Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F11/00—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent
  • C23F11/08—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids
  • C23F11/10—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids using organic inhibitors
  • C23F11/14—Nitrogen-containing compounds
  • C23F11/145—Amides; N-substituted amides
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F11/00—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent
  • C23F11/08—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids
  • C23F11/10—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids using organic inhibitors
  • C23F11/14—Nitrogen-containing compounds
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F11/00—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent
  • C23F11/06—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in markedly alkaline liquids
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F11/00—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent
  • C23F11/08—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids
  • C23F11/10—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids using organic inhibitors
  • C23F11/14—Nitrogen-containing compounds
  • C23F11/144—Aminocarboxylic acids
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F11/00—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent
  • C23F11/08—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids
  • C23F11/10—Inhibiting corrosion of metallic material by applying inhibitors to the surface in danger of corrosion or adding them to the corrosive agent in other liquids using organic inhibitors
  • C23F11/173—Macromolecular compounds

Definitions

• the present invention relates to new and improved corrosion inhibiting compositions, an unexpected and new use of biodegradable corrosion inhibitors, and to improved processes for inhibiting corrosion of ferrous metal surfaces (susceptible to corrosion) in the presence of an aqueous medium. More particularly, this invention relates to corrosion inhibiting a ino acids and processes for the use of such corrosion inhibiting amino acids effective to inhibit corrosion of ferrous metals under use conditions in the presence of an otherwise corrosive aqueous medium.
• aspartic acid the preferred amino acid for use in the present invention
• glutamic acid did not come within the scope of the "tendency 11 .
• amino acids such as aspartic acid, although nontoxic and biodegradable, have been avoided as corrosion inhibitors.
• a process for inhibition of corrosion of ferrous metals by using amino acids having only a single amino group, and having an additional carboxyl group (such as aspartic acid) under conditions wherein such amino acids are fully ionized would represent a surprisingly unexpected discovery while satisfying a long-felt need in the industry.
• a corrosion inhibitor for ferrous metals which would decrease the rate of corrosion, even under increased aqueous fluid movement conditions, would represent a substantial improvement in the art.
• Fig. 1 shows a plot of the impedance spectrum in real versus imaginary coordinates for a mild steel electrode rotating at 200 rpm in an aqueous solution at 90 ⁇ C containing 1000 ppm aspartic acid at a pH of 10.
• Fig. 2 shows a plot of the impedence spectrum in real versus imaginary coordinates for a mild steel electrode rotating at 200 rpm in an aqueous solution at 90 ⁇ C at a pH of 10 without aspartic acid, but with conductivity adjusted with sodium sulfate.
• Fig. 3 shows a plot of the impedance magnitude versus logarithm of frequency for the mild steel electrode in Figs. 1 and 2.
• Fig. 4 shows a plot of the phase angle versus logarithm of the frequency for the mild steel electrode in Figs. 1 and 2.
• amino acids having a single amino group and salts thereof.
• these compounds have an excess of carboxyl groups over "free" amino groups, for example, two 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:
• R 1 [-NH-(-CH-) ⁇ -C-] n -R 2 o wherein R represents H, H 2 N-CH-C- , or H 2 N-(-CH 2 -) y - ; CH 2 COOH
• R 2 represents HO-,HOOC-CH-NH-, or HOOC-(-CH 2 -) y -NH-
• CH 2 COOH R 3 represents H or -COOH ; x and y each independently represents an integer from 1 to 3; and n represents an integer for 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, for example, alkali metal, soluble alkaline earth metal, and C,-C 4 alkylamine salts. These compounds are readily available from a number of sources and can be manufactured either by chemical synthesis or microbial fermentation.
• the corrosion inhibitors of the present invention may be employed (in the aqueous medium) at concentrations as low as 100 parts per million to as high as 5.0 weight percent and above. It is particularly preferred to utilize the corrosion inhibitors of the present invention at a concentration of from about 1000 ppm to about 3.3 weight percent. It is understood, however, that concentrations greater than 5.0 weight percent of the corrosion inhibitors can be utilized, if desired, so long as the higher amounts are not detrimental to the system in which the corrosion inhibitors are employed.
• the corrosion inhibiting effect of the compositions of the present invention can be found at temperatures as low as room temperature or about 25 ⁇ C or below and as high as about 90 ⁇ C and above.
• temperature is known to accelerate the corrosion of metals, it is particularly noted that an increase in temperature does not affect the corrosion inhibiting properties of the present invention beyond whatever effect temperature has on the pH.
• the pH of the system may decrease by 1 unit from the value measured at 25 ⁇ C, compared to that measured at 90"C.
• the pK of the fully ionized form of the amino acid will also decrease with an increase in temperature. However, so long as the temperature does not decrease the pH below the point at which the inhibitors become fully ionized, the compositions of the present invention will remain effective.
• the compositions of the present invention are employed in dynamic, flowing systems.
• the corrosion rate of ferrous metals in such systems does not increase with increasing fluid velocity.
• an increase in fluid velocity from, for example, 200 revolutions per minute (rpm) to about 1000 rpm in a rotating cylinder electrode results in an increase in the corrosion rate of ferrous metals in the presence of such an aqueous medium during a period of at least 24 hours.
• This increase in corrosion rate occurs commonly for steels in water and other aqueous systems because the reduction of oxygen is often the rate limiting step. That is, the rate of mass transfer of oxygen to the corroding surface increases with increasing fluid velocity.
• the pH of the aqueous medium under use conditions for the corrosion inhibiting compositions of the present invention may vary from about 8.9 to about 14, preferably from about 9.5 to about 12, as measured at ambient or room temperatures (about 25*C) . It is particularly preferred to use the compositions of the present invention at a pH of about 10 or greater, as measured at ambient or room temperatures. It is understood, however, as previously noted, that the pH will vary, depending upon the temperature at which it is measured.
• one preferred embodiment of this invention is to employ 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 at above about 9.9-10, at which pH the amino acid exists in the fully ionized (conjugate base) form.
• a suitable amino acid preferably aspartic acid
• the pH of the aqueous medium may be adjusted by addition of any suitable base such as an alkali metal hydroxide, for example, sodium hydroxide and potassium hydroxide.
• suitable bases such as an alkali metal hydroxide, for example, sodium hydroxide and potassium hydroxide.
• Additional bases which my be employed in this invention include alkali metal carbonates, hydrocarbylamines, alkaline earth metal hydroxides, and ammonium hydroxides.
• the pH of a corrosive environment may be inherently alkaline, such as, for example, aqueous solutions in contact with lime deposits, concrete, and fertilizer, and automotive antifreeze solutions.
• corrosion inhibition may be effected by merely adding a suitable amino acid or salt thereof in an amount sufficient to provide in the aqueous medium the concentrations previously described, without having to add extraneous bases.
• the corrosion inhibitors may also be used in aqueous media which contain various inorganic and/or organic materials, particularly all ingredients or substances used by the water-treating industry, the automotive industry, and others such as with antifreeze compositions, metal cleaning compositions, and radiator flush compositions.
• the effectiveness of corrosion inhibition for metal surfaces is commonly determined by measurement of the rate of corrosion of the subject metal under specified conditions. Two modes of measurement of corrosion rate were employed herein. For convenience, these may be referred to as (1) the standard metal coupon mass loss test, also referred to as static immersion test, and (2) electrochemical impedance technique.
• metal coupons of known mass are immersed in an aqueous solution whose corrosion inhibiting properties are to be determined.
• the aqueous media is maintained at a specified set of conditions for a specified period of time.
• the coupons are removed from the aqueous solution, cleaned in an ultrasonic bath with soap solution, rinsed with deionized water, rinsed with acetone, patted dry with a lint-free paper towel, blown with a stream of nitrogen, and weighed to determine mass loss and examined under a stereoscope at suitable magnification to determine penetration of the metal surface due to corrosion.
• Corrosion is an electrochemical process rather than a strictly chemical reaction.
• Electrochemical techniques for example, the electrochemical impedance technique, therefore, provide a useful and convenient indication of corrosion rate.
• the electrochemical impedance technique it is helpful to visualize that a corroding metal surface is comprised of a large number of local anodes and a large number of local cathodes whose sites may actually shift or be at the same location as the corrosion reaction ensues.
• the metal is being oxidized, while at the cathodic site reduction is occurring, reduction of hydrogen ions in acidic solutions.
• This corrosion current density is referred to as the "corrosion rate”.
• corrosion rate is converted to "penetration rate" of corrosion, in mils per year (mpy) , or mass loss, by assuming, for example, two electrons per ionized iron atom.
• the "electrochemical impedance technique” is applied wherein the frequency at an electrode interface is varied, using a small voltage amplitude wave of, for example, 5 to 10 millivolts (mV) .
• the response is used to estimate the corrosion rate and to draw some conclusions about the corrosion mechanism.
• a common proportionality factor for carbon steels is 0.025 volts.
• the polarization resistance is inversely proportional to the corrosion rate, relative degrees of polarization resistance are used to determine the degree to which various compositions will either have lower or higher corrosion rates.
• a polarization resistance of 100 ohm-cm 2 is created by a corrosion rate that is about 100 times faster than a corrosion rate having a polarization resistance of 10,000 ohm- cm 2 .
• a polarization resistance of 100 ohm-cm 2 represents a corrosion rate on the order of about 100 mpy, while that of 1000 ohm-cm 2 represents a corrosion rate on the order of about 10 mpy. Conversion of polarization resistance to corrosion rate (as mpy) can be made by assuming a proportionality constant of 25 V and Faraday's law.
• the cylindrical electrode was fabricated from mild steel (C1018) .
• the electrode was sanded with 600 grit silicon carbide paper prior to immersion in the solution to be investigated. Also, the solution was heated to the desired temperature of 90"C prior to immersing the electrode.
• the electrode was mounted on a cylindrical shaft, then immersed and set to rotate at 200 rpm in order to guarantee turbulent flow conditions.
• the water line was at the center of the upper Rulon ® [graphite-impregnated poly(tetrafluoroethylene) , E.I. du Pont de Nemours " Company] spacer to prevent hydrodynamic and effects from interfering with the results to insure optimal flow and current lines.
• the electrodes for each of Samples A and B were rotated at various velocities over identical exposure times.
• the polarization resistances were determined as described in Silverman and Carrico, Ibid, and were used to estimate the corrosion rates which were converted to the penetration rate or mass loss in mils per year (mpy) .
• Impedance spectra for the steel coupon electrodes were generated at a pH of 10 in each of the aqueous solutions employed for Samples A and B and 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 demonstrates how well the model used to obtain the polarization resistance agrees with the actual results. The localized nature of the attack noted for the static immersion test under comparable conditions (in Runs 4 and 5 of Example 2, below) was absent on the rotating cylinder electrode. This behavior suggests that the presence of a uniform velocity field advantageously enables the aspartic acid to inhibit corrosion more uniformly.
• Sample A corrosion potential is -310 mV (S.C.E)
• Sample B corrosion potential is -630 mV (S.C.E.)
• Tables 1 and 2 indicate, 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 far more active at -630 mV (S.C.E.). This difference between the corrosion potentials suggests that the sodium aspartate has a greater tendency to oxidize the steel surface.
• a sodium salt of aspartic acid performs as a corrosion inhibitor for ferrous metals in an unexpected fashion.
• the impedance spectra themselves were studied as a function of the rotation rate or fluid velocity using the rotating cylinder electrode over a 48 hour period. Plots at 200 rpm and after 24 hours are shown in Figs. 1, 2, 3, and 4.
• the specimens were hung on glass hooks in glass jars, each containing about 600 cm 3 (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 mounted through rubber stoppers which sealed the tops of the jars.
• a gas sparger water was introduced at 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 in which the temperature was maintained at 90*C.
• the coupon exposure times were 5 to 7 days, during which time deionized water was periodically added to the aspartic acid test solution to compensate for water loss via evaporation at the elevated temperatures.
• the pH of each solution was adjusted at the beginning of the test by use of sodium hydroxide and was measured at both room temperature (RT, approximately 25°C) and at the temperature of the test.
• 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 under a stereoscope at between 10X and 3OX magnification after exposure. Corrosion rates were estimated 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 in which corrosion was extremely nonuniform or localized to certain areas on the surface, only the mass loss in grams divided by the total exposure time in hours was reported.
• EXAMPLE 3 The procedure described in Example 2 was employed, 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 the addition of sodium sulfate, thereby limiting the corrosion to that created solely by oxygen contained in the water at the designated pH. The results are set forth in Table 4.
• Steel coupons were fabricated to be used as electrodes in the rotating cylinder electrode apparatus described in Example 1 at three different pH levels (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 estimated using the electrochemical impedance technique described in Example 1. The results are shown in Table 5.
• Electrochemical impedance spectra were generated to 0.01 hertz (hz) after about 30 minutes to obtain an estimate of the corrosion rate at short exposure. Thereafter, spectra were generated to 0.001 hz at 200 rpm each day. In addition, spectra were generated to 0.01 hz at 1000 rpm to obtain estimates of the effect of fluid velocity on corrosion.
• EXAMPLE 5 This Example demonstrates that a precorroded surface can be protected by the corrosion inhibitors of the present invention.
• the results shown in Table 6 were determined by exposing a steel cylinder electrode precorroded in deionized water in the rotating cylinder electrode apparatus described in Example 1 with 2000 ppm of sodium sulfate (to have about the same conductivity as 1000 ppm aspartic acid at a pH of 10) and 50 ppm of sodium chloride. In 24 hours, the electrode suffered a significant mass loss and had a red-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 held under constant rotation.
• 1000 ppm aspartic acid can inhibit corrosion of steel, even precorroded steel, under the proper conditions.
• EXAMPLE 7 Static immersion tests were conducted as described in Example 2, except that polyaspartic acid at concentrations from between 2000 ppm to 3.3 percent and polyaspartyl hydroxamic acid (to show the effects of the absence of the amino group of the amino acid) at 90 ⁇ C were employed in place of aspartic acid.
• the parameters and results are shown in Table 8.
• the 2000 ppm concentration was chosen so that the carboxyl concentration would be similar to that of aspartic acid at 1000 ppm.
• Corrosion inhibition was found for pH values of 9.5 and higher when measured at 25°C (which converts to a pH of about 8.4 at 90°C) .
• Polyaspartyl hydroxamic acid which does not contain an amino group, showed poorer inhibition at the same concentration as aspartic acid.
• EXAMPLE 8 This Example demonstrates that the compositions of the present inventions are effective as corrosion inhibitors at relatively low temperatures.
• compositions and a process for inhibiting corrosion of ferrous metals in the presence of an aqueous medium that fully satisfy the objects and advantages set forth hereinabove. While the invention has been described with respect to various specific examples and embodiments thereof, it is understood that the invention is not limited thereto and many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the invention.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Anti-Oxidant Or Stabilizer Compositions (AREA)

Applications Claiming Priority (5)

Publications (2)

Family

ID=27044813

Family Applications (1)

Country Status (9)

Families Citing this family (11)

Family Cites Families (5)

• 1990
  • 1990-08-06 JP JP2511875A patent/JP2823137B2/ja not_active Expired - Lifetime
  • 1990-08-06 CA CA002072881A patent/CA2072881C/en not_active Expired - Lifetime
  • 1990-08-06 EP EP90912512A patent/EP0514376B1/de not_active Expired - Lifetime
  • 1990-08-06 DK DK90912512T patent/DK0514376T3/da active
  • 1990-08-06 DE DE69033634T patent/DE69033634T2/de not_active Expired - Lifetime
  • 1990-08-06 ES ES90912512T patent/ES2152210T3/es not_active Expired - Lifetime
  • 1990-08-06 AT AT90912512T patent/ATE196661T1/de not_active IP Right Cessation
  • 1990-08-06 WO PCT/US1990/004378 patent/WO1991012354A1/en not_active Ceased
  • 1990-08-06 KR KR1019920701771A patent/KR950000908B1/ko not_active Expired - Fee Related

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events