KR950000908B1 - Corrosion Inhibition Composition and Method of Ferrous Metals - Google Patents

Abstract

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Description

[Name of invention]

Corrosion Inhibition Composition and Method of Ferrous Metals

[Brief Description of Drawings]

1 is a plot of the imaginary coordinates for the actual impedance spectrum for a mild steel electrode rotating at 200 rpm in an aqueous solution at pH 10, 90 ° C. containing 1000 ppm of aspartic acid.

2 is a plot of the actual impedance spectra versus virtual coordinates for a mild steel electrode rotating at 200 rpm in an aqueous solution of pH 10, 90 ° C. without aspartic acid.

FIG. 3 is a plot of the logarithm of impedance magnitude versus frequency for the mild steel electrodes of FIGS.

4 is a plot of the logarithm of phase angle versus frequency for the mild steel electrodes of FIGS.

Detailed description of the invention

[Field of Invention]

The present invention provides a novel and simplified corrosion inhibitor composition, an unexpected new use of biodegradable corrosion inhibitors and an improved suppression of corrosion of ferrous metal surfaces (corrosive) in the presence of an aqueous medium. It is about a method.

More specifically, the present invention relates to corrosion inhibiting amino acids, and also to methods of using such corrosion inhibiting amino acids effective to inhibit corrosion of ferrous metals under conditions in which other corrosive aqueous media are present.

[Description of Prior Art]

An important mechanism for protecting metals against corrosion is achieved through the use of corrosion inhibitors. Unfortunately, some conventional corrosion inhibitors, such as formulations containing nitrogen and aromatic compounds widely used as corrosion inhibitor additives in aqueous heating and cooling systems, have been found to be harmful to public health and the surrounding environment. The removal of such harmful compounds by precipitation or other treatment is not only complicated but expensive. Other corrosion inhibitors, such as chromium salts, are prohibited because they are suspected to be carcinogenic factors. Therefore, it has begun to investigate the corrosion inhibitory properties of biologically suitable salts and / or biodegradable compounds.

Biodegradable, non-toxic compounds that can produce high purity products can be very easy to remove or recycle. As such compounds amino acids have been proposed for limited use.

For example, Japanese Patent J50091546-A, issued July 22, 1975 to Nippon Koho, discloses a variety of ferrous and non-ferrous metals when mixtures of amines and amino acids or salts thereof are dissolved in water to form a 20% aqueous solution. It is disclosed to inhibit the corrosion of non-ferrous metal sheets in the atmosphere. The pH of the water absorbed on the sheet is considered to be about 5.5 or less, based on the relationship of the condensation of water in contact with carbon dioxide (CO ₂ ). See Industrial and Engineering Chemistry of White et al., 16 (7), 655-670 (1924), and Journal of Electronanalytical Chemistry of Herren, 180, 511-526 (1984).

However, a wider study of common amino acids has not demonstrated any good results. For example, V. Hluchan et al., "Amino Acids As Corrosion Inhibitors in Hydrochloric Acid Solution," Warkstoffe und Korrosion, 39, 512-517 (1988). The most common amino acids of the species were investigated for their effectiveness as iron corrosion inhibitors in 1.0 M hydrochloric acid at pH about 0. In general, such amino acids that have inhibitory properties at acidic pH did not exhibit an effective corrosion inhibitory effect that would be readily available for industrial use. Long-chain hydrocarbon amino acids and amino acids with additional amino groups or groups capable of increasing the electron density of amino groups only showed the tendency of effective corrosion inhibitory action.

In particular, the preferred amino acids aspartic acid and glutamic acid used in the present invention were not within the scope of this "trend" in the literature, and it was concluded that these amino acids are poor inhibitors due to the single amino group and the short carbon chain and additional carboxyl groups. .

Moreover, since aspartic acid is known to be corrosive at weakly alkaline pH conditions, it is known to those skilled in the art that it is disadvantageous to use aspartic acid as a reverse agent under such acidic pH conditions. Role of Some Biologically Important Componds on the Corrosion of Mind Steel and Copper in Sodium Chloride by K. Ramakrishnaiah Solutions), Bulletin of Electro Chemistry, 2 (1), 7-10 (1986), discloses that aspartic acid actually promotes corrosion at pH8 (inhibitory effect: -25.4%). In fact, even when used in combination with superior corrosion inhibitors for mild steels such as papaverine, the presence of aspartic acid maintains the corrosiveness of the solution.

A problem in the industrial aspect is the fact that when exposed to an aqueous environment, the corrosion movement to ferrous metal increases due to fluid movement. Thus, the expected corrosive effects of amino acids, such as aspartic acid, in aqueous media are expected to be substantially worse if such amino acids are present in an automatic cooler or heater installed such media is operated.

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

Therefore, under conditions where amino acids are fully ionized, methods of inhibiting the corrosion of ferrous metals using amino acids (such as aspartic acid) with a single amino group and additional carboxyl groups are surprising expectations that meet the long-standing needs of the industry. It would have been a discovery. In addition, corrosion inhibitors of ferrous metals that can reduce the corrosion rate even under conditions of increased aqueous fluid motion would be a significant improvement in the art.

Summary of the Invention

It is a primary object of the present invention to provide a novel and improved corrosion inhibiting composition for ferrous metals which exhibits corrosion inhibitive effect in the presence of an aqueous medium.

It is an object of the present invention to provide a new and improved method for inhibiting corrosion of ferrous metals in the presence of an aqueous medium.

Another object of the present invention is to provide a new and improved method of using amino acids having a single amino group as corrosion inhibitor of ferrous metal in the presence of an aqueous medium.

It is a further object of the present invention to provide a new and improved method for inhibiting corrosion of ferrous metals in the presence of an aqueous medium under static conditions.

It is a further object of the present invention to provide a new and improved method for inhibiting the corrosion of ferrous metals in the presence of an aqueous medium under dynamic fluid motion conditions.

Such objects of the present invention will become apparent from the accompanying drawings and the claims.

In particular, it has been found in the present invention that some amino acids, such as aspartic acid, which have previously been known to promote the corrosion of metals in weakly alkaline aqueous media, surprisingly act effectively as corrosion inhibitors when fully ionized under the conditions of use. Such amino acids reduce the corrosion rate of ferrous metal by 100 to 1000 times. Surprisingly, this corrosion inhibitory effect is further enhanced with increasing fluid velocity.

[Description of the Desirable Sun]

Useful in the present invention are amino acids having a single amino group and salts thereof. While it is suitable that the ratio of carboxyl groups / amino groups is 1, preferably these compounds have excess carboxyl groups relative to “free” amino groups, for example in the case of having two carboxyl groups and one amino group. Suitable amino acids are represented by the following structural formula:

Where R ¹ is or or or x and y are each an integer of 1-3; n is an integer indicating the number of repeating aminoacyl units.

Suitable compounds are glycine, polyglycine, aspartic acid, polyaspartic acid, glutamic acid, polyglutamic acid and salts thereof. Suitable salts include, but are not limited to, for example, alkali metal salts, soluble alkaline earth metal salts and C ₁ -C ₄ alkylamine salts.

These compounds are readily available from many sources and may be prepared through chemical synthesis or microbial fermentation.

The molecular weight (M.W.) of polymers of common monomeric amino acids ranges from about 8,000 to about 10,200, with an average M.W. of about 9,200. Typically, for polyaspartic acid, the average M.W. The n value is 78, so the n value ranges from 1 for (monomer) aspartic acid to 87 for polyamino acids having a molecular weight with an upper limit of about 10,200.

These compounds are ineffective as corrosion inhibitors when in the fully quantized cation form, and are exacerbated by the fact that they actually promote corrosion as the pH rises from acid to alkali. However, it has been found that once fully ionized under sufficient alkali use conditions, they dramatically reverse the corrosion rate of ferrous metals. In general, an alkaline pH value of at least about 8.9 is suitable, depending on the temperature used and the particular compound. The corrosion rate under such conditions is reduced by 100 to 1,000 times compared to the corrosion rate of ferrous metal under equivalent pH conditions in the absence of such compounds.

Corrosion inhibitors of the present invention may be used (in aqueous media) at concentrations ranging from low concentrations of 100 parts per million (ppm) to high concentrations of 5.0% by weight and above. The corrosion inhibitor of the present invention is preferably used at a concentration of about 1,000 ppm to about 3.3% by weight. However, if necessary, the corrosion inhibitor may be used in a concentration of 5.0% by weight or more, so long as it is not harmful to the system in which the corrosion inhibitor is used.

The corrosion inhibitory effect of the compositions of the present invention may be exhibited at room temperature or at low temperatures of about 25 ° C. or less and at high temperatures of about 90 ° C. and above.

Although temperature is known to promote metal corrosion, the fact that the temperature rise has no effect on the corrosion inhibitive properties of the present invention other than the effect of temperature on pH is particularly noteworthy. For example, the value measured at 90 ° C. in the pH of the system will be reduced by one unit than the value measured at 25 ° C. In addition, the pH of the fully ionized form of amino acid will also decrease with increasing temperature. However, the composition of the present invention will still work as long as the pH is not decreased by the temperature below the pH at which the inhibitor begins to fully ionize.

In a particularly preferred embodiment, the compositions of the present invention are used in dynamic flow systems. Surprisingly, the corrosion rate of ferrous metal in such a system does not increase with increasing fluid velocity, and the corrosion rate tends to decrease substantially with the increase of the flow velocity.

Typically, increasing the flow rate of the rotating cylinder electrode from 200 rpm to about 1000 rpm under conditions without the present composition results in an increase in the corrosion rate of the ferrous metal in the aqueous medium for at least 24 hours. Since the decrease in oxygen is often the rate limiting step, this increase in corrosion rate usually occurs for steel in water and other aqueous systems. That is, the mass transfer rate of oxygen as the corrosion surface increases with increasing flow velocity.

Under 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, as measured at ambient or room temperature (about 25 ° C.). . Particular preference is given to using the compositions of the present invention at a pH of about 10 or greater, as measured at ambient or room temperature. However, as described above, the pH value may vary depending on the temperature at the time of pH measurement.

Thus, when the aqueous mediator is originally acidic, one preferred aspect of the present invention is at least about 9.5, and most preferably at least about the total pH of the aqueous mediator being in the form of a fully ionized (conjugate base) amino acid. The inclusion of a suitable amino acid, preferably aspartic acid, in a composition comprising an amount of base effective to increase above 9.9 to 10.

The pH of the aqueous medium can be adjusted by addition of suitable bases such as, for example, sodium hydroxide and potassium hydroxide. Addition bases that can be used in the present invention include alkali metal carbonates, hydrocarbylamines, alkaline earth metal hydroxides and ammonium hydroxides.

The pH of the corrosive environment may be alkaline, such as, for example, automotive antifreeze and solutions in contact with lime eluate, concrete and fertilizer. In such a system, corrosion inhibition can be achieved by adding only a suitable amino acid or salt thereof in an amount sufficient to provide the above-mentioned concentration in the aqueous medium, without the addition of a separate base. Includes all components or materials used in a variety of inorganic and / or organic materials, particularly in the water-treatment industry, automotive industry, and in combination with antifreeze compositions, metal wash compositions, and radiator flush compositions The use of corrosion inhibitors in aqueous media is within the scope of the present invention.

The effect of corrosion inhibition on the metal surface is usually determined by measuring the corrosion rate of the metal under certain conditions. Two measurement methods of corrosion rate are used here. For convenience, these methods are called (1) standard metal coupon mass loss test or static immersion test and (2) electrochemical impedance technique.

In the standard metal coupon mass reduction test, a certain mass of metal coupon is immersed in an aqueous solution to determine the corrosion inhibitive properties of the solution. Aqueous vehicles are maintained under certain rights for a specific period of time. At the end of the exposure time, the coupons are removed from the aqueous solution, washed in an ultrasonic bath with soapy liquid, then rinsed with deionized water, rinsed with acetone, and then patted with lint-free paper towels. After blown with a medium of nitrogen, weighing is carried out to determine the mass loss, and the penetration of the metal surface due to corrosion is determined by inspection with a three-dimensional diameter of a suitable magnification.

However, corrosion is an electrochemical process rather than a chemical reaction. Thus, electrochemical impedance techniques and electrochemical techniques provide a useful and convenient indication of corrosion rate. In electrochemical impedance techniques, it is useful to visualize that the corroded metal surface consists of a large number of localized cathodes and anodes whose position is virtually shifted or in the same position as the corrosion reaction proceeds. Metals are oxidized at the anode site, while reduction occurs at the cathode site, while hydrogen ions are reduced in the acidic solution.

The magnitude of the current, expressed in amperes per square centimeter (A / cm 2), measured relative to the reference electrode, is a measure of the tendency for each response to proceed. This corrosion current density is called "corrosion rate". In many cases, the corrosion rate may be converted into a corrosion “penetration rate” expressed in mills / mpy, or a mass loss calculated by estimating, for example, two electrons per ionized iron atom.

An "electrochemical impedance technique" is used which varies the frequency at the electrode interface, for example using small voltage amplification waves of 5-10 millivolts (mV). Response is used to measure the corrosion rate and draw some conclusions about the corrosion mechanism. Analysis of the impedance spectrum yields a "polarization resistance" value measured in ohm-cm 2, which is inversely proportional to the corrosion current density (corrosion rate). Therefore, the corrosion rate according to Ohm's law (i = V / r _p ) is the proportional factor (relative to metal) measured in volts divided by the polarization resistance.

For example, a typical proportional factor for carbon steel is 0.025 volts. Since the polarization resistance is inversely proportional to the corrosion rate, the relative level of polarization resistance is used to determine the degree of low or high corrosion rate of various compositions. Therefore, the polarization resistance of 100 ohm-cm 2 is caused by the corrosion rate about 100 times faster than the corrosion rate having the polarization resistance of 10,000 ohm-cm 2. A polarization resistance of 100 ohm-cm 2 represents a corrosion rate of about 100 mpy, while a polarization resistance of 1000 ohm-cm 2 represents a corrosion rate of about 10 mpy. The polarization resistance can be ringed as a corrosion rate (mpy) using a 25mV proportional constant and Faraday's law.

For electrochemical impedance technology, the 1986 edition of Houston's Corrosion Engineers International Association, Electro Chemical Techniques for Corrosion Engineering (R. Baboian, ed.), Pp. See Primer on AC Impedance Technique by D. C. Silverman, pp. 73-79.

For a clear understanding of the invention, the following specific examples describe in detail the best method for carrying out the invention. However, the detailed description of the application of the present invention has been given by way of example only to represent preferred embodiments and should not be construed as limiting the present invention, and various changes and modifications are possible within the technical idea of the present invention. It will be apparent to those skilled in the art from the detailed description of the invention.

In the following examples, unless otherwise indicated, all parts are parts by weight, all percentages are weight percentages, all temperatures are degrees Celsius (° C.), pH is values measured at 25 ° C., and “loss of mass” means “penetration rate”. "Means.

Example 1

Electrochemical impedance techniques were used to measure the corrosion rate for the two mild steel (C1080) electrodes labeled Samples A and B. The parameters and results are shown in Table 1 and Table 2.

Steel coupons were made to be used as electrodes in a rotating cylinder electrode device. The apparatus is described in detail in D. C. Silverman's “Rotating Cylinder Electrode for Velocity Sensitivity Testing” described in Corrosion, 40 (5), 220 ° C. 226 (1984). For electrochemical impedance techniques, see DC Silverman and JE Carico's Corrosion, 44 (5), 280-287 (1988), "Electrochemical Impedance Techniques-Practical Tools for Corrosion Prediction". A Practical Tool for Corrosion Prediction.

A cylindrical electrode was prepared using mild steel (C1018). The electrode was polished using 600 grit silicon carbide paper before immersing the electrode in the solution to be irradiated. The solution was also heated to the required temperature of 90 ° C. before immersing the electrode. The electrode was placed on a cylinder shaft and then immersed in solution and allowed to rotate at 200 rpm to ensure turbulent flow conditions. Water line is the upper runlon [Poly (tetrafluoroethylene) containing graphite, product of EI du pont de Nemours & Company] It is located in the center of spacer, so it is optimal flow and current due to the influence of hydraulic pressure. This prevented the securing of current lines.

The data shown in Table 1 (as sample A) was obtained by exposing the mild steel electrode to a sodium aspartate solution at pH 10 in a rotating cylinder apparatus. The concentration of sodium aspartate was approximately 1000 ppm. The pH was measured at 25 ° C., but the temperature was adjusted to 90 ° C.

The data shown in Table 2 were obtained for Sample B in a similar manner except that sodium sulfate (no effect on corrosion) was used in place of sodium aspartate to give the same ionic strength.

By measuring the voltage between the steel electrode and the saturated calomel electrode, the corrosion potential of the steel electrode used for each of Samples A and B was measured. The electrodes of each of Samples A and B were rotated at various speeds during the same exposure time. The polarization resistance was measured as described in Silverman and Carico mentioned above, and this measurement was used to calculate the corrosion rate in terms of penetration or mass loss expressed in mills / may.

Impedance spectra for steel coupon electrodes (Samples A and B) were obtained using a rotating cylinder electrode device under the condition that the pH of each aqueous solution used for Samples A and B was 10 and the rotation rate was 200 rpm. These spectra (curves) are shown in the first, second, third and fourth degrees.

The agreement between the estimated curve and the actual data demonstrates how good the model was used to obtain polarization resistance that matches the actual results. The corrosive localized properties of the immersion test under comparative conditions (performed in 4 and 5 of Example 2 below) did not appear on the rotating cylinder electrode. This phenomenon shows that aspartic acid can inhibit corrosion evenly due to the presence of a uniform velocity field. In addition, this uniform increase in corrosion inhibition shows that the suppression is aided by the flatter 600 grit used for the electrode as compared to 120 grit for the coupons used in the static immersion test. The real result obtained by the smoother smother finish is that the surface topography of the electrode will be more homogeneous than the surface topography of the static immersed coupon. Thus, a more uniform rate and flat steel surface reduced the concentration of aspartic acid needed to uniformly suppress all site corrosion on that surface.

TABLE 1

Corrosion of mild steel in the presence of 1000 ppm sodium aspartate (pH at 25 ° C = 10)

The corrosion potential of Sample A is -310 mV (S.C.E).

TABLE 2

Corrosion of Mild Steel in the Presence of Sodium Hydroxide (pH at 10 ° C. = 10)

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

As shown in Table 1 and Table 2. The corrosion potential of sample A in the presence of sodium aspartate is -310 mV (S.C.E), while the corrosion potential of sample B in the absence of sodium aspartate is much more active at -630 mV (S.C.E). Sodium aspartate between these corrosion potentials shows a greater tendency to oxidize the steel surface.

Nevertheless, the corrosion rate of each sample is inversely related to this oxidation tendency. After the same exposure time, the magnitude of the difference between the corrosion rates of Sample A and Sample B indicates that aspartate inhibits corrosion by 100-1000 times. Although the corrosion starts about the same rate for Sample A (32mpy) and Sample B (45mpy), the corrosion rate decreases rapidly in the presence of sodium aspartate, whereas the corrosion rate in the absence of sodium aspartate Stayed constant.

Moreover, in the absence of aspartate, an increase in turnover or flow rate resulted in an increase in corrosion rate up to at least 24 hours. After 48 hours, there was no change due to corrosion products formed on the surface. This is common for carbon steel and water because the reduction of oxygen is the limiting step in corrosion. That is, the mass transfer rate of oxygen to the corrosion surface often determines the corrosion rate, and this oxygen transfer rate is adversely affected when the corrosive is formed on the surface. However, in the presence of aspartate, the corrosion rate did not increase with increasing flow rate, and in practice the corrosion rate decreased significantly with increasing turnover during operation. Even after the turnover rate has been reduced to 200 rpm, as can be seen from the corrosion rates in Table 1, which have been determined for exposure times of 46 to 48 hours, 49 hours, and 50 hours, the corrosion rate before the flow rate increases to 1000 rpm. Since the sample did not return to 0.13 mpy at 200 rpm, the decrease in corrosion rate obtained by increasing the flow rate is irreversible.

Therefore, sodium salts of aspartic acid under basic conditions act as corrosion inhibitors for ferrous metals in unexpected ways.

The rotational cylinder electrode was used for 48 hours to study the functional relationship between the impedance spectrum and the rotation rate or flow rate. Plots at 200 rpm after 24 hours are shown in FIGS. 1, 2, 3, and 4.

There are two peaks in the phase angle plot for mild steel in contact with sodium aspartate. This is shown in FIG. This phenomenon suggests two relaxation time constants, and also shows that strongly adsorbed media or firmly adhered films are involved in the corrosion mechanism. High frequency peaks are due to adsorbed media on the film, while low frequency peaks are associated with corrosion rates. Thus, while the present invention is not intended to be bound or limited in any way to the theory of corrosion mechanisms, aspartate ions are intended to form some form of an adsorption layer on a steel surface, although the mechanism is not fully understood. It is believed.

Another evidence that some form of adsorption layer is present on the steel surface in the presence of aspartate ions is provided by the phase angle plot for mild steel under comparative conditions without aspartate ions, as shown in FIG. In this plot, there is only one peak showing that only a charge transfer corrosion reaction occurs.

Example 2

Fourteen identical mild steel (C1018) coupon specimens were rubbed using 120 grit silicon carbide paper and rinsed with deionized water, then dried and weighed. The samples were then subjected to a static dipping test. Parameters and results are reported in Table 3. The specimens were hung on glass hooks in glass bottles each containing about 600 cm 3 (or cc) of L-aspartic acid test solution.

The solution was prepared using deionized water and an amount of L-aspartic acid sufficient to provide the desired aspartic acid concentration. The loop was placed through a rubber stopper that sealed the top of the bottle. A gas sparger was introduced on the stopper side to aeration the solution of water-saturated air with carbon dioxide removed. The bottle was placed in a thermostat maintained at 90 ° C. The coupon exposure time was 5-7 days, during which deionized water was periodically added to the aspartic acid test solution to replenish the water reduced due to evaporation at elevated temperatures. The pH of each solution was measured at room temperature (RT, approximately 25 ° C.) and test temperature adjusted at the beginning of the test using sodium hydroxide.

When the coupon exposure time was completed, the coupon was removed from the solution, washed in an ultrasonic bath containing soap solution, rinsed with deionized water, rinsed with acetone, and then dried and weighed. The coupon surface after exposure was irradiated at 10 × and 30 × magnification using a stereoscope. The change in mass (both before and after exposure to aspartic acid solution) was measured to determine the corrosion rate by the method described above, and then the penetration or mass loss as mpy or grams per hour.

If the corrosion occurred extremely non-uniformly or locally for any area on the surface, only the mass loss (grams) divided by the total exposure time (hours) was recorded. The reason is that if corrosion occurs within a very limited area, the average corrosion rate for the entire surface does not accurately describe the degree of corrosion. Nevertheless, the results in Table 3 obtained from the static immersion test compared to the results in Tables 1 and 2 obtained from certain flow systems, require aspartic acid to achieve the same level of corrosion inhibition under fairly steady flow conditions. Demonstrate an increase in concentration.

TABLE 3

Static immersion test results on mild steel / aspartic acid, 90 ° C-air injected

Total Grams per Hour Total Exposure

Example 3

Aspartic acid was not included in the aqueous solution and the process described in Example 2 was used as is except that static immersion tests were applied to only three steel coupons. By adding sodium sulfate to the solution, it was adjusted to have the same conductivity as with aspartic acid, and was limited to the degree of corrosion that can only be produced by oxygen contained in water at the specified pH. The results are shown in Table 4.

TABLE 4

90 ° C static test results for mild steel without aspartic acid

Total Grams per Hour Total Exposure

Comparing the results of Examples 2 and 3 of Table 3 with the results of Example 1 of Table 4, the corrosion rate at pH 8 is much greater in the presence of aspartic acid than in the absence of aspartic acid. This ensures that at pH 8 there is no effective corrosion inhibitory action by aspartic acid; Instead it acts as a corrosion promoter.

The same phenomenon is found for pH 9.5, 3% by weight of aspartic acid (Executive 10 in Table 3). Such a phenomenon is due to the fact that aspartic acid does not exist in a fully or sufficiently ionized (paired) state at a pH of 9.5 or below. This is reliably demonstrated by observing changes in action at low aspartic acid concentrations, such as 1000 ppm, at pH higher than 9.5, for example pH 10 or higher. In fact, all corrosion disappears at pH 10, 1% by weight, under the static test conditions of Table 3 (Example 9). Thus, aspartic acid at a pH between 8.5 and about 9.0 at 90 ° C. or at pH 10 or above at room temperature (approximately 25 ° C.), inhibits corrosion under static conditions while increasing corrosion at lower pH. Or promote it.

The fact that some attack or corrosion occurs at concentrations of 1000 ppm (Examples 4 and 5) and 5000 ppm (Example 8), pH 9.9 to 10, means that aspartic acid does not inhibit corrosion at such concentrations. The large area without any attack shows that aspartic acid actually inhibits corrosion. This apparent inconsistency stems from the fact that aspartic acid cannot be uniformly distributed on steel coupons under static flow or static conditions in immersion test runs.

Example 4

Steel coupons were prepared for use as electrodes in the rotating cylinder electrode device described in Example 1 at six different pH levels (8, 10, 12) for aspartic acid solution containing 1000 ppm of aspartic acid. For the fourth coupon, the same process was performed at pH 10 except that the solution did not contain aspartic acid but the solution was adjusted to the same conductivity with sodium sulfate as when aspartic acid was present. Corrosion was evaluated using the electrochemical impedance technique described in Example 1. The results are shown in Table 5.

In order to calculate the corrosion rate following short exposure, an electrochemical impedance spectrum of 0.01 Hz was obtained after about 30 minutes. Then, a spectrum of 0.001 hz was obtained every day at 200 rpm. We also obtained a spectrum of 0.01 hz at 1000 rpm to measure the effect of flow rate on corrosion. Experiments were performed at pH 8, 10 and pH 12 in the presence of 1000 ppm aspartic acid and at pH 10 in the absence of aspartic acid. The magnitude of the perturbing voltage signal is small (5mV), which confirms the linearity between perturbation and damping.

The steel electrode was weighed before and after the experiment and the corrosion rate was calculated using mass reduction. It should be noted that the mass loss at pH 10 and especially at pH 12 was affected by the leakage of water at the electrode back. Polarization resistance was calculated using the circuit analog shown in FIG. 2 in Silverman and Carico mentioned above.

The results of the rotating cylinder electrode test show that under ideal flow rate conditions, aspartic acid concentrations as low as at least 1000 ppm can reduce the corrosion rate from 0.1 to 0.5 mpy from 50 to 100 mpy in the absence of aspartic acid (at pH 10). Fluid movement in the absence of aspartic acid increases corrosion until the surface corrodes to such an extent that the velocity profile is affected near the surface. This dependency is usually expected for the corrosion of mild and low alloy steels in water. However, if aspartic acid is present at a pH above 9.5, the corrosion is reduced by fluid motion.

TABLE 5

Electrochemical Impedance Results for Mild Steel at 90 ° C

Example 5

This example demonstrates that already corroded surfaces can be protected by the corrosion inhibitor of the present invention. The results shown in Table 6 show the pre-corroded steel in deionized water in the presence of 2000 ppm sodium sulfate (pH 10 to 1000 ppm aspartic acid) and 50 ppm sodium chloride in the rotary cylinder electrode device described in Example 1 Results determined by exposing the cylinder electrode. Within 24 hours the electrode had a significant mass loss and a red-brown rust layer formed. This electrode was placed in an aqueous solution containing 5000 ppm aspartic acid concentration and adjusted to pH 10 with sodium hydroxide and left under constant rotation. The polarization resistance increased rapidly for 24 hours, indicating that the corrosion rate decreased with the exposure time.

The corrosion rate was not reduced to the corrosion rate obtained when the noncorroded electrode was exposed to 1000 ppm of aspartic acid, which can be seen by comparison with the results shown in Table 1, for example. This difference tells us that the concentration was not optimal for this particular system. More importantly, however, under suitable conditions, 1000 ppm of aspartic acid can inhibit corrosion of steel, already corroded steel.

TABLE 6

Electrochemical Impedance on Mild Steel in Aspartic Acid at 90 ° C: Effect on Corrosion

Example 6

Static soaking tests were performed as described in Example 2 except that some amino acids commonly used in glutamic acid, glycine and anti-antifreeze were used in place of aspartic acid. The parameters and results are shown in Table 7. Glutamic acid and glycine each show similar actions and corrosion inhibition aspartic acid. While coupon staining was observed somewhat more, the mass loss was similar to the value obtained by aspartic acid at the same pH. These results also show that aspartic acid has a comparable action at 90 ° C. with a mixture of benzoic acid, sebacic acid and octanoic acid. Since acids such as the acids mentioned above are usually used in anti-floating agents, the results for aspartic acid (Examples 11 and 12 in Table 3 of Example 2) can be used as a suitable substitute for such acids. Tells.

As shown in Table 7, the ammonium salts of aspartic acid do not appear to have the same effect as sodium salts because the pH is reduced to pH 7.7, which is below the pH at which aspartic acid (as a base) can exist in fully ionized form. .

TABLE 7

Static immersion test results for mild steel / aspartic acid at 90 ° C-air injection

Total Grams per Hour Total Exposure

Example 7

Polyaspartic acid at a concentration of 200 ppm-3.3% at 90 ° C. instead of aspartic acid [M.W = 8,000-10,200; Average M.W. = 9,200; static immersion test was performed as described in Example 2 except that n = 78 (relative to mean MW)] and polyaspartyl hydroxamic acid (to show the effect of the amino group absence of amino acids). . The parameters and results are shown in Table 8. The carboxyl concentration was chosen at a 2000 ppm concentration to be the same as for the 1000 ppm aspartic acid.

Corrosion inhibition was observed at pH 9.5 (which translates to pH 8.4 at 90 ° C) and above when measured at 25 ° C.

This is very similar to the threshold pH for aspartic acid, which is higher for total inhibition on all surface areas so that even if a significant degree of inhibition is observed at 2000 ppm, the inhibitory effect also appears on other heterogeneous surfaces. Suggests the need for polyaspartic acid. However, under high flow rates such as those used with rotary electrodes, the corrosion inhibitory properties of polyaspartic acid are expected to increase. Polyaspartyl hydroxamic acid without amino group showed lower inhibition than when using the same concentration of aspartic acid.

TABLE 8

Static immersion test results for mild steel / aspartic acid at 90 ° C

Total Grams per Hour Total Exposure

2M. W. = 8,000-12,000; Average M. W. = 9,200; n = 78 (for mean M. W.)

Example 8

The examples demonstrate that the compositions of the present invention exert an effect as corrosion inhibitors even at relatively low temperatures.

Static immersion test was performed as described in Example 2 with 3% aspartic acid or 3% polyaspartic acid on steel in pH 10 without inhibitor at 30 ° C. The parameters and results are shown in Table 9. Both aspartic acid (n = 1) and polyaspartic acid exhibited significant corrosion inhibitory effects under relatively low temperature conditions (reduced from 10.0 mpy to less than 0.1 mpy without local corrosion).

TABLE 9

Static immersion test results for mild steel / polyaspartic acid at 30 ° C

Total Grams per Hour Total Exposure

It is therefore evident that the present invention provides a composition and method for inhibiting the corrosion of ferrous metals in the presence of an aqueous medium, which satisfactorily satisfies the above objects and advantages. While the invention has been described through various specific embodiments and aspects thereof, it is to be understood that the invention is not so limited and that many changes, modifications and variations will be apparent to those skilled in the art in view of the foregoing description. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and broad scope of the present invention.

Claims (30)

(a) an amino acid of the formula Where R ¹ is or R ¹ is or R ³ is or x and y are each an integer of 1-3; n is an integer indicating the number of repetitions of the aminoacyl unit in an amount effective to suppress corrosion of the ferrous metal; (b) a composition which inhibits the corrosion of ferrous metals in the presence of an aqueous medium comprising an amount of a base effective to provide an amino acid in a fully ionized form under conditions of use. The composition of claim 1, wherein the amino acid is selected from the group consisting of glycine, polyglycine, aspartic acid, polyaspartic acid, glutamic acid, polyglutamic acid and salts thereof. The composition of claim 1 wherein the amino acid is aspartic acid and salts thereof. The composition of claim 1, wherein the amino acid is present in an aqueous medium under conditions of use in an amount sufficient to provide an amino acid concentration of at least 100 ppm to 5.0% by weight. The composition according to claim 4, wherein the amino acid concentration is 1000 ppm to 3.3% by weight. The composition of claim 1 wherein the base is selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, alkali metal hydroxides, ammonium hydroxides and hydrocarbylamines. 7. The composition of claim 6, wherein the base is an alkali metal hydroxide. 8. The composition of claim 7, wherein the alkali metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide. The composition of claim 1 wherein the pH of the aqueous medium under conditions of use is at least 8.9. 10. The composition of claim 9 wherein the pH of the aqueous medium under conditions of use is 8.9-14. The composition of claim 1, wherein the pH of the aqueous medium is 9.9-12, as measured at room temperature. The composition of claim 11 wherein the pH of the aqueous medium is 10-11 as measured at room temperature. A method of inhibiting corrosion of ferrous metal in the presence of an aqueous medium, comprising adding (a) and (b) to the aqueous medium: Where R ¹ is or R ³ is or x and y are each an integer of 1-3; n is an integer representing the repetition number of aminoacyl units in an amount effective to suppress corrosion of the ferrous metal; (b) an amount of base effective to provide an amino acid in its fully ionized form under the conditions of use. The method of claim 13, wherein the amino acid is selected from the group consisting of glycyl, polyglycine, aspartic acid, polyaspartic acid, glutamic acid, polyglutamic acid, and salts thereof. The method of claim 13, wherein the amino acid is aspartic acid and its salts. The method of claim 13, wherein the amino acid is present in an aqueous medium under conditions of use in an amount sufficient to provide an amino acid concentration of at least 100 ppm to 5.0 weight percent. The method of claim 16, wherein the amino acid concentration is 1000 ppm to 3.3% by weight. The method of claim 13, wherein the base is selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, alkali metal hydroxide ammonium hydroxides and hydrocarbylamines. The method of claim 13, wherein the base is an alkali metal hydroxide. 20. The method of claim 19, wherein the alkali metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide. The method of claim 13, wherein the base is added in an amount sufficient to provide at least pH 8.9 in the aqueous medium under the conditions of use. 22. The method of claim 21, wherein the base is added in an amount sufficient to provide a pH of 8.9-14 in the aqueous medium under the conditions of use. The method of claim 13, wherein the pH of the aqueous medium is 9.9-12 when measured at room temperature. The method of claim 23, wherein the pH of the aqueous medium is 10-11 when measured at room temperature. The method of claim 13, wherein the corrosion rate for the ferrous metal is reduced by 100-1000 times when compared to the corrosion rate in the absence of amino acids. The method of claim 13, wherein the aqueous medium is under essentially static conditions. The method of claim 13 wherein the aqueous medium is under dynamic flow conditions. The method of claim 13, wherein the temperature of the aqueous medium under the conditions of use is 25-90 ° C. 29. The method of claim 28, wherein the temperature is 30 ° C. 29. The method of claim 28, wherein the temperature is 90 ° C.