Corrosion inhibitor composition for magnesium or magnesium alloys

Abstract

The present invention relates to novel corrosion inhibitor compositions for magnesium or magnesium alloys and to a process for inhibiting the corrosion of such metals using such compositions. The corrosion inhibitor composition for magnesium or magnesium alloys comprises at least one corrosion inhibiting compound which is capable of forming a complex with at least one of Fe(II), Fe(III), Cu(I), Cu(II) and Ni(II) ions, where the complex with at least one of Fe(II), Fe(III), Cu(I), Cu(II) and Ni(II) ions has a stability constant in aqueous solution at room temperature (about 21 C.) log K of greater than or equal to 3.5 wherein the corrosion inhibiting compound is selected from the group of pyridinedicarboxylic acids, and their salts or esters thereof.

Claims

1. A corrosion inhibitor composition coated on a magnesium or magnesium alloy, the corrosion inhibitor composition comprising at least one corrosion inhibiting compound selected from the group of substituted or unsubstituted pyridine-dicarboxylic acids, their salts, esters or amides, and pyridine-dicarboxaldehyde dioximes, the corrosion inhibitor composition being coated on the magnesium or magnesium alloy.

2. The corrosion inhibitor composition of claim 1, wherein the unsubstituted pyridinedicarboxylic acids, their salts, esters and/or amides are selected from the group consisting of 2,6-pyridinedicarboxylic acid, 2,5-pyridinedicarboxylic acid, 2,4-pyridinedicarboxylic acid, 2,3-pyridinedi-carboxylic acid, 3,5-pyridinedicarboxylic acid, 3,4-pyridine-dicarboxylic acid, and their salts, esters and/or amides.

3. The corrosion inhibitor composition of claim 1, wherein the substituted pyridinedicarboxylic acids, their salts, esters and/or amides are selected from the group consisting of 2-amino-3,5-pyridinedicarboxylic acid, 2-hydroxy-3,5-pyridine-dicarboxylic acid, 2-chloro-3,5-pyridinedicarboxylic acid, 2-methyl-3,5-pyridinedi-carboxylic acid, 2-ethyl-3,5-pyridinedicarboxylic acid, 2-amino-3,4-pyridinedi-carboxylic acid, 2-hydroxy-3,4-pyri-dinedicarboxylic acid, 2-chloro-3,4-pyridinedicarboxylic acid, 2-methyl-3,4-pyridinedicarboxylic acid, 2-ethyl-3,4-pyridinedi-carboxylic acid, 3-amino-2,5-pyridinedi-carboxylic acid, 3-hydroxy-2,5-pyridinedicarboxylic acid, 3-chloro-2,5-pyridinedicarboxylic acid 3-methyl-2,5-pyridinedi-carboxylic acid, 3-ethyl-2,5-pyridine-dicarboxylic acid, 3-amino-2,4-pyridinedicarboxylic acid, 3-hydroxy-2,4-pyridinedicarboxylic acid, 3-chloro-2,4-pyridinedi-carboxylic acid, 3-methyl-2,4-pyridinedi-carboxylic acid, 3-ethyl-2,4-pyridinedicarboxylic acid, 3-amino-2,6-pyridinedicarboxylic acid, 3-hydroxy-2,6-pyridinedi-carboxylic acid, 3-chloro-2,6-pyridinedi-carboxylic acid, 3-methyl-2,6-pyridinedicarboxylic acid, 3-ethyl-2,6-pyridinedicarboxylic acid, 4-amino-2,6-pyridine-dicarboxylic acid, 4-hydroxy-2,6-pyridinedi-carboxylic acid, 4-chloro-2,6-pyridinedicarboxylic acid 4-methyl-2,6-pyridinedicarboxylic acid, 4-ethyl-2,6-pyridinedi-carboxylic acid, 4-amino-2,5-pyridinedi-carboxylic acid, 4-hydroxy-2,5-pyridinedicarboxylic acid, 4-chloro-2,5-pyridinedicarboxylic acid, 4-methyl-2,5-pyridinedi-carboxylic acid, 4-ethyl-2,5-pyridinedicar-boxylic acid, 4-amino-2,3-pyridinedicarboxylic acid, 4-hydroxy-2,3-pyridinedicarboxylic acid, 4-chloro-2,3-pyridinedi-carboxylic acid, 4-methyl-2,3-pyridinedicar-boxylic acid, 4-ethyl-2,3-pyridinedicarboxylic acid, 5-amino-2,3-pyridinedi-carboxylic acid, 5-hydroxy-2,3-pyridinedi-carboxylic acid, 5-chloro-2,3-pyridinedicar-boxylic acid, 5-methyl-2,3-pyridinedicarboxylic acid, 5-ethyl-2,3-pyridinedicarboxylic acid, 5-amino-2,4-pyridinedi-carboxylic acid, 5-hydroxy-2,4-pyridinedi-carboxylic acid, 5-chloro-2,4-pyridinedicarboxylic acid, 5-methyl-2,4-pyridinedicarboxylic acid, 5-ethyl-2,4-pyridinedi-carboxylic acid, 5-amino-2,6-pyridinedi-carboxylic acid, 5-hydroxy-2,6-pyridinedicarboxylic acid, 5-chloro-2,6-pyridinedicarboxylic acid, 5-methyl-2,6-pyridinedi-carboxylic acid, 5-ethyl-2,6-pyridinedicar-boxylic acid, and their salts esters and/or amides.

4. The corrosion inhibitor composition of claim 1, wherein the salts of the substituted or unsubstituted pyridinedicarboxylic acid are selected from their alkali metal, earth alkaline metal salts or ammonium salts.

5. The corrosion inhibitor composition of claim 1, wherein the esters of the substituted or unsubstituted pyridinedicarboxylic acid are selected from their methyl, ethyl, propyl, butyl, or pentyl esters.

6. The corrosion inhibitor composition of claim 1, wherein the pyridine-dicarboxaldehyde dioximes are selected from pyridine-dicarboxaldehyde-2,6-dioxime, dicarboxaldehyde-2,5-dioxime, dicarboxaldehyde-2,4-dioxime, dicarboxaldehyde-2,3-dioxime, dicarboxaldehyde-3,5-dioxime and dicarboxaldehyde-3,4-dioxime.

7. The corrosion inhibitor composition of claim 1 wherein the corrosion inhibiting compound is present in cavities of porous nano- or microparticles distributed within the corrosion inhibitor composition coating the magnesium or magnesium alloy.

8. The corrosion inhibitor composition of claim 1, wherein the corrosion inhibiting compound is present in micro- and nano-pores produced on the surface of the magnesium or magnesium alloy.

9. The corrosion inhibitor composition of claim 1, wherein magnesium or magnesium alloy is selected from the group consisting of high purity Mg, commercial purity Mg, WE43, ZE41, E21, AZ31, AZ91 and AM50.

10. A method for inhibiting the corrosion of magnesium or magnesium alloys comprising the steps of a) providing magnesium or a magnesium alloy and b) coating the magnesium or magnesium alloy with a corrosion inhibiting coating comprising a corrosion inhibitor composition according to claim 1.

11. The method of claim 9, further comprising a step a1) between step a) and b), wherein in step a1) the magnesium or magnesium alloy is pre-treated with a corrosion inhibitor composition according to claim 1.

12. A method of using the corrosion inhibitor composition according to claim 1 comprising coating magnesium or a magnesium alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the results (normalized values) of hydrogen evolution measurements during immersion of HP Mg (high purity Mg containing Fe51 ppm);

(2) FIG. 2 shows the results (normalized values) of hydrogen evolution measurements during immersion of CP Mg (commercial purity Mg containing Fe220 ppm);

(3) FIG. 3 shows the results (normalized values) of hydrogen evolution measurements during immersion of WE43 magnesium alloy;

(4) FIG. 4 shows the results (normalized values) of hydrogen evolution measurements during immersion of ZE41 magnesium;

(5) FIG. 5 shows the results (normalized values) of hydrogen evolution measurements during immersion of Elektron 21 (E21) magnesium alloy;

(6) FIG. 6 shows the results (normalized values) of hydrogen evolution measurements during immersion of AZ31 magnesium alloy;

(7) FIG. 7 shows the results (normalized values) of hydrogen evolution measurements during immersion of AZ91 magnesium alloy; and

(8) FIG. 8 shows the results (normalized values) of hydrogen evolution measurements during immersion of AM50 magnesium alloy.

DETAILED DESCRIPTION

(9) The term stability constant, K, refers herein above and in the claims section to the equilibrium constant for the equilibrium that exists between a metal ion surrounded by water molecule ligands and the same transition metal ion surrounded by a ligand or ligands of another kind in a ligand displacement reaction for the overall displacement reaction.

(10) The term room temperature refers herein above and in the claims section to the temperature as defined in The American Heritage Dictionary of the English Language, which identifies room temperature as around 20 C. to 22 C. (68 F. to 72 F.).

(11) The present inventors found that surprisingly noble impurities like iron, copper and nickel get detached from the magnesium metal by undermining mechanisms and dissolve by forming Fe(II), Fe(III), Cu(I), Cu(II) and Ni(II) ions. Subsequently, these ions are being reduced and re-deposit on the surface of the anode, which accelerates corrosion. Thus, based on this finding, it has been found that prevention of re-deposition of noble impurities significantly decreases the corrosion rate of the metal. Re-deposition of dissolved nickel, copper, and especially iron is effectively avoided by chemically binding said ions by means of complexing agents among which pyridinedicarboxylic acids were found to be excellent ligands for metal ions including iron, copper and nickel.

(12) The pyridinedicarboxylic acids or their salts, esters or amides may be unsubstituted or substituted at the pyridine ring with one or more alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl and/or hexyl groups; alkoxy groups, such as methoxy, ethoxy, propoxy, butoxy, pentoxy and/or hexoxy groups; halogen groups, such as chloro, bromo and/or iodo groups; cyano groups; amino groups; and/or hydroxyl groups or the like.

(13) Suitable pyridinedicarboxylic acids or their salts, esters and/or amides are preferably selected from the group consisting of 2,6-pyridinedicarboxylic acid, 2,5-pyridinedicarboxylic acid, 2,4-pyridinedicarboxylic acid, 2,3-pyridinedicarboxylic acid, 3,5-pyridinedicarboxylic acid, 3,4-pyridinedicarboxylic acid, and their salts, esters and/or amides. The afore-mentioned pyridinedicarboxylic acids, their salts, esters and/or amides may be unsubstituted or substituted with alkyl, e.g. methyl, ethyl, propyl, butyl, pentyl and/or hexyl; alkoxy, e.g. methoxy, ethoxy, propoxy, butoxy, pentoxy and/or hexoxy; halogen, e.g. chloro, bromo and/or iodo; cyano; amino; and/or hydroxyl.

(14) Suitable substituted pyridinedicarboxylic acids or their salts, esters and/or amides include 2-amino-3,5-pyridine-dicarboxylic acid, 2-hydroxy-3,5-pyridinedicarboxylic acid, 2-chloro-3,5-pyridinedicarboxylic acid, 2-methyl-3,5-pyridine-dicarboxylic acid, 2-ethyl-3,5-pyridinedicarboxylic acid, 2-amino-3,4-pyridinedi-carboxylic acid, 2-hydroxy-3,4-pyridine-dicarboxylic acid, 2-chloro-3,4-pyridinedicarboxylic acid, 2-methyl-3,4-pyridinedicarboxylic acid, 2-ethyl-3,4-pyridine-dicarboxylic acid, 3-amino-2,5-pyridinedicarboxylic acid, 3-hydroxy-2,5-pyridinedicarboxylic acid, 3-chloro-2,5-pyridine-dicarboxylic acid 3-methyl-2,5-pyridinedicarboxylic acid, 3-ethyl-2,5-pyridinedicarboxylic acid, 3-amino-2,4-pyridine-dicarboxylic acid, 3-hydroxy-2,4-pyridinedicarboxylic acid, 3-chloro-2,4-pyridinedicarboxylic acid, 3-methyl-2,4-pyridine-dicarboxylic acid, 3-ethyl-2,4-pyridinedicarboxylic acid, 3-amino-2,6-pyridinedicarboxylic acid, 3-hydroxy-2,6-pyridine-dicarboxylic acid, 3-chloro-2,6-pyridinedicarboxylic acid, 3-methyl-2,6-pyridinedicarboxylic acid, 3-ethyl-2,6-pyridine-dicarboxylic acid, 4-amino-2,6-pyridinedicarboxylic acid, 4-hydroxy-2,6-pyridinedicarboxylic acid, 4-chloro-2,6-pyridine-dicarboxylic acid 4-methyl-2,6-pyridinedicarboxylic acid, 4-ethyl-2,6-pyridinedicarboxylic acid, 4-amino-2,5-pyridine-dicarboxylic acid, 4-hydroxy-2,5-pyridinedicarboxylic acid, 4-chloro-2,5-pyridinedicarboxylic acid, 4-methyl-2,5-pyridine-dicarboxylic acid, 4-ethyl-2,5-pyridinedicarboxylic acid, 4-amino-2,3-pyridinedicarboxylic acid, 4-hydroxy-2,3-pyridine-dicarboxylic acid, 4-chloro-2,3-pyridinedicarboxylic acid, 4-methyl-2,3-pyridinedicarboxylic acid, 4-ethyl-2,3-pyridine-dicarboxylic acid, 5-amino-2,3-pyridinedi-carboxylic acid, 5-hydroxy-2,3-pyridinedicarboxylic acid, 5-chloro-2,3-pyridine-dicarboxylic acid, 5-methyl-2,3-pyridinedicarboxylic acid, 5-ethyl-2,3-pyridinedicarboxylic acid, 5-amino-2,4-pyridinedi-carboxylic acid, 5-hydroxy-2,4-pyridinedicarboxylic acid, 5-chloro-2,4-pyridinedicarboxylic acid, 5-methyl-2,4-pyridine-dicarboxylic acid, 5-ethyl-2,4-pyridinedicarboxylic acid, 5-amino-2,6-pyridinedi-carboxylic acid, 5-hydroxy-2,6-pyridine-dicarboxylic acid, 5-chloro-2,6-pyridinedicarboxylic acid, 5-methyl-2,6-pyridinedicarboxylic acid, 5-ethyl-2,6-pyridine-dicarboxylic acid as well as their salts, esters and/or amides.

(15) Most preferred pyridinedicarboxylic acids are selected from 2,6-pyridinedicarboxylic acid, 2,5-pyridinedicarboxylic acid, 2,4-pyridinedicarboxylic acid, 2,3-pyridinedicarboxylic acid, 3,5-pyridinedicarboxylic acid, 3,4-pyridinedicarboxylic acid, their salts, esters and/or amides.

(16) Suitable salts include alkali salts such as lithium, sodium or potassium salts; alkaline earth salts, such as calcium or magnesium salts; and ammonium salts. Suitable esters include alkyl esters, such as methyl, ethyl, propyl, butyl, or pentyl esters.

(17) The pyridine-dicarboxaldehyde dioximes may be unsubstituted or substituted at the pyridine ring with one or more alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl and/or hexyl groups; alkoxy groups, such as methoxy, ethoxy, propoxy, butoxy, pentoxy and/or hexoxy groups; halogen groups, such as chloro, bromo and/or iodo groups; cyano groups; amino groups; and/or hydroxyl groups or the like.

(18) Suitable pyridine-dicarboxaldehyde dioximes include pyridine-dicarboxaldehyde-2,6-dioxime, dicarboxaldehyde-2,5-dioxime, dicarboxaldehyde-2,4-dioxime, dicarboxaldehyde-2,3-dioxime, dicarboxaldehyde-3,5-dioxime and dicarboxaldehyde-3,4-dioxime. The afore-mentioned pyridine-dicarboxaldehyde dioximes may be unsubstituted or substituted with alkyl, e.g. methyl, ethyl, propyl, butyl, pentyl and/or hexyl; alkoxy, e.g. methoxy, ethoxy, propoxy, butoxy, pentoxy and/or hexoxy; halogen, e.g. chloro, bromo and/or iodo; cyano; amino; and/or hydroxyl.

(19) In an embodiment of the present invention the corrosion inhibitor composition according to the present invention is present in a coating on the magnesium metal or magnesium metal alloy. Preferably the corrosion inhibiting compounds are present in cavities of porous nano- or microparticles distributed within the coating or in micro- and nano-pores produced on the surface of magnesium or magnesium alloys.

(20) The magnesium metal or magnesium alloy is not restricted to a specific species. The corrosion inhibitors according to the present invention can be used with a large variety of different magnesium metals and alloys, e.g. high purity magnesium, (HP Mg), magnesium of commercial purity (CP Mg), magnesium alloys such as WE43, ZE41, Elektron 21, AZ31, AZ91 or AM50.

EXAMPLES

(21) Preferred embodiments of the present invention are further illustrated by the following, non-limiting examples by referring to the figures below. Magnesium materials used for hydrogen evolution measurements presented in FIGS. 1 to 8 were as specified in Table 1. The ingots of HP Mg51, WE43, ZE41, E21, AZ31, AZ91 and AM50 were shaved to receive the stripes with the surface area of 180 to 550 cm.sup.2/g. This was done to ensure the identical chemical composition of each portion of the alloy used for testing solutions of different inhibitors. The plates (5.0 cm.sup.2/g) of commercial purity magnesium (CP Mg220) were tested.

(22) TABLE-US-00001 TABLE 1 Noble impurities found* in the materials used for hydrogen evolution tests. Impurity, ppm Material Fe Cu Ni High Purity Mg (HP Mg 51) 51 <1 <2 Commercial Purity Mg (CP Mg 220 5 <2 220) WE43 38 47 46 ZE41 15 19 6 Elektron 21 (E21) 12 20 52 AZ31 18 14 3 AZ91 22 48 <2 AM50 9 13 3 *Analysed by spark emission spectroscopy

(23) Table 1: Noble impurities found* in the materials used for hydrogen evolution tests.

(24) FIG. 1 shows the results (normalized values) of hydrogen evolution measurements during immersion of HP Mg (high purity Mg containing Fe51 ppm) in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt marked as 2,3 PDCA, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt marked as 2,5 PDCA, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt marked as 2,6 PDCA, 0.038M of 3,4-pyridinedicarboylic acid sodium salt marked as 3,4 PDCA, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(25) FIG. 2 shows the results (normalized values) of hydrogen evolution measurements during immersion of CP Mg (commercial purity Mg containing Fe220 ppm) in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt, 0.038M of 3,4-pyridinedicarboxylic acid sodium salt, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(26) FIG. 3 shows the results (normalized values) of hydrogen evolution measurements during immersion of WE43 magnesium alloy in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt, 0.038M of 3,4-pyridinedicarboxylic acid sodium salt, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(27) FIG. 4 shows the results (normalized values) of hydrogen evolution measurements during immersion of ZE41 magnesium alloy in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt, 0.038M of 3,4-pyridinedicarboxylic acid sodium salt, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(28) FIG. 5 shows the results (normalized values) of hydrogen evolution measurements during immersion of Elektron 21 (E21) magnesium alloy in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt, 0.038M of 3,4-pyridinedicarboxylic acid sodium salt, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(29) FIG. 6 shows the results (normalized values) of hydrogen evolution measurements during immersion of AZ31 magnesium alloy in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt, 0.038M of 3,4-pyridinedicarboxylic acid sodium salt, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(30) FIG. 7 shows the results (normalized values) of hydrogen evolution measurements during immersion of AZ91 magnesium alloy in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt, 0.038M of 3,4-pyridinedicarboxylic acid sodium salt, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(31) FIG. 8 shows the results (normalized values) of hydrogen evolution measurements during immersion of AM50 magnesium alloy in 0.5% NaCl containing 0.05 M of 2,3-pyridinedicarboxylic acid sodium salt, 0.05M of 2,5-pyridinedicarboxylic acid sodium salt, 0.03M of 2,6-pyridinedicarboxylic acid sodium salt, 0.038M of 3,4-pyridinedicarboxylic acid sodium salt, 0.05M of 3,5-dimethylpyrazole (comparative), 0.05M of toluic acid sodium salt (comparative) and pure 0.5% NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was in the range of 6.3 to 7.3.

(32) In the examples shown in FIGS. 1 to 8 it is shown that the corrosion inhibiting compounds according to the present invention efficiently protect the magnesium and magnesium alloys from corrosion and show significantly improved corrosion inhibiting efficiencies compared to toluic acid and 3,5-dimethylpyrazole as known from U.S. Pat. No. 6,569,264 B1, US 2007/080319 A1, and EP 1 683 894 A1.

(33) As can also be seen in FIGS. 1 to 8, the corrosion inhibiting compounds according to the present invention show a corrosion inhibiting efficiency of more than 30% (in most of the cases, inhibiting efficiency is higher than 60%) that is at least 25% better than toluic acid and at least 45% better than 3,5-dimethylpyrazole known from the prior art.

(34) The values of the inhibiting efficiency (IE) were calculated using the following equation:

(35) $\mathrm{IE}=\frac{V_{H_2}^0-V_{H_2}^{\mathrm{Inh}}}{V_{H_2}^0}*100\%$
where V.sub.H.sub.2.sup.0 and V.sub.H.sub.2.sup.Inh are the amounts of H.sub.2 (ml) evolved at 20 hours of immersion in pure NaCl solution and in NaCl solution containing one of the corrosion inhibitors.

(36) TABLE-US-00002 TABLE 2 Inhibiting efficiency of sodium salts of pyridinedicarboxylic acids and comparative inhibitors known from previous state of the art. Pure magnesium or alloy Inhibitor CP Mg HP Mg WE43 ZE41 E21 AZ31 AZ91 AM50 2,3-Pyridinedicarb-oxylic 66.1 70.1 52.4 32.4 50.1 62.5 48.7 48.0 acid (0.05M) 2,5-Pyridinedicarb-oxylic 90.6 90.1 83.3 49.1 81.3 83.5 65.1 72.1 acid (0.05M) 2,6-Pyridinedicarb-oxylic 82.2 75.3 73.4 70.9 69.0 79.2 65.4 68.6 acid (0.03M) 3,4-Pyridinedicarb-oxylic 80.3 63.9 71.3 57.2 81.3 82.2 66.5 74.0 acid (0.038M) Toluic acid (0.05M) 48.6 204.1 24.9 5.6 194.4 20.9 39.1 19.5 3,5-Dimethylpyrazole 0.1 19.5 7.8 66.5 6.4 42.7 27.3 42.0 (0.05M)

(37) As it is evident from FIGS. 1 to 8, the corrosion inhibiting effect of the novel corrosion inhibiting compounds is not restricted to a specific magnesium metal or magnesium alloy, but present for a large variety of different magnesium metals and alloys, e.g. high purity magnesium, commercial impurity magnesium containing noble impurities, WE43, ZE41, Elektron 21, AZ31, AZ91 or AM50. Independently of the magnesium metal or alloy, the novel corrosion inhibiting compounds show a significantly improved corrosion inhibiting effect compared to toluic acid and 3,5-dimethylpyrazole known from the prior art.