "mass loss" means "penetration rate".

EXAMPLE 1

The electrochemical impedance technique was used to determine the corrosion of two (C1018) mild steel electrodes, designated as samples A and B. The parameters and the results are given in Table 1 and Table 2.

Steel coupons were prepared and used as electrodes in a rotating cylinder electrode apparatus. The apparatus is described in detail in DC Silverman, "Rotating Cylinder Electrode for Velocity Sensitivity Testing," in orrosion, 41(5), 220-226 (1984). The electrochemical impedance technique is described in detail in DC Silverman and JE Carrico, "Electrochemical Impedance Technique - A Practical Tool for Corrosion Prediction," in Corrosion, 44(5), 280-287 (1988).

The cylindrical electrode was made of mild steel (C1018). The electrode was ground with 600 grit silicon carbide paper before being immersed in the solution under study. The solution was also heated to the desired temperature of 90ºC before the electrode was immersed. The electrode was mounted on a cylindrical shaft, then immersed and rotated at 200 rpm to ensure turbulent flow conditions. The water line was located in the center of the upper Rulon® spacer [graphite-impregnated poly(tetrafluoroethylene), E.I. du Pont de Nemours & Company] to prevent hydrodynamic effects and effects from interfering with the results to ensure optimal flow and streamlines.

In situ data reported in Table 1 (as Sample A) were obtained by exposing the mild steel electrode to a sodium aspartate solution at pH 10 in the rotating cylinder apparatus. The pH of the sodium aspartate was about 1000 ppm. The temperature was set at 90ºC, although the pH was measured at 25ºC.

Similarly, in situ data presented in Table 2 were obtained for Sample B, except that sodium aspartate was not present and the same ionic strength was achieved by using sodium sulfate (which has a nuclear effect on corrosion) instead.

Corrosion potentials were measured for the steel electrode used for each of Samples A and B by measuring the voltage between the steel electrode and a saturated calomel electrode. The electrodes for each of Samples A and B were rotated at various speeds for identical exposure times. Polarization resistances were determined as described in Silverman and Carrico, supra, and were used to evaluate corrosion rates, which were converted to penetration rate or mass loss in millunches per year (mpy).

Impedance spectra were generated for the steel coupon electrodes (samples A and B) at pH 10 in each of the aqueous solutions used for samples A and B at 200 rpm using the rotating cylinder electrode apparatus. These spectra (curves) are shown in Figs. 1, 2, 3 and 4. The agreement between the calculated curve and the actual Data shows how well the model used to obtain polarization resistance agrees with the actual results. The point nature of attack noted for the static immersion test under comparable conditions (in runs #4 and 5 of Example 2 below) was absent for the rotating cylinder electrode. This behavior suggests that the presence of a uniform velocity field advantageously allows the aspartic acid to inhibit corrosion more uniformly. Furthermore, the increased uniform inhibition suggests that the process is aided by the finer 600 grit finish used for the electrode, compared to a 120 grit finish used for the coupons used in the static immersion tests. The net result of the smoother finish is that the surface topography of the electrode was less heterogeneous than that of the static immersion coupon. By using a more uniform speed and a smoother steel surface, the concentration of aspartic acid required to inhibit corrosion uniformly on all parts of the surface could be reduced. TABLE 1 CORROSION OF MILD STEEL WITH 1000 PPM SODIUM ASPARTATE (pH = 10, adjusted at 25ºC)

The corrosion potential of sample A is -310 mV(SCE) TABLE 2 CORROSION OF MILD STEEL WITH SODIUM HYDROXIDE (pH = 10, adjusted at 25ºC).

The corrosion potential of sample B is -630 mV (S. C. E.)

As shown in Tables 1 and 2, the corrosion potential of sample A with sodium aspartate is -310 mV (S.C.E.), while the corrosion potential of sample B without sodium aspartate is much more active at -630 mV (S.C.E.). The difference between the corrosion potentials suggests that the sodium aspartate has a greater tendency to oxidize the steel surface.

Nevertheless, the corrosion rates of the respective samples show an inverse relationship to this oxidation tendency. The magnitude of the difference between the corrosion rates of sample A versus sample B after identical exposure times indicates that the aspartate inhibits corrosion by 100 to 1000-fold. Although corrosion began at approximately the same rate for both sample A (32 mpy) and sample B (45 mpy), the rate decreased rapidly in the presence of sodium aspartate, while remaining very constant in its absence.

Furthermore, in the absence of aspartate, an increase in rotation rate or fluid velocity leads to an increase in the corrosion rate, at least in the first 24 hours of the run. After 48 hours, no change was observed, most likely because corrosion product was present on the surface. This behavior is normal for carbon steel and water since the reduction of oxygen is the rate limiting step. The rate of mass transfer of oxygen to the corroding surface often determines the corrosion rate, and this oxygen transfer rate can be adversely affected if corrosion products build up on the surface. However, in the presence of the aspartate, the corrosion rate did not increase with increasing speed. In fact, there was a significant decrease in the corrosion rate with increasing rotation rate consistently in all experiments. The decrease in corrosion rate achieved by increasing rotation rate appears to be irreversible since even after the rotation rate is subsequently decreased to 200 rpm, as can be seen from the rates in Table 1 determined at exposure times of 46 to 48, 49, and 50 hours, the corrosion rate did not return to the 0.13 mpy rate at 200 rpm that the sample exhibited before the fluid velocity was increased to 1000 rpm.

Accordingly, a sodium salt of aspartic acid acts unexpectedly as a corrosion inhibitor for iron or iron-containing metals under basic conditions.

The impedance spectra themselves were studied as a function of rotation rate or fluid velocity using the rotating cylinder electrode over a period of 48 hours. Plots at 200 rpm and after 24 hours are shown in Figs. 1, 2, 3 and 4.

Two peaks exist in the phase angle plots for mild steel in contact with sodium aspartate. This is shown in Fig. 4. Such behavior suggests two relaxation time constants, which in turn suggest that either a strongly adsorbed intermediate or a self-adhering film is involved in the corrosion mechanism. The peak at the high frequency is attributed to the adsorbed intermediate on the film, while the peak at the low frequency is related to the corrosion rate. Accordingly, although we do not wish to be bound by any theory for the corrosion mechanism or to limit the present invention in any way, it is believed that the aspartate ions form some sort of adsorbed layer on the steel surface, even though the mechanism is not fully understood. Further evidence for the existence of some sort of adsorbed layer on the steel surface in the presence of aspartate ions is provided by the phase angle plot for mild steel under comparable conditions, but in the absence of aspartate ions. In such a representation, which is also shown in Fig. 4, there is only one peak, which suggests that only the charge-transfer corrosion reaction takes place.

EXAMPLE 22

Fourteen identical coupon samples made of mild steel (C1018) were tested.

The samples were ground using 120 grit silicon carbide paper, washed with deionized water, dried and weighed. The samples were then subjected to static immersion tests. The parameters and results are given in Table 3 below. The samples were hung on glass hooks in glass jars, each containing approximately 600 cm3 (or cc) of the L-aspartic acid test solution. The solutions were prepared using deionized water and L-aspartic acid in an amount sufficient to provide the desired aspartic acid concentration. The hooks were attached through rubber stoppers that closed the top of the jars. A gas/water sparger was inserted into the side of the stopper for aeration of the solutions with water-saturated air from which carbon dioxide had been removed. The jars were placed in constant temperature baths where the temperature was maintained at 90ºC. Coupon exposure times were 5-7 days; during this time, deionized water was periodically added to the aspartic acid test solution to compensate for water loss by evaporation at the elevated temperatures. The pH of each solution was adjusted at the start of the test using sodium hydroxide and measured at both room temperature (RT, approximately 25ºC) and at the temperature of the test.

At the end of the exposure times, the coupons were removed from the solutions, cleaned in an ultrasonic bath with soap solution, rinsed with deionized water, rinsed with acetone, dried, and weighed. The coupon surfaces were examined after exposure under a stereoscope at between 10x and 30x magnification. Corrosion rates were evaluated in the manner previously explained by measuring the weight change (both before and after exposure to the aspartic acid solution) and then calculating the penetration rate or mass loss in either mpy or grams per hour. In those cases where the corrosion was extremely non-uniform or localized to certain areas on the surface, only the mass loss in grams divided by the exposure time in hours was reported. The reason for this is that the The corrosion rate determined over the entire surface does not accurately describe the extent of corrosion when corrosion occurs in very narrow areas. Nevertheless, the results in Table 3 from the static immersion test, compared to the results in Tables 1 and 2 from the constant flow system, show that under completely stagnant flow conditions the concentration of aspartic acid required to achieve an equivalent level of corrosion inhibition increases. TABLE 3 RESULTS OF STATIC IMMERSION TEST FOR MILD STEEL/ASPARAGIC ACID AT 90ºC - VENTILATED

EXAMPLE 3

The procedure described in Example 2 was used, except that the solutions did not contain aspartic acid and only three steel coupons were subjected to the static immersion test. The solutions were adjusted to have the same conductivity as those containing aspartic acid by adding sodium sulfate, thus limiting the corrosion to that produced solely by oxygen present in the water at a given pH. The results are given in Table 4 below. TABLE 4 RESULTS OF THE STATIC IMMERSION TEST FOR MILD STEEL WITHOUT INHIBITOR AT 90ºC

¹Total grams per total exposure time in hours.

At a pH of 8, the corrosion rate in the presence of aspartic acid is higher than in its absence, comparing the results of Experiments 2 and 3 of Table 3 with those of Experiment 1 of Table 4. This seems to confirm that at a pH of 8, aspartic acid does not provide a beneficial corrosion inhibition; instead, it behaves as a corrosion accelerator. The same behavior is found at a pH of 9.5 and a concentration of 3 wt% aspartic acid (Experiment 10 of Table 3). Such behavior is attributed to the fact that at a pH of 9.5 or less, aspartic acid does not in the fully or totally ionized form (corresponding base). This is clearly demonstrated by an observed change in behavior at a pH of > 9.5, for example at a pH of 10 or greater, even at concentrations of aspartic acid less than 1000 ppm. In fact, under the conditions of the static test of Table 3, at concentrations of 1 wt.% at a pH of 10 (Test 9), all corrosion disappears. Thus, at pH values between 8.5 and about 9.0, measured at 90ºC, or at a pH of 10 or greater at room temperature (about 25ºC), aspartic acid prevents corrosion under stable conditions, whereas at a lower pH it increases or accelerates corrosion.

The fact that some attack or corrosion is observed at concentrations of 1000 ppm (tests 4 and 5) and 5000 ppm (test 8) at a pH of 9.9 to 10 does not mean that aspartic acid does not inhibit corrosion at these concentrations. The large areas of unattacked strongly suggest that aspartic acid does indeed inhibit corrosion. This apparent contradiction arises from the fact that aspartic acid cannot be uniformly distributed on the steel coupon under the stagnant flowing or static conditions in the immersion test tests.

EXAMPLE 4

Steel coupons were prepared to be used as electrodes in the rotating cylinder electrode apparatus described in Example 1 at three different pH values (8, 10 and 12) for aspartic acid solutions containing 1000 ppm aspartic acid. A fourth coupon was subjected to the same procedure (for comparison purposes) at a pH of 10, except that aspartic acid was omitted and the solution was adjusted with sodium sulfate to have the same conductivity as if aspartic acid were present. Corrosion was determined using the electrochemical impedance technique described in Example 1. The results are given in Table 5.

Electrochemical impedance spectra were generated at 0.01 hertz (hz) after approximately 30 minutes to obtain an assessment of the corrosion rate during short exposure. Thereafter, spectra were generated at 0.001 hertz at 200 rpm daily. In addition, spectra were generated at 0.01 hertz at 1000 rpm to obtain values for the effect of fluid velocity on corrosion. The tests were conducted at pH values of 8, 10 and 12 at 1000 ppm aspartic acid and at a pH of 10 in the absence of aspartic acid. The amplitude of the noise signal was low (5 mV) to ensure linearity between deflection and response.

The steel electrodes were weighed both before and after the test.

Mass loss was used for an additional assessment of corrosion rate. It should be noted that at pH 10 and especially 12, mass losses were affected by water seepage behind the electrode. Polarization resistances were evaluated using the circuit analogs of Silverman and Carrico, supra, given in Fig. 2.

The results of the rotating cylinder electrode tests show that under ideal fluid velocity conditions, aspartic acid concentrations of 1000 ppm or less can reduce the corrosion rate to the order of 0.1 to 0.5 mpy, starting from 50 to 100 mpy in the absence of aspartic acid (at a pH of 10). In the absence of aspartic acid, corrosion is increased by fluid movement until the surface is corroded to such an extent that the near-surface velocity profile is compromised. This dependence is expected for corrosion of mild and low-alloy steels in water. However, in the presence of aspartic acid at a pH above 9.5 (measured at room temperature), corrosion is reduced by fluid movement. TABLE 5 ELECTROCHEMICAL IMPEDANCE RESULTS FOR MILD STEEL AT 90ºC

EXAMPLE 5

This example demonstrates that a pre-corroded surface can be protected by the corrosion inhibitors of the present invention. The results reported in Table 6 were obtained by exposing a steel cylinder electrode that had been pre-corroded in deionized water in the rotating cylinder electrode apparatus described in Example 1 with 2000 ppm sodium sulfate (to obtain approximately the same conductivity as 1000 ppm aspartic acid at a pH of 10) and 50 ppm sodium chloride. In 24 hours, the electrode had suffered a significant mass loss and exhibited a reddish-brown rust layer. This electrode was placed in an aqueous solution having an aspartic acid concentration of 5000 ppm and adjusted to a pH of about 10 with sodium hydroxide and maintained under constant rotation. The polarization resistance increased rapidly over a 24 hour period, showing that the corrosion rate decreased with exposure time. The corrosion rate never dropped to the level of an electrode that was not pre-corroded and exposed to 1000 ppm aspartic acid. For example, compare the results given in Table 1. This difference shows that the concentration was not optimized for this particular system. Of greater importance, however, is the observation that even 1000 ppm aspartic acid can inhibit the corrosion of steel, even pre-corroded steel, under the appropriate conditions. TABLE 6 ELECTROCHEMICAL IMPEDANCE FOR MILD STEEL IN ASPARAGIC ACID AT 90ºC: EFFECT OF PRE-CORROSION ON CORROSION INHIBITING PROPERTIES

EXAMPLE 6 (COMPARISON EXAMPLE)

Static immersion tests were conducted as described in Example 2, except that glutamic acid, glycine and certain acids commonly used in antifreeze formulations were substituted for aspartic acid. The parameters and results are given in Table 7. As can be seen, glutamic acid and glycine, respectively, exhibit behavior and corrosion inhibition similar to that of aspartic acid. Although slightly more staining was observed on the coupon, the mass loss was similar to that with aspartic acid at the same pH. Furthermore, these results show that aspartic acid behaves similarly to a mixture of benzoic acid, sebacic acid and octanoic acid at 90°C. Since acids such as the latter are commonly used in antifreeze formulations, the results for aspartic acid (Runs 11 and 12 in Table 3 of Example 2) show that aspartic acid can be used as a suitable substitute for such acids. The ammonium salt of aspartic acid does not appear to work as well as the sodium salt, as shown in Table 7, since the pH drops to 7.7, a pH which is below the point at which aspartic acid (as the corresponding base) can exist in the fully ionized form. TABLE 7 RESULTS OF STATIC IMMERSION TEST FOR MILD STEEL/ASPARAGIC ACID AT 90ºC - VENTILATED

EXAMPLE 7

Static immersion tests were carried out as described in Example 2, except that polyaspartic acid at concentrations of between 2000 ppm and 3.3% and polyaspartylhydroxamic acid (to show the effects of the absence of the amino group of the amino acid) at 90ºC were used instead of aspartic acid. The parameters and results are given in Table 8. The concentration of 2000 ppm was chosen so that the carboxyl concentration is similar to that of aspartic acid at 1000 ppm: corrosion inhibition was found for pH values of 9.5 and higher when measured at 25ºC (which corresponds to a pH of about 8.4 at 90ºC). This is very similar to the pH threshold for aspartic acid and suggests that a higher loading of polyaspartic acid might be required for total inhibition of all surface sites occurring on heterogeneous surfaces, although a significant level of inhibition was observed at 2000 ppm. However, it is expected that at higher fluid velocities such as those used in the rotating electrode, the corrosion inhibiting properties of polyaspartic acid would increase.

Polyaspartylhydroxamic acid, which does not contain an amino group, showed poorer inhibition at the same concentration as aspartic acid. TABLE 8 RESULTS OF STATIC IMMERSION TEST FOR MILD STEEL/POLYASPARAGIC ACID AT 90ºC

EXAMPLE 8

This example shows that the compositions of the invention are effective as corrosion inhibitors at relatively low temperatures.

Static immersion tests were conducted on steel in water at pH 10 as described in Example 2 using no inhibitor, 3% aspartic acid, or 3% polyaspartic acid at 30ºC. The parameters and results are given in Table 9. Both the aspartic acid and the polyaspartic acid imparted significant corrosion inhibition under these relatively low temperature conditions (10.0 mpy decreased to less than 0.1 mpy with no localized corrosion occurring). TABLE 9 RESULTS OF STATIC IMMERSION TEST FOR MILD STEEL/POLYASPARAGIC ACID AT 30ºC

It is thus apparent that according to the present invention, there have been provided compositions and a method for inhibiting corrosion of ferrous or ferrous-containing metals in the presence of an aqueous medium which fully satisfy the objects and advantages set forth hereinbefore.

Claims (20)

1. A composition for inhibiting corrosion of ferrous metals in the presence of an aqueous medium, the composition comprising: a) an amino acid selected from the group consisting of aspartic acid, polyaspartic acid and salts thereof, in an amount sufficient to provide an amino acid concentration of 100 ppm to 5.0 wt.% in the aqueous medium under conditions of use and b) a base in an amount which provides a pH of at least 8.9 in the aqueous medium under conditions of use. 2. The composition of claim 1, wherein the amino acid is present in an amount sufficient to provide an amino acid concentration of 1000 ppm to 3.3% by weight in the aqueous medium under conditions of use. 3. The composition of claim 1, wherein the base is selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, alkaline earth metal hydroxides, ammonium hydroxides and hydrocarbylamines. 4. A composition according to claim 3, wherein the base is an alkali metal hydroxide. 5. A composition according to claim 1, wherein the pH in the aqueous medium under conditions of use is from 8.9 to 14. 6. The composition of claim 1, wherein the pH in the aqueous medium, when measured at room temperature, is from 9.9 to 12. 7. A composition according to claim 6, wherein the pH in the aqueous medium, when measured at room temperature, is from 10 to 11. 8. A process for inhibiting corrosion of ferrous metals in the presence of an aqueous medium, the process comprising adding to the aqueous medium a) adding an amino acid selected from the group consisting of aspartic acid, polyaspartic acid and salts thereof, in an amount sufficient to provide an amino acid concentration of 100 ppm to 5.0 wt.% in the aqueous medium under conditions of use, b) adding a base in an amount sufficient to provide a pH of at least 8.9 in the aqueous medium under conditions of use. 9. The method of claim 8, wherein the amino acid is added in an amount sufficient to provide an amino acid concentration of 1000 ppm to 3.3% by weight in the aqueous medium under conditions of use. 10. The process of claim 1, wherein the base is selected from the group consisting of alkali metal hydroxides, picolin metal carbonates, alkaline earth metal hydroxides, ammonium hydroxides and hydrocarbylamines. 11. The process of claim 8, wherein the base is an alkali metal hydroxide. 12. The method of claim 8, wherein the base is added in an amount sufficient to provide a pH of 8.9 to 14 in the aqueous medium under conditions of use. 13. The method of claim 12, wherein the pH in the aqueous medium, when measured at room temperature, is from 9.9 to 12. 14. A composition according to claim 13, wherein the pH in the aqueous medium, when measured at room temperature, is from 10 to 11. 15. The method of claim 8, wherein the corrosion rate for the iron metal is reduced by 100 to 1000 times compared to the corrosion rate in the absence of the amino acid. 16. The method of claim 8, wherein the aqueous medium is under substantially static conditions. 17. The method of claim 8, wherein the aqueous medium is under dynamic fluid conditions. 18. The method of claim 8, wherein the aqueous medium is at a temperature of 25°C to 90°C under use conditions. 19. The method of claim 18, wherein the temperature is 30°C. 20. The method of claim 18, wherein the temperature is about 90°C.